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//! Code shared by trait and projection goals for candidate assembly.
use super::{EvalCtxt, SolverMode};
use crate::traits::coherence;
use rustc_hir::def_id::DefId;
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::Reveal;
use rustc_middle::traits::solve::inspect::ProbeKind;
use rustc_middle::traits::solve::{
CandidateSource, CanonicalResponse, Certainty, Goal, QueryResult,
};
use rustc_middle::traits::BuiltinImplSource;
use rustc_middle::ty::fast_reject::{SimplifiedType, TreatParams};
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::ty::{fast_reject, TypeFoldable};
use rustc_middle::ty::{ToPredicate, TypeVisitableExt};
use rustc_span::ErrorGuaranteed;
use std::fmt::Debug;
pub(super) mod structural_traits;
/// A candidate is a possible way to prove a goal.
///
/// It consists of both the `source`, which describes how that goal would be proven,
/// and the `result` when using the given `source`.
#[derive(Debug, Clone)]
pub(super) struct Candidate<'tcx> {
pub(super) source: CandidateSource,
pub(super) result: CanonicalResponse<'tcx>,
}
/// Methods used to assemble candidates for either trait or projection goals.
pub(super) trait GoalKind<'tcx>:
TypeFoldable<TyCtxt<'tcx>> + Copy + Eq + std::fmt::Display
{
fn self_ty(self) -> Ty<'tcx>;
fn trait_ref(self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx>;
fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self;
fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId;
/// Try equating an assumption predicate against a goal's predicate. If it
/// holds, then execute the `then` callback, which should do any additional
/// work, then produce a response (typically by executing
/// [`EvalCtxt::evaluate_added_goals_and_make_canonical_response`]).
fn probe_and_match_goal_against_assumption(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
assumption: ty::Clause<'tcx>,
then: impl FnOnce(&mut EvalCtxt<'_, 'tcx>) -> QueryResult<'tcx>,
) -> QueryResult<'tcx>;
/// Consider a clause, which consists of a "assumption" and some "requirements",
/// to satisfy a goal. If the requirements hold, then attempt to satisfy our
/// goal by equating it with the assumption.
fn consider_implied_clause(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
assumption: ty::Clause<'tcx>,
requirements: impl IntoIterator<Item = Goal<'tcx, ty::Predicate<'tcx>>>,
) -> QueryResult<'tcx> {
Self::probe_and_match_goal_against_assumption(ecx, goal, assumption, |ecx| {
ecx.add_goals(requirements);
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
})
}
/// Consider a bound originating from the item bounds of an alias. For this we
/// require that the well-formed requirements of the self type of the goal
/// are "satisfied from the param-env".
/// See [`EvalCtxt::validate_alias_bound_self_from_param_env`].
fn consider_alias_bound_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
assumption: ty::Clause<'tcx>,
) -> QueryResult<'tcx> {
Self::probe_and_match_goal_against_assumption(ecx, goal, assumption, |ecx| {
ecx.validate_alias_bound_self_from_param_env(goal)
})
}
/// Consider a clause specifically for a `dyn Trait` self type. This requires
/// additionally checking all of the supertraits and object bounds to hold,
/// since they're not implied by the well-formedness of the object type.
fn consider_object_bound_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
assumption: ty::Clause<'tcx>,
) -> QueryResult<'tcx> {
Self::probe_and_match_goal_against_assumption(ecx, goal, assumption, |ecx| {
let tcx = ecx.tcx();
let ty::Dynamic(bounds, _, _) = *goal.predicate.self_ty().kind() else {
bug!("expected object type in `consider_object_bound_candidate`");
};
ecx.add_goals(structural_traits::predicates_for_object_candidate(
&ecx,
goal.param_env,
goal.predicate.trait_ref(tcx),
bounds,
));
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
})
}
fn consider_impl_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
impl_def_id: DefId,
) -> QueryResult<'tcx>;
/// If the predicate contained an error, we want to avoid emitting unnecessary trait
/// errors but still want to emit errors for other trait goals. We have some special
/// handling for this case.
///
/// Trait goals always hold while projection goals never do. This is a bit arbitrary
/// but prevents incorrect normalization while hiding any trait errors.
fn consider_error_guaranteed_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
guar: ErrorGuaranteed,
) -> QueryResult<'tcx>;
/// A type implements an `auto trait` if its components do as well.
///
/// These components are given by built-in rules from
/// [`structural_traits::instantiate_constituent_tys_for_auto_trait`].
fn consider_auto_trait_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A trait alias holds if the RHS traits and `where` clauses hold.
fn consider_trait_alias_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A type is `Copy` or `Clone` if its components are `Sized`.
///
/// These components are given by built-in rules from
/// [`structural_traits::instantiate_constituent_tys_for_sized_trait`].
fn consider_builtin_sized_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A type is `Copy` or `Clone` if its components are `Copy` or `Clone`.
///
/// These components are given by built-in rules from
/// [`structural_traits::instantiate_constituent_tys_for_copy_clone_trait`].
fn consider_builtin_copy_clone_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A type is `PointerLike` if we can compute its layout, and that layout
/// matches the layout of `usize`.
fn consider_builtin_pointer_like_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A type is a `FnPtr` if it is of `FnPtr` type.
fn consider_builtin_fn_ptr_trait_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A callable type (a closure, fn def, or fn ptr) is known to implement the `Fn<A>`
/// family of traits where `A` is given by the signature of the type.
fn consider_builtin_fn_trait_candidates(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
kind: ty::ClosureKind,
) -> QueryResult<'tcx>;
/// `Tuple` is implemented if the `Self` type is a tuple.
fn consider_builtin_tuple_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// `Pointee` is always implemented.
///
/// See the projection implementation for the `Metadata` types for all of
/// the built-in types. For structs, the metadata type is given by the struct
/// tail.
fn consider_builtin_pointee_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A generator (that comes from an `async` desugaring) is known to implement
/// `Future<Output = O>`, where `O` is given by the generator's return type
/// that was computed during type-checking.
fn consider_builtin_future_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// A generator (that doesn't come from an `async` desugaring) is known to
/// implement `Generator<R, Yield = Y, Return = O>`, given the resume, yield,
/// and return types of the generator computed during type-checking.
fn consider_builtin_generator_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
fn consider_builtin_discriminant_kind_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
fn consider_builtin_destruct_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
fn consider_builtin_transmute_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
/// Consider (possibly several) candidates to upcast or unsize a type to another
/// type, excluding the coercion of a sized type into a `dyn Trait`.
///
/// We return the `BuiltinImplSource` for each candidate as it is needed
/// for unsize coercion in hir typeck and because it is difficult to
/// otherwise recompute this for codegen. This is a bit of a mess but the
/// easiest way to maintain the existing behavior for now.
fn consider_structural_builtin_unsize_candidates(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)>;
/// Consider the `Unsize` candidate corresponding to coercing a sized type
/// into a `dyn Trait`.
///
/// This is computed separately from the rest of the `Unsize` candidates
/// since it is only done once per self type, and not once per
/// *normalization step* (in `assemble_candidates_via_self_ty`).
fn consider_unsize_to_dyn_candidate(
ecx: &mut EvalCtxt<'_, 'tcx>,
goal: Goal<'tcx, Self>,
) -> QueryResult<'tcx>;
}
impl<'tcx> EvalCtxt<'_, 'tcx> {
pub(super) fn assemble_and_evaluate_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
) -> Vec<Candidate<'tcx>> {
debug_assert_eq!(goal, self.resolve_vars_if_possible(goal));
if let Some(ambig) = self.assemble_self_ty_infer_ambiguity_response(goal) {
return ambig;
}
let mut candidates = self.assemble_candidates_via_self_ty(goal, 0);
self.assemble_unsize_to_dyn_candidate(goal, &mut candidates);
self.assemble_blanket_impl_candidates(goal, &mut candidates);
self.assemble_param_env_candidates(goal, &mut candidates);
self.assemble_coherence_unknowable_candidates(goal, &mut candidates);
candidates
}
/// `?0: Trait` is ambiguous, because it may be satisfied via a builtin rule,
/// object bound, alias bound, etc. We are unable to determine this until we can at
/// least structurally resolve the type one layer.
///
/// It would also require us to consider all impls of the trait, which is both pretty
/// bad for perf and would also constrain the self type if there is just a single impl.
fn assemble_self_ty_infer_ambiguity_response<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
) -> Option<Vec<Candidate<'tcx>>> {
goal.predicate.self_ty().is_ty_var().then(|| {
vec![Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
result: self
.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
.unwrap(),
}]
})
}
/// Assemble candidates which apply to the self type. This only looks at candidate which
/// apply to the specific self type and ignores all others.
///
/// Returns `None` if the self type is still ambiguous.
fn assemble_candidates_via_self_ty<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
num_steps: usize,
) -> Vec<Candidate<'tcx>> {
debug_assert_eq!(goal, self.resolve_vars_if_possible(goal));
if let Some(ambig) = self.assemble_self_ty_infer_ambiguity_response(goal) {
return ambig;
}
let mut candidates = Vec::new();
self.assemble_non_blanket_impl_candidates(goal, &mut candidates);
self.assemble_builtin_impl_candidates(goal, &mut candidates);
self.assemble_alias_bound_candidates(goal, &mut candidates);
self.assemble_object_bound_candidates(goal, &mut candidates);
self.assemble_candidates_after_normalizing_self_ty(goal, &mut candidates, num_steps);
candidates
}
/// If the self type of a goal is an alias we first try to normalize the self type
/// and compute the candidates for the normalized self type in case that succeeds.
///
/// These candidates are used in addition to the ones with the alias as a self type.
/// We do this to simplify both builtin candidates and for better performance.
///
/// We generate the builtin candidates on the fly by looking at the self type, e.g.
/// add `FnPtr` candidates if the self type is a function pointer. Handling builtin
/// candidates while the self type is still an alias seems difficult. This is similar
/// to `try_structurally_resolve_type` during hir typeck (FIXME once implemented).
///
/// Looking at all impls for some trait goal is prohibitively expensive. We therefore
/// only look at implementations with a matching self type. Because of this function,
/// we can avoid looking at all existing impls if the self type is an alias.
#[instrument(level = "debug", skip_all)]
fn assemble_candidates_after_normalizing_self_ty<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
num_steps: usize,
) {
let tcx = self.tcx();
let &ty::Alias(_, projection_ty) = goal.predicate.self_ty().kind() else { return };
candidates.extend(self.probe(|_| ProbeKind::NormalizedSelfTyAssembly).enter(|ecx| {
if num_steps < ecx.local_overflow_limit() {
let normalized_ty = ecx.next_ty_infer();
let normalizes_to_goal = goal.with(
tcx,
ty::ProjectionPredicate { projection_ty, term: normalized_ty.into() },
);
ecx.add_goal(normalizes_to_goal);
if let Err(NoSolution) = ecx.try_evaluate_added_goals() {
debug!("self type normalization failed");
return vec![];
}
let normalized_ty = ecx.resolve_vars_if_possible(normalized_ty);
debug!(?normalized_ty, "self type normalized");
// NOTE: Alternatively we could call `evaluate_goal` here and only
// have a `Normalized` candidate. This doesn't work as long as we
// use `CandidateSource` in winnowing.
let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
ecx.assemble_candidates_via_self_ty(goal, num_steps + 1)
} else {
match ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW) {
Ok(result) => vec![Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
result,
}],
Err(NoSolution) => vec![],
}
}
}));
}
#[instrument(level = "debug", skip_all)]
fn assemble_non_blanket_impl_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let tcx = self.tcx();
let self_ty = goal.predicate.self_ty();
let trait_impls = tcx.trait_impls_of(goal.predicate.trait_def_id(tcx));
let mut consider_impls_for_simplified_type = |simp| {
if let Some(impls_for_type) = trait_impls.non_blanket_impls().get(&simp) {
for &impl_def_id in impls_for_type {
match G::consider_impl_candidate(self, goal, impl_def_id) {
Ok(result) => candidates
.push(Candidate { source: CandidateSource::Impl(impl_def_id), result }),
Err(NoSolution) => (),
}
}
}
};
match self_ty.kind() {
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::Never
| ty::Tuple(_) => {
let simp =
fast_reject::simplify_type(tcx, self_ty, TreatParams::ForLookup).unwrap();
consider_impls_for_simplified_type(simp);
}
// HACK: For integer and float variables we have to manually look at all impls
// which have some integer or float as a self type.
ty::Infer(ty::IntVar(_)) => {
use ty::IntTy::*;
use ty::UintTy::*;
// This causes a compiler error if any new integer kinds are added.
let (I8 | I16 | I32 | I64 | I128 | Isize): ty::IntTy;
let (U8 | U16 | U32 | U64 | U128 | Usize): ty::UintTy;
let possible_integers = [
// signed integers
SimplifiedType::Int(I8),
SimplifiedType::Int(I16),
SimplifiedType::Int(I32),
SimplifiedType::Int(I64),
SimplifiedType::Int(I128),
SimplifiedType::Int(Isize),
// unsigned integers
SimplifiedType::Uint(U8),
SimplifiedType::Uint(U16),
SimplifiedType::Uint(U32),
SimplifiedType::Uint(U64),
SimplifiedType::Uint(U128),
SimplifiedType::Uint(Usize),
];
for simp in possible_integers {
consider_impls_for_simplified_type(simp);
}
}
ty::Infer(ty::FloatVar(_)) => {
// This causes a compiler error if any new float kinds are added.
let (ty::FloatTy::F32 | ty::FloatTy::F64);
let possible_floats = [
SimplifiedType::Float(ty::FloatTy::F32),
SimplifiedType::Float(ty::FloatTy::F64),
];
for simp in possible_floats {
consider_impls_for_simplified_type(simp);
}
}
// The only traits applying to aliases and placeholders are blanket impls.
//
// Impls which apply to an alias after normalization are handled by
// `assemble_candidates_after_normalizing_self_ty`.
ty::Alias(_, _) | ty::Placeholder(..) | ty::Error(_) => (),
// FIXME: These should ideally not exist as a self type. It would be nice for
// the builtin auto trait impls of generators to instead directly recurse
// into the witness.
ty::GeneratorWitness(..) => (),
// These variants should not exist as a self type.
ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
| ty::Param(_)
| ty::Bound(_, _) => bug!("unexpected self type: {self_ty}"),
}
}
fn assemble_unsize_to_dyn_candidate<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let tcx = self.tcx();
if tcx.lang_items().unsize_trait() == Some(goal.predicate.trait_def_id(tcx)) {
match G::consider_unsize_to_dyn_candidate(self, goal) {
Ok(result) => candidates.push(Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
result,
}),
Err(NoSolution) => (),
}
}
}
fn assemble_blanket_impl_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let tcx = self.tcx();
let trait_impls = tcx.trait_impls_of(goal.predicate.trait_def_id(tcx));
for &impl_def_id in trait_impls.blanket_impls() {
match G::consider_impl_candidate(self, goal, impl_def_id) {
Ok(result) => candidates
.push(Candidate { source: CandidateSource::Impl(impl_def_id), result }),
Err(NoSolution) => (),
}
}
}
#[instrument(level = "debug", skip_all)]
fn assemble_builtin_impl_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let tcx = self.tcx();
let lang_items = tcx.lang_items();
let trait_def_id = goal.predicate.trait_def_id(tcx);
// N.B. When assembling built-in candidates for lang items that are also
// `auto` traits, then the auto trait candidate that is assembled in
// `consider_auto_trait_candidate` MUST be disqualified to remain sound.
//
// Instead of adding the logic here, it's a better idea to add it in
// `EvalCtxt::disqualify_auto_trait_candidate_due_to_possible_impl` in
// `solve::trait_goals` instead.
let result = if let Err(guar) = goal.predicate.error_reported() {
G::consider_error_guaranteed_candidate(self, guar)
} else if tcx.trait_is_auto(trait_def_id) {
G::consider_auto_trait_candidate(self, goal)
} else if tcx.trait_is_alias(trait_def_id) {
G::consider_trait_alias_candidate(self, goal)
} else if lang_items.sized_trait() == Some(trait_def_id) {
G::consider_builtin_sized_candidate(self, goal)
} else if lang_items.copy_trait() == Some(trait_def_id)
|| lang_items.clone_trait() == Some(trait_def_id)
{
G::consider_builtin_copy_clone_candidate(self, goal)
} else if lang_items.pointer_like() == Some(trait_def_id) {
G::consider_builtin_pointer_like_candidate(self, goal)
} else if lang_items.fn_ptr_trait() == Some(trait_def_id) {
G::consider_builtin_fn_ptr_trait_candidate(self, goal)
} else if let Some(kind) = self.tcx().fn_trait_kind_from_def_id(trait_def_id) {
G::consider_builtin_fn_trait_candidates(self, goal, kind)
} else if lang_items.tuple_trait() == Some(trait_def_id) {
G::consider_builtin_tuple_candidate(self, goal)
} else if lang_items.pointee_trait() == Some(trait_def_id) {
G::consider_builtin_pointee_candidate(self, goal)
} else if lang_items.future_trait() == Some(trait_def_id) {
G::consider_builtin_future_candidate(self, goal)
} else if lang_items.gen_trait() == Some(trait_def_id) {
G::consider_builtin_generator_candidate(self, goal)
} else if lang_items.discriminant_kind_trait() == Some(trait_def_id) {
G::consider_builtin_discriminant_kind_candidate(self, goal)
} else if lang_items.destruct_trait() == Some(trait_def_id) {
G::consider_builtin_destruct_candidate(self, goal)
} else if lang_items.transmute_trait() == Some(trait_def_id) {
G::consider_builtin_transmute_candidate(self, goal)
} else {
Err(NoSolution)
};
match result {
Ok(result) => candidates.push(Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
result,
}),
Err(NoSolution) => (),
}
// There may be multiple unsize candidates for a trait with several supertraits:
// `trait Foo: Bar<A> + Bar<B>` and `dyn Foo: Unsize<dyn Bar<_>>`
if lang_items.unsize_trait() == Some(trait_def_id) {
for (result, source) in G::consider_structural_builtin_unsize_candidates(self, goal) {
candidates.push(Candidate { source: CandidateSource::BuiltinImpl(source), result });
}
}
}
#[instrument(level = "debug", skip_all)]
fn assemble_param_env_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
for (i, assumption) in goal.param_env.caller_bounds().iter().enumerate() {
match G::consider_implied_clause(self, goal, assumption, []) {
Ok(result) => {
candidates.push(Candidate { source: CandidateSource::ParamEnv(i), result })
}
Err(NoSolution) => (),
}
}
}
#[instrument(level = "debug", skip_all)]
fn assemble_alias_bound_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let alias_ty = match goal.predicate.self_ty().kind() {
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::Tuple(_)
| ty::Param(_)
| ty::Placeholder(..)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Alias(ty::Inherent, _)
| ty::Alias(ty::Weak, _)
| ty::Error(_) => return,
ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
| ty::Bound(..) => bug!("unexpected self type for `{goal:?}`"),
// Excluding IATs and type aliases here as they don't have meaningful item bounds.
ty::Alias(ty::Projection | ty::Opaque, alias_ty) => alias_ty,
};
for assumption in
self.tcx().item_bounds(alias_ty.def_id).instantiate(self.tcx(), alias_ty.args)
{
match G::consider_alias_bound_candidate(self, goal, assumption) {
Ok(result) => {
candidates.push(Candidate { source: CandidateSource::AliasBound, result })
}
Err(NoSolution) => (),
}
}
}
/// Check that we are allowed to use an alias bound originating from the self
/// type of this goal. This means something different depending on the self type's
/// alias kind.
///
/// * Projection: Given a goal with a self type such as `<Ty as Trait>::Assoc`,
/// we require that the bound `Ty: Trait` can be proven using either a nested alias
/// bound candidate, or a param-env candidate.
///
/// * Opaque: The param-env must be in `Reveal::UserFacing` mode. Otherwise,
/// the goal should be proven by using the hidden type instead.
#[instrument(level = "debug", skip(self), ret)]
pub(super) fn validate_alias_bound_self_from_param_env<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
) -> QueryResult<'tcx> {
match *goal.predicate.self_ty().kind() {
ty::Alias(ty::Projection, projection_ty) => {
let mut param_env_candidates = vec![];
let self_trait_ref = projection_ty.trait_ref(self.tcx());
if self_trait_ref.self_ty().is_ty_var() {
return self
.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS);
}
let trait_goal: Goal<'_, ty::TraitPredicate<'tcx>> = goal.with(
self.tcx(),
ty::TraitPredicate {
trait_ref: self_trait_ref,
polarity: ty::ImplPolarity::Positive,
},
);
self.assemble_param_env_candidates(trait_goal, &mut param_env_candidates);
// FIXME: We probably need some sort of recursion depth check here.
// Can't come up with an example yet, though, and the worst case
// we can have is a compiler stack overflow...
self.assemble_alias_bound_candidates(trait_goal, &mut param_env_candidates);
// FIXME: We must also consider alias-bound candidates for a peculiar
// class of built-in candidates that I'll call "defaulted" built-ins.
//
// For example, we always know that `T: Pointee` is implemented, but
// we do not always know what `<T as Pointee>::Metadata` actually is,
// similar to if we had a user-defined impl with a `default type ...`.
// For these traits, since we're not able to always normalize their
// associated types to a concrete type, we must consider their alias bounds
// instead, so we can prove bounds such as `<T as Pointee>::Metadata: Copy`.
self.assemble_alias_bound_candidates_for_builtin_impl_default_items(
trait_goal,
&mut param_env_candidates,
);
self.merge_candidates(param_env_candidates)
}
ty::Alias(ty::Opaque, _opaque_ty) => match goal.param_env.reveal() {
Reveal::UserFacing => {
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
Reveal::All => return Err(NoSolution),
},
_ => bug!("only expected to be called on alias tys"),
}
}
/// Assemble a subset of builtin impl candidates for a class of candidates called
/// "defaulted" built-in traits.
///
/// For example, we always know that `T: Pointee` is implemented, but we do not
/// always know what `<T as Pointee>::Metadata` actually is! See the comment in
/// [`EvalCtxt::validate_alias_bound_self_from_param_env`] for more detail.
#[instrument(level = "debug", skip_all)]
fn assemble_alias_bound_candidates_for_builtin_impl_default_items<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let lang_items = self.tcx().lang_items();
let trait_def_id = goal.predicate.trait_def_id(self.tcx());
// You probably shouldn't add anything to this list unless you
// know what you're doing.
let result = if lang_items.pointee_trait() == Some(trait_def_id) {
G::consider_builtin_pointee_candidate(self, goal)
} else if lang_items.discriminant_kind_trait() == Some(trait_def_id) {
G::consider_builtin_discriminant_kind_candidate(self, goal)
} else {
Err(NoSolution)
};
match result {
Ok(result) => candidates.push(Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
result,
}),
Err(NoSolution) => (),
}
}
#[instrument(level = "debug", skip_all)]
fn assemble_object_bound_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let tcx = self.tcx();
if !tcx.trait_def(goal.predicate.trait_def_id(tcx)).implement_via_object {
return;
}
let self_ty = goal.predicate.self_ty();
let bounds = match *self_ty.kind() {
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::Alias(..)
| ty::Closure(..)
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Never
| ty::Tuple(_)
| ty::Param(_)
| ty::Placeholder(..)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Error(_) => return,
ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
| ty::Bound(..) => bug!("unexpected self type for `{goal:?}`"),
ty::Dynamic(bounds, ..) => bounds,
};
// Do not consider built-in object impls for non-object-safe types.
if bounds.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
return;
}
// Consider all of the auto-trait and projection bounds, which don't
// need to be recorded as a `BuiltinImplSource::Object` since they don't
// really have a vtable base...
for bound in bounds {
match bound.skip_binder() {
ty::ExistentialPredicate::Trait(_) => {
// Skip principal
}
ty::ExistentialPredicate::Projection(_)
| ty::ExistentialPredicate::AutoTrait(_) => {
match G::consider_object_bound_candidate(
self,
goal,
bound.with_self_ty(tcx, self_ty),
) {
Ok(result) => candidates.push(Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
result,
}),
Err(NoSolution) => (),
}
}
}
}
// FIXME: We only need to do *any* of this if we're considering a trait goal,
// since we don't need to look at any supertrait or anything if we are doing
// a projection goal.
if let Some(principal) = bounds.principal() {
let principal_trait_ref = principal.with_self_ty(tcx, self_ty);
self.walk_vtable(principal_trait_ref, |ecx, assumption, vtable_base, _| {
match G::consider_object_bound_candidate(ecx, goal, assumption.to_predicate(tcx)) {
Ok(result) => candidates.push(Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Object {
vtable_base,
}),
result,
}),
Err(NoSolution) => (),
}
});
}
}
#[instrument(level = "debug", skip_all)]
fn assemble_coherence_unknowable_candidates<G: GoalKind<'tcx>>(
&mut self,
goal: Goal<'tcx, G>,
candidates: &mut Vec<Candidate<'tcx>>,
) {
let tcx = self.tcx();
match self.solver_mode() {
SolverMode::Normal => return,
SolverMode::Coherence => {}
};
let result = self.probe_misc_candidate("coherence unknowable").enter(|ecx| {
let trait_ref = goal.predicate.trait_ref(tcx);
#[derive(Debug)]
enum FailureKind {
Overflow,
NoSolution(NoSolution),
}
let lazily_normalize_ty = |ty| match ecx.try_normalize_ty(goal.param_env, ty) {
Ok(Some(ty)) => Ok(ty),
Ok(None) => Err(FailureKind::Overflow),
Err(e) => Err(FailureKind::NoSolution(e)),
};
match coherence::trait_ref_is_knowable(tcx, trait_ref, lazily_normalize_ty) {
Err(FailureKind::Overflow) => {
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW)
}
Err(FailureKind::NoSolution(NoSolution)) | Ok(Ok(())) => Err(NoSolution),
Ok(Err(_)) => {
ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
}
}
});
match result {
Ok(result) => candidates.push(Candidate {
source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
result,
}),
Err(NoSolution) => {}
}
}
/// If there are multiple ways to prove a trait or projection goal, we have
/// to somehow try to merge the candidates into one. If that fails, we return
/// ambiguity.
#[instrument(level = "debug", skip(self), ret)]
pub(super) fn merge_candidates(
&mut self,
mut candidates: Vec<Candidate<'tcx>>,
) -> QueryResult<'tcx> {
// First try merging all candidates. This is complete and fully sound.
let responses = candidates.iter().map(|c| c.result).collect::<Vec<_>>();
if let Some(result) = self.try_merge_responses(&responses) {
return Ok(result);
}
// We then check whether we should prioritize `ParamEnv` candidates.
//
// Doing so is incomplete and would therefore be unsound during coherence.
match self.solver_mode() {
SolverMode::Coherence => (),
// Prioritize `ParamEnv` candidates only if they do not guide inference.
//
// This is still incomplete as we may add incorrect region bounds.
SolverMode::Normal => {
let param_env_responses = candidates
.iter()
.filter(|c| {
matches!(
c.source,
CandidateSource::ParamEnv(_) | CandidateSource::AliasBound
)
})
.map(|c| c.result)
.collect::<Vec<_>>();
if let Some(result) = self.try_merge_responses(¶m_env_responses) {
// We strongly prefer alias and param-env bounds here, even if they affect inference.
// See https://github.com/rust-lang/trait-system-refactor-initiative/issues/11.
return Ok(result);
}
}
}
self.flounder(&responses)
}
}