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//! Candidate selection. See the [rustc dev guide] for more information on how this works.
//!
//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
use self::EvaluationResult::*;
use self::SelectionCandidate::*;
use super::coherence::{self, Conflict};
use super::const_evaluatable;
use super::project;
use super::project::normalize_with_depth_to;
use super::project::ProjectionTyObligation;
use super::util;
use super::util::closure_trait_ref_and_return_type;
use super::wf;
use super::{
ErrorReporting, ImplDerivedObligation, ImplDerivedObligationCause, Normalized, Obligation,
ObligationCause, ObligationCauseCode, Overflow, PolyTraitObligation, PredicateObligation,
Selection, SelectionError, SelectionResult, TraitQueryMode,
};
use crate::infer::{InferCtxt, InferOk, TypeFreshener};
use crate::solve::InferCtxtSelectExt;
use crate::traits::error_reporting::TypeErrCtxtExt;
use crate::traits::project::try_normalize_with_depth_to;
use crate::traits::project::ProjectAndUnifyResult;
use crate::traits::project::ProjectionCacheKeyExt;
use crate::traits::ProjectionCacheKey;
use crate::traits::Unimplemented;
use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_errors::Diagnostic;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::DefineOpaqueTypes;
use rustc_infer::infer::LateBoundRegionConversionTime;
use rustc_infer::traits::TraitObligation;
use rustc_middle::dep_graph::dep_kinds;
use rustc_middle::dep_graph::DepNodeIndex;
use rustc_middle::mir::interpret::ErrorHandled;
use rustc_middle::traits::DefiningAnchor;
use rustc_middle::ty::_match::MatchAgainstFreshVars;
use rustc_middle::ty::abstract_const::NotConstEvaluatable;
use rustc_middle::ty::fold::BottomUpFolder;
use rustc_middle::ty::relate::TypeRelation;
use rustc_middle::ty::GenericArgsRef;
use rustc_middle::ty::{self, EarlyBinder, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
use rustc_span::symbol::sym;
use std::cell::{Cell, RefCell};
use std::cmp;
use std::fmt::{self, Display};
use std::iter;
pub use rustc_middle::traits::select::*;
use rustc_middle::ty::print::with_no_trimmed_paths;
mod candidate_assembly;
mod confirmation;
#[derive(Clone, Debug, Eq, PartialEq, Hash)]
pub enum IntercrateAmbiguityCause {
DownstreamCrate { trait_desc: String, self_desc: Option<String> },
UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> },
ReservationImpl { message: String },
}
impl IntercrateAmbiguityCause {
/// Emits notes when the overlap is caused by complex intercrate ambiguities.
/// See #23980 for details.
pub fn add_intercrate_ambiguity_hint(&self, err: &mut Diagnostic) {
err.note(self.intercrate_ambiguity_hint());
}
pub fn intercrate_ambiguity_hint(&self) -> String {
match self {
IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc } => {
let self_desc = if let Some(ty) = self_desc {
format!(" for type `{ty}`")
} else {
String::new()
};
format!("downstream crates may implement trait `{trait_desc}`{self_desc}")
}
IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc } => {
let self_desc = if let Some(ty) = self_desc {
format!(" for type `{ty}`")
} else {
String::new()
};
format!(
"upstream crates may add a new impl of trait `{trait_desc}`{self_desc} \
in future versions"
)
}
IntercrateAmbiguityCause::ReservationImpl { message } => message.clone(),
}
}
}
pub struct SelectionContext<'cx, 'tcx> {
pub infcx: &'cx InferCtxt<'tcx>,
/// Freshener used specifically for entries on the obligation
/// stack. This ensures that all entries on the stack at one time
/// will have the same set of placeholder entries, which is
/// important for checking for trait bounds that recursively
/// require themselves.
freshener: TypeFreshener<'cx, 'tcx>,
/// If `intercrate` is set, we remember predicates which were
/// considered ambiguous because of impls potentially added in other crates.
/// This is used in coherence to give improved diagnostics.
/// We don't do his until we detect a coherence error because it can
/// lead to false overflow results (#47139) and because always
/// computing it may negatively impact performance.
intercrate_ambiguity_causes: Option<FxIndexSet<IntercrateAmbiguityCause>>,
/// The mode that trait queries run in, which informs our error handling
/// policy. In essence, canonicalized queries need their errors propagated
/// rather than immediately reported because we do not have accurate spans.
query_mode: TraitQueryMode,
treat_inductive_cycle: TreatInductiveCycleAs,
}
// A stack that walks back up the stack frame.
struct TraitObligationStack<'prev, 'tcx> {
obligation: &'prev PolyTraitObligation<'tcx>,
/// The trait predicate from `obligation` but "freshened" with the
/// selection-context's freshener. Used to check for recursion.
fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
/// Starts out equal to `depth` -- if, during evaluation, we
/// encounter a cycle, then we will set this flag to the minimum
/// depth of that cycle for all participants in the cycle. These
/// participants will then forego caching their results. This is
/// not the most efficient solution, but it addresses #60010. The
/// problem we are trying to prevent:
///
/// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
/// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
/// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
///
/// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
/// is `EvaluatedToOk`; this is because they were only considered
/// ok on the premise that if `A: AutoTrait` held, but we indeed
/// encountered a problem (later on) with `A: AutoTrait`. So we
/// currently set a flag on the stack node for `B: AutoTrait` (as
/// well as the second instance of `A: AutoTrait`) to suppress
/// caching.
///
/// This is a simple, targeted fix. A more-performant fix requires
/// deeper changes, but would permit more caching: we could
/// basically defer caching until we have fully evaluated the
/// tree, and then cache the entire tree at once. In any case, the
/// performance impact here shouldn't be so horrible: every time
/// this is hit, we do cache at least one trait, so we only
/// evaluate each member of a cycle up to N times, where N is the
/// length of the cycle. This means the performance impact is
/// bounded and we shouldn't have any terrible worst-cases.
reached_depth: Cell<usize>,
previous: TraitObligationStackList<'prev, 'tcx>,
/// The number of parent frames plus one (thus, the topmost frame has depth 1).
depth: usize,
/// The depth-first number of this node in the search graph -- a
/// pre-order index. Basically, a freshly incremented counter.
dfn: usize,
}
struct SelectionCandidateSet<'tcx> {
/// A list of candidates that definitely apply to the current
/// obligation (meaning: types unify).
vec: Vec<SelectionCandidate<'tcx>>,
/// If `true`, then there were candidates that might or might
/// not have applied, but we couldn't tell. This occurs when some
/// of the input types are type variables, in which case there are
/// various "builtin" rules that might or might not trigger.
ambiguous: bool,
}
#[derive(PartialEq, Eq, Debug, Clone)]
struct EvaluatedCandidate<'tcx> {
candidate: SelectionCandidate<'tcx>,
evaluation: EvaluationResult,
}
/// When does the builtin impl for `T: Trait` apply?
#[derive(Debug)]
enum BuiltinImplConditions<'tcx> {
/// The impl is conditional on `T1, T2, ...: Trait`.
Where(ty::Binder<'tcx, Vec<Ty<'tcx>>>),
/// There is no built-in impl. There may be some other
/// candidate (a where-clause or user-defined impl).
None,
/// It is unknown whether there is an impl.
Ambiguous,
}
#[derive(Copy, Clone)]
pub enum TreatInductiveCycleAs {
/// This is the previous behavior, where `Recur` represents an inductive
/// cycle that is known not to hold. This is not forwards-compatible with
/// coinduction, and will be deprecated. This is the default behavior
/// of the old trait solver due to back-compat reasons.
Recur,
/// This is the behavior of the new trait solver, where inductive cycles
/// are treated as ambiguous and possibly holding.
Ambig,
}
impl From<TreatInductiveCycleAs> for EvaluationResult {
fn from(treat: TreatInductiveCycleAs) -> EvaluationResult {
match treat {
TreatInductiveCycleAs::Ambig => EvaluatedToUnknown,
TreatInductiveCycleAs::Recur => EvaluatedToRecur,
}
}
}
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
SelectionContext {
infcx,
freshener: infcx.freshener(),
intercrate_ambiguity_causes: None,
query_mode: TraitQueryMode::Standard,
treat_inductive_cycle: TreatInductiveCycleAs::Recur,
}
}
// Sets the `TreatInductiveCycleAs` mode temporarily in the selection context
pub fn with_treat_inductive_cycle_as<T>(
&mut self,
treat_inductive_cycle: TreatInductiveCycleAs,
f: impl FnOnce(&mut Self) -> T,
) -> T {
// Should be executed in a context where caching is disabled,
// otherwise the cache is poisoned with the temporary result.
assert!(self.is_intercrate());
let treat_inductive_cycle =
std::mem::replace(&mut self.treat_inductive_cycle, treat_inductive_cycle);
let value = f(self);
self.treat_inductive_cycle = treat_inductive_cycle;
value
}
pub fn with_query_mode(
infcx: &'cx InferCtxt<'tcx>,
query_mode: TraitQueryMode,
) -> SelectionContext<'cx, 'tcx> {
debug!(?query_mode, "with_query_mode");
SelectionContext { query_mode, ..SelectionContext::new(infcx) }
}
/// Enables tracking of intercrate ambiguity causes. See
/// the documentation of [`Self::intercrate_ambiguity_causes`] for more.
pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
assert!(self.is_intercrate());
assert!(self.intercrate_ambiguity_causes.is_none());
self.intercrate_ambiguity_causes = Some(FxIndexSet::default());
debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
}
/// Gets the intercrate ambiguity causes collected since tracking
/// was enabled and disables tracking at the same time. If
/// tracking is not enabled, just returns an empty vector.
pub fn take_intercrate_ambiguity_causes(&mut self) -> FxIndexSet<IntercrateAmbiguityCause> {
assert!(self.is_intercrate());
self.intercrate_ambiguity_causes.take().unwrap_or_default()
}
pub fn tcx(&self) -> TyCtxt<'tcx> {
self.infcx.tcx
}
pub fn is_intercrate(&self) -> bool {
self.infcx.intercrate
}
///////////////////////////////////////////////////////////////////////////
// Selection
//
// The selection phase tries to identify *how* an obligation will
// be resolved. For example, it will identify which impl or
// parameter bound is to be used. The process can be inconclusive
// if the self type in the obligation is not fully inferred. Selection
// can result in an error in one of two ways:
//
// 1. If no applicable impl or parameter bound can be found.
// 2. If the output type parameters in the obligation do not match
// those specified by the impl/bound. For example, if the obligation
// is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
// `impl<T> Iterable<T> for Vec<T>`, than an error would result.
/// Attempts to satisfy the obligation. If successful, this will affect the surrounding
/// type environment by performing unification.
#[instrument(level = "debug", skip(self), ret)]
pub fn poly_select(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
) -> SelectionResult<'tcx, Selection<'tcx>> {
if self.infcx.next_trait_solver() {
return self.infcx.select_in_new_trait_solver(obligation);
}
let candidate = match self.select_from_obligation(obligation) {
Err(SelectionError::Overflow(OverflowError::Canonical)) => {
// In standard mode, overflow must have been caught and reported
// earlier.
assert!(self.query_mode == TraitQueryMode::Canonical);
return Err(SelectionError::Overflow(OverflowError::Canonical));
}
Err(e) => {
return Err(e);
}
Ok(None) => {
return Ok(None);
}
Ok(Some(candidate)) => candidate,
};
match self.confirm_candidate(obligation, candidate) {
Err(SelectionError::Overflow(OverflowError::Canonical)) => {
assert!(self.query_mode == TraitQueryMode::Canonical);
Err(SelectionError::Overflow(OverflowError::Canonical))
}
Err(e) => Err(e),
Ok(candidate) => Ok(Some(candidate)),
}
}
pub fn select(
&mut self,
obligation: &TraitObligation<'tcx>,
) -> SelectionResult<'tcx, Selection<'tcx>> {
self.poly_select(&Obligation {
cause: obligation.cause.clone(),
param_env: obligation.param_env,
predicate: ty::Binder::dummy(obligation.predicate),
recursion_depth: obligation.recursion_depth,
})
}
fn select_from_obligation(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
debug_assert!(!obligation.predicate.has_escaping_bound_vars());
let pec = &ProvisionalEvaluationCache::default();
let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
self.candidate_from_obligation(&stack)
}
#[instrument(level = "debug", skip(self), ret)]
fn candidate_from_obligation<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
debug_assert!(!self.infcx.next_trait_solver());
// Watch out for overflow. This intentionally bypasses (and does
// not update) the cache.
self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
// Check the cache. Note that we freshen the trait-ref
// separately rather than using `stack.fresh_trait_ref` --
// this is because we want the unbound variables to be
// replaced with fresh types starting from index 0.
let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
debug!(?cache_fresh_trait_pred);
debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
if let Some(c) =
self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
{
debug!("CACHE HIT");
return c;
}
// If no match, compute result and insert into cache.
//
// FIXME(nikomatsakis) -- this cache is not taking into
// account cycles that may have occurred in forming the
// candidate. I don't know of any specific problems that
// result but it seems awfully suspicious.
let (candidate, dep_node) =
self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
debug!("CACHE MISS");
self.insert_candidate_cache(
stack.obligation.param_env,
cache_fresh_trait_pred,
dep_node,
candidate.clone(),
);
candidate
}
fn candidate_from_obligation_no_cache<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
if let Err(conflict) = self.is_knowable(stack) {
debug!("coherence stage: not knowable");
if self.intercrate_ambiguity_causes.is_some() {
debug!("evaluate_stack: intercrate_ambiguity_causes is some");
// Heuristics: show the diagnostics when there are no candidates in crate.
if let Ok(candidate_set) = self.assemble_candidates(stack) {
let mut no_candidates_apply = true;
for c in candidate_set.vec.iter() {
if self.evaluate_candidate(stack, &c)?.may_apply() {
no_candidates_apply = false;
break;
}
}
if !candidate_set.ambiguous && no_candidates_apply {
let trait_ref = self.infcx.resolve_vars_if_possible(
stack.obligation.predicate.skip_binder().trait_ref,
);
if !trait_ref.references_error() {
let self_ty = trait_ref.self_ty();
let (trait_desc, self_desc) = with_no_trimmed_paths!({
let trait_desc = trait_ref.print_only_trait_path().to_string();
let self_desc =
self_ty.has_concrete_skeleton().then(|| self_ty.to_string());
(trait_desc, self_desc)
});
let cause = if let Conflict::Upstream = conflict {
IntercrateAmbiguityCause::UpstreamCrateUpdate {
trait_desc,
self_desc,
}
} else {
IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
};
debug!(?cause, "evaluate_stack: pushing cause");
self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
}
}
}
}
return Ok(None);
}
let candidate_set = self.assemble_candidates(stack)?;
if candidate_set.ambiguous {
debug!("candidate set contains ambig");
return Ok(None);
}
let candidates = candidate_set.vec;
debug!(?stack, ?candidates, "assembled {} candidates", candidates.len());
// At this point, we know that each of the entries in the
// candidate set is *individually* applicable. Now we have to
// figure out if they contain mutual incompatibilities. This
// frequently arises if we have an unconstrained input type --
// for example, we are looking for `$0: Eq` where `$0` is some
// unconstrained type variable. In that case, we'll get a
// candidate which assumes $0 == int, one that assumes `$0 ==
// usize`, etc. This spells an ambiguity.
let mut candidates = self.filter_impls(candidates, stack.obligation);
// If there is more than one candidate, first winnow them down
// by considering extra conditions (nested obligations and so
// forth). We don't winnow if there is exactly one
// candidate. This is a relatively minor distinction but it
// can lead to better inference and error-reporting. An
// example would be if there was an impl:
//
// impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
//
// and we were to see some code `foo.push_clone()` where `boo`
// is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
// we were to winnow, we'd wind up with zero candidates.
// Instead, we select the right impl now but report "`Bar` does
// not implement `Clone`".
if candidates.len() == 1 {
return self.filter_reservation_impls(candidates.pop().unwrap(), stack.obligation);
}
// Winnow, but record the exact outcome of evaluation, which
// is needed for specialization. Propagate overflow if it occurs.
let mut candidates = candidates
.into_iter()
.map(|c| match self.evaluate_candidate(stack, &c) {
Ok(eval) if eval.may_apply() => {
Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
}
Ok(_) => Ok(None),
Err(OverflowError::Canonical) => Err(Overflow(OverflowError::Canonical)),
Err(OverflowError::ErrorReporting) => Err(ErrorReporting),
Err(OverflowError::Error(e)) => Err(Overflow(OverflowError::Error(e))),
})
.flat_map(Result::transpose)
.collect::<Result<Vec<_>, _>>()?;
debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
let has_non_region_infer = stack.obligation.predicate.has_non_region_infer();
// If there are STILL multiple candidates, we can further
// reduce the list by dropping duplicates -- including
// resolving specializations.
if candidates.len() > 1 {
let mut i = 0;
while i < candidates.len() {
let should_drop_i = (0..candidates.len()).filter(|&j| i != j).any(|j| {
self.candidate_should_be_dropped_in_favor_of(
&candidates[i],
&candidates[j],
has_non_region_infer,
) == DropVictim::Yes
});
if should_drop_i {
debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
candidates.swap_remove(i);
} else {
debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
i += 1;
// If there are *STILL* multiple candidates, give up
// and report ambiguity.
if i > 1 {
debug!("multiple matches, ambig");
return Ok(None);
}
}
}
}
// If there are *NO* candidates, then there are no impls --
// that we know of, anyway. Note that in the case where there
// are unbound type variables within the obligation, it might
// be the case that you could still satisfy the obligation
// from another crate by instantiating the type variables with
// a type from another crate that does have an impl. This case
// is checked for in `evaluate_stack` (and hence users
// who might care about this case, like coherence, should use
// that function).
if candidates.is_empty() {
// If there's an error type, 'downgrade' our result from
// `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
// emitting additional spurious errors, since we're guaranteed
// to have emitted at least one.
if stack.obligation.predicate.references_error() {
debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous");
return Ok(None);
}
return Err(Unimplemented);
}
// Just one candidate left.
self.filter_reservation_impls(candidates.pop().unwrap().candidate, stack.obligation)
}
///////////////////////////////////////////////////////////////////////////
// EVALUATION
//
// Tests whether an obligation can be selected or whether an impl
// can be applied to particular types. It skips the "confirmation"
// step and hence completely ignores output type parameters.
//
// The result is "true" if the obligation *may* hold and "false" if
// we can be sure it does not.
/// Evaluates whether the obligation `obligation` can be satisfied
/// and returns an `EvaluationResult`. This is meant for the
/// *initial* call.
///
/// Do not use this directly, use `infcx.evaluate_obligation` instead.
pub fn evaluate_root_obligation(
&mut self,
obligation: &PredicateObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug_assert!(!self.infcx.next_trait_solver());
self.evaluation_probe(|this| {
let goal =
this.infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
let mut result = this.evaluate_predicate_recursively(
TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
obligation.clone(),
)?;
// If the predicate has done any inference, then downgrade the
// result to ambiguous.
if this.infcx.shallow_resolve(goal) != goal {
result = result.max(EvaluatedToAmbig);
}
Ok(result)
})
}
fn evaluation_probe(
&mut self,
op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
) -> Result<EvaluationResult, OverflowError> {
self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
let outer_universe = self.infcx.universe();
let result = op(self)?;
match self.infcx.leak_check(outer_universe, Some(snapshot)) {
Ok(()) => {}
Err(_) => return Ok(EvaluatedToErr),
}
if self.infcx.opaque_types_added_in_snapshot(snapshot) {
return Ok(result.max(EvaluatedToOkModuloOpaqueTypes));
}
if self.infcx.region_constraints_added_in_snapshot(snapshot) {
Ok(result.max(EvaluatedToOkModuloRegions))
} else {
Ok(result)
}
})
}
/// Evaluates the predicates in `predicates` recursively. Note that
/// this applies projections in the predicates, and therefore
/// is run within an inference probe.
#[instrument(skip(self, stack), level = "debug")]
fn evaluate_predicates_recursively<'o, I>(
&mut self,
stack: TraitObligationStackList<'o, 'tcx>,
predicates: I,
) -> Result<EvaluationResult, OverflowError>
where
I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
{
let mut result = EvaluatedToOk;
for mut obligation in predicates {
obligation.set_depth_from_parent(stack.depth());
let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
if let EvaluatedToErr = eval {
// fast-path - EvaluatedToErr is the top of the lattice,
// so we don't need to look on the other predicates.
return Ok(EvaluatedToErr);
} else {
result = cmp::max(result, eval);
}
}
Ok(result)
}
#[instrument(
level = "debug",
skip(self, previous_stack),
fields(previous_stack = ?previous_stack.head())
ret,
)]
fn evaluate_predicate_recursively<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
obligation: PredicateObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug_assert!(!self.infcx.next_trait_solver());
// `previous_stack` stores a `PolyTraitObligation`, while `obligation` is
// a `PredicateObligation`. These are distinct types, so we can't
// use any `Option` combinator method that would force them to be
// the same.
match previous_stack.head() {
Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
None => self.check_recursion_limit(&obligation, &obligation)?,
}
ensure_sufficient_stack(|| {
let bound_predicate = obligation.predicate.kind();
match bound_predicate.skip_binder() {
ty::PredicateKind::Clause(ty::ClauseKind::Trait(t)) => {
let t = bound_predicate.rebind(t);
debug_assert!(!t.has_escaping_bound_vars());
let obligation = obligation.with(self.tcx(), t);
self.evaluate_trait_predicate_recursively(previous_stack, obligation)
}
ty::PredicateKind::Subtype(p) => {
let p = bound_predicate.rebind(p);
// Does this code ever run?
match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
Ok(Ok(InferOk { obligations, .. })) => {
self.evaluate_predicates_recursively(previous_stack, obligations)
}
Ok(Err(_)) => Ok(EvaluatedToErr),
Err(..) => Ok(EvaluatedToAmbig),
}
}
ty::PredicateKind::Coerce(p) => {
let p = bound_predicate.rebind(p);
// Does this code ever run?
match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
Ok(Ok(InferOk { obligations, .. })) => {
self.evaluate_predicates_recursively(previous_stack, obligations)
}
Ok(Err(_)) => Ok(EvaluatedToErr),
Err(..) => Ok(EvaluatedToAmbig),
}
}
ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
// So, there is a bit going on here. First, `WellFormed` predicates
// are coinductive, like trait predicates with auto traits.
// This means that we need to detect if we have recursively
// evaluated `WellFormed(X)`. Otherwise, we would run into
// a "natural" overflow error.
//
// Now, the next question is whether we need to do anything
// special with caching. Considering the following tree:
// - `WF(Foo<T>)`
// - `Bar<T>: Send`
// - `WF(Foo<T>)`
// - `Foo<T>: Trait`
// In this case, the innermost `WF(Foo<T>)` should return
// `EvaluatedToOk`, since it's coinductive. Then if
// `Bar<T>: Send` is resolved to `EvaluatedToOk`, it can be
// inserted into a cache (because without thinking about `WF`
// goals, it isn't in a cycle). If `Foo<T>: Trait` later doesn't
// hold, then `Bar<T>: Send` shouldn't hold. Therefore, we
// *do* need to keep track of coinductive cycles.
let cache = previous_stack.cache;
let dfn = cache.next_dfn();
for stack_arg in previous_stack.cache.wf_args.borrow().iter().rev() {
if stack_arg.0 != arg {
continue;
}
debug!("WellFormed({:?}) on stack", arg);
if let Some(stack) = previous_stack.head {
// Okay, let's imagine we have two different stacks:
// `T: NonAutoTrait -> WF(T) -> T: NonAutoTrait`
// `WF(T) -> T: NonAutoTrait -> WF(T)`
// Because of this, we need to check that all
// predicates between the WF goals are coinductive.
// Otherwise, we can say that `T: NonAutoTrait` is
// true.
// Let's imagine we have a predicate stack like
// `Foo: Bar -> WF(T) -> T: NonAutoTrait -> T: Auto`
// depth ^1 ^2 ^3
// and the current predicate is `WF(T)`. `wf_args`
// would contain `(T, 1)`. We want to check all
// trait predicates greater than `1`. The previous
// stack would be `T: Auto`.
let cycle = stack.iter().take_while(|s| s.depth > stack_arg.1);
let tcx = self.tcx();
let cycle =
cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
if self.coinductive_match(cycle) {
stack.update_reached_depth(stack_arg.1);
return Ok(EvaluatedToOk);
} else {
return Ok(self.treat_inductive_cycle.into());
}
}
return Ok(EvaluatedToOk);
}
match wf::obligations(
self.infcx,
obligation.param_env,
obligation.cause.body_id,
obligation.recursion_depth + 1,
arg,
obligation.cause.span,
) {
Some(obligations) => {
cache.wf_args.borrow_mut().push((arg, previous_stack.depth()));
let result =
self.evaluate_predicates_recursively(previous_stack, obligations);
cache.wf_args.borrow_mut().pop();
let result = result?;
if !result.must_apply_modulo_regions() {
cache.on_failure(dfn);
}
cache.on_completion(dfn);
Ok(result)
}
None => Ok(EvaluatedToAmbig),
}
}
ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(pred)) => {
// A global type with no free lifetimes or generic parameters
// outlives anything.
if pred.0.has_free_regions()
|| pred.0.has_late_bound_regions()
|| pred.0.has_non_region_infer()
|| pred.0.has_non_region_infer()
{
Ok(EvaluatedToOkModuloRegions)
} else {
Ok(EvaluatedToOk)
}
}
ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(..)) => {
// We do not consider region relationships when evaluating trait matches.
Ok(EvaluatedToOkModuloRegions)
}
ty::PredicateKind::ObjectSafe(trait_def_id) => {
if self.tcx().check_is_object_safe(trait_def_id) {
Ok(EvaluatedToOk)
} else {
Ok(EvaluatedToErr)
}
}
ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) => {
let data = bound_predicate.rebind(data);
let project_obligation = obligation.with(self.tcx(), data);
match project::poly_project_and_unify_type(self, &project_obligation) {
ProjectAndUnifyResult::Holds(mut subobligations) => {
'compute_res: {
// If we've previously marked this projection as 'complete', then
// use the final cached result (either `EvaluatedToOk` or
// `EvaluatedToOkModuloRegions`), and skip re-evaluating the
// sub-obligations.
if let Some(key) =
ProjectionCacheKey::from_poly_projection_predicate(self, data)
{
if let Some(cached_res) = self
.infcx
.inner
.borrow_mut()
.projection_cache()
.is_complete(key)
{
break 'compute_res Ok(cached_res);
}
}
// Need to explicitly set the depth of nested goals here as
// projection obligations can cycle by themselves and in
// `evaluate_predicates_recursively` we only add the depth
// for parent trait goals because only these get added to the
// `TraitObligationStackList`.
for subobligation in subobligations.iter_mut() {
subobligation.set_depth_from_parent(obligation.recursion_depth);
}
let res = self.evaluate_predicates_recursively(
previous_stack,
subobligations,
);
if let Ok(eval_rslt) = res
&& (eval_rslt == EvaluatedToOk || eval_rslt == EvaluatedToOkModuloRegions)
&& let Some(key) =
ProjectionCacheKey::from_poly_projection_predicate(
self, data,
)
{
// If the result is something that we can cache, then mark this
// entry as 'complete'. This will allow us to skip evaluating the
// subobligations at all the next time we evaluate the projection
// predicate.
self.infcx
.inner
.borrow_mut()
.projection_cache()
.complete(key, eval_rslt);
}
res
}
}
ProjectAndUnifyResult::FailedNormalization => Ok(EvaluatedToAmbig),
ProjectAndUnifyResult::Recursive => Ok(self.treat_inductive_cycle.into()),
ProjectAndUnifyResult::MismatchedProjectionTypes(_) => Ok(EvaluatedToErr),
}
}
ty::PredicateKind::ClosureKind(_, closure_args, kind) => {
match self.infcx.closure_kind(closure_args) {
Some(closure_kind) => {
if closure_kind.extends(kind) {
Ok(EvaluatedToOk)
} else {
Ok(EvaluatedToErr)
}
}
None => Ok(EvaluatedToAmbig),
}
}
ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(uv)) => {
match const_evaluatable::is_const_evaluatable(
self.infcx,
uv,
obligation.param_env,
obligation.cause.span,
) {
Ok(()) => Ok(EvaluatedToOk),
Err(NotConstEvaluatable::MentionsInfer) => Ok(EvaluatedToAmbig),
Err(NotConstEvaluatable::MentionsParam) => Ok(EvaluatedToErr),
Err(_) => Ok(EvaluatedToErr),
}
}
ty::PredicateKind::ConstEquate(c1, c2) => {
let tcx = self.tcx();
assert!(
tcx.features().generic_const_exprs,
"`ConstEquate` without a feature gate: {c1:?} {c2:?}",
);
{
let c1 = tcx.expand_abstract_consts(c1);
let c2 = tcx.expand_abstract_consts(c2);
debug!(
"evaluate_predicate_recursively: equating consts:\nc1= {:?}\nc2= {:?}",
c1, c2
);
use rustc_hir::def::DefKind;
use ty::Unevaluated;
match (c1.kind(), c2.kind()) {
(Unevaluated(a), Unevaluated(b))
if a.def == b.def && tcx.def_kind(a.def) == DefKind::AssocConst =>
{
if let Ok(InferOk { obligations, value: () }) = self
.infcx
.at(&obligation.cause, obligation.param_env)
.trace(c1, c2)
.eq(DefineOpaqueTypes::No, a.args, b.args)
{
return self.evaluate_predicates_recursively(
previous_stack,
obligations,
);
}
}
(_, Unevaluated(_)) | (Unevaluated(_), _) => (),
(_, _) => {
if let Ok(InferOk { obligations, value: () }) = self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(DefineOpaqueTypes::No, c1, c2)
{
return self.evaluate_predicates_recursively(
previous_stack,
obligations,
);
}
}
}
}
let evaluate = |c: ty::Const<'tcx>| {
if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
match self.infcx.try_const_eval_resolve(
obligation.param_env,
unevaluated,
c.ty(),
Some(obligation.cause.span),
) {
Ok(val) => Ok(val),
Err(e) => Err(e),
}
} else {
Ok(c)
}
};
match (evaluate(c1), evaluate(c2)) {
(Ok(c1), Ok(c2)) => {
match self.infcx.at(&obligation.cause, obligation.param_env).eq(
DefineOpaqueTypes::No,
c1,
c2,
) {
Ok(inf_ok) => self.evaluate_predicates_recursively(
previous_stack,
inf_ok.into_obligations(),
),
Err(_) => Ok(EvaluatedToErr),
}
}
(Err(ErrorHandled::Reported(..)), _)
| (_, Err(ErrorHandled::Reported(..))) => Ok(EvaluatedToErr),
(Err(ErrorHandled::TooGeneric(..)), _)
| (_, Err(ErrorHandled::TooGeneric(..))) => {
if c1.has_non_region_infer() || c2.has_non_region_infer() {
Ok(EvaluatedToAmbig)
} else {
// Two different constants using generic parameters ~> error.
Ok(EvaluatedToErr)
}
}
}
}
ty::PredicateKind::AliasRelate(..)
if matches!(self.infcx.defining_use_anchor, DefiningAnchor::Bubble) =>
{
Ok(EvaluatedToAmbig)
}
ty::PredicateKind::AliasRelate(a, b, relate) => match relate {
ty::AliasRelationDirection::Equate => match self
.infcx
.at(&obligation.cause, obligation.param_env)
.eq(DefineOpaqueTypes::Yes, a, b)
{
Ok(inf_ok) => self.evaluate_predicates_recursively(
previous_stack,
inf_ok.into_obligations(),
),
Err(_) => Ok(EvaluatedToErr),
},
ty::AliasRelationDirection::Subtype => {
bug!("AliasRelate subtyping is only used for new solver")
}
},
ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
match self.infcx.at(&obligation.cause, obligation.param_env).eq(
DefineOpaqueTypes::No,
ct.ty(),
ty,
) {
Ok(inf_ok) => self.evaluate_predicates_recursively(
previous_stack,
inf_ok.into_obligations(),
),
Err(_) => Ok(EvaluatedToErr),
}
}
}
})
}
#[instrument(skip(self, previous_stack), level = "debug", ret)]
fn evaluate_trait_predicate_recursively<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
mut obligation: PolyTraitObligation<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
if !self.is_intercrate()
&& obligation.is_global()
&& obligation.param_env.caller_bounds().iter().all(|bound| bound.has_param())
{
// If a param env has no global bounds, global obligations do not
// depend on its particular value in order to work, so we can clear
// out the param env and get better caching.
debug!("in global");
obligation.param_env = obligation.param_env.without_caller_bounds();
}
let stack = self.push_stack(previous_stack, &obligation);
let fresh_trait_pred = stack.fresh_trait_pred;
let param_env = obligation.param_env;
debug!(?fresh_trait_pred);
// If a trait predicate is in the (local or global) evaluation cache,
// then we know it holds without cycles.
if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
debug!("CACHE HIT");
return Ok(result);
}
if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
debug!("PROVISIONAL CACHE HIT");
stack.update_reached_depth(result.reached_depth);
return Ok(result.result);
}
// Check if this is a match for something already on the
// stack. If so, we don't want to insert the result into the
// main cache (it is cycle dependent) nor the provisional
// cache (which is meant for things that have completed but
// for a "backedge" -- this result *is* the backedge).
if let Some(cycle_result) = self.check_evaluation_cycle(&stack) {
return Ok(cycle_result);
}
let (result, dep_node) = self.in_task(|this| {
let mut result = this.evaluate_stack(&stack)?;
// fix issue #103563, we don't normalize
// nested obligations which produced by `TraitDef` candidate
// (i.e. using bounds on assoc items as assumptions).
// because we don't have enough information to
// normalize these obligations before evaluating.
// so we will try to normalize the obligation and evaluate again.
// we will replace it with new solver in the future.
if EvaluationResult::EvaluatedToErr == result
&& fresh_trait_pred.has_projections()
&& fresh_trait_pred.is_global()
{
let mut nested_obligations = Vec::new();
let predicate = try_normalize_with_depth_to(
this,
param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.predicate,
&mut nested_obligations,
);
if predicate != obligation.predicate {
let mut nested_result = EvaluationResult::EvaluatedToOk;
for obligation in nested_obligations {
nested_result = cmp::max(
this.evaluate_predicate_recursively(previous_stack, obligation)?,
nested_result,
);
}
if nested_result.must_apply_modulo_regions() {
let obligation = obligation.with(this.tcx(), predicate);
result = cmp::max(
nested_result,
this.evaluate_trait_predicate_recursively(previous_stack, obligation)?,
);
}
}
}
Ok::<_, OverflowError>(result)
});
let result = result?;
if !result.must_apply_modulo_regions() {
stack.cache().on_failure(stack.dfn);
}
let reached_depth = stack.reached_depth.get();
if reached_depth >= stack.depth {
debug!("CACHE MISS");
self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
stack.cache().on_completion(stack.dfn);
} else {
debug!("PROVISIONAL");
debug!(
"caching provisionally because {:?} \
is a cycle participant (at depth {}, reached depth {})",
fresh_trait_pred, stack.depth, reached_depth,
);
stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
}
Ok(result)
}
/// If there is any previous entry on the stack that precisely
/// matches this obligation, then we can assume that the
/// obligation is satisfied for now (still all other conditions
/// must be met of course). One obvious case this comes up is
/// marker traits like `Send`. Think of a linked list:
///
/// struct List<T> { data: T, next: Option<Box<List<T>>> }
///
/// `Box<List<T>>` will be `Send` if `T` is `Send` and
/// `Option<Box<List<T>>>` is `Send`, and in turn
/// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
/// `Send`.
///
/// Note that we do this comparison using the `fresh_trait_ref`
/// fields. Because these have all been freshened using
/// `self.freshener`, we can be sure that (a) this will not
/// affect the inferencer state and (b) that if we see two
/// fresh regions with the same index, they refer to the same
/// unbound type variable.
fn check_evaluation_cycle(
&mut self,
stack: &TraitObligationStack<'_, 'tcx>,
) -> Option<EvaluationResult> {
if let Some(cycle_depth) = stack
.iter()
.skip(1) // Skip top-most frame.
.find(|prev| {
stack.obligation.param_env == prev.obligation.param_env
&& stack.fresh_trait_pred == prev.fresh_trait_pred
})
.map(|stack| stack.depth)
{
debug!("evaluate_stack --> recursive at depth {}", cycle_depth);
// If we have a stack like `A B C D E A`, where the top of
// the stack is the final `A`, then this will iterate over
// `A, E, D, C, B` -- i.e., all the participants apart
// from the cycle head. We mark them as participating in a
// cycle. This suppresses caching for those nodes. See
// `in_cycle` field for more details.
stack.update_reached_depth(cycle_depth);
// Subtle: when checking for a coinductive cycle, we do
// not compare using the "freshened trait refs" (which
// have erased regions) but rather the fully explicit
// trait refs. This is important because it's only a cycle
// if the regions match exactly.
let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
let tcx = self.tcx();
let cycle = cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
if self.coinductive_match(cycle) {
debug!("evaluate_stack --> recursive, coinductive");
Some(EvaluatedToOk)
} else {
debug!("evaluate_stack --> recursive, inductive");
Some(self.treat_inductive_cycle.into())
}
} else {
None
}
}
fn evaluate_stack<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> Result<EvaluationResult, OverflowError> {
debug_assert!(!self.infcx.next_trait_solver());
// In intercrate mode, whenever any of the generics are unbound,
// there can always be an impl. Even if there are no impls in
// this crate, perhaps the type would be unified with
// something from another crate that does provide an impl.
//
// In intra mode, we must still be conservative. The reason is
// that we want to avoid cycles. Imagine an impl like:
//
// impl<T:Eq> Eq for Vec<T>
//
// and a trait reference like `$0 : Eq` where `$0` is an
// unbound variable. When we evaluate this trait-reference, we
// will unify `$0` with `Vec<$1>` (for some fresh variable
// `$1`), on the condition that `$1 : Eq`. We will then wind
// up with many candidates (since that are other `Eq` impls
// that apply) and try to winnow things down. This results in
// a recursive evaluation that `$1 : Eq` -- as you can
// imagine, this is just where we started. To avoid that, we
// check for unbound variables and return an ambiguous (hence possible)
// match if we've seen this trait before.
//
// This suffices to allow chains like `FnMut` implemented in
// terms of `Fn` etc, but we could probably make this more
// precise still.
let unbound_input_types =
stack.fresh_trait_pred.skip_binder().trait_ref.args.types().any(|ty| ty.is_fresh());
if unbound_input_types
&& stack.iter().skip(1).any(|prev| {
stack.obligation.param_env == prev.obligation.param_env
&& self.match_fresh_trait_refs(
stack.fresh_trait_pred,
prev.fresh_trait_pred,
prev.obligation.param_env,
)
})
{
debug!("evaluate_stack --> unbound argument, recursive --> giving up",);
return Ok(EvaluatedToUnknown);
}
match self.candidate_from_obligation(stack) {
Ok(Some(c)) => self.evaluate_candidate(stack, &c),
Ok(None) => Ok(EvaluatedToAmbig),
Err(Overflow(OverflowError::Canonical)) => Err(OverflowError::Canonical),
Err(ErrorReporting) => Err(OverflowError::ErrorReporting),
Err(..) => Ok(EvaluatedToErr),
}
}
/// For defaulted traits, we use a co-inductive strategy to solve, so
/// that recursion is ok. This routine returns `true` if the top of the
/// stack (`cycle[0]`):
///
/// - is a defaulted trait,
/// - it also appears in the backtrace at some position `X`,
/// - all the predicates at positions `X..` between `X` and the top are
/// also defaulted traits.
pub(crate) fn coinductive_match<I>(&mut self, mut cycle: I) -> bool
where
I: Iterator<Item = ty::Predicate<'tcx>>,
{
cycle.all(|predicate| predicate.is_coinductive(self.tcx()))
}
/// Further evaluates `candidate` to decide whether all type parameters match and whether nested
/// obligations are met. Returns whether `candidate` remains viable after this further
/// scrutiny.
#[instrument(
level = "debug",
skip(self, stack),
fields(depth = stack.obligation.recursion_depth),
ret
)]
fn evaluate_candidate<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
candidate: &SelectionCandidate<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
let mut result = self.evaluation_probe(|this| {
let candidate = (*candidate).clone();
match this.confirm_candidate(stack.obligation, candidate) {
Ok(selection) => {
debug!(?selection);
this.evaluate_predicates_recursively(
stack.list(),
selection.nested_obligations().into_iter(),
)
}
Err(..) => Ok(EvaluatedToErr),
}
})?;
// If we erased any lifetimes, then we want to use
// `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
// as your final result. The result will be cached using
// the freshened trait predicate as a key, so we need
// our result to be correct by *any* choice of original lifetimes,
// not just the lifetime choice for this particular (non-erased)
// predicate.
// See issue #80691
if stack.fresh_trait_pred.has_erased_regions() {
result = result.max(EvaluatedToOkModuloRegions);
}
Ok(result)
}
fn check_evaluation_cache(
&self,
param_env: ty::ParamEnv<'tcx>,
trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> Option<EvaluationResult> {
// Neither the global nor local cache is aware of intercrate
// mode, so don't do any caching. In particular, we might
// re-use the same `InferCtxt` with both an intercrate
// and non-intercrate `SelectionContext`
if self.is_intercrate() {
return None;
}
let tcx = self.tcx();
if self.can_use_global_caches(param_env) {
if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) {
return Some(res);
}
}
self.infcx.evaluation_cache.get(&(param_env, trait_pred), tcx)
}
fn insert_evaluation_cache(
&mut self,
param_env: ty::ParamEnv<'tcx>,
trait_pred: ty::PolyTraitPredicate<'tcx>,
dep_node: DepNodeIndex,
result: EvaluationResult,
) {
// Avoid caching results that depend on more than just the trait-ref
// - the stack can create recursion.
if result.is_stack_dependent() {
return;
}
// Neither the global nor local cache is aware of intercrate
// mode, so don't do any caching. In particular, we might
// re-use the same `InferCtxt` with both an intercrate
// and non-intercrate `SelectionContext`
if self.is_intercrate() {
return;
}
if self.can_use_global_caches(param_env) {
if !trait_pred.has_infer() {
debug!(?trait_pred, ?result, "insert_evaluation_cache global");
// This may overwrite the cache with the same value
// FIXME: Due to #50507 this overwrites the different values
// This should be changed to use HashMapExt::insert_same
// when that is fixed
self.tcx().evaluation_cache.insert((param_env, trait_pred), dep_node, result);
return;
}
}
debug!(?trait_pred, ?result, "insert_evaluation_cache");
self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
}
fn check_recursion_depth<T>(
&self,
depth: usize,
error_obligation: &Obligation<'tcx, T>,
) -> Result<(), OverflowError>
where
T: ToPredicate<'tcx> + Clone,
{
if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
match self.query_mode {
TraitQueryMode::Standard => {
if let Some(e) = self.infcx.tainted_by_errors() {
return Err(OverflowError::Error(e));
}
self.infcx.err_ctxt().report_overflow_obligation(error_obligation, true);
}
TraitQueryMode::Canonical => {
return Err(OverflowError::Canonical);
}
}
}
Ok(())
}
/// Checks that the recursion limit has not been exceeded.
///
/// The weird return type of this function allows it to be used with the `try` (`?`)
/// operator within certain functions.
#[inline(always)]
fn check_recursion_limit<T: Display + TypeFoldable<TyCtxt<'tcx>>, V>(
&self,
obligation: &Obligation<'tcx, T>,
error_obligation: &Obligation<'tcx, V>,
) -> Result<(), OverflowError>
where
V: ToPredicate<'tcx> + Clone,
{
self.check_recursion_depth(obligation.recursion_depth, error_obligation)
}
fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
where
OP: FnOnce(&mut Self) -> R,
{
let (result, dep_node) =
self.tcx().dep_graph.with_anon_task(self.tcx(), dep_kinds::TraitSelect, || op(self));
self.tcx().dep_graph.read_index(dep_node);
(result, dep_node)
}
/// filter_impls filters candidates that have a positive impl for a negative
/// goal and a negative impl for a positive goal
#[instrument(level = "debug", skip(self, candidates))]
fn filter_impls(
&mut self,
candidates: Vec<SelectionCandidate<'tcx>>,
obligation: &PolyTraitObligation<'tcx>,
) -> Vec<SelectionCandidate<'tcx>> {
trace!("{candidates:#?}");
let tcx = self.tcx();
let mut result = Vec::with_capacity(candidates.len());
for candidate in candidates {
if let ImplCandidate(def_id) = candidate {
if ty::ImplPolarity::Reservation == tcx.impl_polarity(def_id)
|| obligation.polarity() == tcx.impl_polarity(def_id)
{
result.push(candidate);
}
} else {
result.push(candidate);
}
}
trace!("{result:#?}");
result
}
/// filter_reservation_impls filter reservation impl for any goal as ambiguous
#[instrument(level = "debug", skip(self))]
fn filter_reservation_impls(
&mut self,
candidate: SelectionCandidate<'tcx>,
obligation: &PolyTraitObligation<'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
let tcx = self.tcx();
// Treat reservation impls as ambiguity.
if let ImplCandidate(def_id) = candidate {
if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
let value = tcx
.get_attr(def_id, sym::rustc_reservation_impl)
.and_then(|a| a.value_str());
if let Some(value) = value {
debug!(
"filter_reservation_impls: \
reservation impl ambiguity on {:?}",
def_id
);
intercrate_ambiguity_clauses.insert(
IntercrateAmbiguityCause::ReservationImpl {
message: value.to_string(),
},
);
}
}
return Ok(None);
}
}
Ok(Some(candidate))
}
fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
debug!("is_knowable(intercrate={:?})", self.is_intercrate());
if !self.is_intercrate() {
return Ok(());
}
let obligation = &stack.obligation;
let predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
// Okay to skip binder because of the nature of the
// trait-ref-is-knowable check, which does not care about
// bound regions.
let trait_ref = predicate.skip_binder().trait_ref;
coherence::trait_ref_is_knowable::<!>(self.tcx(), trait_ref, |ty| Ok(ty)).unwrap()
}
/// Returns `true` if the global caches can be used.
fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
// If there are any inference variables in the `ParamEnv`, then we
// always use a cache local to this particular scope. Otherwise, we
// switch to a global cache.
if param_env.has_infer() {
return false;
}
// Avoid using the master cache during coherence and just rely
// on the local cache. This effectively disables caching
// during coherence. It is really just a simplification to
// avoid us having to fear that coherence results "pollute"
// the master cache. Since coherence executes pretty quickly,
// it's not worth going to more trouble to increase the
// hit-rate, I don't think.
if self.is_intercrate() {
return false;
}
// Otherwise, we can use the global cache.
true
}
fn check_candidate_cache(
&mut self,
param_env: ty::ParamEnv<'tcx>,
cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
// Neither the global nor local cache is aware of intercrate
// mode, so don't do any caching. In particular, we might
// re-use the same `InferCtxt` with both an intercrate
// and non-intercrate `SelectionContext`
if self.is_intercrate() {
return None;
}
let tcx = self.tcx();
let pred = cache_fresh_trait_pred.skip_binder();
if self.can_use_global_caches(param_env) {
if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
return Some(res);
}
}
self.infcx.selection_cache.get(&(param_env, pred), tcx)
}
/// Determines whether can we safely cache the result
/// of selecting an obligation. This is almost always `true`,
/// except when dealing with certain `ParamCandidate`s.
///
/// Ordinarily, a `ParamCandidate` will contain no inference variables,
/// since it was usually produced directly from a `DefId`. However,
/// certain cases (currently only librustdoc's blanket impl finder),
/// a `ParamEnv` may be explicitly constructed with inference types.
/// When this is the case, we do *not* want to cache the resulting selection
/// candidate. This is due to the fact that it might not always be possible
/// to equate the obligation's trait ref and the candidate's trait ref,
/// if more constraints end up getting added to an inference variable.
///
/// Because of this, we always want to re-run the full selection
/// process for our obligation the next time we see it, since
/// we might end up picking a different `SelectionCandidate` (or none at all).
fn can_cache_candidate(
&self,
result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>,
) -> bool {
// Neither the global nor local cache is aware of intercrate
// mode, so don't do any caching. In particular, we might
// re-use the same `InferCtxt` with both an intercrate
// and non-intercrate `SelectionContext`
if self.is_intercrate() {
return false;
}
match result {
Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.has_infer(),
_ => true,
}
}
#[instrument(skip(self, param_env, cache_fresh_trait_pred, dep_node), level = "debug")]
fn insert_candidate_cache(
&mut self,
param_env: ty::ParamEnv<'tcx>,
cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
dep_node: DepNodeIndex,
candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
) {
let tcx = self.tcx();
let pred = cache_fresh_trait_pred.skip_binder();
if !self.can_cache_candidate(&candidate) {
debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
return;
}
if self.can_use_global_caches(param_env) {
if let Err(Overflow(OverflowError::Canonical)) = candidate {
// Don't cache overflow globally; we only produce this in certain modes.
} else if !pred.has_infer() {
if !candidate.has_infer() {
debug!(?pred, ?candidate, "insert_candidate_cache global");
// This may overwrite the cache with the same value.
tcx.selection_cache.insert((param_env, pred), dep_node, candidate);
return;
}
}
}
debug!(?pred, ?candidate, "insert_candidate_cache local");
self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
}
/// Matches a predicate against the bounds of its self type.
///
/// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
/// a projection, look at the bounds of `T::Bar`, see if we can find a
/// `Baz` bound. We return indexes into the list returned by
/// `tcx.item_bounds` for any applicable bounds.
#[instrument(level = "debug", skip(self), ret)]
fn match_projection_obligation_against_definition_bounds(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
) -> smallvec::SmallVec<[usize; 2]> {
let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
let placeholder_trait_predicate =
self.infcx.instantiate_binder_with_placeholders(poly_trait_predicate);
debug!(?placeholder_trait_predicate);
let tcx = self.infcx.tcx;
let (def_id, args) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
ty::Alias(ty::Projection | ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
(def_id, args)
}
_ => {
span_bug!(
obligation.cause.span,
"match_projection_obligation_against_definition_bounds() called \
but self-ty is not a projection: {:?}",
placeholder_trait_predicate.trait_ref.self_ty()
);
}
};
let bounds = tcx.item_bounds(def_id).instantiate(tcx, args);
// The bounds returned by `item_bounds` may contain duplicates after
// normalization, so try to deduplicate when possible to avoid
// unnecessary ambiguity.
let mut distinct_normalized_bounds = FxHashSet::default();
bounds
.iter()
.enumerate()
.filter_map(|(idx, bound)| {
let bound_predicate = bound.kind();
if let ty::ClauseKind::Trait(pred) = bound_predicate.skip_binder() {
let bound = bound_predicate.rebind(pred.trait_ref);
if self.infcx.probe(|_| {
match self.match_normalize_trait_ref(
obligation,
bound,
placeholder_trait_predicate.trait_ref,
) {
Ok(None) => true,
Ok(Some(normalized_trait))
if distinct_normalized_bounds.insert(normalized_trait) =>
{
true
}
_ => false,
}
}) {
return Some(idx);
}
}
None
})
.collect()
}
/// Equates the trait in `obligation` with trait bound. If the two traits
/// can be equated and the normalized trait bound doesn't contain inference
/// variables or placeholders, the normalized bound is returned.
fn match_normalize_trait_ref(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
trait_bound: ty::PolyTraitRef<'tcx>,
placeholder_trait_ref: ty::TraitRef<'tcx>,
) -> Result<Option<ty::PolyTraitRef<'tcx>>, ()> {
debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
if placeholder_trait_ref.def_id != trait_bound.def_id() {
// Avoid unnecessary normalization
return Err(());
}
let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
project::normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
trait_bound,
)
});
self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(DefineOpaqueTypes::No, ty::Binder::dummy(placeholder_trait_ref), trait_bound)
.map(|InferOk { obligations: _, value: () }| {
// This method is called within a probe, so we can't have
// inference variables and placeholders escape.
if !trait_bound.has_infer() && !trait_bound.has_placeholders() {
Some(trait_bound)
} else {
None
}
})
.map_err(|_| ())
}
fn where_clause_may_apply<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<EvaluationResult, OverflowError> {
self.evaluation_probe(|this| {
match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
Err(()) => Ok(EvaluatedToErr),
}
})
}
/// Return `Yes` if the obligation's predicate type applies to the env_predicate, and
/// `No` if it does not. Return `Ambiguous` in the case that the projection type is a GAT,
/// and applying this env_predicate constrains any of the obligation's GAT substitutions.
///
/// This behavior is a somewhat of a hack to prevent over-constraining inference variables
/// in cases like #91762.
pub(super) fn match_projection_projections(
&mut self,
obligation: &ProjectionTyObligation<'tcx>,
env_predicate: PolyProjectionPredicate<'tcx>,
potentially_unnormalized_candidates: bool,
) -> ProjectionMatchesProjection {
let mut nested_obligations = Vec::new();
let infer_predicate = self.infcx.instantiate_binder_with_fresh_vars(
obligation.cause.span,
LateBoundRegionConversionTime::HigherRankedType,
env_predicate,
);
let infer_projection = if potentially_unnormalized_candidates {
ensure_sufficient_stack(|| {
project::normalize_with_depth_to(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
infer_predicate.projection_ty,
&mut nested_obligations,
)
})
} else {
infer_predicate.projection_ty
};
let is_match = self
.infcx
.at(&obligation.cause, obligation.param_env)
.sup(DefineOpaqueTypes::No, obligation.predicate, infer_projection)
.is_ok_and(|InferOk { obligations, value: () }| {
self.evaluate_predicates_recursively(
TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
nested_obligations.into_iter().chain(obligations),
)
.is_ok_and(|res| res.may_apply())
});
if is_match {
let generics = self.tcx().generics_of(obligation.predicate.def_id);
// FIXME(generic-associated-types): Addresses aggressive inference in #92917.
// If this type is a GAT, and of the GAT args resolve to something new,
// that means that we must have newly inferred something about the GAT.
// We should give up in that case.
if !generics.params.is_empty()
&& obligation.predicate.args[generics.parent_count..]
.iter()
.any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
{
ProjectionMatchesProjection::Ambiguous
} else {
ProjectionMatchesProjection::Yes
}
} else {
ProjectionMatchesProjection::No
}
}
}
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
enum DropVictim {
Yes,
No,
}
impl DropVictim {
fn drop_if(should_drop: bool) -> DropVictim {
if should_drop { DropVictim::Yes } else { DropVictim::No }
}
}
/// ## Winnowing
///
/// Winnowing is the process of attempting to resolve ambiguity by
/// probing further. During the winnowing process, we unify all
/// type variables and then we also attempt to evaluate recursive
/// bounds to see if they are satisfied.
impl<'tcx> SelectionContext<'_, 'tcx> {
/// Returns `DropVictim::Yes` if `victim` should be dropped in favor of
/// `other`. Generally speaking we will drop duplicate
/// candidates and prefer where-clause candidates.
///
/// See the comment for "SelectionCandidate" for more details.
#[instrument(level = "debug", skip(self))]
fn candidate_should_be_dropped_in_favor_of(
&mut self,
victim: &EvaluatedCandidate<'tcx>,
other: &EvaluatedCandidate<'tcx>,
has_non_region_infer: bool,
) -> DropVictim {
if victim.candidate == other.candidate {
return DropVictim::Yes;
}
// Check if a bound would previously have been removed when normalizing
// the param_env so that it can be given the lowest priority. See
// #50825 for the motivation for this.
let is_global =
|cand: &ty::PolyTraitPredicate<'tcx>| cand.is_global() && !cand.has_late_bound_vars();
// (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
// `DiscriminantKindCandidate`, `ConstDestructCandidate`
// to anything else.
//
// This is a fix for #53123 and prevents winnowing from accidentally extending the
// lifetime of a variable.
match (&other.candidate, &victim.candidate) {
// FIXME(@jswrenn): this should probably be more sophisticated
(TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => DropVictim::No,
// (*)
(BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => {
DropVictim::Yes
}
(_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => {
DropVictim::No
}
(ParamCandidate(other), ParamCandidate(victim)) => {
let same_except_bound_vars = other.skip_binder().trait_ref
== victim.skip_binder().trait_ref
&& other.skip_binder().polarity == victim.skip_binder().polarity
&& !other.skip_binder().trait_ref.has_escaping_bound_vars();
if same_except_bound_vars {
// See issue #84398. In short, we can generate multiple ParamCandidates which are
// the same except for unused bound vars. Just pick the one with the fewest bound vars
// or the current one if tied (they should both evaluate to the same answer). This is
// probably best characterized as a "hack", since we might prefer to just do our
// best to *not* create essentially duplicate candidates in the first place.
DropVictim::drop_if(other.bound_vars().len() <= victim.bound_vars().len())
} else {
DropVictim::No
}
}
// Drop otherwise equivalent non-const fn pointer candidates
(FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => DropVictim::Yes,
(
ParamCandidate(ref other_cand),
ImplCandidate(..)
| AutoImplCandidate
| ClosureCandidate { .. }
| GeneratorCandidate
| FutureCandidate
| FnPointerCandidate { .. }
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { .. }
| TraitAliasCandidate
| ObjectCandidate(_)
| ProjectionCandidate(..),
) => {
// We have a where clause so don't go around looking
// for impls. Arbitrarily give param candidates priority
// over projection and object candidates.
//
// Global bounds from the where clause should be ignored
// here (see issue #50825).
DropVictim::drop_if(!is_global(other_cand))
}
(ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref victim_cand)) => {
// Prefer these to a global where-clause bound
// (see issue #50825).
if is_global(victim_cand) { DropVictim::Yes } else { DropVictim::No }
}
(
ImplCandidate(_)
| AutoImplCandidate
| ClosureCandidate { .. }
| GeneratorCandidate
| FutureCandidate
| FnPointerCandidate { .. }
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { has_nested: true }
| TraitAliasCandidate,
ParamCandidate(ref victim_cand),
) => {
// Prefer these to a global where-clause bound
// (see issue #50825).
DropVictim::drop_if(
is_global(victim_cand) && other.evaluation.must_apply_modulo_regions(),
)
}
(ProjectionCandidate(i, _), ProjectionCandidate(j, _))
| (ObjectCandidate(i), ObjectCandidate(j)) => {
// Arbitrarily pick the lower numbered candidate for backwards
// compatibility reasons. Don't let this affect inference.
DropVictim::drop_if(i < j && !has_non_region_infer)
}
(ObjectCandidate(_), ProjectionCandidate(..))
| (ProjectionCandidate(..), ObjectCandidate(_)) => {
bug!("Have both object and projection candidate")
}
// Arbitrarily give projection and object candidates priority.
(
ObjectCandidate(_) | ProjectionCandidate(..),
ImplCandidate(..)
| AutoImplCandidate
| ClosureCandidate { .. }
| GeneratorCandidate
| FutureCandidate
| FnPointerCandidate { .. }
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { .. }
| TraitAliasCandidate,
) => DropVictim::Yes,
(
ImplCandidate(..)
| AutoImplCandidate
| ClosureCandidate { .. }
| GeneratorCandidate
| FutureCandidate
| FnPointerCandidate { .. }
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { .. }
| TraitAliasCandidate,
ObjectCandidate(_) | ProjectionCandidate(..),
) => DropVictim::No,
(&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
// See if we can toss out `victim` based on specialization.
// While this requires us to know *for sure* that the `other` impl applies
// we still use modulo regions here.
//
// This is fine as specialization currently assumes that specializing
// impls have to be always applicable, meaning that the only allowed
// region constraints may be constraints also present on the default impl.
let tcx = self.tcx();
if other.evaluation.must_apply_modulo_regions() {
if tcx.specializes((other_def, victim_def)) {
return DropVictim::Yes;
}
}
match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
// For #33140 the impl headers must be exactly equal, the trait must not have
// any associated items and there are no where-clauses.
//
// We can just arbitrarily drop one of the impls.
Some(ty::ImplOverlapKind::Issue33140) => {
assert_eq!(other.evaluation, victim.evaluation);
DropVictim::Yes
}
// For candidates which already reference errors it doesn't really
// matter what we do 🤷
Some(ty::ImplOverlapKind::Permitted { marker: false }) => {
DropVictim::drop_if(other.evaluation.must_apply_considering_regions())
}
Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
// Subtle: If the predicate we are evaluating has inference
// variables, do *not* allow discarding candidates due to
// marker trait impls.
//
// Without this restriction, we could end up accidentally
// constraining inference variables based on an arbitrarily
// chosen trait impl.
//
// Imagine we have the following code:
//
// ```rust
// #[marker] trait MyTrait {}
// impl MyTrait for u8 {}
// impl MyTrait for bool {}
// ```
//
// And we are evaluating the predicate `<_#0t as MyTrait>`.
//
// During selection, we will end up with one candidate for each
// impl of `MyTrait`. If we were to discard one impl in favor
// of the other, we would be left with one candidate, causing
// us to "successfully" select the predicate, unifying
// _#0t with (for example) `u8`.
//
// However, we have no reason to believe that this unification
// is correct - we've essentially just picked an arbitrary
// *possibility* for _#0t, and required that this be the *only*
// possibility.
//
// Eventually, we will either:
// 1) Unify all inference variables in the predicate through
// some other means (e.g. type-checking of a function). We will
// then be in a position to drop marker trait candidates
// without constraining inference variables (since there are
// none left to constrain)
// 2) Be left with some unconstrained inference variables. We
// will then correctly report an inference error, since the
// existence of multiple marker trait impls tells us nothing
// about which one should actually apply.
DropVictim::drop_if(
!has_non_region_infer
&& other.evaluation.must_apply_considering_regions(),
)
}
None => DropVictim::No,
}
}
(AutoImplCandidate, ImplCandidate(_)) | (ImplCandidate(_), AutoImplCandidate) => {
DropVictim::No
}
(AutoImplCandidate, _) | (_, AutoImplCandidate) => {
bug!(
"default implementations shouldn't be recorded \
when there are other global candidates: {:?} {:?}",
other,
victim
);
}
// Everything else is ambiguous
(
ImplCandidate(_)
| ClosureCandidate { .. }
| GeneratorCandidate
| FutureCandidate
| FnPointerCandidate { .. }
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { has_nested: true }
| TraitAliasCandidate,
ImplCandidate(_)
| ClosureCandidate { .. }
| GeneratorCandidate
| FutureCandidate
| FnPointerCandidate { .. }
| BuiltinObjectCandidate
| BuiltinUnsizeCandidate
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { has_nested: true }
| TraitAliasCandidate,
) => DropVictim::No,
}
}
}
impl<'tcx> SelectionContext<'_, 'tcx> {
fn sized_conditions(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
) -> BuiltinImplConditions<'tcx> {
use self::BuiltinImplConditions::{Ambiguous, None, Where};
// NOTE: binder moved to (*)
let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
match self_ty.kind() {
ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Uint(_)
| ty::Int(_)
| ty::Bool
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::RawPtr(..)
| ty::Char
| ty::Ref(..)
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Array(..)
| ty::Closure(..)
| ty::Never
| ty::Dynamic(_, _, ty::DynStar)
| ty::Error(_) => {
// safe for everything
Where(ty::Binder::dummy(Vec::new()))
}
ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
ty::Tuple(tys) => Where(
obligation.predicate.rebind(tys.last().map_or_else(Vec::new, |&last| vec![last])),
),
ty::Adt(def, args) => {
let sized_crit = def.sized_constraint(self.tcx());
// (*) binder moved here
Where(
obligation
.predicate
.rebind(sized_crit.iter_instantiated(self.tcx(), args).collect()),
)
}
ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) => None,
ty::Infer(ty::TyVar(_)) => Ambiguous,
// We can make this an ICE if/once we actually instantiate the trait obligation eagerly.
ty::Bound(..) => None,
ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
}
}
}
fn copy_clone_conditions(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
) -> BuiltinImplConditions<'tcx> {
// NOTE: binder moved to (*)
let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
use self::BuiltinImplConditions::{Ambiguous, None, Where};
match *self_ty.kind() {
ty::FnDef(..) | ty::FnPtr(_) | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
ty::Uint(_)
| ty::Int(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Bool
| ty::Float(_)
| ty::Char
| ty::RawPtr(..)
| ty::Never
| ty::Ref(_, _, hir::Mutability::Not)
| ty::Array(..) => {
// Implementations provided in libcore
None
}
ty::Dynamic(..)
| ty::Str
| ty::Slice(..)
| ty::Generator(_, _, hir::Movability::Static)
| ty::Foreign(..)
| ty::Ref(_, _, hir::Mutability::Mut) => None,
ty::Tuple(tys) => {
// (*) binder moved here
Where(obligation.predicate.rebind(tys.iter().collect()))
}
ty::Generator(_, args, hir::Movability::Movable) => {
if self.tcx().features().generator_clone {
let resolved_upvars =
self.infcx.shallow_resolve(args.as_generator().tupled_upvars_ty());
let resolved_witness =
self.infcx.shallow_resolve(args.as_generator().witness());
if resolved_upvars.is_ty_var() || resolved_witness.is_ty_var() {
// Not yet resolved.
Ambiguous
} else {
let all = args
.as_generator()
.upvar_tys()
.iter()
.chain([args.as_generator().witness()])
.collect::<Vec<_>>();
Where(obligation.predicate.rebind(all))
}
} else {
None
}
}
ty::GeneratorWitness(def_id, ref args) => {
let hidden_types = bind_generator_hidden_types_above(
self.infcx,
def_id,
args,
obligation.predicate.bound_vars(),
);
Where(hidden_types)
}
ty::Closure(_, args) => {
// (*) binder moved here
let ty = self.infcx.shallow_resolve(args.as_closure().tupled_upvars_ty());
if let ty::Infer(ty::TyVar(_)) = ty.kind() {
// Not yet resolved.
Ambiguous
} else {
Where(obligation.predicate.rebind(args.as_closure().upvar_tys().to_vec()))
}
}
ty::Adt(..) | ty::Alias(..) | ty::Param(..) | ty::Placeholder(..) => {
// Fallback to whatever user-defined impls exist in this case.
None
}
ty::Infer(ty::TyVar(_)) => {
// Unbound type variable. Might or might not have
// applicable impls and so forth, depending on what
// those type variables wind up being bound to.
Ambiguous
}
// We can make this an ICE if/once we actually instantiate the trait obligation eagerly.
ty::Bound(..) => None,
ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
}
}
}
/// For default impls, we need to break apart a type into its
/// "constituent types" -- meaning, the types that it contains.
///
/// Here are some (simple) examples:
///
/// ```ignore (illustrative)
/// (i32, u32) -> [i32, u32]
/// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
/// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
/// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
/// ```
#[instrument(level = "debug", skip(self), ret)]
fn constituent_types_for_ty(
&self,
t: ty::Binder<'tcx, Ty<'tcx>>,
) -> Result<ty::Binder<'tcx, Vec<Ty<'tcx>>>, SelectionError<'tcx>> {
Ok(match *t.skip_binder().kind() {
ty::Uint(_)
| ty::Int(_)
| ty::Bool
| ty::Float(_)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Error(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Never
| ty::Char => ty::Binder::dummy(Vec::new()),
// Treat this like `struct str([u8]);`
ty::Str => ty::Binder::dummy(vec![Ty::new_slice(self.tcx(), self.tcx().types.u8)]),
ty::Placeholder(..)
| ty::Dynamic(..)
| ty::Param(..)
| ty::Foreign(..)
| ty::Alias(ty::Projection | ty::Inherent | ty::Weak, ..)
| ty::Bound(..)
| ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!("asked to assemble constituent types of unexpected type: {:?}", t);
}
ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
t.rebind(vec![element_ty])
}
ty::Array(element_ty, _) | ty::Slice(element_ty) => t.rebind(vec![element_ty]),
ty::Tuple(ref tys) => {
// (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
t.rebind(tys.iter().collect())
}
ty::Closure(_, ref args) => {
let ty = self.infcx.shallow_resolve(args.as_closure().tupled_upvars_ty());
t.rebind(vec![ty])
}
ty::Generator(_, ref args, _) => {
let ty = self.infcx.shallow_resolve(args.as_generator().tupled_upvars_ty());
let witness = args.as_generator().witness();
t.rebind([ty].into_iter().chain(iter::once(witness)).collect())
}
ty::GeneratorWitness(def_id, ref args) => {
bind_generator_hidden_types_above(self.infcx, def_id, args, t.bound_vars())
}
// For `PhantomData<T>`, we pass `T`.
ty::Adt(def, args) if def.is_phantom_data() => t.rebind(args.types().collect()),
ty::Adt(def, args) => {
t.rebind(def.all_fields().map(|f| f.ty(self.tcx(), args)).collect())
}
ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => {
// We can resolve the `impl Trait` to its concrete type,
// which enforces a DAG between the functions requiring
// the auto trait bounds in question.
match self.tcx().type_of_opaque(def_id) {
Ok(ty) => t.rebind(vec![ty.instantiate(self.tcx(), args)]),
Err(_) => {
return Err(SelectionError::OpaqueTypeAutoTraitLeakageUnknown(def_id));
}
}
}
})
}
fn collect_predicates_for_types(
&mut self,
param_env: ty::ParamEnv<'tcx>,
cause: ObligationCause<'tcx>,
recursion_depth: usize,
trait_def_id: DefId,
types: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
) -> Vec<PredicateObligation<'tcx>> {
// Because the types were potentially derived from
// higher-ranked obligations they may reference late-bound
// regions. For example, `for<'a> Foo<&'a i32> : Copy` would
// yield a type like `for<'a> &'a i32`. In general, we
// maintain the invariant that we never manipulate bound
// regions, so we have to process these bound regions somehow.
//
// The strategy is to:
//
// 1. Instantiate those regions to placeholder regions (e.g.,
// `for<'a> &'a i32` becomes `&0 i32`.
// 2. Produce something like `&'0 i32 : Copy`
// 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
types
.as_ref()
.skip_binder() // binder moved -\
.iter()
.flat_map(|ty| {
let ty: ty::Binder<'tcx, Ty<'tcx>> = types.rebind(*ty); // <----/
let placeholder_ty = self.infcx.instantiate_binder_with_placeholders(ty);
let Normalized { value: normalized_ty, mut obligations } =
ensure_sufficient_stack(|| {
project::normalize_with_depth(
self,
param_env,
cause.clone(),
recursion_depth,
placeholder_ty,
)
});
let obligation = Obligation::new(
self.tcx(),
cause.clone(),
param_env,
ty::TraitRef::new(self.tcx(), trait_def_id, [normalized_ty]),
);
obligations.push(obligation);
obligations
})
.collect()
}
///////////////////////////////////////////////////////////////////////////
// Matching
//
// Matching is a common path used for both evaluation and
// confirmation. It basically unifies types that appear in impls
// and traits. This does affect the surrounding environment;
// therefore, when used during evaluation, match routines must be
// run inside of a `probe()` so that their side-effects are
// contained.
fn rematch_impl(
&mut self,
impl_def_id: DefId,
obligation: &PolyTraitObligation<'tcx>,
) -> Normalized<'tcx, GenericArgsRef<'tcx>> {
let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
match self.match_impl(impl_def_id, impl_trait_ref, obligation) {
Ok(args) => args,
Err(()) => {
// FIXME: A rematch may fail when a candidate cache hit occurs
// on the freshened form of the trait predicate, but the match
// fails for some reason that is not captured in the freshened
// cache key. For example, equating an impl trait ref against
// the placeholder trait ref may fail due the Generalizer relation
// raising a CyclicalTy error due to a sub_root_var relation
// for a variable being generalized...
let guar = self.infcx.tcx.sess.delay_span_bug(
obligation.cause.span,
format!(
"Impl {impl_def_id:?} was matchable against {obligation:?} but now is not"
),
);
let value = self.infcx.fresh_args_for_item(obligation.cause.span, impl_def_id);
let err = Ty::new_error(self.tcx(), guar);
let value = value.fold_with(&mut BottomUpFolder {
tcx: self.tcx(),
ty_op: |_| err,
lt_op: |l| l,
ct_op: |c| c,
});
Normalized { value, obligations: vec![] }
}
}
}
#[instrument(level = "debug", skip(self), ret)]
fn match_impl(
&mut self,
impl_def_id: DefId,
impl_trait_ref: EarlyBinder<ty::TraitRef<'tcx>>,
obligation: &PolyTraitObligation<'tcx>,
) -> Result<Normalized<'tcx, GenericArgsRef<'tcx>>, ()> {
let placeholder_obligation =
self.infcx.instantiate_binder_with_placeholders(obligation.predicate);
let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
let impl_args = self.infcx.fresh_args_for_item(obligation.cause.span, impl_def_id);
let impl_trait_ref = impl_trait_ref.instantiate(self.tcx(), impl_args);
if impl_trait_ref.references_error() {
return Err(());
}
debug!(?impl_trait_ref);
let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
ensure_sufficient_stack(|| {
project::normalize_with_depth(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
impl_trait_ref,
)
});
debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
let cause = ObligationCause::new(
obligation.cause.span,
obligation.cause.body_id,
ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
);
let InferOk { obligations, .. } = self
.infcx
.at(&cause, obligation.param_env)
.eq(DefineOpaqueTypes::No, placeholder_obligation_trait_ref, impl_trait_ref)
.map_err(|e| {
debug!("match_impl: failed eq_trait_refs due to `{}`", e.to_string(self.tcx()))
})?;
nested_obligations.extend(obligations);
if !self.is_intercrate()
&& self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
{
debug!("reservation impls only apply in intercrate mode");
return Err(());
}
Ok(Normalized { value: impl_args, obligations: nested_obligations })
}
fn match_upcast_principal(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
unnormalized_upcast_principal: ty::PolyTraitRef<'tcx>,
a_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
a_region: ty::Region<'tcx>,
b_region: ty::Region<'tcx>,
) -> SelectionResult<'tcx, Vec<PredicateObligation<'tcx>>> {
let tcx = self.tcx();
let mut nested = vec![];
let upcast_principal = normalize_with_depth_to(
self,
obligation.param_env,
obligation.cause.clone(),
obligation.recursion_depth + 1,
unnormalized_upcast_principal,
&mut nested,
);
for bound in b_data {
match bound.skip_binder() {
// Check that a_ty's supertrait (upcast_principal) is compatible
// with the target (b_ty).
ty::ExistentialPredicate::Trait(target_principal) => {
nested.extend(
self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(
DefineOpaqueTypes::No,
upcast_principal.map_bound(|trait_ref| {
ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
}),
bound.rebind(target_principal),
)
.map_err(|_| SelectionError::Unimplemented)?
.into_obligations(),
);
}
// Check that b_ty's projection is satisfied by exactly one of
// a_ty's projections. First, we look through the list to see if
// any match. If not, error. Then, if *more* than one matches, we
// return ambiguity. Otherwise, if exactly one matches, equate
// it with b_ty's projection.
ty::ExistentialPredicate::Projection(target_projection) => {
let target_projection = bound.rebind(target_projection);
let mut matching_projections =
a_data.projection_bounds().filter(|source_projection| {
// Eager normalization means that we can just use can_eq
// here instead of equating and processing obligations.
source_projection.item_def_id() == target_projection.item_def_id()
&& self.infcx.can_eq(
obligation.param_env,
*source_projection,
target_projection,
)
});
let Some(source_projection) = matching_projections.next() else {
return Err(SelectionError::Unimplemented);
};
if matching_projections.next().is_some() {
return Ok(None);
}
nested.extend(
self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(DefineOpaqueTypes::No, source_projection, target_projection)
.map_err(|_| SelectionError::Unimplemented)?
.into_obligations(),
);
}
// Check that b_ty's auto traits are present in a_ty's bounds.
ty::ExistentialPredicate::AutoTrait(def_id) => {
if !a_data.auto_traits().any(|source_def_id| source_def_id == def_id) {
return Err(SelectionError::Unimplemented);
}
}
}
}
nested.push(Obligation::with_depth(
tcx,
obligation.cause.clone(),
obligation.recursion_depth + 1,
obligation.param_env,
ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region)),
));
Ok(Some(nested))
}
/// Normalize `where_clause_trait_ref` and try to match it against
/// `obligation`. If successful, return any predicates that
/// result from the normalization.
fn match_where_clause_trait_ref(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
self.match_poly_trait_ref(obligation, where_clause_trait_ref)
}
/// Returns `Ok` if `poly_trait_ref` being true implies that the
/// obligation is satisfied.
#[instrument(skip(self), level = "debug")]
fn match_poly_trait_ref(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
poly_trait_ref: ty::PolyTraitRef<'tcx>,
) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
self.infcx
.at(&obligation.cause, obligation.param_env)
.sup(DefineOpaqueTypes::No, obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
.map(|InferOk { obligations, .. }| obligations)
.map_err(|_| ())
}
///////////////////////////////////////////////////////////////////////////
// Miscellany
fn match_fresh_trait_refs(
&self,
previous: ty::PolyTraitPredicate<'tcx>,
current: ty::PolyTraitPredicate<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> bool {
let mut matcher = MatchAgainstFreshVars::new(self.tcx(), param_env);
matcher.relate(previous, current).is_ok()
}
fn push_stack<'o>(
&mut self,
previous_stack: TraitObligationStackList<'o, 'tcx>,
obligation: &'o PolyTraitObligation<'tcx>,
) -> TraitObligationStack<'o, 'tcx> {
let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
let dfn = previous_stack.cache.next_dfn();
let depth = previous_stack.depth() + 1;
TraitObligationStack {
obligation,
fresh_trait_pred,
reached_depth: Cell::new(depth),
previous: previous_stack,
dfn,
depth,
}
}
#[instrument(skip(self), level = "debug")]
fn closure_trait_ref_unnormalized(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
args: GenericArgsRef<'tcx>,
) -> ty::PolyTraitRef<'tcx> {
let closure_sig = args.as_closure().sig();
debug!(?closure_sig);
// NOTE: The self-type is an unboxed closure type and hence is
// in fact unparameterized (or at least does not reference any
// regions bound in the obligation).
let self_ty = obligation
.predicate
.self_ty()
.no_bound_vars()
.expect("unboxed closure type should not capture bound vars from the predicate");
closure_trait_ref_and_return_type(
self.tcx(),
obligation.predicate.def_id(),
self_ty,
closure_sig,
util::TupleArgumentsFlag::No,
)
.map_bound(|(trait_ref, _)| trait_ref)
}
/// Returns the obligations that are implied by instantiating an
/// impl or trait. The obligations are substituted and fully
/// normalized. This is used when confirming an impl or default
/// impl.
#[instrument(level = "debug", skip(self, cause, param_env))]
fn impl_or_trait_obligations(
&mut self,
cause: &ObligationCause<'tcx>,
recursion_depth: usize,
param_env: ty::ParamEnv<'tcx>,
def_id: DefId, // of impl or trait
args: GenericArgsRef<'tcx>, // for impl or trait
parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
) -> Vec<PredicateObligation<'tcx>> {
let tcx = self.tcx();
// To allow for one-pass evaluation of the nested obligation,
// each predicate must be preceded by the obligations required
// to normalize it.
// for example, if we have:
// impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
// the impl will have the following predicates:
// <V as Iterator>::Item = U,
// U: Iterator, U: Sized,
// V: Iterator, V: Sized,
// <U as Iterator>::Item: Copy
// When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
// obligation will normalize to `<$0 as Iterator>::Item = $1` and
// `$1: Copy`, so we must ensure the obligations are emitted in
// that order.
let predicates = tcx.predicates_of(def_id);
assert_eq!(predicates.parent, None);
let predicates = predicates.instantiate_own(tcx, args);
let mut obligations = Vec::with_capacity(predicates.len());
for (index, (predicate, span)) in predicates.into_iter().enumerate() {
let cause =
if Some(parent_trait_pred.def_id()) == tcx.lang_items().coerce_unsized_trait() {
cause.clone()
} else {
cause.clone().derived_cause(parent_trait_pred, |derived| {
ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
derived,
impl_or_alias_def_id: def_id,
impl_def_predicate_index: Some(index),
span,
}))
})
};
let clause = normalize_with_depth_to(
self,
param_env,
cause.clone(),
recursion_depth,
predicate,
&mut obligations,
);
obligations.push(Obligation {
cause,
recursion_depth,
param_env,
predicate: clause.as_predicate(),
});
}
obligations
}
}
impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
TraitObligationStackList::with(self)
}
fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
self.previous.cache
}
fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
self.list()
}
/// Indicates that attempting to evaluate this stack entry
/// required accessing something from the stack at depth `reached_depth`.
fn update_reached_depth(&self, reached_depth: usize) {
assert!(
self.depth >= reached_depth,
"invoked `update_reached_depth` with something under this stack: \
self.depth={} reached_depth={}",
self.depth,
reached_depth,
);
debug!(reached_depth, "update_reached_depth");
let mut p = self;
while reached_depth < p.depth {
debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
p.reached_depth.set(p.reached_depth.get().min(reached_depth));
p = p.previous.head.unwrap();
}
}
}
/// The "provisional evaluation cache" is used to store intermediate cache results
/// when solving auto traits. Auto traits are unusual in that they can support
/// cycles. So, for example, a "proof tree" like this would be ok:
///
/// - `Foo<T>: Send` :-
/// - `Bar<T>: Send` :-
/// - `Foo<T>: Send` -- cycle, but ok
/// - `Baz<T>: Send`
///
/// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
/// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
/// For non-auto traits, this cycle would be an error, but for auto traits (because
/// they are coinductive) it is considered ok.
///
/// However, there is a complication: at the point where we have
/// "proven" `Bar<T>: Send`, we have in fact only proven it
/// *provisionally*. In particular, we proved that `Bar<T>: Send`
/// *under the assumption* that `Foo<T>: Send`. But what if we later
/// find out this assumption is wrong? Specifically, we could
/// encounter some kind of error proving `Baz<T>: Send`. In that case,
/// `Bar<T>: Send` didn't turn out to be true.
///
/// In Issue #60010, we found a bug in rustc where it would cache
/// these intermediate results. This was fixed in #60444 by disabling
/// *all* caching for things involved in a cycle -- in our example,
/// that would mean we don't cache that `Bar<T>: Send`. But this led
/// to large slowdowns.
///
/// Specifically, imagine this scenario, where proving `Baz<T>: Send`
/// first requires proving `Bar<T>: Send` (which is true:
///
/// - `Foo<T>: Send` :-
/// - `Bar<T>: Send` :-
/// - `Foo<T>: Send` -- cycle, but ok
/// - `Baz<T>: Send`
/// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
/// - `*const T: Send` -- but what if we later encounter an error?
///
/// The *provisional evaluation cache* resolves this issue. It stores
/// cache results that we've proven but which were involved in a cycle
/// in some way. We track the minimal stack depth (i.e., the
/// farthest from the top of the stack) that we are dependent on.
/// The idea is that the cache results within are all valid -- so long as
/// none of the nodes in between the current node and the node at that minimum
/// depth result in an error (in which case the cached results are just thrown away).
///
/// During evaluation, we consult this provisional cache and rely on
/// it. Accessing a cached value is considered equivalent to accessing
/// a result at `reached_depth`, so it marks the *current* solution as
/// provisional as well. If an error is encountered, we toss out any
/// provisional results added from the subtree that encountered the
/// error. When we pop the node at `reached_depth` from the stack, we
/// can commit all the things that remain in the provisional cache.
struct ProvisionalEvaluationCache<'tcx> {
/// next "depth first number" to issue -- just a counter
dfn: Cell<usize>,
/// Map from cache key to the provisionally evaluated thing.
/// The cache entries contain the result but also the DFN in which they
/// were added. The DFN is used to clear out values on failure.
///
/// Imagine we have a stack like:
///
/// - `A B C` and we add a cache for the result of C (DFN 2)
/// - Then we have a stack `A B D` where `D` has DFN 3
/// - We try to solve D by evaluating E: `A B D E` (DFN 4)
/// - `E` generates various cache entries which have cyclic dependencies on `B`
/// - `A B D E F` and so forth
/// - the DFN of `F` for example would be 5
/// - then we determine that `E` is in error -- we will then clear
/// all cache values whose DFN is >= 4 -- in this case, that
/// means the cached value for `F`.
map: RefCell<FxIndexMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
/// The stack of args that we assume to be true because a `WF(arg)` predicate
/// is on the stack above (and because of wellformedness is coinductive).
/// In an "ideal" world, this would share a stack with trait predicates in
/// `TraitObligationStack`. However, trait predicates are *much* hotter than
/// `WellFormed` predicates, and it's very likely that the additional matches
/// will have a perf effect. The value here is the well-formed `GenericArg`
/// and the depth of the trait predicate *above* that well-formed predicate.
wf_args: RefCell<Vec<(ty::GenericArg<'tcx>, usize)>>,
}
/// A cache value for the provisional cache: contains the depth-first
/// number (DFN) and result.
#[derive(Copy, Clone, Debug)]
struct ProvisionalEvaluation {
from_dfn: usize,
reached_depth: usize,
result: EvaluationResult,
}
impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
fn default() -> Self {
Self { dfn: Cell::new(0), map: Default::default(), wf_args: Default::default() }
}
}
impl<'tcx> ProvisionalEvaluationCache<'tcx> {
/// Get the next DFN in sequence (basically a counter).
fn next_dfn(&self) -> usize {
let result = self.dfn.get();
self.dfn.set(result + 1);
result
}
/// Check the provisional cache for any result for
/// `fresh_trait_ref`. If there is a hit, then you must consider
/// it an access to the stack slots at depth
/// `reached_depth` (from the returned value).
fn get_provisional(
&self,
fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> Option<ProvisionalEvaluation> {
debug!(
?fresh_trait_pred,
"get_provisional = {:#?}",
self.map.borrow().get(&fresh_trait_pred),
);
Some(*self.map.borrow().get(&fresh_trait_pred)?)
}
/// Insert a provisional result into the cache. The result came
/// from the node with the given DFN. It accessed a minimum depth
/// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
/// and resulted in `result`.
fn insert_provisional(
&self,
from_dfn: usize,
reached_depth: usize,
fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
result: EvaluationResult,
) {
debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
let mut map = self.map.borrow_mut();
// Subtle: when we complete working on the DFN `from_dfn`, anything
// that remains in the provisional cache must be dependent on some older
// stack entry than `from_dfn`. We have to update their depth with our transitive
// depth in that case or else it would be referring to some popped note.
//
// Example:
// A (reached depth 0)
// ...
// B // depth 1 -- reached depth = 0
// C // depth 2 -- reached depth = 1 (should be 0)
// B
// A // depth 0
// D (reached depth 1)
// C (cache -- reached depth = 2)
for (_k, v) in &mut *map {
if v.from_dfn >= from_dfn {
v.reached_depth = reached_depth.min(v.reached_depth);
}
}
map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
}
/// Invoked when the node with dfn `dfn` does not get a successful
/// result. This will clear out any provisional cache entries
/// that were added since `dfn` was created. This is because the
/// provisional entries are things which must assume that the
/// things on the stack at the time of their creation succeeded --
/// since the failing node is presently at the top of the stack,
/// these provisional entries must either depend on it or some
/// ancestor of it.
fn on_failure(&self, dfn: usize) {
debug!(?dfn, "on_failure");
self.map.borrow_mut().retain(|key, eval| {
if !eval.from_dfn >= dfn {
debug!("on_failure: removing {:?}", key);
false
} else {
true
}
});
}
/// Invoked when the node at depth `depth` completed without
/// depending on anything higher in the stack (if that completion
/// was a failure, then `on_failure` should have been invoked
/// already).
///
/// Note that we may still have provisional cache items remaining
/// in the cache when this is done. For example, if there is a
/// cycle:
///
/// * A depends on...
/// * B depends on A
/// * C depends on...
/// * D depends on C
/// * ...
///
/// Then as we complete the C node we will have a provisional cache
/// with results for A, B, C, and D. This method would clear out
/// the C and D results, but leave A and B provisional.
///
/// This is determined based on the DFN: we remove any provisional
/// results created since `dfn` started (e.g., in our example, dfn
/// would be 2, representing the C node, and hence we would
/// remove the result for D, which has DFN 3, but not the results for
/// A and B, which have DFNs 0 and 1 respectively).
///
/// Note that we *do not* attempt to cache these cycle participants
/// in the evaluation cache. Doing so would require carefully computing
/// the correct `DepNode` to store in the cache entry:
/// cycle participants may implicitly depend on query results
/// related to other participants in the cycle, due to our logic
/// which examines the evaluation stack.
///
/// We used to try to perform this caching,
/// but it lead to multiple incremental compilation ICEs
/// (see #92987 and #96319), and was very hard to understand.
/// Fortunately, removing the caching didn't seem to
/// have a performance impact in practice.
fn on_completion(&self, dfn: usize) {
debug!(?dfn, "on_completion");
self.map.borrow_mut().retain(|fresh_trait_pred, eval| {
if eval.from_dfn >= dfn {
debug!(?fresh_trait_pred, ?eval, "on_completion");
return false;
}
true
});
}
}
#[derive(Copy, Clone)]
struct TraitObligationStackList<'o, 'tcx> {
cache: &'o ProvisionalEvaluationCache<'tcx>,
head: Option<&'o TraitObligationStack<'o, 'tcx>>,
}
impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
TraitObligationStackList { cache, head: None }
}
fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
TraitObligationStackList { cache: r.cache(), head: Some(r) }
}
fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
self.head
}
fn depth(&self) -> usize {
if let Some(head) = self.head { head.depth } else { 0 }
}
}
impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
type Item = &'o TraitObligationStack<'o, 'tcx>;
fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
let o = self.head?;
*self = o.previous;
Some(o)
}
}
impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "TraitObligationStack({:?})", self.obligation)
}
}
pub enum ProjectionMatchesProjection {
Yes,
Ambiguous,
No,
}
/// Replace all regions inside the generator interior with late bound regions.
/// Note that each region slot in the types gets a new fresh late bound region, which means that
/// none of the regions inside relate to any other, even if typeck had previously found constraints
/// that would cause them to be related.
#[instrument(level = "trace", skip(infcx), ret)]
fn bind_generator_hidden_types_above<'tcx>(
infcx: &InferCtxt<'tcx>,
def_id: DefId,
args: ty::GenericArgsRef<'tcx>,
bound_vars: &ty::List<ty::BoundVariableKind>,
) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
let tcx = infcx.tcx;
let mut seen_tys = FxHashSet::default();
let considering_regions = infcx.considering_regions;
let num_bound_variables = bound_vars.len() as u32;
let mut counter = num_bound_variables;
let hidden_types: Vec<_> = tcx
.generator_hidden_types(def_id)
// Deduplicate tys to avoid repeated work.
.filter(|bty| seen_tys.insert(*bty))
.map(|mut bty| {
// Only remap erased regions if we use them.
if considering_regions {
bty = bty.map_bound(|ty| {
tcx.fold_regions(ty, |r, current_depth| match r.kind() {
ty::ReErased => {
let br = ty::BoundRegion {
var: ty::BoundVar::from_u32(counter),
kind: ty::BrAnon,
};
counter += 1;
ty::Region::new_late_bound(tcx, current_depth, br)
}
r => bug!("unexpected region: {r:?}"),
})
})
}
bty.instantiate(tcx, args)
})
.collect();
let bound_vars =
tcx.mk_bound_variable_kinds_from_iter(bound_vars.iter().chain(
(num_bound_variables..counter).map(|_| ty::BoundVariableKind::Region(ty::BrAnon)),
));
ty::Binder::bind_with_vars(hidden_types, bound_vars)
}