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//! Candidate assembly.
//!
//! The selection process begins by examining all in-scope impls,
//! caller obligations, and so forth and assembling a list of
//! candidates. See the [rustc dev guide] for more details.
//!
//! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
use hir::def_id::DefId;
use hir::LangItem;
use rustc_hir as hir;
use rustc_infer::traits::ObligationCause;
use rustc_infer::traits::{Obligation, PolyTraitObligation, SelectionError};
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
use rustc_middle::ty::{self, Ty, TypeVisitableExt};
use crate::traits;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::util;
use super::BuiltinImplConditions;
use super::SelectionCandidate::*;
use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack};
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
#[instrument(skip(self, stack), level = "debug")]
pub(super) fn assemble_candidates<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
let TraitObligationStack { obligation, .. } = *stack;
let obligation = &Obligation {
param_env: obligation.param_env,
cause: obligation.cause.clone(),
recursion_depth: obligation.recursion_depth,
predicate: self.infcx.resolve_vars_if_possible(obligation.predicate),
};
if obligation.predicate.skip_binder().self_ty().is_ty_var() {
debug!(ty = ?obligation.predicate.skip_binder().self_ty(), "ambiguous inference var or opaque type");
// Self is a type variable (e.g., `_: AsRef<str>`).
//
// This is somewhat problematic, as the current scheme can't really
// handle it turning to be a projection. This does end up as truly
// ambiguous in most cases anyway.
//
// Take the fast path out - this also improves
// performance by preventing assemble_candidates_from_impls from
// matching every impl for this trait.
return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
}
let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
// The only way to prove a NotImplemented(T: Foo) predicate is via a negative impl.
// There are no compiler built-in rules for this.
if obligation.polarity() == ty::ImplPolarity::Negative {
self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
self.assemble_candidates_from_impls(obligation, &mut candidates);
self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
} else {
self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
// Other bounds. Consider both in-scope bounds from fn decl
// and applicable impls. There is a certain set of precedence rules here.
let def_id = obligation.predicate.def_id();
let lang_items = self.tcx().lang_items();
if lang_items.copy_trait() == Some(def_id) {
debug!(obligation_self_ty = ?obligation.predicate.skip_binder().self_ty());
// User-defined copy impls are permitted, but only for
// structs and enums.
self.assemble_candidates_from_impls(obligation, &mut candidates);
// For other types, we'll use the builtin rules.
let copy_conditions = self.copy_clone_conditions(obligation);
self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates);
} else if lang_items.discriminant_kind_trait() == Some(def_id) {
// `DiscriminantKind` is automatically implemented for every type.
candidates.vec.push(BuiltinCandidate { has_nested: false });
} else if lang_items.pointee_trait() == Some(def_id) {
// `Pointee` is automatically implemented for every type.
candidates.vec.push(BuiltinCandidate { has_nested: false });
} else if lang_items.sized_trait() == Some(def_id) {
// Sized is never implementable by end-users, it is
// always automatically computed.
let sized_conditions = self.sized_conditions(obligation);
self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates);
} else if lang_items.unsize_trait() == Some(def_id) {
self.assemble_candidates_for_unsizing(obligation, &mut candidates);
} else if lang_items.destruct_trait() == Some(def_id) {
self.assemble_const_destruct_candidates(obligation, &mut candidates);
} else if lang_items.transmute_trait() == Some(def_id) {
// User-defined transmutability impls are permitted.
self.assemble_candidates_from_impls(obligation, &mut candidates);
self.assemble_candidates_for_transmutability(obligation, &mut candidates);
} else if lang_items.tuple_trait() == Some(def_id) {
self.assemble_candidate_for_tuple(obligation, &mut candidates);
} else if lang_items.pointer_like() == Some(def_id) {
self.assemble_candidate_for_pointer_like(obligation, &mut candidates);
} else if lang_items.fn_ptr_trait() == Some(def_id) {
self.assemble_candidates_for_fn_ptr_trait(obligation, &mut candidates);
} else {
if lang_items.clone_trait() == Some(def_id) {
// Same builtin conditions as `Copy`, i.e., every type which has builtin support
// for `Copy` also has builtin support for `Clone`, and tuples/arrays of `Clone`
// types have builtin support for `Clone`.
let clone_conditions = self.copy_clone_conditions(obligation);
self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates);
}
if lang_items.gen_trait() == Some(def_id) {
self.assemble_generator_candidates(obligation, &mut candidates);
} else if lang_items.future_trait() == Some(def_id) {
self.assemble_future_candidates(obligation, &mut candidates);
}
self.assemble_closure_candidates(obligation, &mut candidates);
self.assemble_fn_pointer_candidates(obligation, &mut candidates);
self.assemble_candidates_from_impls(obligation, &mut candidates);
self.assemble_candidates_from_object_ty(obligation, &mut candidates);
}
self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
self.assemble_candidates_from_auto_impls(obligation, &mut candidates);
}
debug!("candidate list size: {}", candidates.vec.len());
Ok(candidates)
}
#[instrument(level = "debug", skip(self, candidates))]
fn assemble_candidates_from_projected_tys(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// Before we go into the whole placeholder thing, just
// quickly check if the self-type is a projection at all.
match obligation.predicate.skip_binder().trait_ref.self_ty().kind() {
// Excluding IATs and type aliases here as they don't have meaningful item bounds.
ty::Alias(ty::Projection | ty::Opaque, _) => {}
ty::Infer(ty::TyVar(_)) => {
span_bug!(
obligation.cause.span,
"Self=_ should have been handled by assemble_candidates"
);
}
_ => return,
}
let result = self
.infcx
.probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));
// FIXME(effects) proper constness needed?
candidates.vec.extend(
result.into_iter().map(|idx| ProjectionCandidate(idx, ty::BoundConstness::NotConst)),
);
}
/// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
/// supplied to find out whether it is listed among them.
///
/// Never affects the inference environment.
#[instrument(level = "debug", skip(self, stack, candidates))]
fn assemble_candidates_from_caller_bounds<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
debug!(?stack.obligation);
let all_bounds = stack
.obligation
.param_env
.caller_bounds()
.iter()
.filter(|p| !p.references_error())
.filter_map(|p| p.as_trait_clause());
// Micro-optimization: filter out predicates relating to different traits.
let matching_bounds =
all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
// Keep only those bounds which may apply, and propagate overflow if it occurs.
for bound in matching_bounds {
if bound.skip_binder().polarity != stack.obligation.predicate.skip_binder().polarity {
continue;
}
// FIXME(oli-obk): it is suspicious that we are dropping the constness and
// polarity here.
let wc = self.where_clause_may_apply(stack, bound.map_bound(|t| t.trait_ref))?;
if wc.may_apply() {
candidates.vec.push(ParamCandidate(bound));
}
}
Ok(())
}
fn assemble_generator_candidates(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// Okay to skip binder because the args on generator types never
// touch bound regions, they just capture the in-scope
// type/region parameters.
let self_ty = obligation.self_ty().skip_binder();
match self_ty.kind() {
// async constructs get lowered to a special kind of generator that
// should *not* `impl Generator`.
ty::Generator(did, ..) if !self.tcx().generator_is_async(*did) => {
debug!(?self_ty, ?obligation, "assemble_generator_candidates",);
candidates.vec.push(GeneratorCandidate);
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_generator_candidates: ambiguous self-type");
candidates.ambiguous = true;
}
_ => {}
}
}
fn assemble_future_candidates(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
let self_ty = obligation.self_ty().skip_binder();
if let ty::Generator(did, ..) = self_ty.kind() {
// async constructs get lowered to a special kind of generator that
// should directly `impl Future`.
if self.tcx().generator_is_async(*did) {
debug!(?self_ty, ?obligation, "assemble_future_candidates",);
candidates.vec.push(FutureCandidate);
}
}
}
/// Checks for the artificial impl that the compiler will create for an obligation like `X :
/// FnMut<..>` where `X` is a closure type.
///
/// Note: the type parameters on a closure candidate are modeled as *output* type
/// parameters and hence do not affect whether this trait is a match or not. They will be
/// unified during the confirmation step.
fn assemble_closure_candidates(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
let Some(kind) = self.tcx().fn_trait_kind_from_def_id(obligation.predicate.def_id()) else {
return;
};
// Okay to skip binder because the args on closure types never
// touch bound regions, they just capture the in-scope
// type/region parameters
match *obligation.self_ty().skip_binder().kind() {
ty::Closure(def_id, closure_args) => {
let is_const = self.tcx().is_const_fn_raw(def_id);
debug!(?kind, ?obligation, "assemble_unboxed_candidates");
match self.infcx.closure_kind(closure_args) {
Some(closure_kind) => {
debug!(?closure_kind, "assemble_unboxed_candidates");
if closure_kind.extends(kind) {
candidates.vec.push(ClosureCandidate { is_const });
}
}
None => {
debug!("assemble_unboxed_candidates: closure_kind not yet known");
candidates.vec.push(ClosureCandidate { is_const });
}
}
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
candidates.ambiguous = true;
}
_ => {}
}
}
/// Implements one of the `Fn()` family for a fn pointer.
fn assemble_fn_pointer_candidates(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// We provide impl of all fn traits for fn pointers.
if !self.tcx().is_fn_trait(obligation.predicate.def_id()) {
return;
}
// Keep this function in sync with extract_tupled_inputs_and_output_from_callable
// until the old solver (and thus this function) is removed.
// Okay to skip binder because what we are inspecting doesn't involve bound regions.
let self_ty = obligation.self_ty().skip_binder();
match *self_ty.kind() {
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_fn_pointer_candidates: ambiguous self-type");
candidates.ambiguous = true; // Could wind up being a fn() type.
}
// Provide an impl, but only for suitable `fn` pointers.
ty::FnPtr(sig) => {
if sig.is_fn_trait_compatible() {
candidates.vec.push(FnPointerCandidate { is_const: false });
}
}
// Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396).
ty::FnDef(def_id, _) => {
if self.tcx().fn_sig(def_id).skip_binder().is_fn_trait_compatible()
&& self.tcx().codegen_fn_attrs(def_id).target_features.is_empty()
{
candidates
.vec
.push(FnPointerCandidate { is_const: self.tcx().is_const_fn(def_id) });
}
}
_ => {}
}
}
/// Searches for impls that might apply to `obligation`.
#[instrument(level = "debug", skip(self, candidates))]
fn assemble_candidates_from_impls(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// Essentially any user-written impl will match with an error type,
// so creating `ImplCandidates` isn't useful. However, we might
// end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized`)
// This helps us avoid overflow: see issue #72839
// Since compilation is already guaranteed to fail, this is just
// to try to show the 'nicest' possible errors to the user.
// We don't check for errors in the `ParamEnv` - in practice,
// it seems to cause us to be overly aggressive in deciding
// to give up searching for candidates, leading to spurious errors.
if obligation.predicate.references_error() {
return;
}
let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
let obligation_args = obligation.predicate.skip_binder().trait_ref.args;
self.tcx().for_each_relevant_impl(
obligation.predicate.def_id(),
obligation.predicate.skip_binder().trait_ref.self_ty(),
|impl_def_id| {
// Before we create the substitutions and everything, first
// consider a "quick reject". This avoids creating more types
// and so forth that we need to.
let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
if !drcx.args_refs_may_unify(obligation_args, impl_trait_ref.skip_binder().args) {
return;
}
if self.reject_fn_ptr_impls(
impl_def_id,
obligation,
impl_trait_ref.skip_binder().self_ty(),
) {
return;
}
self.infcx.probe(|_| {
if let Ok(_args) = self.match_impl(impl_def_id, impl_trait_ref, obligation) {
candidates.vec.push(ImplCandidate(impl_def_id));
}
});
},
);
}
/// The various `impl<T: FnPtr> Trait for T` in libcore are more like builtin impls for all function items
/// and function pointers and less like blanket impls. Rejecting them when they can't possibly apply (because
/// the obligation's self-type does not implement `FnPtr`) avoids reporting that the self type does not implement
/// `FnPtr`, when we wanted to report that it doesn't implement `Trait`.
#[instrument(level = "trace", skip(self), ret)]
fn reject_fn_ptr_impls(
&mut self,
impl_def_id: DefId,
obligation: &PolyTraitObligation<'tcx>,
impl_self_ty: Ty<'tcx>,
) -> bool {
// Let `impl<T: FnPtr> Trait for Vec<T>` go through the normal rejection path.
if !matches!(impl_self_ty.kind(), ty::Param(..)) {
return false;
}
let Some(fn_ptr_trait) = self.tcx().lang_items().fn_ptr_trait() else {
return false;
};
for &(predicate, _) in self.tcx().predicates_of(impl_def_id).predicates {
let ty::ClauseKind::Trait(pred) = predicate.kind().skip_binder() else { continue };
if fn_ptr_trait != pred.trait_ref.def_id {
continue;
}
trace!(?pred);
// Not the bound we're looking for
if pred.self_ty() != impl_self_ty {
continue;
}
match obligation.self_ty().skip_binder().kind() {
// Fast path to avoid evaluating an obligation that trivially holds.
// There may be more bounds, but these are checked by the regular path.
ty::FnPtr(..) => return false,
// These may potentially implement `FnPtr`
ty::Placeholder(..)
| ty::Dynamic(_, _, _)
| ty::Alias(_, _)
| ty::Infer(_)
| ty::Param(..)
| ty::Bound(_, _) => {}
// These can't possibly implement `FnPtr` as they are concrete types
// and not `FnPtr`
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Adt(_, _)
| ty::Foreign(_)
| ty::Str
| ty::Array(_, _)
| ty::Slice(_)
| ty::RawPtr(_)
| ty::Ref(_, _, _)
| ty::Closure(_, _)
| ty::Generator(_, _, _)
| ty::GeneratorWitness(..)
| ty::Never
| ty::Tuple(_)
| ty::Error(_) => return true,
// FIXME: Function definitions could actually implement `FnPtr` by
// casting the ZST function def to a function pointer.
ty::FnDef(_, _) => return true,
}
// Generic params can implement `FnPtr` if the predicate
// holds within its own environment.
let obligation = Obligation::new(
self.tcx(),
obligation.cause.clone(),
obligation.param_env,
self.tcx().mk_predicate(obligation.predicate.map_bound(|mut pred| {
pred.trait_ref =
ty::TraitRef::new(self.tcx(), fn_ptr_trait, [pred.trait_ref.self_ty()]);
ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred))
})),
);
if let Ok(r) = self.evaluate_root_obligation(&obligation) {
if !r.may_apply() {
return true;
}
}
}
false
}
fn assemble_candidates_from_auto_impls(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// Okay to skip binder here because the tests we do below do not involve bound regions.
let self_ty = obligation.self_ty().skip_binder();
debug!(?self_ty, "assemble_candidates_from_auto_impls");
let def_id = obligation.predicate.def_id();
if self.tcx().trait_is_auto(def_id) {
match self_ty.kind() {
ty::Dynamic(..) => {
// For object types, we don't know what the closed
// over types are. This means we conservatively
// say nothing; a candidate may be added by
// `assemble_candidates_from_object_ty`.
}
ty::Foreign(..) => {
// Since the contents of foreign types is unknown,
// we don't add any `..` impl. Default traits could
// still be provided by a manual implementation for
// this trait and type.
}
ty::Param(..)
| ty::Alias(ty::Projection | ty::Inherent, ..)
| ty::Placeholder(..)
| ty::Bound(..) => {
// In these cases, we don't know what the actual
// type is. Therefore, we cannot break it down
// into its constituent types. So we don't
// consider the `..` impl but instead just add no
// candidates: this means that typeck will only
// succeed if there is another reason to believe
// that this obligation holds. That could be a
// where-clause or, in the case of an object type,
// it could be that the object type lists the
// trait (e.g., `Foo+Send : Send`). See
// `ui/typeck/typeck-default-trait-impl-send-param.rs`
// for an example of a test case that exercises
// this path.
}
ty::Infer(ty::TyVar(_) | ty::IntVar(_) | ty::FloatVar(_)) => {
// The auto impl might apply; we don't know.
candidates.ambiguous = true;
}
ty::Generator(_, _, movability)
if self.tcx().lang_items().unpin_trait() == Some(def_id) =>
{
match movability {
hir::Movability::Static => {
// Immovable generators are never `Unpin`, so
// suppress the normal auto-impl candidate for it.
}
hir::Movability::Movable => {
// Movable generators are always `Unpin`, so add an
// unconditional builtin candidate.
candidates.vec.push(BuiltinCandidate { has_nested: false });
}
}
}
ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
bug!(
"asked to assemble auto trait candidates of unexpected type: {:?}",
self_ty
);
}
ty::Alias(_, _)
if candidates.vec.iter().any(|c| matches!(c, ProjectionCandidate(..))) =>
{
// We do not generate an auto impl candidate for `impl Trait`s which already
// reference our auto trait.
//
// For example during candidate assembly for `impl Send: Send`, we don't have
// to look at the constituent types for this opaque types to figure out that this
// trivially holds.
//
// Note that this is only sound as projection candidates of opaque types
// are always applicable for auto traits.
}
ty::Alias(_, _) => candidates.vec.push(AutoImplCandidate),
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Str
| ty::Array(_, _)
| ty::Slice(_)
| ty::Adt(..)
| ty::RawPtr(_)
| ty::Ref(..)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Closure(_, _)
| ty::Generator(..)
| ty::Never
| ty::Tuple(_)
| ty::GeneratorWitness(..) => {
// Only consider auto impls if there are no manual impls for the root of `self_ty`.
//
// For example, we only consider auto candidates for `&i32: Auto` if no explicit impl
// for `&SomeType: Auto` exists. Due to E0321 the only crate where impls
// for `&SomeType: Auto` can be defined is the crate where `Auto` has been defined.
//
// Generally, we have to guarantee that for all `SimplifiedType`s the only crate
// which may define impls for that type is either the crate defining the type
// or the trait. This should be guaranteed by the orphan check.
let mut has_impl = false;
self.tcx().for_each_relevant_impl(def_id, self_ty, |_| has_impl = true);
if !has_impl {
candidates.vec.push(AutoImplCandidate)
}
}
ty::Error(_) => {} // do not add an auto trait impl for `ty::Error` for now.
}
}
}
/// Searches for impls that might apply to `obligation`.
fn assemble_candidates_from_object_ty(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
debug!(
self_ty = ?obligation.self_ty().skip_binder(),
"assemble_candidates_from_object_ty",
);
if !self.tcx().trait_def(obligation.predicate.def_id()).implement_via_object {
return;
}
self.infcx.probe(|_snapshot| {
if obligation.has_non_region_late_bound() {
return;
}
// The code below doesn't care about regions, and the
// self-ty here doesn't escape this probe, so just erase
// any LBR.
let self_ty = self.tcx().erase_late_bound_regions(obligation.self_ty());
let poly_trait_ref = match self_ty.kind() {
ty::Dynamic(ref data, ..) => {
if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
debug!(
"assemble_candidates_from_object_ty: matched builtin bound, \
pushing candidate"
);
candidates.vec.push(BuiltinObjectCandidate);
return;
}
if let Some(principal) = data.principal() {
if !self.infcx.tcx.features().object_safe_for_dispatch {
principal.with_self_ty(self.tcx(), self_ty)
} else if self.tcx().check_is_object_safe(principal.def_id()) {
principal.with_self_ty(self.tcx(), self_ty)
} else {
return;
}
} else {
// Only auto trait bounds exist.
return;
}
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_candidates_from_object_ty: ambiguous");
candidates.ambiguous = true; // could wind up being an object type
return;
}
_ => return,
};
debug!(?poly_trait_ref, "assemble_candidates_from_object_ty");
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);
// Count only those upcast versions that match the trait-ref
// we are looking for. Specifically, do not only check for the
// correct trait, but also the correct type parameters.
// For example, we may be trying to upcast `Foo` to `Bar<i32>`,
// but `Foo` is declared as `trait Foo: Bar<u32>`.
let candidate_supertraits = util::supertraits(self.tcx(), poly_trait_ref)
.enumerate()
.filter(|&(_, upcast_trait_ref)| {
self.infcx.probe(|_| {
self.match_normalize_trait_ref(
obligation,
upcast_trait_ref,
placeholder_trait_predicate.trait_ref,
)
.is_ok()
})
})
.map(|(idx, _)| ObjectCandidate(idx));
candidates.vec.extend(candidate_supertraits);
})
}
/// Temporary migration for #89190
fn need_migrate_deref_output_trait_object(
&mut self,
ty: Ty<'tcx>,
param_env: ty::ParamEnv<'tcx>,
cause: &ObligationCause<'tcx>,
) -> Option<ty::PolyExistentialTraitRef<'tcx>> {
let tcx = self.tcx();
if tcx.features().trait_upcasting {
return None;
}
// <ty as Deref>
let trait_ref = ty::TraitRef::new(tcx, tcx.lang_items().deref_trait()?, [ty]);
let obligation =
traits::Obligation::new(tcx, cause.clone(), param_env, ty::Binder::dummy(trait_ref));
if !self.infcx.predicate_may_hold(&obligation) {
return None;
}
self.infcx.probe(|_| {
let ty = traits::normalize_projection_type(
self,
param_env,
tcx.mk_alias_ty(tcx.lang_items().deref_target()?, trait_ref.args),
cause.clone(),
0,
// We're *intentionally* throwing these away,
// since we don't actually use them.
&mut vec![],
)
.ty()
.unwrap();
if let ty::Dynamic(data, ..) = ty.kind() { data.principal() } else { None }
})
}
/// Searches for unsizing that might apply to `obligation`.
fn assemble_candidates_for_unsizing(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// We currently never consider higher-ranked obligations e.g.
// `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
// because they are a priori invalid, and we could potentially add support
// for them later, it's just that there isn't really a strong need for it.
// A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
// impl, and those are generally applied to concrete types.
//
// That said, one might try to write a fn with a where clause like
// for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
// where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
// Still, you'd be more likely to write that where clause as
// T: Trait
// so it seems ok if we (conservatively) fail to accept that `Unsize`
// obligation above. Should be possible to extend this in the future.
let Some(source) = obligation.self_ty().no_bound_vars() else {
// Don't add any candidates if there are bound regions.
return;
};
let target = obligation.predicate.skip_binder().trait_ref.args.type_at(1);
debug!(?source, ?target, "assemble_candidates_for_unsizing");
match (source.kind(), target.kind()) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(
&ty::Dynamic(ref a_data, a_region, ty::Dyn),
&ty::Dynamic(ref b_data, b_region, ty::Dyn),
) => {
// Upcast coercions permit several things:
//
// 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
// 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
// 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar`
//
// Note that neither of the first two of these changes requires any
// change at runtime. The third needs to change pointer metadata at runtime.
//
// We always perform upcasting coercions when we can because of reason
// #2 (region bounds).
let auto_traits_compatible = b_data
.auto_traits()
// All of a's auto traits need to be in b's auto traits.
.all(|b| a_data.auto_traits().any(|a| a == b));
if auto_traits_compatible {
let principal_def_id_a = a_data.principal_def_id();
let principal_def_id_b = b_data.principal_def_id();
if principal_def_id_a == principal_def_id_b {
// no cyclic
candidates.vec.push(BuiltinUnsizeCandidate);
} else if principal_def_id_a.is_some() && principal_def_id_b.is_some() {
// not casual unsizing, now check whether this is trait upcasting coercion.
let principal_a = a_data.principal().unwrap();
let target_trait_did = principal_def_id_b.unwrap();
let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
if let Some(deref_trait_ref) = self.need_migrate_deref_output_trait_object(
source,
obligation.param_env,
&obligation.cause,
) {
if deref_trait_ref.def_id() == target_trait_did {
return;
}
}
for (idx, upcast_trait_ref) in
util::supertraits(self.tcx(), source_trait_ref).enumerate()
{
self.infcx.probe(|_| {
if upcast_trait_ref.def_id() == target_trait_did
&& let Ok(nested) = self.match_upcast_principal(
obligation,
upcast_trait_ref,
a_data,
b_data,
a_region,
b_region,
)
{
if nested.is_none() {
candidates.ambiguous = true;
}
candidates.vec.push(TraitUpcastingUnsizeCandidate(idx));
}
})
}
}
}
}
// `T` -> `Trait`
(_, &ty::Dynamic(_, _, ty::Dyn)) => {
candidates.vec.push(BuiltinUnsizeCandidate);
}
// Ambiguous handling is below `T` -> `Trait`, because inference
// variables can still implement `Unsize<Trait>` and nested
// obligations will have the final say (likely deferred).
(&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
debug!("assemble_candidates_for_unsizing: ambiguous");
candidates.ambiguous = true;
}
// `[T; n]` -> `[T]`
(&ty::Array(..), &ty::Slice(_)) => {
candidates.vec.push(BuiltinUnsizeCandidate);
}
// `Struct<T>` -> `Struct<U>`
(&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
if def_id_a == def_id_b {
candidates.vec.push(BuiltinUnsizeCandidate);
}
}
// `(.., T)` -> `(.., U)`
(&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
if tys_a.len() == tys_b.len() {
candidates.vec.push(BuiltinUnsizeCandidate);
}
}
_ => {}
};
}
#[instrument(level = "debug", skip(self, obligation, candidates))]
fn assemble_candidates_for_transmutability(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
if obligation.predicate.has_non_region_param() {
return;
}
if obligation.has_non_region_infer() {
candidates.ambiguous = true;
return;
}
candidates.vec.push(TransmutabilityCandidate);
}
#[instrument(level = "debug", skip(self, obligation, candidates))]
fn assemble_candidates_for_trait_alias(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// Okay to skip binder here because the tests we do below do not involve bound regions.
let self_ty = obligation.self_ty().skip_binder();
debug!(?self_ty);
let def_id = obligation.predicate.def_id();
if self.tcx().is_trait_alias(def_id) {
candidates.vec.push(TraitAliasCandidate);
}
}
/// Assembles the trait which are built-in to the language itself:
/// `Copy`, `Clone` and `Sized`.
#[instrument(level = "debug", skip(self, candidates))]
fn assemble_builtin_bound_candidates(
&mut self,
conditions: BuiltinImplConditions<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
match conditions {
BuiltinImplConditions::Where(nested) => {
candidates
.vec
.push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
}
BuiltinImplConditions::None => {}
BuiltinImplConditions::Ambiguous => {
candidates.ambiguous = true;
}
}
}
fn assemble_const_destruct_candidates(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// If the predicate is `~const Destruct` in a non-const environment, we don't actually need
// to check anything. We'll short-circuit checking any obligations in confirmation, too.
// FIXME(effects)
if true {
candidates.vec.push(ConstDestructCandidate(None));
return;
}
let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
match self_ty.skip_binder().kind() {
ty::Alias(..)
| ty::Dynamic(..)
| ty::Error(_)
| ty::Bound(..)
| ty::Param(_)
| ty::Placeholder(_) => {
// We don't know if these are `~const Destruct`, at least
// not structurally... so don't push a candidate.
}
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Infer(ty::IntVar(_))
| ty::Infer(ty::FloatVar(_))
| ty::Str
| ty::RawPtr(_)
| ty::Ref(..)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Never
| ty::Foreign(_)
| ty::Array(..)
| ty::Slice(_)
| ty::Closure(..)
| ty::Generator(..)
| ty::Tuple(_)
| ty::GeneratorWitness(..) => {
// These are built-in, and cannot have a custom `impl const Destruct`.
candidates.vec.push(ConstDestructCandidate(None));
}
ty::Adt(..) => {
let mut relevant_impl = None;
self.tcx().for_each_relevant_impl(
self.tcx().require_lang_item(LangItem::Drop, None),
obligation.predicate.skip_binder().trait_ref.self_ty(),
|impl_def_id| {
if let Some(old_impl_def_id) = relevant_impl {
self.tcx()
.sess
.struct_span_err(
self.tcx().def_span(impl_def_id),
"multiple drop impls found",
)
.span_note(self.tcx().def_span(old_impl_def_id), "other impl here")
.delay_as_bug();
}
relevant_impl = Some(impl_def_id);
},
);
if let Some(impl_def_id) = relevant_impl {
// Check that `impl Drop` is actually const, if there is a custom impl
if self.tcx().constness(impl_def_id) == hir::Constness::Const {
candidates.vec.push(ConstDestructCandidate(Some(impl_def_id)));
}
} else {
// Otherwise check the ADT like a built-in type (structurally)
candidates.vec.push(ConstDestructCandidate(None));
}
}
ty::Infer(_) => {
candidates.ambiguous = true;
}
}
}
fn assemble_candidate_for_tuple(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
match self_ty.kind() {
ty::Tuple(_) => {
candidates.vec.push(BuiltinCandidate { has_nested: false });
}
ty::Infer(ty::TyVar(_)) => {
candidates.ambiguous = true;
}
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Adt(_, _)
| ty::Foreign(_)
| ty::Str
| ty::Array(_, _)
| ty::Slice(_)
| ty::RawPtr(_)
| ty::Ref(_, _, _)
| ty::FnDef(_, _)
| ty::FnPtr(_)
| ty::Dynamic(_, _, _)
| ty::Closure(_, _)
| ty::Generator(_, _, _)
| ty::GeneratorWitness(..)
| ty::Never
| ty::Alias(..)
| ty::Param(_)
| ty::Bound(_, _)
| ty::Error(_)
| ty::Infer(_)
| ty::Placeholder(_) => {}
}
}
fn assemble_candidate_for_pointer_like(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// The regions of a type don't affect the size of the type
let tcx = self.tcx();
let self_ty = tcx.erase_late_bound_regions(obligation.predicate.self_ty());
// We should erase regions from both the param-env and type, since both
// may have infer regions. Specifically, after canonicalizing and instantiating,
// early bound regions turn into region vars in both the new and old solver.
let key = tcx.erase_regions(obligation.param_env.and(self_ty));
// But if there are inference variables, we have to wait until it's resolved.
if key.has_non_region_infer() {
candidates.ambiguous = true;
return;
}
if let Ok(layout) = tcx.layout_of(key)
&& layout.layout.is_pointer_like(&tcx.data_layout)
{
candidates.vec.push(BuiltinCandidate { has_nested: false });
}
}
fn assemble_candidates_for_fn_ptr_trait(
&mut self,
obligation: &PolyTraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
match self_ty.skip_binder().kind() {
ty::FnPtr(_) => candidates.vec.push(BuiltinCandidate { has_nested: false }),
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Adt(..)
| ty::Foreign(..)
| ty::Str
| ty::Array(..)
| ty::Slice(_)
| ty::RawPtr(_)
| ty::Ref(..)
| ty::FnDef(..)
| ty::Placeholder(..)
| ty::Dynamic(..)
| ty::Closure(..)
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Never
| ty::Tuple(..)
| ty::Alias(..)
| ty::Param(..)
| ty::Bound(..)
| ty::Error(_)
| ty::Infer(
ty::InferTy::IntVar(_)
| ty::InferTy::FloatVar(_)
| ty::InferTy::FreshIntTy(_)
| ty::InferTy::FreshFloatTy(_),
) => {}
ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)) => {
candidates.ambiguous = true;
}
}
}
}