1 //! Candidate selection. See the [rustc guide] for more information on how this works.
3 //! [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/resolution.html#selection
5 use self::EvaluationResult::*;
6 use self::SelectionCandidate::*;
8 use super::coherence::{self, Conflict};
10 use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey};
12 use super::DerivedObligationCause;
14 use super::SelectionResult;
15 use super::TraitNotObjectSafe;
16 use super::{BuiltinDerivedObligation, ImplDerivedObligation, ObligationCauseCode};
17 use super::{IntercrateMode, TraitQueryMode};
18 use super::{ObjectCastObligation, Obligation};
19 use super::{ObligationCause, PredicateObligation, TraitObligation};
20 use super::{OutputTypeParameterMismatch, Overflow, SelectionError, Unimplemented};
22 VtableAutoImpl, VtableBuiltin, VtableClosure, VtableFnPointer, VtableGenerator, VtableImpl,
23 VtableObject, VtableParam, VtableTraitAlias,
26 VtableAutoImplData, VtableBuiltinData, VtableClosureData, VtableFnPointerData,
27 VtableGeneratorData, VtableImplData, VtableObjectData, VtableTraitAliasData,
30 use dep_graph::{DepKind, DepNodeIndex};
31 use hir::def_id::DefId;
32 use infer::{InferCtxt, InferOk, TypeFreshener};
33 use middle::lang_items;
34 use mir::interpret::GlobalId;
36 use ty::relate::{TypeRelation, TraitObjectMode};
37 use ty::subst::{Subst, Substs};
38 use ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable};
41 use rustc_data_structures::bit_set::GrowableBitSet;
42 use rustc_data_structures::sync::Lock;
43 use rustc_target::spec::abi::Abi;
45 use std::fmt::{self, Display};
48 use util::nodemap::{FxHashMap, FxHashSet};
50 pub struct SelectionContext<'cx, 'gcx: 'cx + 'tcx, 'tcx: 'cx> {
51 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
53 /// Freshener used specifically for entries on the obligation
54 /// stack. This ensures that all entries on the stack at one time
55 /// will have the same set of placeholder entries, which is
56 /// important for checking for trait bounds that recursively
57 /// require themselves.
58 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
60 /// If `true`, indicates that the evaluation should be conservative
61 /// and consider the possibility of types outside this crate.
62 /// This comes up primarily when resolving ambiguity. Imagine
63 /// there is some trait reference `$0: Bar` where `$0` is an
64 /// inference variable. If `intercrate` is true, then we can never
65 /// say for sure that this reference is not implemented, even if
66 /// there are *no impls at all for `Bar`*, because `$0` could be
67 /// bound to some type that in a downstream crate that implements
68 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
69 /// though, we set this to false, because we are only interested
70 /// in types that the user could actually have written --- in
71 /// other words, we consider `$0: Bar` to be unimplemented if
72 /// there is no type that the user could *actually name* that
73 /// would satisfy it. This avoids crippling inference, basically.
74 intercrate: Option<IntercrateMode>,
76 intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
78 /// Controls whether or not to filter out negative impls when selecting.
79 /// This is used in librustdoc to distinguish between the lack of an impl
80 /// and a negative impl
81 allow_negative_impls: bool,
83 /// The mode that trait queries run in, which informs our error handling
84 /// policy. In essence, canonicalized queries need their errors propagated
85 /// rather than immediately reported because we do not have accurate spans.
86 query_mode: TraitQueryMode,
89 #[derive(Clone, Debug)]
90 pub enum IntercrateAmbiguityCause {
93 self_desc: Option<String>,
97 self_desc: Option<String>,
101 impl IntercrateAmbiguityCause {
102 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
103 /// See #23980 for details.
104 pub fn add_intercrate_ambiguity_hint<'a, 'tcx>(
106 err: &mut ::errors::DiagnosticBuilder<'_>,
108 err.note(&self.intercrate_ambiguity_hint());
111 pub fn intercrate_ambiguity_hint(&self) -> String {
113 &IntercrateAmbiguityCause::DownstreamCrate {
117 let self_desc = if let &Some(ref ty) = self_desc {
118 format!(" for type `{}`", ty)
123 "downstream crates may implement trait `{}`{}",
124 trait_desc, self_desc
127 &IntercrateAmbiguityCause::UpstreamCrateUpdate {
131 let self_desc = if let &Some(ref ty) = self_desc {
132 format!(" for type `{}`", ty)
137 "upstream crates may add new impl of trait `{}`{} \
139 trait_desc, self_desc
146 // A stack that walks back up the stack frame.
147 struct TraitObligationStack<'prev, 'tcx: 'prev> {
148 obligation: &'prev TraitObligation<'tcx>,
150 /// Trait ref from `obligation` but "freshened" with the
151 /// selection-context's freshener. Used to check for recursion.
152 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
154 previous: TraitObligationStackList<'prev, 'tcx>,
157 #[derive(Clone, Default)]
158 pub struct SelectionCache<'tcx> {
160 FxHashMap<ty::TraitRef<'tcx>, WithDepNode<SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
164 /// The selection process begins by considering all impls, where
165 /// clauses, and so forth that might resolve an obligation. Sometimes
166 /// we'll be able to say definitively that (e.g.) an impl does not
167 /// apply to the obligation: perhaps it is defined for `usize` but the
168 /// obligation is for `int`. In that case, we drop the impl out of the
169 /// list. But the other cases are considered *candidates*.
171 /// For selection to succeed, there must be exactly one matching
172 /// candidate. If the obligation is fully known, this is guaranteed
173 /// by coherence. However, if the obligation contains type parameters
174 /// or variables, there may be multiple such impls.
176 /// It is not a real problem if multiple matching impls exist because
177 /// of type variables - it just means the obligation isn't sufficiently
178 /// elaborated. In that case we report an ambiguity, and the caller can
179 /// try again after more type information has been gathered or report a
180 /// "type annotations required" error.
182 /// However, with type parameters, this can be a real problem - type
183 /// parameters don't unify with regular types, but they *can* unify
184 /// with variables from blanket impls, and (unless we know its bounds
185 /// will always be satisfied) picking the blanket impl will be wrong
186 /// for at least *some* substitutions. To make this concrete, if we have
188 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
189 /// impl<T: fmt::Debug> AsDebug for T {
191 /// fn debug(self) -> fmt::Debug { self }
193 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
195 /// we can't just use the impl to resolve the <T as AsDebug> obligation
196 /// - a type from another crate (that doesn't implement fmt::Debug) could
197 /// implement AsDebug.
199 /// Because where-clauses match the type exactly, multiple clauses can
200 /// only match if there are unresolved variables, and we can mostly just
201 /// report this ambiguity in that case. This is still a problem - we can't
202 /// *do anything* with ambiguities that involve only regions. This is issue
205 /// If a single where-clause matches and there are no inference
206 /// variables left, then it definitely matches and we can just select
209 /// In fact, we even select the where-clause when the obligation contains
210 /// inference variables. The can lead to inference making "leaps of logic",
211 /// for example in this situation:
213 /// pub trait Foo<T> { fn foo(&self) -> T; }
214 /// impl<T> Foo<()> for T { fn foo(&self) { } }
215 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
217 /// pub fn foo<T>(t: T) where T: Foo<bool> {
218 /// println!("{:?}", <T as Foo<_>>::foo(&t));
220 /// fn main() { foo(false); }
222 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
223 /// impl and the where-clause. We select the where-clause and unify $0=bool,
224 /// so the program prints "false". However, if the where-clause is omitted,
225 /// the blanket impl is selected, we unify $0=(), and the program prints
228 /// Exactly the same issues apply to projection and object candidates, except
229 /// that we can have both a projection candidate and a where-clause candidate
230 /// for the same obligation. In that case either would do (except that
231 /// different "leaps of logic" would occur if inference variables are
232 /// present), and we just pick the where-clause. This is, for example,
233 /// required for associated types to work in default impls, as the bounds
234 /// are visible both as projection bounds and as where-clauses from the
235 /// parameter environment.
236 #[derive(PartialEq, Eq, Debug, Clone)]
237 enum SelectionCandidate<'tcx> {
238 /// If has_nested is false, there are no *further* obligations
242 ParamCandidate(ty::PolyTraitRef<'tcx>),
243 ImplCandidate(DefId),
244 AutoImplCandidate(DefId),
246 /// This is a trait matching with a projected type as `Self`, and
247 /// we found an applicable bound in the trait definition.
250 /// Implementation of a `Fn`-family trait by one of the anonymous types
251 /// generated for a `||` expression.
254 /// Implementation of a `Generator` trait by one of the anonymous types
255 /// generated for a generator.
258 /// Implementation of a `Fn`-family trait by one of the anonymous
259 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
262 TraitAliasCandidate(DefId),
266 BuiltinObjectCandidate,
268 BuiltinUnsizeCandidate,
271 impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> {
272 type Lifted = SelectionCandidate<'tcx>;
273 fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
275 BuiltinCandidate { has_nested } => BuiltinCandidate { has_nested },
276 ImplCandidate(def_id) => ImplCandidate(def_id),
277 AutoImplCandidate(def_id) => AutoImplCandidate(def_id),
278 ProjectionCandidate => ProjectionCandidate,
279 ClosureCandidate => ClosureCandidate,
280 GeneratorCandidate => GeneratorCandidate,
281 FnPointerCandidate => FnPointerCandidate,
282 TraitAliasCandidate(def_id) => TraitAliasCandidate(def_id),
283 ObjectCandidate => ObjectCandidate,
284 BuiltinObjectCandidate => BuiltinObjectCandidate,
285 BuiltinUnsizeCandidate => BuiltinUnsizeCandidate,
287 ParamCandidate(ref trait_ref) => {
288 return tcx.lift(trait_ref).map(ParamCandidate);
294 struct SelectionCandidateSet<'tcx> {
295 // a list of candidates that definitely apply to the current
296 // obligation (meaning: types unify).
297 vec: Vec<SelectionCandidate<'tcx>>,
299 // if this is true, then there were candidates that might or might
300 // not have applied, but we couldn't tell. This occurs when some
301 // of the input types are type variables, in which case there are
302 // various "builtin" rules that might or might not trigger.
306 #[derive(PartialEq, Eq, Debug, Clone)]
307 struct EvaluatedCandidate<'tcx> {
308 candidate: SelectionCandidate<'tcx>,
309 evaluation: EvaluationResult,
312 /// When does the builtin impl for `T: Trait` apply?
313 enum BuiltinImplConditions<'tcx> {
314 /// The impl is conditional on T1,T2,.. : Trait
315 Where(ty::Binder<Vec<Ty<'tcx>>>),
316 /// There is no built-in impl. There may be some other
317 /// candidate (a where-clause or user-defined impl).
319 /// It is unknown whether there is an impl.
323 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
324 /// The result of trait evaluation. The order is important
325 /// here as the evaluation of a list is the maximum of the
328 /// The evaluation results are ordered:
329 /// - `EvaluatedToOk` implies `EvaluatedToOkModuloRegions`
330 /// implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
331 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
332 /// - the "union" of evaluation results is equal to their maximum -
333 /// all the "potential success" candidates can potentially succeed,
334 /// so they are no-ops when unioned with a definite error, and within
335 /// the categories it's easy to see that the unions are correct.
336 pub enum EvaluationResult {
337 /// Evaluation successful
339 /// Evaluation successful, but there were unevaluated region obligations
340 EvaluatedToOkModuloRegions,
341 /// Evaluation is known to be ambiguous - it *might* hold for some
342 /// assignment of inference variables, but it might not.
344 /// While this has the same meaning as `EvaluatedToUnknown` - we can't
345 /// know whether this obligation holds or not - it is the result we
346 /// would get with an empty stack, and therefore is cacheable.
348 /// Evaluation failed because of recursion involving inference
349 /// variables. We are somewhat imprecise there, so we don't actually
350 /// know the real result.
352 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
354 /// Evaluation failed because we encountered an obligation we are already
355 /// trying to prove on this branch.
357 /// We know this branch can't be a part of a minimal proof-tree for
358 /// the "root" of our cycle, because then we could cut out the recursion
359 /// and maintain a valid proof tree. However, this does not mean
360 /// that all the obligations on this branch do not hold - it's possible
361 /// that we entered this branch "speculatively", and that there
362 /// might be some other way to prove this obligation that does not
363 /// go through this cycle - so we can't cache this as a failure.
365 /// For example, suppose we have this:
367 /// ```rust,ignore (pseudo-Rust)
368 /// pub trait Trait { fn xyz(); }
369 /// // This impl is "useless", but we can still have
370 /// // an `impl Trait for SomeUnsizedType` somewhere.
371 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
373 /// pub fn foo<T: Trait + ?Sized>() {
374 /// <T as Trait>::xyz();
378 /// When checking `foo`, we have to prove `T: Trait`. This basically
379 /// translates into this:
382 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
385 /// When we try to prove it, we first go the first option, which
386 /// recurses. This shows us that the impl is "useless" - it won't
387 /// tell us that `T: Trait` unless it already implemented `Trait`
388 /// by some other means. However, that does not prevent `T: Trait`
389 /// does not hold, because of the bound (which can indeed be satisfied
390 /// by `SomeUnsizedType` from another crate).
392 /// FIXME: when an `EvaluatedToRecur` goes past its parent root, we
393 /// ought to convert it to an `EvaluatedToErr`, because we know
394 /// there definitely isn't a proof tree for that obligation. Not
395 /// doing so is still sound - there isn't any proof tree, so the
396 /// branch still can't be a part of a minimal one - but does not
397 /// re-enable caching.
399 /// Evaluation failed
403 impl EvaluationResult {
404 /// True if this evaluation result is known to apply, even
405 /// considering outlives constraints.
406 pub fn must_apply_considering_regions(self) -> bool {
407 self == EvaluatedToOk
410 /// True if this evaluation result is known to apply, ignoring
411 /// outlives constraints.
412 pub fn must_apply_modulo_regions(self) -> bool {
413 self <= EvaluatedToOkModuloRegions
416 pub fn may_apply(self) -> bool {
418 EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToUnknown => {
422 EvaluatedToErr | EvaluatedToRecur => false,
426 fn is_stack_dependent(self) -> bool {
428 EvaluatedToUnknown | EvaluatedToRecur => true,
430 EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToErr => false,
435 impl_stable_hash_for!(enum self::EvaluationResult {
437 EvaluatedToOkModuloRegions,
444 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
445 /// Indicates that trait evaluation caused overflow.
446 pub struct OverflowError;
448 impl_stable_hash_for!(struct OverflowError {});
450 impl<'tcx> From<OverflowError> for SelectionError<'tcx> {
451 fn from(OverflowError: OverflowError) -> SelectionError<'tcx> {
452 SelectionError::Overflow
456 #[derive(Clone, Default)]
457 pub struct EvaluationCache<'tcx> {
458 hashmap: Lock<FxHashMap<ty::PolyTraitRef<'tcx>, WithDepNode<EvaluationResult>>>,
461 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
462 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
465 freshener: infcx.freshener(),
467 intercrate_ambiguity_causes: None,
468 allow_negative_impls: false,
469 query_mode: TraitQueryMode::Standard,
474 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
475 mode: IntercrateMode,
476 ) -> SelectionContext<'cx, 'gcx, 'tcx> {
477 debug!("intercrate({:?})", mode);
480 freshener: infcx.freshener(),
481 intercrate: Some(mode),
482 intercrate_ambiguity_causes: None,
483 allow_negative_impls: false,
484 query_mode: TraitQueryMode::Standard,
488 pub fn with_negative(
489 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
490 allow_negative_impls: bool,
491 ) -> SelectionContext<'cx, 'gcx, 'tcx> {
492 debug!("with_negative({:?})", allow_negative_impls);
495 freshener: infcx.freshener(),
497 intercrate_ambiguity_causes: None,
498 allow_negative_impls,
499 query_mode: TraitQueryMode::Standard,
503 pub fn with_query_mode(
504 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
505 query_mode: TraitQueryMode,
506 ) -> SelectionContext<'cx, 'gcx, 'tcx> {
507 debug!("with_query_mode({:?})", query_mode);
510 freshener: infcx.freshener(),
512 intercrate_ambiguity_causes: None,
513 allow_negative_impls: false,
518 /// Enables tracking of intercrate ambiguity causes. These are
519 /// used in coherence to give improved diagnostics. We don't do
520 /// this until we detect a coherence error because it can lead to
521 /// false overflow results (#47139) and because it costs
522 /// computation time.
523 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
524 assert!(self.intercrate.is_some());
525 assert!(self.intercrate_ambiguity_causes.is_none());
526 self.intercrate_ambiguity_causes = Some(vec![]);
527 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
530 /// Gets the intercrate ambiguity causes collected since tracking
531 /// was enabled and disables tracking at the same time. If
532 /// tracking is not enabled, just returns an empty vector.
533 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
534 assert!(self.intercrate.is_some());
535 self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
538 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
542 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
546 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
550 ///////////////////////////////////////////////////////////////////////////
553 // The selection phase tries to identify *how* an obligation will
554 // be resolved. For example, it will identify which impl or
555 // parameter bound is to be used. The process can be inconclusive
556 // if the self type in the obligation is not fully inferred. Selection
557 // can result in an error in one of two ways:
559 // 1. If no applicable impl or parameter bound can be found.
560 // 2. If the output type parameters in the obligation do not match
561 // those specified by the impl/bound. For example, if the obligation
562 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
563 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
565 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
566 /// type environment by performing unification.
569 obligation: &TraitObligation<'tcx>,
570 ) -> SelectionResult<'tcx, Selection<'tcx>> {
571 debug!("select({:?})", obligation);
572 debug_assert!(!obligation.predicate.has_escaping_bound_vars());
574 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
576 let candidate = match self.candidate_from_obligation(&stack) {
577 Err(SelectionError::Overflow) => {
578 // In standard mode, overflow must have been caught and reported
580 assert!(self.query_mode == TraitQueryMode::Canonical);
581 return Err(SelectionError::Overflow);
589 Ok(Some(candidate)) => candidate,
592 match self.confirm_candidate(obligation, candidate) {
593 Err(SelectionError::Overflow) => {
594 assert!(self.query_mode == TraitQueryMode::Canonical);
595 Err(SelectionError::Overflow)
598 Ok(candidate) => Ok(Some(candidate)),
602 ///////////////////////////////////////////////////////////////////////////
605 // Tests whether an obligation can be selected or whether an impl
606 // can be applied to particular types. It skips the "confirmation"
607 // step and hence completely ignores output type parameters.
609 // The result is "true" if the obligation *may* hold and "false" if
610 // we can be sure it does not.
612 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
613 pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
614 debug!("predicate_may_hold_fatal({:?})", obligation);
616 // This fatal query is a stopgap that should only be used in standard mode,
617 // where we do not expect overflow to be propagated.
618 assert!(self.query_mode == TraitQueryMode::Standard);
620 self.evaluate_obligation_recursively(obligation)
621 .expect("Overflow should be caught earlier in standard query mode")
625 /// Evaluates whether the obligation `obligation` can be satisfied and returns
626 /// an `EvaluationResult`.
627 pub fn evaluate_obligation_recursively(
629 obligation: &PredicateObligation<'tcx>,
630 ) -> Result<EvaluationResult, OverflowError> {
631 self.evaluation_probe(|this| {
632 this.evaluate_predicate_recursively(TraitObligationStackList::empty(),
639 op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
640 ) -> Result<EvaluationResult, OverflowError> {
641 self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
642 let result = op(self)?;
643 match self.infcx.region_constraints_added_in_snapshot(snapshot) {
645 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
650 /// Evaluates the predicates in `predicates` recursively. Note that
651 /// this applies projections in the predicates, and therefore
652 /// is run within an inference probe.
653 fn evaluate_predicates_recursively<'a, 'o, I>(
655 stack: TraitObligationStackList<'o, 'tcx>,
657 ) -> Result<EvaluationResult, OverflowError>
659 I: IntoIterator<Item = PredicateObligation<'tcx>>,
662 let mut result = EvaluatedToOk;
663 for obligation in predicates {
664 let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
666 "evaluate_predicate_recursively({:?}) = {:?}",
669 if let EvaluatedToErr = eval {
670 // fast-path - EvaluatedToErr is the top of the lattice,
671 // so we don't need to look on the other predicates.
672 return Ok(EvaluatedToErr);
674 result = cmp::max(result, eval);
680 fn evaluate_predicate_recursively<'o>(
682 previous_stack: TraitObligationStackList<'o, 'tcx>,
683 obligation: PredicateObligation<'tcx>,
684 ) -> Result<EvaluationResult, OverflowError> {
685 debug!("evaluate_predicate_recursively(previous_stack={:?}, obligation={:?})",
686 previous_stack.head(), obligation);
688 // Previous_stack stores a TraitObligatiom, while 'obligation' is
689 // a PredicateObligation. These are distinct types, so we can't
690 // use any Option combinator method that would force them to be
692 match previous_stack.head() {
693 Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
694 None => self.check_recursion_limit(&obligation, &obligation)?
697 match obligation.predicate {
698 ty::Predicate::Trait(ref t) => {
699 debug_assert!(!t.has_escaping_bound_vars());
700 let mut obligation = obligation.with(t.clone());
701 obligation.recursion_depth += 1;
702 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
705 ty::Predicate::Subtype(ref p) => {
706 // does this code ever run?
708 .subtype_predicate(&obligation.cause, obligation.param_env, p)
710 Some(Ok(InferOk { mut obligations, .. })) => {
711 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
712 self.evaluate_predicates_recursively(previous_stack,obligations.into_iter())
714 Some(Err(_)) => Ok(EvaluatedToErr),
715 None => Ok(EvaluatedToAmbig),
719 ty::Predicate::WellFormed(ty) => match ty::wf::obligations(
721 obligation.param_env,
722 obligation.cause.body_id,
724 obligation.cause.span,
726 Some(mut obligations) => {
727 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
728 self.evaluate_predicates_recursively(previous_stack, obligations.into_iter())
730 None => Ok(EvaluatedToAmbig),
733 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
734 // we do not consider region relationships when
735 // evaluating trait matches
736 Ok(EvaluatedToOkModuloRegions)
739 ty::Predicate::ObjectSafe(trait_def_id) => {
740 if self.tcx().is_object_safe(trait_def_id) {
747 ty::Predicate::Projection(ref data) => {
748 let project_obligation = obligation.with(data.clone());
749 match project::poly_project_and_unify_type(self, &project_obligation) {
750 Ok(Some(mut subobligations)) => {
751 self.add_depth(subobligations.iter_mut(), obligation.recursion_depth);
752 let result = self.evaluate_predicates_recursively(
754 subobligations.into_iter(),
757 ProjectionCacheKey::from_poly_projection_predicate(self, data)
759 self.infcx.projection_cache.borrow_mut().complete(key);
763 Ok(None) => Ok(EvaluatedToAmbig),
764 Err(_) => Ok(EvaluatedToErr),
768 ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
769 match self.infcx.closure_kind(closure_def_id, closure_substs) {
770 Some(closure_kind) => {
771 if closure_kind.extends(kind) {
777 None => Ok(EvaluatedToAmbig),
781 ty::Predicate::ConstEvaluatable(def_id, substs) => {
782 let tcx = self.tcx();
783 match tcx.lift_to_global(&(obligation.param_env, substs)) {
784 Some((param_env, substs)) => {
786 ty::Instance::resolve(tcx.global_tcx(), param_env, def_id, substs);
787 if let Some(instance) = instance {
792 match self.tcx().const_eval(param_env.and(cid)) {
793 Ok(_) => Ok(EvaluatedToOk),
794 Err(_) => Ok(EvaluatedToErr),
801 // Inference variables still left in param_env or substs.
809 fn evaluate_trait_predicate_recursively<'o>(
811 previous_stack: TraitObligationStackList<'o, 'tcx>,
812 mut obligation: TraitObligation<'tcx>,
813 ) -> Result<EvaluationResult, OverflowError> {
814 debug!("evaluate_trait_predicate_recursively({:?})", obligation);
816 if self.intercrate.is_none() && obligation.is_global()
821 .all(|bound| bound.needs_subst())
823 // If a param env has no global bounds, global obligations do not
824 // depend on its particular value in order to work, so we can clear
825 // out the param env and get better caching.
827 "evaluate_trait_predicate_recursively({:?}) - in global",
830 obligation.param_env = obligation.param_env.without_caller_bounds();
833 let stack = self.push_stack(previous_stack, &obligation);
834 let fresh_trait_ref = stack.fresh_trait_ref;
835 if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
836 debug!("CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result);
840 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
841 let result = result?;
843 debug!("CACHE MISS: EVAL({:?})={:?}", fresh_trait_ref, result);
844 self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
849 fn evaluate_stack<'o>(
851 stack: &TraitObligationStack<'o, 'tcx>,
852 ) -> Result<EvaluationResult, OverflowError> {
853 // In intercrate mode, whenever any of the types are unbound,
854 // there can always be an impl. Even if there are no impls in
855 // this crate, perhaps the type would be unified with
856 // something from another crate that does provide an impl.
858 // In intra mode, we must still be conservative. The reason is
859 // that we want to avoid cycles. Imagine an impl like:
861 // impl<T:Eq> Eq for Vec<T>
863 // and a trait reference like `$0 : Eq` where `$0` is an
864 // unbound variable. When we evaluate this trait-reference, we
865 // will unify `$0` with `Vec<$1>` (for some fresh variable
866 // `$1`), on the condition that `$1 : Eq`. We will then wind
867 // up with many candidates (since that are other `Eq` impls
868 // that apply) and try to winnow things down. This results in
869 // a recursive evaluation that `$1 : Eq` -- as you can
870 // imagine, this is just where we started. To avoid that, we
871 // check for unbound variables and return an ambiguous (hence possible)
872 // match if we've seen this trait before.
874 // This suffices to allow chains like `FnMut` implemented in
875 // terms of `Fn` etc, but we could probably make this more
877 let unbound_input_types = stack
881 .any(|ty| ty.is_fresh());
882 // this check was an imperfect workaround for a bug n the old
883 // intercrate mode, it should be removed when that goes away.
884 if unbound_input_types && self.intercrate == Some(IntercrateMode::Issue43355) {
886 "evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
887 stack.fresh_trait_ref
889 // Heuristics: show the diagnostics when there are no candidates in crate.
890 if self.intercrate_ambiguity_causes.is_some() {
891 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
892 if let Ok(candidate_set) = self.assemble_candidates(stack) {
893 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
894 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
895 let self_ty = trait_ref.self_ty();
896 let cause = IntercrateAmbiguityCause::DownstreamCrate {
897 trait_desc: trait_ref.to_string(),
898 self_desc: if self_ty.has_concrete_skeleton() {
899 Some(self_ty.to_string())
904 debug!("evaluate_stack: pushing cause = {:?}", cause);
905 self.intercrate_ambiguity_causes
912 return Ok(EvaluatedToAmbig);
914 if unbound_input_types && stack.iter().skip(1).any(|prev| {
915 stack.obligation.param_env == prev.obligation.param_env
916 && self.match_fresh_trait_refs(&stack.fresh_trait_ref, &prev.fresh_trait_ref)
919 "evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
920 stack.fresh_trait_ref
922 return Ok(EvaluatedToUnknown);
925 // If there is any previous entry on the stack that precisely
926 // matches this obligation, then we can assume that the
927 // obligation is satisfied for now (still all other conditions
928 // must be met of course). One obvious case this comes up is
929 // marker traits like `Send`. Think of a linked list:
931 // struct List<T> { data: T, next: Option<Box<List<T>>> }
933 // `Box<List<T>>` will be `Send` if `T` is `Send` and
934 // `Option<Box<List<T>>>` is `Send`, and in turn
935 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
938 // Note that we do this comparison using the `fresh_trait_ref`
939 // fields. Because these have all been freshened using
940 // `self.freshener`, we can be sure that (a) this will not
941 // affect the inferencer state and (b) that if we see two
942 // fresh regions with the same index, they refer to the same
943 // unbound type variable.
944 if let Some(rec_index) = stack.iter()
945 .skip(1) // skip top-most frame
946 .position(|prev| stack.obligation.param_env == prev.obligation.param_env &&
947 stack.fresh_trait_ref == prev.fresh_trait_ref)
949 debug!("evaluate_stack({:?}) --> recursive", stack.fresh_trait_ref);
951 // Subtle: when checking for a coinductive cycle, we do
952 // not compare using the "freshened trait refs" (which
953 // have erased regions) but rather the fully explicit
954 // trait refs. This is important because it's only a cycle
955 // if the regions match exactly.
956 let cycle = stack.iter().skip(1).take(rec_index + 1);
957 let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate));
958 if self.coinductive_match(cycle) {
960 "evaluate_stack({:?}) --> recursive, coinductive",
961 stack.fresh_trait_ref
963 return Ok(EvaluatedToOk);
966 "evaluate_stack({:?}) --> recursive, inductive",
967 stack.fresh_trait_ref
969 return Ok(EvaluatedToRecur);
973 match self.candidate_from_obligation(stack) {
974 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
975 Ok(None) => Ok(EvaluatedToAmbig),
976 Err(Overflow) => Err(OverflowError),
977 Err(..) => Ok(EvaluatedToErr),
981 /// For defaulted traits, we use a co-inductive strategy to solve, so
982 /// that recursion is ok. This routine returns true if the top of the
983 /// stack (`cycle[0]`):
985 /// - is a defaulted trait, and
986 /// - it also appears in the backtrace at some position `X`; and,
987 /// - all the predicates at positions `X..` between `X` an the top are
988 /// also defaulted traits.
989 pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
991 I: Iterator<Item = ty::Predicate<'tcx>>,
993 let mut cycle = cycle;
994 cycle.all(|predicate| self.coinductive_predicate(predicate))
997 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
998 let result = match predicate {
999 ty::Predicate::Trait(ref data) => self.tcx().trait_is_auto(data.def_id()),
1002 debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
1006 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
1007 /// obligations are met. Returns true if `candidate` remains viable after this further
1009 fn evaluate_candidate<'o>(
1011 stack: &TraitObligationStack<'o, 'tcx>,
1012 candidate: &SelectionCandidate<'tcx>,
1013 ) -> Result<EvaluationResult, OverflowError> {
1015 "evaluate_candidate: depth={} candidate={:?}",
1016 stack.obligation.recursion_depth, candidate
1018 let result = self.evaluation_probe(|this| {
1019 let candidate = (*candidate).clone();
1020 match this.confirm_candidate(stack.obligation, candidate) {
1021 Ok(selection) => this.evaluate_predicates_recursively(
1023 selection.nested_obligations().into_iter()
1025 Err(..) => Ok(EvaluatedToErr),
1029 "evaluate_candidate: depth={} result={:?}",
1030 stack.obligation.recursion_depth, result
1035 fn check_evaluation_cache(
1037 param_env: ty::ParamEnv<'tcx>,
1038 trait_ref: ty::PolyTraitRef<'tcx>,
1039 ) -> Option<EvaluationResult> {
1040 let tcx = self.tcx();
1041 if self.can_use_global_caches(param_env) {
1042 let cache = tcx.evaluation_cache.hashmap.borrow();
1043 if let Some(cached) = cache.get(&trait_ref) {
1044 return Some(cached.get(tcx));
1052 .map(|v| v.get(tcx))
1055 fn insert_evaluation_cache(
1057 param_env: ty::ParamEnv<'tcx>,
1058 trait_ref: ty::PolyTraitRef<'tcx>,
1059 dep_node: DepNodeIndex,
1060 result: EvaluationResult,
1062 // Avoid caching results that depend on more than just the trait-ref
1063 // - the stack can create recursion.
1064 if result.is_stack_dependent() {
1068 if self.can_use_global_caches(param_env) {
1069 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
1071 "insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global",
1074 // This may overwrite the cache with the same value
1075 // FIXME: Due to #50507 this overwrites the different values
1076 // This should be changed to use HashMapExt::insert_same
1077 // when that is fixed
1082 .insert(trait_ref, WithDepNode::new(dep_node, result));
1088 "insert_evaluation_cache(trait_ref={:?}, candidate={:?})",
1095 .insert(trait_ref, WithDepNode::new(dep_node, result));
1098 // Due to caching of projection results, it's possible for a subobligation
1099 // to have a *lower* recursion_depth than the obligation used to create it.
1100 // To ensure that obligation_depth never decreasees, we force all subobligations
1101 // to have at least the depth of the original obligation.
1102 fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(&self, it: I,
1104 it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
1107 // Check that the recursion limit has not been exceeded.
1109 // The weird return type of this function allows it to be used with the 'try' (?)
1110 // operator within certain functions
1111 fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>(
1113 obligation: &Obligation<'tcx, T>,
1114 error_obligation: &Obligation<'tcx, V>
1115 ) -> Result<(), OverflowError> {
1116 let recursion_limit = *self.infcx.tcx.sess.recursion_limit.get();
1117 if obligation.recursion_depth >= recursion_limit {
1118 match self.query_mode {
1119 TraitQueryMode::Standard => {
1120 self.infcx().report_overflow_error(error_obligation, true);
1122 TraitQueryMode::Canonical => {
1123 return Err(OverflowError);
1130 ///////////////////////////////////////////////////////////////////////////
1131 // CANDIDATE ASSEMBLY
1133 // The selection process begins by examining all in-scope impls,
1134 // caller obligations, and so forth and assembling a list of
1135 // candidates. See the [rustc guide] for more details.
1138 // https://rust-lang.github.io/rustc-guide/traits/resolution.html#candidate-assembly
1140 fn candidate_from_obligation<'o>(
1142 stack: &TraitObligationStack<'o, 'tcx>,
1143 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1144 // Watch out for overflow. This intentionally bypasses (and does
1145 // not update) the cache.
1146 self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
1149 // Check the cache. Note that we freshen the trait-ref
1150 // separately rather than using `stack.fresh_trait_ref` --
1151 // this is because we want the unbound variables to be
1152 // replaced with fresh types starting from index 0.
1153 let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate.clone());
1155 "candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
1156 cache_fresh_trait_pred, stack
1158 debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
1161 self.check_candidate_cache(stack.obligation.param_env, &cache_fresh_trait_pred)
1163 debug!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c);
1167 // If no match, compute result and insert into cache.
1168 let (candidate, dep_node) =
1169 self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
1172 "CACHE MISS: SELECT({:?})={:?}",
1173 cache_fresh_trait_pred, candidate
1175 self.insert_candidate_cache(
1176 stack.obligation.param_env,
1177 cache_fresh_trait_pred,
1184 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1186 OP: FnOnce(&mut Self) -> R,
1188 let (result, dep_node) = self.tcx()
1190 .with_anon_task(DepKind::TraitSelect, || op(self));
1191 self.tcx().dep_graph.read_index(dep_node);
1195 // Treat negative impls as unimplemented
1196 fn filter_negative_impls(
1198 candidate: SelectionCandidate<'tcx>,
1199 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1200 if let ImplCandidate(def_id) = candidate {
1201 if !self.allow_negative_impls
1202 && self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative
1204 return Err(Unimplemented);
1210 fn candidate_from_obligation_no_cache<'o>(
1212 stack: &TraitObligationStack<'o, 'tcx>,
1213 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1214 if stack.obligation.predicate.references_error() {
1215 // If we encounter a `Error`, we generally prefer the
1216 // most "optimistic" result in response -- that is, the
1217 // one least likely to report downstream errors. But
1218 // because this routine is shared by coherence and by
1219 // trait selection, there isn't an obvious "right" choice
1220 // here in that respect, so we opt to just return
1221 // ambiguity and let the upstream clients sort it out.
1225 if let Some(conflict) = self.is_knowable(stack) {
1226 debug!("coherence stage: not knowable");
1227 if self.intercrate_ambiguity_causes.is_some() {
1228 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
1229 // Heuristics: show the diagnostics when there are no candidates in crate.
1230 if let Ok(candidate_set) = self.assemble_candidates(stack) {
1231 let mut no_candidates_apply = true;
1233 let evaluated_candidates = candidate_set
1236 .map(|c| self.evaluate_candidate(stack, &c));
1238 for ec in evaluated_candidates {
1242 no_candidates_apply = false;
1246 Err(e) => return Err(e.into()),
1251 if !candidate_set.ambiguous && no_candidates_apply {
1252 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
1253 let self_ty = trait_ref.self_ty();
1254 let trait_desc = trait_ref.to_string();
1255 let self_desc = if self_ty.has_concrete_skeleton() {
1256 Some(self_ty.to_string())
1260 let cause = if let Conflict::Upstream = conflict {
1261 IntercrateAmbiguityCause::UpstreamCrateUpdate {
1266 IntercrateAmbiguityCause::DownstreamCrate {
1271 debug!("evaluate_stack: pushing cause = {:?}", cause);
1272 self.intercrate_ambiguity_causes
1282 let candidate_set = self.assemble_candidates(stack)?;
1284 if candidate_set.ambiguous {
1285 debug!("candidate set contains ambig");
1289 let mut candidates = candidate_set.vec;
1292 "assembled {} candidates for {:?}: {:?}",
1298 // At this point, we know that each of the entries in the
1299 // candidate set is *individually* applicable. Now we have to
1300 // figure out if they contain mutual incompatibilities. This
1301 // frequently arises if we have an unconstrained input type --
1302 // for example, we are looking for $0:Eq where $0 is some
1303 // unconstrained type variable. In that case, we'll get a
1304 // candidate which assumes $0 == int, one that assumes $0 ==
1305 // usize, etc. This spells an ambiguity.
1307 // If there is more than one candidate, first winnow them down
1308 // by considering extra conditions (nested obligations and so
1309 // forth). We don't winnow if there is exactly one
1310 // candidate. This is a relatively minor distinction but it
1311 // can lead to better inference and error-reporting. An
1312 // example would be if there was an impl:
1314 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
1316 // and we were to see some code `foo.push_clone()` where `boo`
1317 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
1318 // we were to winnow, we'd wind up with zero candidates.
1319 // Instead, we select the right impl now but report `Bar does
1320 // not implement Clone`.
1321 if candidates.len() == 1 {
1322 return self.filter_negative_impls(candidates.pop().unwrap());
1325 // Winnow, but record the exact outcome of evaluation, which
1326 // is needed for specialization. Propagate overflow if it occurs.
1327 let mut candidates = candidates
1329 .map(|c| match self.evaluate_candidate(stack, &c) {
1330 Ok(eval) if eval.may_apply() => Ok(Some(EvaluatedCandidate {
1335 Err(OverflowError) => Err(Overflow),
1337 .flat_map(Result::transpose)
1338 .collect::<Result<Vec<_>, _>>()?;
1341 "winnowed to {} candidates for {:?}: {:?}",
1347 // If there are STILL multiple candidates, we can further
1348 // reduce the list by dropping duplicates -- including
1349 // resolving specializations.
1350 if candidates.len() > 1 {
1352 while i < candidates.len() {
1353 let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
1354 self.candidate_should_be_dropped_in_favor_of(&candidates[i], &candidates[j])
1358 "Dropping candidate #{}/{}: {:?}",
1363 candidates.swap_remove(i);
1366 "Retaining candidate #{}/{}: {:?}",
1373 // If there are *STILL* multiple candidates, give up
1374 // and report ambiguity.
1376 debug!("multiple matches, ambig");
1383 // If there are *NO* candidates, then there are no impls --
1384 // that we know of, anyway. Note that in the case where there
1385 // are unbound type variables within the obligation, it might
1386 // be the case that you could still satisfy the obligation
1387 // from another crate by instantiating the type variables with
1388 // a type from another crate that does have an impl. This case
1389 // is checked for in `evaluate_stack` (and hence users
1390 // who might care about this case, like coherence, should use
1392 if candidates.is_empty() {
1393 return Err(Unimplemented);
1396 // Just one candidate left.
1397 self.filter_negative_impls(candidates.pop().unwrap().candidate)
1400 fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
1401 debug!("is_knowable(intercrate={:?})", self.intercrate);
1403 if !self.intercrate.is_some() {
1407 let obligation = &stack.obligation;
1408 let predicate = self.infcx()
1409 .resolve_type_vars_if_possible(&obligation.predicate);
1411 // OK to skip binder because of the nature of the
1412 // trait-ref-is-knowable check, which does not care about
1414 let trait_ref = predicate.skip_binder().trait_ref;
1416 let result = coherence::trait_ref_is_knowable(self.tcx(), trait_ref);
1418 Some(Conflict::Downstream {
1419 used_to_be_broken: true,
1421 Some(IntercrateMode::Issue43355),
1422 ) = (result, self.intercrate)
1424 debug!("is_knowable: IGNORING conflict to be bug-compatible with #43355");
1431 /// Returns true if the global caches can be used.
1432 /// Do note that if the type itself is not in the
1433 /// global tcx, the local caches will be used.
1434 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1435 // If there are any where-clauses in scope, then we always use
1436 // a cache local to this particular scope. Otherwise, we
1437 // switch to a global cache. We used to try and draw
1438 // finer-grained distinctions, but that led to a serious of
1439 // annoying and weird bugs like #22019 and #18290. This simple
1440 // rule seems to be pretty clearly safe and also still retains
1441 // a very high hit rate (~95% when compiling rustc).
1442 if !param_env.caller_bounds.is_empty() {
1446 // Avoid using the master cache during coherence and just rely
1447 // on the local cache. This effectively disables caching
1448 // during coherence. It is really just a simplification to
1449 // avoid us having to fear that coherence results "pollute"
1450 // the master cache. Since coherence executes pretty quickly,
1451 // it's not worth going to more trouble to increase the
1452 // hit-rate I don't think.
1453 if self.intercrate.is_some() {
1457 // Same idea as the above, but for alt trait object modes. These
1458 // should only be used in intercrate mode - better safe than sorry.
1459 if self.infcx.trait_object_mode() != TraitObjectMode::NoSquash {
1460 bug!("using squashing TraitObjectMode outside of intercrate mode? param_env={:?}",
1464 // Otherwise, we can use the global cache.
1468 fn check_candidate_cache(
1470 param_env: ty::ParamEnv<'tcx>,
1471 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>,
1472 ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1473 let tcx = self.tcx();
1474 let trait_ref = &cache_fresh_trait_pred.skip_binder().trait_ref;
1475 if self.can_use_global_caches(param_env) {
1476 let cache = tcx.selection_cache.hashmap.borrow();
1477 if let Some(cached) = cache.get(&trait_ref) {
1478 return Some(cached.get(tcx));
1486 .map(|v| v.get(tcx))
1489 /// Determines whether can we safely cache the result
1490 /// of selecting an obligation. This is almost always 'true',
1491 /// except when dealing with certain ParamCandidates.
1493 /// Ordinarily, a ParamCandidate will contain no inference variables,
1494 /// since it was usually produced directly from a DefId. However,
1495 /// certain cases (currently only librustdoc's blanket impl finder),
1496 /// a ParamEnv may be explicitly constructed with inference types.
1497 /// When this is the case, we do *not* want to cache the resulting selection
1498 /// candidate. This is due to the fact that it might not always be possible
1499 /// to equate the obligation's trait ref and the candidate's trait ref,
1500 /// if more constraints end up getting added to an inference variable.
1502 /// Because of this, we always want to re-run the full selection
1503 /// process for our obligation the next time we see it, since
1504 /// we might end up picking a different SelectionCandidate (or none at all)
1505 fn can_cache_candidate(&self,
1506 result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>
1509 Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => {
1510 !trait_ref.skip_binder().input_types().any(|t| t.walk().any(|t_| t_.is_ty_infer()))
1516 fn insert_candidate_cache(
1518 param_env: ty::ParamEnv<'tcx>,
1519 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1520 dep_node: DepNodeIndex,
1521 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1523 let tcx = self.tcx();
1524 let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref;
1526 if !self.can_cache_candidate(&candidate) {
1527 debug!("insert_candidate_cache(trait_ref={:?}, candidate={:?} -\
1528 candidate is not cacheable", trait_ref, candidate);
1533 if self.can_use_global_caches(param_env) {
1534 if let Err(Overflow) = candidate {
1535 // Don't cache overflow globally; we only produce this
1536 // in certain modes.
1537 } else if let Some(trait_ref) = tcx.lift_to_global(&trait_ref) {
1538 if let Some(candidate) = tcx.lift_to_global(&candidate) {
1540 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) global",
1541 trait_ref, candidate,
1543 // This may overwrite the cache with the same value
1547 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1554 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) local",
1555 trait_ref, candidate,
1561 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1564 fn assemble_candidates<'o>(
1566 stack: &TraitObligationStack<'o, 'tcx>,
1567 ) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
1568 let TraitObligationStack { obligation, .. } = *stack;
1569 let ref obligation = Obligation {
1570 param_env: obligation.param_env,
1571 cause: obligation.cause.clone(),
1572 recursion_depth: obligation.recursion_depth,
1573 predicate: self.infcx()
1574 .resolve_type_vars_if_possible(&obligation.predicate),
1577 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1578 // Self is a type variable (e.g., `_: AsRef<str>`).
1580 // This is somewhat problematic, as the current scheme can't really
1581 // handle it turning to be a projection. This does end up as truly
1582 // ambiguous in most cases anyway.
1584 // Take the fast path out - this also improves
1585 // performance by preventing assemble_candidates_from_impls from
1586 // matching every impl for this trait.
1587 return Ok(SelectionCandidateSet {
1593 let mut candidates = SelectionCandidateSet {
1598 self.assemble_candidates_for_trait_alias(obligation, &mut candidates)?;
1600 // Other bounds. Consider both in-scope bounds from fn decl
1601 // and applicable impls. There is a certain set of precedence rules here.
1602 let def_id = obligation.predicate.def_id();
1603 let lang_items = self.tcx().lang_items();
1605 if lang_items.copy_trait() == Some(def_id) {
1607 "obligation self ty is {:?}",
1608 obligation.predicate.skip_binder().self_ty()
1611 // User-defined copy impls are permitted, but only for
1612 // structs and enums.
1613 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1615 // For other types, we'll use the builtin rules.
1616 let copy_conditions = self.copy_clone_conditions(obligation);
1617 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1618 } else if lang_items.sized_trait() == Some(def_id) {
1619 // Sized is never implementable by end-users, it is
1620 // always automatically computed.
1621 let sized_conditions = self.sized_conditions(obligation);
1622 self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates)?;
1623 } else if lang_items.unsize_trait() == Some(def_id) {
1624 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1626 if lang_items.clone_trait() == Some(def_id) {
1627 // Same builtin conditions as `Copy`, i.e., every type which has builtin support
1628 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
1629 // types have builtin support for `Clone`.
1630 let clone_conditions = self.copy_clone_conditions(obligation);
1631 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
1634 self.assemble_generator_candidates(obligation, &mut candidates)?;
1635 self.assemble_closure_candidates(obligation, &mut candidates)?;
1636 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1637 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1638 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1641 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1642 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1643 // Auto implementations have lower priority, so we only
1644 // consider triggering a default if there is no other impl that can apply.
1645 if candidates.vec.is_empty() {
1646 self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?;
1648 debug!("candidate list size: {}", candidates.vec.len());
1652 fn assemble_candidates_from_projected_tys(
1654 obligation: &TraitObligation<'tcx>,
1655 candidates: &mut SelectionCandidateSet<'tcx>,
1657 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1659 // before we go into the whole placeholder thing, just
1660 // quickly check if the self-type is a projection at all.
1661 match obligation.predicate.skip_binder().trait_ref.self_ty().sty {
1662 ty::Projection(_) | ty::Opaque(..) => {}
1663 ty::Infer(ty::TyVar(_)) => {
1665 obligation.cause.span,
1666 "Self=_ should have been handled by assemble_candidates"
1672 let result = self.infcx.probe(|_| {
1673 self.match_projection_obligation_against_definition_bounds(obligation)
1677 candidates.vec.push(ProjectionCandidate);
1681 fn match_projection_obligation_against_definition_bounds(
1683 obligation: &TraitObligation<'tcx>,
1685 let poly_trait_predicate = self.infcx()
1686 .resolve_type_vars_if_possible(&obligation.predicate);
1687 let (skol_trait_predicate, _) = self.infcx()
1688 .replace_bound_vars_with_placeholders(&poly_trait_predicate);
1690 "match_projection_obligation_against_definition_bounds: \
1691 skol_trait_predicate={:?}",
1692 skol_trait_predicate,
1695 let (def_id, substs) = match skol_trait_predicate.trait_ref.self_ty().sty {
1696 ty::Projection(ref data) => (data.trait_ref(self.tcx()).def_id, data.substs),
1697 ty::Opaque(def_id, substs) => (def_id, substs),
1700 obligation.cause.span,
1701 "match_projection_obligation_against_definition_bounds() called \
1702 but self-ty is not a projection: {:?}",
1703 skol_trait_predicate.trait_ref.self_ty()
1708 "match_projection_obligation_against_definition_bounds: \
1709 def_id={:?}, substs={:?}",
1713 let predicates_of = self.tcx().predicates_of(def_id);
1714 let bounds = predicates_of.instantiate(self.tcx(), substs);
1716 "match_projection_obligation_against_definition_bounds: \
1721 let matching_bound = util::elaborate_predicates(self.tcx(), bounds.predicates)
1724 self.infcx.probe(|_| {
1725 self.match_projection(
1728 skol_trait_predicate.trait_ref.clone(),
1734 "match_projection_obligation_against_definition_bounds: \
1735 matching_bound={:?}",
1738 match matching_bound {
1741 // Repeat the successful match, if any, this time outside of a probe.
1742 let result = self.match_projection(
1745 skol_trait_predicate.trait_ref.clone(),
1754 fn match_projection(
1756 obligation: &TraitObligation<'tcx>,
1757 trait_bound: ty::PolyTraitRef<'tcx>,
1758 skol_trait_ref: ty::TraitRef<'tcx>,
1760 debug_assert!(!skol_trait_ref.has_escaping_bound_vars());
1762 .at(&obligation.cause, obligation.param_env)
1763 .sup(ty::Binder::dummy(skol_trait_ref), trait_bound)
1767 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1768 /// supplied to find out whether it is listed among them.
1770 /// Never affects inference environment.
1771 fn assemble_candidates_from_caller_bounds<'o>(
1773 stack: &TraitObligationStack<'o, 'tcx>,
1774 candidates: &mut SelectionCandidateSet<'tcx>,
1775 ) -> Result<(), SelectionError<'tcx>> {
1777 "assemble_candidates_from_caller_bounds({:?})",
1781 let all_bounds = stack
1786 .filter_map(|o| o.to_opt_poly_trait_ref());
1788 // micro-optimization: filter out predicates relating to different
1790 let matching_bounds =
1791 all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
1793 // keep only those bounds which may apply, and propagate overflow if it occurs
1794 let mut param_candidates = vec![];
1795 for bound in matching_bounds {
1796 let wc = self.evaluate_where_clause(stack, bound.clone())?;
1798 param_candidates.push(ParamCandidate(bound));
1802 candidates.vec.extend(param_candidates);
1807 fn evaluate_where_clause<'o>(
1809 stack: &TraitObligationStack<'o, 'tcx>,
1810 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1811 ) -> Result<EvaluationResult, OverflowError> {
1812 self.evaluation_probe(|this| {
1813 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1814 Ok(obligations) => {
1815 this.evaluate_predicates_recursively(stack.list(), obligations.into_iter())
1817 Err(()) => Ok(EvaluatedToErr),
1822 fn assemble_generator_candidates(
1824 obligation: &TraitObligation<'tcx>,
1825 candidates: &mut SelectionCandidateSet<'tcx>,
1826 ) -> Result<(), SelectionError<'tcx>> {
1827 if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
1831 // OK to skip binder because the substs on generator types never
1832 // touch bound regions, they just capture the in-scope
1833 // type/region parameters
1834 let self_ty = *obligation.self_ty().skip_binder();
1836 ty::Generator(..) => {
1838 "assemble_generator_candidates: self_ty={:?} obligation={:?}",
1842 candidates.vec.push(GeneratorCandidate);
1844 ty::Infer(ty::TyVar(_)) => {
1845 debug!("assemble_generator_candidates: ambiguous self-type");
1846 candidates.ambiguous = true;
1854 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1855 /// FnMut<..>` where `X` is a closure type.
1857 /// Note: the type parameters on a closure candidate are modeled as *output* type
1858 /// parameters and hence do not affect whether this trait is a match or not. They will be
1859 /// unified during the confirmation step.
1860 fn assemble_closure_candidates(
1862 obligation: &TraitObligation<'tcx>,
1863 candidates: &mut SelectionCandidateSet<'tcx>,
1864 ) -> Result<(), SelectionError<'tcx>> {
1865 let kind = match self.tcx()
1867 .fn_trait_kind(obligation.predicate.def_id())
1875 // OK to skip binder because the substs on closure types never
1876 // touch bound regions, they just capture the in-scope
1877 // type/region parameters
1878 match obligation.self_ty().skip_binder().sty {
1879 ty::Closure(closure_def_id, closure_substs) => {
1881 "assemble_unboxed_candidates: kind={:?} obligation={:?}",
1884 match self.infcx.closure_kind(closure_def_id, closure_substs) {
1885 Some(closure_kind) => {
1887 "assemble_unboxed_candidates: closure_kind = {:?}",
1890 if closure_kind.extends(kind) {
1891 candidates.vec.push(ClosureCandidate);
1895 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1896 candidates.vec.push(ClosureCandidate);
1900 ty::Infer(ty::TyVar(_)) => {
1901 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1902 candidates.ambiguous = true;
1910 /// Implement one of the `Fn()` family for a fn pointer.
1911 fn assemble_fn_pointer_candidates(
1913 obligation: &TraitObligation<'tcx>,
1914 candidates: &mut SelectionCandidateSet<'tcx>,
1915 ) -> Result<(), SelectionError<'tcx>> {
1916 // We provide impl of all fn traits for fn pointers.
1919 .fn_trait_kind(obligation.predicate.def_id())
1925 // OK to skip binder because what we are inspecting doesn't involve bound regions
1926 let self_ty = *obligation.self_ty().skip_binder();
1928 ty::Infer(ty::TyVar(_)) => {
1929 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1930 candidates.ambiguous = true; // could wind up being a fn() type
1932 // provide an impl, but only for suitable `fn` pointers
1933 ty::FnDef(..) | ty::FnPtr(_) => {
1935 unsafety: hir::Unsafety::Normal,
1939 } = self_ty.fn_sig(self.tcx()).skip_binder()
1941 candidates.vec.push(FnPointerCandidate);
1950 /// Search for impls that might apply to `obligation`.
1951 fn assemble_candidates_from_impls(
1953 obligation: &TraitObligation<'tcx>,
1954 candidates: &mut SelectionCandidateSet<'tcx>,
1955 ) -> Result<(), SelectionError<'tcx>> {
1957 "assemble_candidates_from_impls(obligation={:?})",
1961 self.tcx().for_each_relevant_impl(
1962 obligation.predicate.def_id(),
1963 obligation.predicate.skip_binder().trait_ref.self_ty(),
1965 self.infcx.probe(|_| {
1966 if let Ok(_substs) = self.match_impl(impl_def_id, obligation)
1968 candidates.vec.push(ImplCandidate(impl_def_id));
1977 fn assemble_candidates_from_auto_impls(
1979 obligation: &TraitObligation<'tcx>,
1980 candidates: &mut SelectionCandidateSet<'tcx>,
1981 ) -> Result<(), SelectionError<'tcx>> {
1982 // OK to skip binder here because the tests we do below do not involve bound regions
1983 let self_ty = *obligation.self_ty().skip_binder();
1984 debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
1986 let def_id = obligation.predicate.def_id();
1988 if self.tcx().trait_is_auto(def_id) {
1990 ty::Dynamic(..) => {
1991 // For object types, we don't know what the closed
1992 // over types are. This means we conservatively
1993 // say nothing; a candidate may be added by
1994 // `assemble_candidates_from_object_ty`.
1996 ty::Foreign(..) => {
1997 // Since the contents of foreign types is unknown,
1998 // we don't add any `..` impl. Default traits could
1999 // still be provided by a manual implementation for
2000 // this trait and type.
2002 ty::Param(..) | ty::Projection(..) => {
2003 // In these cases, we don't know what the actual
2004 // type is. Therefore, we cannot break it down
2005 // into its constituent types. So we don't
2006 // consider the `..` impl but instead just add no
2007 // candidates: this means that typeck will only
2008 // succeed if there is another reason to believe
2009 // that this obligation holds. That could be a
2010 // where-clause or, in the case of an object type,
2011 // it could be that the object type lists the
2012 // trait (e.g., `Foo+Send : Send`). See
2013 // `compile-fail/typeck-default-trait-impl-send-param.rs`
2014 // for an example of a test case that exercises
2017 ty::Infer(ty::TyVar(_)) => {
2018 // the auto impl might apply, we don't know
2019 candidates.ambiguous = true;
2021 _ => candidates.vec.push(AutoImplCandidate(def_id.clone())),
2028 /// Search for impls that might apply to `obligation`.
2029 fn assemble_candidates_from_object_ty(
2031 obligation: &TraitObligation<'tcx>,
2032 candidates: &mut SelectionCandidateSet<'tcx>,
2035 "assemble_candidates_from_object_ty(self_ty={:?})",
2036 obligation.self_ty().skip_binder()
2039 self.infcx.probe(|_snapshot| {
2040 // The code below doesn't care about regions, and the
2041 // self-ty here doesn't escape this probe, so just erase
2043 let self_ty = self.tcx().erase_late_bound_regions(&obligation.self_ty());
2044 let poly_trait_ref = match self_ty.sty {
2045 ty::Dynamic(ref data, ..) => {
2046 if data.auto_traits()
2047 .any(|did| did == obligation.predicate.def_id())
2050 "assemble_candidates_from_object_ty: matched builtin bound, \
2053 candidates.vec.push(BuiltinObjectCandidate);
2057 data.principal().with_self_ty(self.tcx(), self_ty)
2059 ty::Infer(ty::TyVar(_)) => {
2060 debug!("assemble_candidates_from_object_ty: ambiguous");
2061 candidates.ambiguous = true; // could wind up being an object type
2068 "assemble_candidates_from_object_ty: poly_trait_ref={:?}",
2072 // Count only those upcast versions that match the trait-ref
2073 // we are looking for. Specifically, do not only check for the
2074 // correct trait, but also the correct type parameters.
2075 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
2076 // but `Foo` is declared as `trait Foo : Bar<u32>`.
2077 let upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref)
2078 .filter(|upcast_trait_ref| {
2079 self.infcx.probe(|_| {
2080 let upcast_trait_ref = upcast_trait_ref.clone();
2081 self.match_poly_trait_ref(obligation, upcast_trait_ref)
2087 if upcast_trait_refs > 1 {
2088 // Can be upcast in many ways; need more type information.
2089 candidates.ambiguous = true;
2090 } else if upcast_trait_refs == 1 {
2091 candidates.vec.push(ObjectCandidate);
2096 /// Search for unsizing that might apply to `obligation`.
2097 fn assemble_candidates_for_unsizing(
2099 obligation: &TraitObligation<'tcx>,
2100 candidates: &mut SelectionCandidateSet<'tcx>,
2102 // We currently never consider higher-ranked obligations e.g.
2103 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
2104 // because they are a priori invalid, and we could potentially add support
2105 // for them later, it's just that there isn't really a strong need for it.
2106 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
2107 // impl, and those are generally applied to concrete types.
2109 // That said, one might try to write a fn with a where clause like
2110 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
2111 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
2112 // Still, you'd be more likely to write that where clause as
2114 // so it seems ok if we (conservatively) fail to accept that `Unsize`
2115 // obligation above. Should be possible to extend this in the future.
2116 let source = match obligation.self_ty().no_bound_vars() {
2119 // Don't add any candidates if there are bound regions.
2123 let target = obligation
2131 "assemble_candidates_for_unsizing(source={:?}, target={:?})",
2135 let may_apply = match (&source.sty, &target.sty) {
2136 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2137 (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
2138 // Upcasts permit two things:
2140 // 1. Dropping builtin bounds, e.g., `Foo+Send` to `Foo`
2141 // 2. Tightening the region bound, e.g., `Foo+'a` to `Foo+'b` if `'a : 'b`
2143 // Note that neither of these changes requires any
2144 // change at runtime. Eventually this will be
2147 // We always upcast when we can because of reason
2148 // #2 (region bounds).
2149 data_a.principal().def_id() == data_b.principal().def_id()
2150 && data_b.auto_traits()
2151 // All of a's auto traits need to be in b's auto traits.
2152 .all(|b| data_a.auto_traits().any(|a| a == b))
2156 (_, &ty::Dynamic(..)) => true,
2158 // Ambiguous handling is below T -> Trait, because inference
2159 // variables can still implement Unsize<Trait> and nested
2160 // obligations will have the final say (likely deferred).
2161 (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
2162 debug!("assemble_candidates_for_unsizing: ambiguous");
2163 candidates.ambiguous = true;
2168 (&ty::Array(..), &ty::Slice(_)) => true,
2170 // Struct<T> -> Struct<U>.
2171 (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
2172 def_id_a == def_id_b
2175 // (.., T) -> (.., U).
2176 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
2182 candidates.vec.push(BuiltinUnsizeCandidate);
2186 fn assemble_candidates_for_trait_alias(
2188 obligation: &TraitObligation<'tcx>,
2189 candidates: &mut SelectionCandidateSet<'tcx>,
2190 ) -> Result<(), SelectionError<'tcx>> {
2191 // OK to skip binder here because the tests we do below do not involve bound regions
2192 let self_ty = *obligation.self_ty().skip_binder();
2193 debug!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty);
2195 let def_id = obligation.predicate.def_id();
2197 if ty::is_trait_alias(self.tcx(), def_id) {
2198 candidates.vec.push(TraitAliasCandidate(def_id.clone()));
2204 ///////////////////////////////////////////////////////////////////////////
2207 // Winnowing is the process of attempting to resolve ambiguity by
2208 // probing further. During the winnowing process, we unify all
2209 // type variables and then we also attempt to evaluate recursive
2210 // bounds to see if they are satisfied.
2212 /// Returns true if `victim` should be dropped in favor of
2213 /// `other`. Generally speaking we will drop duplicate
2214 /// candidates and prefer where-clause candidates.
2216 /// See the comment for "SelectionCandidate" for more details.
2217 fn candidate_should_be_dropped_in_favor_of<'o>(
2219 victim: &EvaluatedCandidate<'tcx>,
2220 other: &EvaluatedCandidate<'tcx>,
2222 if victim.candidate == other.candidate {
2226 // Check if a bound would previously have been removed when normalizing
2227 // the param_env so that it can be given the lowest priority. See
2228 // #50825 for the motivation for this.
2230 |cand: &ty::PolyTraitRef<'_>| cand.is_global() && !cand.has_late_bound_regions();
2232 match other.candidate {
2233 // Prefer BuiltinCandidate { has_nested: false } to anything else.
2234 // This is a fix for #53123 and prevents winnowing from accidentally extending the
2235 // lifetime of a variable.
2236 BuiltinCandidate { has_nested: false } => true,
2237 ParamCandidate(ref cand) => match victim.candidate {
2238 AutoImplCandidate(..) => {
2240 "default implementations shouldn't be recorded \
2241 when there are other valid candidates"
2244 // Prefer BuiltinCandidate { has_nested: false } to anything else.
2245 // This is a fix for #53123 and prevents winnowing from accidentally extending the
2246 // lifetime of a variable.
2247 BuiltinCandidate { has_nested: false } => false,
2250 | GeneratorCandidate
2251 | FnPointerCandidate
2252 | BuiltinObjectCandidate
2253 | BuiltinUnsizeCandidate
2254 | BuiltinCandidate { .. }
2255 | TraitAliasCandidate(..) => {
2256 // Global bounds from the where clause should be ignored
2257 // here (see issue #50825). Otherwise, we have a where
2258 // clause so don't go around looking for impls.
2261 ObjectCandidate | ProjectionCandidate => {
2262 // Arbitrarily give param candidates priority
2263 // over projection and object candidates.
2266 ParamCandidate(..) => false,
2268 ObjectCandidate | ProjectionCandidate => match victim.candidate {
2269 AutoImplCandidate(..) => {
2271 "default implementations shouldn't be recorded \
2272 when there are other valid candidates"
2275 // Prefer BuiltinCandidate { has_nested: false } to anything else.
2276 // This is a fix for #53123 and prevents winnowing from accidentally extending the
2277 // lifetime of a variable.
2278 BuiltinCandidate { has_nested: false } => false,
2281 | GeneratorCandidate
2282 | FnPointerCandidate
2283 | BuiltinObjectCandidate
2284 | BuiltinUnsizeCandidate
2285 | BuiltinCandidate { .. }
2286 | TraitAliasCandidate(..) => true,
2287 ObjectCandidate | ProjectionCandidate => {
2288 // Arbitrarily give param candidates priority
2289 // over projection and object candidates.
2292 ParamCandidate(ref cand) => is_global(cand),
2294 ImplCandidate(other_def) => {
2295 // See if we can toss out `victim` based on specialization.
2296 // This requires us to know *for sure* that the `other` impl applies
2297 // i.e., EvaluatedToOk:
2298 if other.evaluation.must_apply_modulo_regions() {
2299 match victim.candidate {
2300 ImplCandidate(victim_def) => {
2301 let tcx = self.tcx().global_tcx();
2302 return tcx.specializes((other_def, victim_def))
2303 || tcx.impls_are_allowed_to_overlap(other_def, victim_def);
2305 ParamCandidate(ref cand) => {
2306 // Prefer the impl to a global where clause candidate.
2307 return is_global(cand);
2316 | GeneratorCandidate
2317 | FnPointerCandidate
2318 | BuiltinObjectCandidate
2319 | BuiltinUnsizeCandidate
2320 | BuiltinCandidate { has_nested: true } => {
2321 match victim.candidate {
2322 ParamCandidate(ref cand) => {
2323 // Prefer these to a global where-clause bound
2324 // (see issue #50825)
2325 is_global(cand) && other.evaluation.must_apply_modulo_regions()
2334 ///////////////////////////////////////////////////////////////////////////
2337 // These cover the traits that are built-in to the language
2338 // itself: `Copy`, `Clone` and `Sized`.
2340 fn assemble_builtin_bound_candidates<'o>(
2342 conditions: BuiltinImplConditions<'tcx>,
2343 candidates: &mut SelectionCandidateSet<'tcx>,
2344 ) -> Result<(), SelectionError<'tcx>> {
2346 BuiltinImplConditions::Where(nested) => {
2347 debug!("builtin_bound: nested={:?}", nested);
2348 candidates.vec.push(BuiltinCandidate {
2349 has_nested: nested.skip_binder().len() > 0,
2352 BuiltinImplConditions::None => {}
2353 BuiltinImplConditions::Ambiguous => {
2354 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
2355 candidates.ambiguous = true;
2362 fn sized_conditions(
2364 obligation: &TraitObligation<'tcx>,
2365 ) -> BuiltinImplConditions<'tcx> {
2366 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2368 // NOTE: binder moved to (*)
2369 let self_ty = self.infcx
2370 .shallow_resolve(obligation.predicate.skip_binder().self_ty());
2373 ty::Infer(ty::IntVar(_))
2374 | ty::Infer(ty::FloatVar(_))
2385 | ty::GeneratorWitness(..)
2390 // safe for everything
2391 Where(ty::Binder::dummy(Vec::new()))
2394 ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
2396 ty::Tuple(tys) => Where(ty::Binder::bind(tys.last().into_iter().cloned().collect())),
2398 ty::Adt(def, substs) => {
2399 let sized_crit = def.sized_constraint(self.tcx());
2400 // (*) binder moved here
2401 Where(ty::Binder::bind(
2404 .map(|ty| ty.subst(self.tcx(), substs))
2409 ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None,
2410 ty::Infer(ty::TyVar(_)) => Ambiguous,
2412 ty::UnnormalizedProjection(..)
2413 | ty::Placeholder(..)
2415 | ty::Infer(ty::FreshTy(_))
2416 | ty::Infer(ty::FreshIntTy(_))
2417 | ty::Infer(ty::FreshFloatTy(_)) => {
2419 "asked to assemble builtin bounds of unexpected type: {:?}",
2426 fn copy_clone_conditions(
2428 obligation: &TraitObligation<'tcx>,
2429 ) -> BuiltinImplConditions<'tcx> {
2430 // NOTE: binder moved to (*)
2431 let self_ty = self.infcx
2432 .shallow_resolve(obligation.predicate.skip_binder().self_ty());
2434 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2437 ty::Infer(ty::IntVar(_))
2438 | ty::Infer(ty::FloatVar(_))
2441 | ty::Error => Where(ty::Binder::dummy(Vec::new())),
2450 | ty::Ref(_, _, hir::MutImmutable) => {
2451 // Implementations provided in libcore
2459 | ty::GeneratorWitness(..)
2461 | ty::Ref(_, _, hir::MutMutable) => None,
2463 ty::Array(element_ty, _) => {
2464 // (*) binder moved here
2465 Where(ty::Binder::bind(vec![element_ty]))
2469 // (*) binder moved here
2470 Where(ty::Binder::bind(tys.to_vec()))
2473 ty::Closure(def_id, substs) => {
2474 let trait_id = obligation.predicate.def_id();
2475 let is_copy_trait = Some(trait_id) == self.tcx().lang_items().copy_trait();
2476 let is_clone_trait = Some(trait_id) == self.tcx().lang_items().clone_trait();
2477 if is_copy_trait || is_clone_trait {
2478 Where(ty::Binder::bind(
2479 substs.upvar_tys(def_id, self.tcx()).collect(),
2486 ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => {
2487 // Fallback to whatever user-defined impls exist in this case.
2491 ty::Infer(ty::TyVar(_)) => {
2492 // Unbound type variable. Might or might not have
2493 // applicable impls and so forth, depending on what
2494 // those type variables wind up being bound to.
2498 ty::UnnormalizedProjection(..)
2499 | ty::Placeholder(..)
2501 | ty::Infer(ty::FreshTy(_))
2502 | ty::Infer(ty::FreshIntTy(_))
2503 | ty::Infer(ty::FreshFloatTy(_)) => {
2505 "asked to assemble builtin bounds of unexpected type: {:?}",
2512 /// For default impls, we need to break apart a type into its
2513 /// "constituent types" -- meaning, the types that it contains.
2515 /// Here are some (simple) examples:
2518 /// (i32, u32) -> [i32, u32]
2519 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2520 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2521 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2523 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
2533 | ty::Infer(ty::IntVar(_))
2534 | ty::Infer(ty::FloatVar(_))
2536 | ty::Char => Vec::new(),
2538 ty::UnnormalizedProjection(..)
2539 | ty::Placeholder(..)
2543 | ty::Projection(..)
2545 | ty::Infer(ty::TyVar(_))
2546 | ty::Infer(ty::FreshTy(_))
2547 | ty::Infer(ty::FreshIntTy(_))
2548 | ty::Infer(ty::FreshFloatTy(_)) => {
2550 "asked to assemble constituent types of unexpected type: {:?}",
2555 ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
2559 ty::Array(element_ty, _) | ty::Slice(element_ty) => vec![element_ty],
2561 ty::Tuple(ref tys) => {
2562 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2566 ty::Closure(def_id, ref substs) => substs.upvar_tys(def_id, self.tcx()).collect(),
2568 ty::Generator(def_id, ref substs, _) => {
2569 let witness = substs.witness(def_id, self.tcx());
2571 .upvar_tys(def_id, self.tcx())
2572 .chain(iter::once(witness))
2576 ty::GeneratorWitness(types) => {
2577 // This is sound because no regions in the witness can refer to
2578 // the binder outside the witness. So we'll effectivly reuse
2579 // the implicit binder around the witness.
2580 types.skip_binder().to_vec()
2583 // for `PhantomData<T>`, we pass `T`
2584 ty::Adt(def, substs) if def.is_phantom_data() => substs.types().collect(),
2586 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect(),
2588 ty::Opaque(def_id, substs) => {
2589 // We can resolve the `impl Trait` to its concrete type,
2590 // which enforces a DAG between the functions requiring
2591 // the auto trait bounds in question.
2592 vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
2597 fn collect_predicates_for_types(
2599 param_env: ty::ParamEnv<'tcx>,
2600 cause: ObligationCause<'tcx>,
2601 recursion_depth: usize,
2602 trait_def_id: DefId,
2603 types: ty::Binder<Vec<Ty<'tcx>>>,
2604 ) -> Vec<PredicateObligation<'tcx>> {
2605 // Because the types were potentially derived from
2606 // higher-ranked obligations they may reference late-bound
2607 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
2608 // yield a type like `for<'a> &'a int`. In general, we
2609 // maintain the invariant that we never manipulate bound
2610 // regions, so we have to process these bound regions somehow.
2612 // The strategy is to:
2614 // 1. Instantiate those regions to placeholder regions (e.g.,
2615 // `for<'a> &'a int` becomes `&0 int`.
2616 // 2. Produce something like `&'0 int : Copy`
2617 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
2624 let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/
2626 self.infcx.in_snapshot(|_| {
2627 let (skol_ty, _) = self.infcx
2628 .replace_bound_vars_with_placeholders(&ty);
2630 value: normalized_ty,
2632 } = project::normalize_with_depth(
2639 let skol_obligation = self.tcx().predicate_for_trait_def(
2647 obligations.push(skol_obligation);
2654 ///////////////////////////////////////////////////////////////////////////
2657 // Confirmation unifies the output type parameters of the trait
2658 // with the values found in the obligation, possibly yielding a
2659 // type error. See the [rustc guide] for more details.
2662 // https://rust-lang.github.io/rustc-guide/traits/resolution.html#confirmation
2664 fn confirm_candidate(
2666 obligation: &TraitObligation<'tcx>,
2667 candidate: SelectionCandidate<'tcx>,
2668 ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
2669 debug!("confirm_candidate({:?}, {:?})", obligation, candidate);
2672 BuiltinCandidate { has_nested } => {
2673 let data = self.confirm_builtin_candidate(obligation, has_nested);
2674 Ok(VtableBuiltin(data))
2677 ParamCandidate(param) => {
2678 let obligations = self.confirm_param_candidate(obligation, param);
2679 Ok(VtableParam(obligations))
2682 ImplCandidate(impl_def_id) => Ok(VtableImpl(self.confirm_impl_candidate(
2687 AutoImplCandidate(trait_def_id) => {
2688 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
2689 Ok(VtableAutoImpl(data))
2692 ProjectionCandidate => {
2693 self.confirm_projection_candidate(obligation);
2694 Ok(VtableParam(Vec::new()))
2697 ClosureCandidate => {
2698 let vtable_closure = self.confirm_closure_candidate(obligation)?;
2699 Ok(VtableClosure(vtable_closure))
2702 GeneratorCandidate => {
2703 let vtable_generator = self.confirm_generator_candidate(obligation)?;
2704 Ok(VtableGenerator(vtable_generator))
2707 FnPointerCandidate => {
2708 let data = self.confirm_fn_pointer_candidate(obligation)?;
2709 Ok(VtableFnPointer(data))
2712 TraitAliasCandidate(alias_def_id) => {
2713 let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
2714 Ok(VtableTraitAlias(data))
2717 ObjectCandidate => {
2718 let data = self.confirm_object_candidate(obligation);
2719 Ok(VtableObject(data))
2722 BuiltinObjectCandidate => {
2723 // This indicates something like `(Trait+Send) :
2724 // Send`. In this case, we know that this holds
2725 // because that's what the object type is telling us,
2726 // and there's really no additional obligations to
2727 // prove and no types in particular to unify etc.
2728 Ok(VtableParam(Vec::new()))
2731 BuiltinUnsizeCandidate => {
2732 let data = self.confirm_builtin_unsize_candidate(obligation)?;
2733 Ok(VtableBuiltin(data))
2738 fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) {
2739 self.infcx.in_snapshot(|_| {
2741 self.match_projection_obligation_against_definition_bounds(obligation);
2746 fn confirm_param_candidate(
2748 obligation: &TraitObligation<'tcx>,
2749 param: ty::PolyTraitRef<'tcx>,
2750 ) -> Vec<PredicateObligation<'tcx>> {
2751 debug!("confirm_param_candidate({:?},{:?})", obligation, param);
2753 // During evaluation, we already checked that this
2754 // where-clause trait-ref could be unified with the obligation
2755 // trait-ref. Repeat that unification now without any
2756 // transactional boundary; it should not fail.
2757 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2758 Ok(obligations) => obligations,
2761 "Where clause `{:?}` was applicable to `{:?}` but now is not",
2769 fn confirm_builtin_candidate(
2771 obligation: &TraitObligation<'tcx>,
2773 ) -> VtableBuiltinData<PredicateObligation<'tcx>> {
2775 "confirm_builtin_candidate({:?}, {:?})",
2776 obligation, has_nested
2779 let lang_items = self.tcx().lang_items();
2780 let obligations = if has_nested {
2781 let trait_def = obligation.predicate.def_id();
2782 let conditions = if Some(trait_def) == lang_items.sized_trait() {
2783 self.sized_conditions(obligation)
2784 } else if Some(trait_def) == lang_items.copy_trait() {
2785 self.copy_clone_conditions(obligation)
2786 } else if Some(trait_def) == lang_items.clone_trait() {
2787 self.copy_clone_conditions(obligation)
2789 bug!("unexpected builtin trait {:?}", trait_def)
2791 let nested = match conditions {
2792 BuiltinImplConditions::Where(nested) => nested,
2794 "obligation {:?} had matched a builtin impl but now doesn't",
2799 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2800 self.collect_predicates_for_types(
2801 obligation.param_env,
2803 obligation.recursion_depth + 1,
2811 debug!("confirm_builtin_candidate: obligations={:?}", obligations);
2814 nested: obligations,
2818 /// This handles the case where a `auto trait Foo` impl is being used.
2819 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2821 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2822 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2823 fn confirm_auto_impl_candidate(
2825 obligation: &TraitObligation<'tcx>,
2826 trait_def_id: DefId,
2827 ) -> VtableAutoImplData<PredicateObligation<'tcx>> {
2829 "confirm_auto_impl_candidate({:?}, {:?})",
2830 obligation, trait_def_id
2833 let types = obligation.predicate.map_bound(|inner| {
2834 let self_ty = self.infcx.shallow_resolve(inner.self_ty());
2835 self.constituent_types_for_ty(self_ty)
2837 self.vtable_auto_impl(obligation, trait_def_id, types)
2840 /// See `confirm_auto_impl_candidate`.
2841 fn vtable_auto_impl(
2843 obligation: &TraitObligation<'tcx>,
2844 trait_def_id: DefId,
2845 nested: ty::Binder<Vec<Ty<'tcx>>>,
2846 ) -> VtableAutoImplData<PredicateObligation<'tcx>> {
2847 debug!("vtable_auto_impl: nested={:?}", nested);
2849 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2850 let mut obligations = self.collect_predicates_for_types(
2851 obligation.param_env,
2853 obligation.recursion_depth + 1,
2858 let trait_obligations: Vec<PredicateObligation<'_>> = self.infcx.in_snapshot(|_| {
2859 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2860 let (trait_ref, _) = self.infcx
2861 .replace_bound_vars_with_placeholders(&poly_trait_ref);
2862 let cause = obligation.derived_cause(ImplDerivedObligation);
2863 self.impl_or_trait_obligations(
2865 obligation.recursion_depth + 1,
2866 obligation.param_env,
2872 // Adds the predicates from the trait. Note that this contains a `Self: Trait`
2873 // predicate as usual. It won't have any effect since auto traits are coinductive.
2874 obligations.extend(trait_obligations);
2876 debug!("vtable_auto_impl: obligations={:?}", obligations);
2878 VtableAutoImplData {
2880 nested: obligations,
2884 fn confirm_impl_candidate(
2886 obligation: &TraitObligation<'tcx>,
2888 ) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
2889 debug!("confirm_impl_candidate({:?},{:?})", obligation, impl_def_id);
2891 // First, create the substitutions by matching the impl again,
2892 // this time not in a probe.
2893 self.infcx.in_snapshot(|_| {
2894 let substs = self.rematch_impl(impl_def_id, obligation);
2895 debug!("confirm_impl_candidate: substs={:?}", substs);
2896 let cause = obligation.derived_cause(ImplDerivedObligation);
2901 obligation.recursion_depth + 1,
2902 obligation.param_env,
2910 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2911 cause: ObligationCause<'tcx>,
2912 recursion_depth: usize,
2913 param_env: ty::ParamEnv<'tcx>,
2914 ) -> VtableImplData<'tcx, PredicateObligation<'tcx>> {
2916 "vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={})",
2917 impl_def_id, substs, recursion_depth,
2920 let mut impl_obligations = self.impl_or_trait_obligations(
2929 "vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2930 impl_def_id, impl_obligations
2933 // Because of RFC447, the impl-trait-ref and obligations
2934 // are sufficient to determine the impl substs, without
2935 // relying on projections in the impl-trait-ref.
2937 // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2938 impl_obligations.append(&mut substs.obligations);
2942 substs: substs.value,
2943 nested: impl_obligations,
2947 fn confirm_object_candidate(
2949 obligation: &TraitObligation<'tcx>,
2950 ) -> VtableObjectData<'tcx, PredicateObligation<'tcx>> {
2951 debug!("confirm_object_candidate({:?})", obligation);
2953 // FIXME(nmatsakis) skipping binder here seems wrong -- we should
2954 // probably flatten the binder from the obligation and the binder
2955 // from the object. Have to try to make a broken test case that
2957 let self_ty = self.infcx
2958 .shallow_resolve(*obligation.self_ty().skip_binder());
2959 let poly_trait_ref = match self_ty.sty {
2960 ty::Dynamic(ref data, ..) => data.principal().with_self_ty(self.tcx(), self_ty),
2961 _ => span_bug!(obligation.cause.span, "object candidate with non-object"),
2964 let mut upcast_trait_ref = None;
2965 let mut nested = vec![];
2969 let tcx = self.tcx();
2971 // We want to find the first supertrait in the list of
2972 // supertraits that we can unify with, and do that
2973 // unification. We know that there is exactly one in the list
2974 // where we can unify because otherwise select would have
2975 // reported an ambiguity. (When we do find a match, also
2976 // record it for later.)
2977 let nonmatching = util::supertraits(tcx, poly_trait_ref).take_while(
2978 |&t| match self.infcx.commit_if_ok(|_| self.match_poly_trait_ref(obligation, t)) {
2979 Ok(obligations) => {
2980 upcast_trait_ref = Some(t);
2981 nested.extend(obligations);
2988 // Additionally, for each of the nonmatching predicates that
2989 // we pass over, we sum up the set of number of vtable
2990 // entries, so that we can compute the offset for the selected
2992 vtable_base = nonmatching.map(|t| tcx.count_own_vtable_entries(t)).sum();
2996 upcast_trait_ref: upcast_trait_ref.unwrap(),
3002 fn confirm_fn_pointer_candidate(
3004 obligation: &TraitObligation<'tcx>,
3005 ) -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3006 debug!("confirm_fn_pointer_candidate({:?})", obligation);
3008 // OK to skip binder; it is reintroduced below
3009 let self_ty = self.infcx
3010 .shallow_resolve(*obligation.self_ty().skip_binder());
3011 let sig = self_ty.fn_sig(self.tcx());
3012 let trait_ref = self.tcx()
3013 .closure_trait_ref_and_return_type(
3014 obligation.predicate.def_id(),
3017 util::TupleArgumentsFlag::Yes,
3019 .map_bound(|(trait_ref, _)| trait_ref);
3024 } = project::normalize_with_depth(
3026 obligation.param_env,
3027 obligation.cause.clone(),
3028 obligation.recursion_depth + 1,
3032 self.confirm_poly_trait_refs(
3033 obligation.cause.clone(),
3034 obligation.param_env,
3035 obligation.predicate.to_poly_trait_ref(),
3038 Ok(VtableFnPointerData {
3040 nested: obligations,
3044 fn confirm_trait_alias_candidate(
3046 obligation: &TraitObligation<'tcx>,
3047 alias_def_id: DefId,
3048 ) -> VtableTraitAliasData<'tcx, PredicateObligation<'tcx>> {
3050 "confirm_trait_alias_candidate({:?}, {:?})",
3051 obligation, alias_def_id
3054 self.infcx.in_snapshot(|_| {
3055 let (predicate, _) = self.infcx()
3056 .replace_bound_vars_with_placeholders(&obligation.predicate);
3057 let trait_ref = predicate.trait_ref;
3058 let trait_def_id = trait_ref.def_id;
3059 let substs = trait_ref.substs;
3061 let trait_obligations = self.impl_or_trait_obligations(
3062 obligation.cause.clone(),
3063 obligation.recursion_depth,
3064 obligation.param_env,
3070 "confirm_trait_alias_candidate: trait_def_id={:?} trait_obligations={:?}",
3071 trait_def_id, trait_obligations
3074 VtableTraitAliasData {
3077 nested: trait_obligations,
3082 fn confirm_generator_candidate(
3084 obligation: &TraitObligation<'tcx>,
3085 ) -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3086 // OK to skip binder because the substs on generator types never
3087 // touch bound regions, they just capture the in-scope
3088 // type/region parameters
3089 let self_ty = self.infcx
3090 .shallow_resolve(obligation.self_ty().skip_binder());
3091 let (generator_def_id, substs) = match self_ty.sty {
3092 ty::Generator(id, substs, _) => (id, substs),
3093 _ => bug!("closure candidate for non-closure {:?}", obligation),
3097 "confirm_generator_candidate({:?},{:?},{:?})",
3098 obligation, generator_def_id, substs
3101 let trait_ref = self.generator_trait_ref_unnormalized(obligation, generator_def_id, substs);
3105 } = normalize_with_depth(
3107 obligation.param_env,
3108 obligation.cause.clone(),
3109 obligation.recursion_depth + 1,
3114 "confirm_generator_candidate(generator_def_id={:?}, \
3115 trait_ref={:?}, obligations={:?})",
3116 generator_def_id, trait_ref, obligations
3119 obligations.extend(self.confirm_poly_trait_refs(
3120 obligation.cause.clone(),
3121 obligation.param_env,
3122 obligation.predicate.to_poly_trait_ref(),
3126 Ok(VtableGeneratorData {
3127 generator_def_id: generator_def_id,
3128 substs: substs.clone(),
3129 nested: obligations,
3133 fn confirm_closure_candidate(
3135 obligation: &TraitObligation<'tcx>,
3136 ) -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3137 debug!("confirm_closure_candidate({:?})", obligation);
3139 let kind = self.tcx()
3141 .fn_trait_kind(obligation.predicate.def_id())
3142 .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation));
3144 // OK to skip binder because the substs on closure types never
3145 // touch bound regions, they just capture the in-scope
3146 // type/region parameters
3147 let self_ty = self.infcx
3148 .shallow_resolve(obligation.self_ty().skip_binder());
3149 let (closure_def_id, substs) = match self_ty.sty {
3150 ty::Closure(id, substs) => (id, substs),
3151 _ => bug!("closure candidate for non-closure {:?}", obligation),
3154 let trait_ref = self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs);
3158 } = normalize_with_depth(
3160 obligation.param_env,
3161 obligation.cause.clone(),
3162 obligation.recursion_depth + 1,
3167 "confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
3168 closure_def_id, trait_ref, obligations
3171 obligations.extend(self.confirm_poly_trait_refs(
3172 obligation.cause.clone(),
3173 obligation.param_env,
3174 obligation.predicate.to_poly_trait_ref(),
3179 if !self.tcx().sess.opts.debugging_opts.chalk {
3180 obligations.push(Obligation::new(
3181 obligation.cause.clone(),
3182 obligation.param_env,
3183 ty::Predicate::ClosureKind(closure_def_id, substs, kind),
3187 Ok(VtableClosureData {
3189 substs: substs.clone(),
3190 nested: obligations,
3194 /// In the case of closure types and fn pointers,
3195 /// we currently treat the input type parameters on the trait as
3196 /// outputs. This means that when we have a match we have only
3197 /// considered the self type, so we have to go back and make sure
3198 /// to relate the argument types too. This is kind of wrong, but
3199 /// since we control the full set of impls, also not that wrong,
3200 /// and it DOES yield better error messages (since we don't report
3201 /// errors as if there is no applicable impl, but rather report
3202 /// errors are about mismatched argument types.
3204 /// Here is an example. Imagine we have a closure expression
3205 /// and we desugared it so that the type of the expression is
3206 /// `Closure`, and `Closure` expects an int as argument. Then it
3207 /// is "as if" the compiler generated this impl:
3209 /// impl Fn(int) for Closure { ... }
3211 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
3212 /// we have matched the self-type `Closure`. At this point we'll
3213 /// compare the `int` to `usize` and generate an error.
3215 /// Note that this checking occurs *after* the impl has selected,
3216 /// because these output type parameters should not affect the
3217 /// selection of the impl. Therefore, if there is a mismatch, we
3218 /// report an error to the user.
3219 fn confirm_poly_trait_refs(
3221 obligation_cause: ObligationCause<'tcx>,
3222 obligation_param_env: ty::ParamEnv<'tcx>,
3223 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
3224 expected_trait_ref: ty::PolyTraitRef<'tcx>,
3225 ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3226 let obligation_trait_ref = obligation_trait_ref.clone();
3228 .at(&obligation_cause, obligation_param_env)
3229 .sup(obligation_trait_ref, expected_trait_ref)
3230 .map(|InferOk { obligations, .. }| obligations)
3231 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
3234 fn confirm_builtin_unsize_candidate(
3236 obligation: &TraitObligation<'tcx>,
3237 ) -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
3238 let tcx = self.tcx();
3240 // assemble_candidates_for_unsizing should ensure there are no late bound
3241 // regions here. See the comment there for more details.
3242 let source = self.infcx
3243 .shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
3244 let target = obligation
3250 let target = self.infcx.shallow_resolve(target);
3253 "confirm_builtin_unsize_candidate(source={:?}, target={:?})",
3257 let mut nested = vec![];
3258 match (&source.sty, &target.sty) {
3259 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
3260 (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
3261 // See assemble_candidates_for_unsizing for more info.
3262 let existential_predicates = data_a.map_bound(|data_a| {
3263 let iter = iter::once(ty::ExistentialPredicate::Trait(data_a.principal()))
3266 .projection_bounds()
3267 .map(|x| ty::ExistentialPredicate::Projection(x)),
3272 .map(ty::ExistentialPredicate::AutoTrait),
3274 tcx.mk_existential_predicates(iter)
3276 let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
3277 let InferOk { obligations, .. } = self.infcx
3278 .at(&obligation.cause, obligation.param_env)
3279 .sup(target, source_trait)
3280 .map_err(|_| Unimplemented)?;
3281 nested.extend(obligations);
3283 // Register one obligation for 'a: 'b.
3284 let cause = ObligationCause::new(
3285 obligation.cause.span,
3286 obligation.cause.body_id,
3287 ObjectCastObligation(target),
3289 let outlives = ty::OutlivesPredicate(r_a, r_b);
3290 nested.push(Obligation::with_depth(
3292 obligation.recursion_depth + 1,
3293 obligation.param_env,
3294 ty::Binder::bind(outlives).to_predicate(),
3299 (_, &ty::Dynamic(ref data, r)) => {
3300 let mut object_dids = data.auto_traits()
3301 .chain(iter::once(data.principal().def_id()));
3302 if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) {
3303 return Err(TraitNotObjectSafe(did));
3306 let cause = ObligationCause::new(
3307 obligation.cause.span,
3308 obligation.cause.body_id,
3309 ObjectCastObligation(target),
3312 let predicate_to_obligation = |predicate| {
3313 Obligation::with_depth(
3315 obligation.recursion_depth + 1,
3316 obligation.param_env,
3321 // Create obligations:
3322 // - Casting T to Trait
3323 // - For all the various builtin bounds attached to the object cast. (In other
3324 // words, if the object type is Foo+Send, this would create an obligation for the
3326 // - Projection predicates
3329 .map(|d| predicate_to_obligation(d.with_self_ty(tcx, source))),
3332 // We can only make objects from sized types.
3333 let tr = ty::TraitRef {
3334 def_id: tcx.require_lang_item(lang_items::SizedTraitLangItem),
3335 substs: tcx.mk_substs_trait(source, &[]),
3337 nested.push(predicate_to_obligation(tr.to_predicate()));
3339 // If the type is `Foo+'a`, ensures that the type
3340 // being cast to `Foo+'a` outlives `'a`:
3341 let outlives = ty::OutlivesPredicate(source, r);
3342 nested.push(predicate_to_obligation(
3343 ty::Binder::dummy(outlives).to_predicate(),
3348 (&ty::Array(a, _), &ty::Slice(b)) => {
3349 let InferOk { obligations, .. } = self.infcx
3350 .at(&obligation.cause, obligation.param_env)
3352 .map_err(|_| Unimplemented)?;
3353 nested.extend(obligations);
3356 // Struct<T> -> Struct<U>.
3357 (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => {
3358 let fields = def.all_fields()
3359 .map(|f| tcx.type_of(f.did))
3360 .collect::<Vec<_>>();
3362 // The last field of the structure has to exist and contain type parameters.
3363 let field = if let Some(&field) = fields.last() {
3366 return Err(Unimplemented);
3368 let mut ty_params = GrowableBitSet::new_empty();
3369 let mut found = false;
3370 for ty in field.walk() {
3371 if let ty::Param(p) = ty.sty {
3372 ty_params.insert(p.idx as usize);
3377 return Err(Unimplemented);
3380 // Replace type parameters used in unsizing with
3381 // Error and ensure they do not affect any other fields.
3382 // This could be checked after type collection for any struct
3383 // with a potentially unsized trailing field.
3384 let params = substs_a.iter().enumerate().map(|(i, &k)| {
3385 if ty_params.contains(i) {
3386 tcx.types.err.into()
3391 let substs = tcx.mk_substs(params);
3392 for &ty in fields.split_last().unwrap().1 {
3393 if ty.subst(tcx, substs).references_error() {
3394 return Err(Unimplemented);
3398 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
3399 let inner_source = field.subst(tcx, substs_a);
3400 let inner_target = field.subst(tcx, substs_b);
3402 // Check that the source struct with the target's
3403 // unsized parameters is equal to the target.
3404 let params = substs_a.iter().enumerate().map(|(i, &k)| {
3405 if ty_params.contains(i) {
3406 substs_b.type_at(i).into()
3411 let new_struct = tcx.mk_adt(def, tcx.mk_substs(params));
3412 let InferOk { obligations, .. } = self.infcx
3413 .at(&obligation.cause, obligation.param_env)
3414 .eq(target, new_struct)
3415 .map_err(|_| Unimplemented)?;
3416 nested.extend(obligations);
3418 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
3419 nested.push(tcx.predicate_for_trait_def(
3420 obligation.param_env,
3421 obligation.cause.clone(),
3422 obligation.predicate.def_id(),
3423 obligation.recursion_depth + 1,
3425 &[inner_target.into()],
3429 // (.., T) -> (.., U).
3430 (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
3431 assert_eq!(tys_a.len(), tys_b.len());
3433 // The last field of the tuple has to exist.
3434 let (&a_last, a_mid) = if let Some(x) = tys_a.split_last() {
3437 return Err(Unimplemented);
3439 let &b_last = tys_b.last().unwrap();
3441 // Check that the source tuple with the target's
3442 // last element is equal to the target.
3443 let new_tuple = tcx.mk_tup(a_mid.iter().cloned().chain(iter::once(b_last)));
3444 let InferOk { obligations, .. } = self.infcx
3445 .at(&obligation.cause, obligation.param_env)
3446 .eq(target, new_tuple)
3447 .map_err(|_| Unimplemented)?;
3448 nested.extend(obligations);
3450 // Construct the nested T: Unsize<U> predicate.
3451 nested.push(tcx.predicate_for_trait_def(
3452 obligation.param_env,
3453 obligation.cause.clone(),
3454 obligation.predicate.def_id(),
3455 obligation.recursion_depth + 1,
3464 Ok(VtableBuiltinData { nested })
3467 ///////////////////////////////////////////////////////////////////////////
3470 // Matching is a common path used for both evaluation and
3471 // confirmation. It basically unifies types that appear in impls
3472 // and traits. This does affect the surrounding environment;
3473 // therefore, when used during evaluation, match routines must be
3474 // run inside of a `probe()` so that their side-effects are
3480 obligation: &TraitObligation<'tcx>,
3481 ) -> Normalized<'tcx, &'tcx Substs<'tcx>> {
3482 match self.match_impl(impl_def_id, obligation) {
3483 Ok(substs) => substs,
3486 "Impl {:?} was matchable against {:?} but now is not",
3497 obligation: &TraitObligation<'tcx>,
3498 ) -> Result<Normalized<'tcx, &'tcx Substs<'tcx>>, ()> {
3499 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
3501 // Before we create the substitutions and everything, first
3502 // consider a "quick reject". This avoids creating more types
3503 // and so forth that we need to.
3504 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
3508 let (skol_obligation, _) = self.infcx()
3509 .replace_bound_vars_with_placeholders(&obligation.predicate);
3510 let skol_obligation_trait_ref = skol_obligation.trait_ref;
3512 let impl_substs = self.infcx
3513 .fresh_substs_for_item(obligation.cause.span, impl_def_id);
3515 let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
3518 value: impl_trait_ref,
3519 obligations: mut nested_obligations,
3520 } = project::normalize_with_depth(
3522 obligation.param_env,
3523 obligation.cause.clone(),
3524 obligation.recursion_depth + 1,
3529 "match_impl(impl_def_id={:?}, obligation={:?}, \
3530 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
3531 impl_def_id, obligation, impl_trait_ref, skol_obligation_trait_ref
3534 let InferOk { obligations, .. } = self.infcx
3535 .at(&obligation.cause, obligation.param_env)
3536 .eq(skol_obligation_trait_ref, impl_trait_ref)
3537 .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
3538 nested_obligations.extend(obligations);
3540 debug!("match_impl: success impl_substs={:?}", impl_substs);
3543 obligations: nested_obligations,
3547 fn fast_reject_trait_refs(
3549 obligation: &TraitObligation<'_>,
3550 impl_trait_ref: &ty::TraitRef<'_>,
3552 // We can avoid creating type variables and doing the full
3553 // substitution if we find that any of the input types, when
3554 // simplified, do not match.
3560 .zip(impl_trait_ref.input_types())
3561 .any(|(obligation_ty, impl_ty)| {
3562 let simplified_obligation_ty =
3563 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
3564 let simplified_impl_ty = fast_reject::simplify_type(self.tcx(), impl_ty, false);
3566 simplified_obligation_ty.is_some()
3567 && simplified_impl_ty.is_some()
3568 && simplified_obligation_ty != simplified_impl_ty
3572 /// Normalize `where_clause_trait_ref` and try to match it against
3573 /// `obligation`. If successful, return any predicates that
3574 /// result from the normalization. Normalization is necessary
3575 /// because where-clauses are stored in the parameter environment
3577 fn match_where_clause_trait_ref(
3579 obligation: &TraitObligation<'tcx>,
3580 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
3581 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
3582 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
3585 /// Returns `Ok` if `poly_trait_ref` being true implies that the
3586 /// obligation is satisfied.
3587 fn match_poly_trait_ref(
3589 obligation: &TraitObligation<'tcx>,
3590 poly_trait_ref: ty::PolyTraitRef<'tcx>,
3591 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
3593 "match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
3594 obligation, poly_trait_ref
3598 .at(&obligation.cause, obligation.param_env)
3599 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
3600 .map(|InferOk { obligations, .. }| obligations)
3604 ///////////////////////////////////////////////////////////////////////////
3607 fn match_fresh_trait_refs(
3609 previous: &ty::PolyTraitRef<'tcx>,
3610 current: &ty::PolyTraitRef<'tcx>,
3612 let mut matcher = ty::_match::Match::new(
3613 self.tcx(), self.infcx.trait_object_mode());
3614 matcher.relate(previous, current).is_ok()
3617 fn push_stack<'o, 's: 'o>(
3619 previous_stack: TraitObligationStackList<'s, 'tcx>,
3620 obligation: &'o TraitObligation<'tcx>,
3621 ) -> TraitObligationStack<'o, 'tcx> {
3622 let fresh_trait_ref = obligation
3624 .to_poly_trait_ref()
3625 .fold_with(&mut self.freshener);
3627 TraitObligationStack {
3630 previous: previous_stack,
3634 fn closure_trait_ref_unnormalized(
3636 obligation: &TraitObligation<'tcx>,
3637 closure_def_id: DefId,
3638 substs: ty::ClosureSubsts<'tcx>,
3639 ) -> ty::PolyTraitRef<'tcx> {
3641 "closure_trait_ref_unnormalized(obligation={:?}, closure_def_id={:?}, substs={:?})",
3642 obligation, closure_def_id, substs,
3644 let closure_type = self.infcx.closure_sig(closure_def_id, substs);
3647 "closure_trait_ref_unnormalized: closure_type = {:?}",
3651 // (1) Feels icky to skip the binder here, but OTOH we know
3652 // that the self-type is an unboxed closure type and hence is
3653 // in fact unparameterized (or at least does not reference any
3654 // regions bound in the obligation). Still probably some
3655 // refactoring could make this nicer.
3657 .closure_trait_ref_and_return_type(
3658 obligation.predicate.def_id(),
3659 obligation.predicate.skip_binder().self_ty(), // (1)
3661 util::TupleArgumentsFlag::No,
3663 .map_bound(|(trait_ref, _)| trait_ref)
3666 fn generator_trait_ref_unnormalized(
3668 obligation: &TraitObligation<'tcx>,
3669 closure_def_id: DefId,
3670 substs: ty::GeneratorSubsts<'tcx>,
3671 ) -> ty::PolyTraitRef<'tcx> {
3672 let gen_sig = substs.poly_sig(closure_def_id, self.tcx());
3674 // (1) Feels icky to skip the binder here, but OTOH we know
3675 // that the self-type is an generator type and hence is
3676 // in fact unparameterized (or at least does not reference any
3677 // regions bound in the obligation). Still probably some
3678 // refactoring could make this nicer.
3681 .generator_trait_ref_and_outputs(
3682 obligation.predicate.def_id(),
3683 obligation.predicate.skip_binder().self_ty(), // (1)
3686 .map_bound(|(trait_ref, ..)| trait_ref)
3689 /// Returns the obligations that are implied by instantiating an
3690 /// impl or trait. The obligations are substituted and fully
3691 /// normalized. This is used when confirming an impl or default
3693 fn impl_or_trait_obligations(
3695 cause: ObligationCause<'tcx>,
3696 recursion_depth: usize,
3697 param_env: ty::ParamEnv<'tcx>,
3698 def_id: DefId, // of impl or trait
3699 substs: &Substs<'tcx>, // for impl or trait
3700 ) -> Vec<PredicateObligation<'tcx>> {
3701 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
3702 let tcx = self.tcx();
3704 // To allow for one-pass evaluation of the nested obligation,
3705 // each predicate must be preceded by the obligations required
3707 // for example, if we have:
3708 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
3709 // the impl will have the following predicates:
3710 // <V as Iterator>::Item = U,
3711 // U: Iterator, U: Sized,
3712 // V: Iterator, V: Sized,
3713 // <U as Iterator>::Item: Copy
3714 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
3715 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
3716 // `$1: Copy`, so we must ensure the obligations are emitted in
3718 let predicates = tcx.predicates_of(def_id);
3719 assert_eq!(predicates.parent, None);
3720 let mut predicates: Vec<_> = predicates
3723 .flat_map(|(predicate, _)| {
3724 let predicate = normalize_with_depth(
3729 &predicate.subst(tcx, substs),
3731 predicate.obligations.into_iter().chain(Some(Obligation {
3732 cause: cause.clone(),
3735 predicate: predicate.value,
3740 // We are performing deduplication here to avoid exponential blowups
3741 // (#38528) from happening, but the real cause of the duplication is
3742 // unknown. What we know is that the deduplication avoids exponential
3743 // amount of predicates being propagated when processing deeply nested
3746 // This code is hot enough that it's worth avoiding the allocation
3747 // required for the FxHashSet when possible. Special-casing lengths 0,
3748 // 1 and 2 covers roughly 75--80% of the cases.
3749 if predicates.len() <= 1 {
3750 // No possibility of duplicates.
3751 } else if predicates.len() == 2 {
3752 // Only two elements. Drop the second if they are equal.
3753 if predicates[0] == predicates[1] {
3754 predicates.truncate(1);
3757 // Three or more elements. Use a general deduplication process.
3758 let mut seen = FxHashSet::default();
3759 predicates.retain(|i| seen.insert(i.clone()));
3766 impl<'tcx> TraitObligation<'tcx> {
3767 #[allow(unused_comparisons)]
3768 pub fn derived_cause(
3770 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
3771 ) -> ObligationCause<'tcx> {
3773 * Creates a cause for obligations that are derived from
3774 * `obligation` by a recursive search (e.g., for a builtin
3775 * bound, or eventually a `auto trait Foo`). If `obligation`
3776 * is itself a derived obligation, this is just a clone, but
3777 * otherwise we create a "derived obligation" cause so as to
3778 * keep track of the original root obligation for error
3782 let obligation = self;
3784 // NOTE(flaper87): As of now, it keeps track of the whole error
3785 // chain. Ideally, we should have a way to configure this either
3786 // by using -Z verbose or just a CLI argument.
3787 if obligation.recursion_depth >= 0 {
3788 let derived_cause = DerivedObligationCause {
3789 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
3790 parent_code: Rc::new(obligation.cause.code.clone()),
3792 let derived_code = variant(derived_cause);
3793 ObligationCause::new(
3794 obligation.cause.span,
3795 obligation.cause.body_id,
3799 obligation.cause.clone()
3804 impl<'tcx> SelectionCache<'tcx> {
3805 /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
3806 pub fn clear(&self) {
3807 *self.hashmap.borrow_mut() = Default::default();
3811 impl<'tcx> EvaluationCache<'tcx> {
3812 /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
3813 pub fn clear(&self) {
3814 *self.hashmap.borrow_mut() = Default::default();
3818 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
3819 fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
3820 TraitObligationStackList::with(self)
3823 fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
3828 #[derive(Copy, Clone)]
3829 struct TraitObligationStackList<'o, 'tcx: 'o> {
3830 head: Option<&'o TraitObligationStack<'o, 'tcx>>,
3833 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
3834 fn empty() -> TraitObligationStackList<'o, 'tcx> {
3835 TraitObligationStackList { head: None }
3838 fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
3839 TraitObligationStackList { head: Some(r) }
3842 fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
3847 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
3848 type Item = &'o TraitObligationStack<'o, 'tcx>;
3850 fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
3861 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
3862 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3863 write!(f, "TraitObligationStack({:?})", self.obligation)
3867 #[derive(Clone, Eq, PartialEq)]
3868 pub struct WithDepNode<T> {
3869 dep_node: DepNodeIndex,
3873 impl<T: Clone> WithDepNode<T> {
3874 pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self {
3881 pub fn get(&self, tcx: TyCtxt<'_, '_, '_>) -> T {
3882 tcx.dep_graph.read_index(self.dep_node);
3883 self.cached_value.clone()