1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! See [rustc guide] for more info on how this works.
13 //! [rustc guide]: https://rust-lang-nursery.github.io/rustc-guide/trait-resolution.html#selection
15 use self::SelectionCandidate::*;
16 use self::EvaluationResult::*;
18 use super::coherence::{self, Conflict};
19 use super::DerivedObligationCause;
20 use super::IntercrateMode;
22 use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey};
23 use super::{PredicateObligation, TraitObligation, ObligationCause};
24 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation};
25 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch};
26 use super::{ObjectCastObligation, Obligation};
27 use super::TraitNotObjectSafe;
29 use super::SelectionResult;
30 use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure, VtableGenerator,
31 VtableFnPointer, VtableObject, VtableAutoImpl};
32 use super::{VtableImplData, VtableObjectData, VtableBuiltinData, VtableGeneratorData,
33 VtableClosureData, VtableAutoImplData, VtableFnPointerData};
36 use dep_graph::{DepNodeIndex, DepKind};
37 use hir::def_id::DefId;
39 use infer::{InferCtxt, InferOk, TypeFreshener};
40 use ty::subst::{Kind, Subst, Substs};
41 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
43 use ty::relate::TypeRelation;
44 use middle::lang_items;
45 use mir::interpret::{GlobalId};
47 use rustc_data_structures::bitvec::BitVector;
49 use std::cell::RefCell;
56 use util::nodemap::{FxHashMap, FxHashSet};
59 pub struct SelectionContext<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
60 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
62 /// Freshener used specifically for skolemizing entries on the
63 /// obligation stack. This ensures that all entries on the stack
64 /// at one time will have the same set of skolemized entries,
65 /// which is important for checking for trait bounds that
66 /// recursively require themselves.
67 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
69 /// If true, indicates that the evaluation should be conservative
70 /// and consider the possibility of types outside this crate.
71 /// This comes up primarily when resolving ambiguity. Imagine
72 /// there is some trait reference `$0 : Bar` where `$0` is an
73 /// inference variable. If `intercrate` is true, then we can never
74 /// say for sure that this reference is not implemented, even if
75 /// there are *no impls at all for `Bar`*, because `$0` could be
76 /// bound to some type that in a downstream crate that implements
77 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
78 /// though, we set this to false, because we are only interested
79 /// in types that the user could actually have written --- in
80 /// other words, we consider `$0 : Bar` to be unimplemented if
81 /// there is no type that the user could *actually name* that
82 /// would satisfy it. This avoids crippling inference, basically.
83 intercrate: Option<IntercrateMode>,
85 intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
87 /// Controls whether or not to filter out negative impls when selecting.
88 /// This is used in librustdoc to distinguish between the lack of an impl
89 /// and a negative impl
90 allow_negative_impls: bool
93 #[derive(Clone, Debug)]
94 pub enum IntercrateAmbiguityCause {
97 self_desc: Option<String>,
101 self_desc: Option<String>,
105 impl IntercrateAmbiguityCause {
106 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
107 /// See #23980 for details.
108 pub fn add_intercrate_ambiguity_hint<'a, 'tcx>(&self,
109 err: &mut ::errors::DiagnosticBuilder) {
110 err.note(&self.intercrate_ambiguity_hint());
113 pub fn intercrate_ambiguity_hint(&self) -> String {
115 &IntercrateAmbiguityCause::DownstreamCrate { ref trait_desc, ref self_desc } => {
116 let self_desc = if let &Some(ref ty) = self_desc {
117 format!(" for type `{}`", ty)
118 } else { "".to_string() };
119 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
121 &IntercrateAmbiguityCause::UpstreamCrateUpdate { ref trait_desc, ref self_desc } => {
122 let self_desc = if let &Some(ref ty) = self_desc {
123 format!(" for type `{}`", ty)
124 } else { "".to_string() };
125 format!("upstream crates may add new impl of trait `{}`{} \
127 trait_desc, self_desc)
133 // A stack that walks back up the stack frame.
134 struct TraitObligationStack<'prev, 'tcx: 'prev> {
135 obligation: &'prev TraitObligation<'tcx>,
137 /// Trait ref from `obligation` but skolemized with the
138 /// selection-context's freshener. Used to check for recursion.
139 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
141 previous: TraitObligationStackList<'prev, 'tcx>,
145 pub struct SelectionCache<'tcx> {
146 hashmap: RefCell<FxHashMap<ty::TraitRef<'tcx>,
147 WithDepNode<SelectionResult<'tcx, SelectionCandidate<'tcx>>>>>,
150 /// The selection process begins by considering all impls, where
151 /// clauses, and so forth that might resolve an obligation. Sometimes
152 /// we'll be able to say definitively that (e.g.) an impl does not
153 /// apply to the obligation: perhaps it is defined for `usize` but the
154 /// obligation is for `int`. In that case, we drop the impl out of the
155 /// list. But the other cases are considered *candidates*.
157 /// For selection to succeed, there must be exactly one matching
158 /// candidate. If the obligation is fully known, this is guaranteed
159 /// by coherence. However, if the obligation contains type parameters
160 /// or variables, there may be multiple such impls.
162 /// It is not a real problem if multiple matching impls exist because
163 /// of type variables - it just means the obligation isn't sufficiently
164 /// elaborated. In that case we report an ambiguity, and the caller can
165 /// try again after more type information has been gathered or report a
166 /// "type annotations required" error.
168 /// However, with type parameters, this can be a real problem - type
169 /// parameters don't unify with regular types, but they *can* unify
170 /// with variables from blanket impls, and (unless we know its bounds
171 /// will always be satisfied) picking the blanket impl will be wrong
172 /// for at least *some* substitutions. To make this concrete, if we have
174 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
175 /// impl<T: fmt::Debug> AsDebug for T {
177 /// fn debug(self) -> fmt::Debug { self }
179 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
181 /// we can't just use the impl to resolve the <T as AsDebug> obligation
182 /// - a type from another crate (that doesn't implement fmt::Debug) could
183 /// implement AsDebug.
185 /// Because where-clauses match the type exactly, multiple clauses can
186 /// only match if there are unresolved variables, and we can mostly just
187 /// report this ambiguity in that case. This is still a problem - we can't
188 /// *do anything* with ambiguities that involve only regions. This is issue
191 /// If a single where-clause matches and there are no inference
192 /// variables left, then it definitely matches and we can just select
195 /// In fact, we even select the where-clause when the obligation contains
196 /// inference variables. The can lead to inference making "leaps of logic",
197 /// for example in this situation:
199 /// pub trait Foo<T> { fn foo(&self) -> T; }
200 /// impl<T> Foo<()> for T { fn foo(&self) { } }
201 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
203 /// pub fn foo<T>(t: T) where T: Foo<bool> {
204 /// println!("{:?}", <T as Foo<_>>::foo(&t));
206 /// fn main() { foo(false); }
208 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
209 /// impl and the where-clause. We select the where-clause and unify $0=bool,
210 /// so the program prints "false". However, if the where-clause is omitted,
211 /// the blanket impl is selected, we unify $0=(), and the program prints
214 /// Exactly the same issues apply to projection and object candidates, except
215 /// that we can have both a projection candidate and a where-clause candidate
216 /// for the same obligation. In that case either would do (except that
217 /// different "leaps of logic" would occur if inference variables are
218 /// present), and we just pick the where-clause. This is, for example,
219 /// required for associated types to work in default impls, as the bounds
220 /// are visible both as projection bounds and as where-clauses from the
221 /// parameter environment.
222 #[derive(PartialEq,Eq,Debug,Clone)]
223 enum SelectionCandidate<'tcx> {
224 BuiltinCandidate { has_nested: bool },
225 ParamCandidate(ty::PolyTraitRef<'tcx>),
226 ImplCandidate(DefId),
227 AutoImplCandidate(DefId),
229 /// This is a trait matching with a projected type as `Self`, and
230 /// we found an applicable bound in the trait definition.
233 /// Implementation of a `Fn`-family trait by one of the anonymous types
234 /// generated for a `||` expression.
237 /// Implementation of a `Generator` trait by one of the anonymous types
238 /// generated for a generator.
241 /// Implementation of a `Fn`-family trait by one of the anonymous
242 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
247 BuiltinObjectCandidate,
249 BuiltinUnsizeCandidate,
252 impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> {
253 type Lifted = SelectionCandidate<'tcx>;
254 fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
256 BuiltinCandidate { has_nested } => {
261 ImplCandidate(def_id) => ImplCandidate(def_id),
262 AutoImplCandidate(def_id) => AutoImplCandidate(def_id),
263 ProjectionCandidate => ProjectionCandidate,
264 FnPointerCandidate => FnPointerCandidate,
265 ObjectCandidate => ObjectCandidate,
266 BuiltinObjectCandidate => BuiltinObjectCandidate,
267 BuiltinUnsizeCandidate => BuiltinUnsizeCandidate,
268 ClosureCandidate => ClosureCandidate,
269 GeneratorCandidate => GeneratorCandidate,
271 ParamCandidate(ref trait_ref) => {
272 return tcx.lift(trait_ref).map(ParamCandidate);
278 struct SelectionCandidateSet<'tcx> {
279 // a list of candidates that definitely apply to the current
280 // obligation (meaning: types unify).
281 vec: Vec<SelectionCandidate<'tcx>>,
283 // if this is true, then there were candidates that might or might
284 // not have applied, but we couldn't tell. This occurs when some
285 // of the input types are type variables, in which case there are
286 // various "builtin" rules that might or might not trigger.
290 #[derive(PartialEq,Eq,Debug,Clone)]
291 struct EvaluatedCandidate<'tcx> {
292 candidate: SelectionCandidate<'tcx>,
293 evaluation: EvaluationResult,
296 /// When does the builtin impl for `T: Trait` apply?
297 enum BuiltinImplConditions<'tcx> {
298 /// The impl is conditional on T1,T2,.. : Trait
299 Where(ty::Binder<Vec<Ty<'tcx>>>),
300 /// There is no built-in impl. There may be some other
301 /// candidate (a where-clause or user-defined impl).
303 /// There is *no* impl for this, builtin or not. Ignore
304 /// all where-clauses.
306 /// It is unknown whether there is an impl.
310 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
311 /// The result of trait evaluation. The order is important
312 /// here as the evaluation of a list is the maximum of the
315 /// The evaluation results are ordered:
316 /// - `EvaluatedToOk` implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
317 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
318 /// - the "union" of evaluation results is equal to their maximum -
319 /// all the "potential success" candidates can potentially succeed,
320 /// so they are no-ops when unioned with a definite error, and within
321 /// the categories it's easy to see that the unions are correct.
322 enum EvaluationResult {
323 /// Evaluation successful
325 /// Evaluation is known to be ambiguous - it *might* hold for some
326 /// assignment of inference variables, but it might not.
328 /// While this has the same meaning as `EvaluatedToUnknown` - we can't
329 /// know whether this obligation holds or not - it is the result we
330 /// would get with an empty stack, and therefore is cacheable.
332 /// Evaluation failed because of recursion involving inference
333 /// variables. We are somewhat imprecise there, so we don't actually
334 /// know the real result.
336 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
338 /// Evaluation failed because we encountered an obligation we are already
339 /// trying to prove on this branch.
341 /// We know this branch can't be a part of a minimal proof-tree for
342 /// the "root" of our cycle, because then we could cut out the recursion
343 /// and maintain a valid proof tree. However, this does not mean
344 /// that all the obligations on this branch do not hold - it's possible
345 /// that we entered this branch "speculatively", and that there
346 /// might be some other way to prove this obligation that does not
347 /// go through this cycle - so we can't cache this as a failure.
349 /// For example, suppose we have this:
351 /// ```rust,ignore (pseudo-Rust)
352 /// pub trait Trait { fn xyz(); }
353 /// // This impl is "useless", but we can still have
354 /// // an `impl Trait for SomeUnsizedType` somewhere.
355 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
357 /// pub fn foo<T: Trait + ?Sized>() {
358 /// <T as Trait>::xyz();
362 /// When checking `foo`, we have to prove `T: Trait`. This basically
363 /// translates into this:
366 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
369 /// When we try to prove it, we first go the first option, which
370 /// recurses. This shows us that the impl is "useless" - it won't
371 /// tell us that `T: Trait` unless it already implemented `Trait`
372 /// by some other means. However, that does not prevent `T: Trait`
373 /// does not hold, because of the bound (which can indeed be satisfied
374 /// by `SomeUnsizedType` from another crate).
376 /// FIXME: when an `EvaluatedToRecur` goes past its parent root, we
377 /// ought to convert it to an `EvaluatedToErr`, because we know
378 /// there definitely isn't a proof tree for that obligation. Not
379 /// doing so is still sound - there isn't any proof tree, so the
380 /// branch still can't be a part of a minimal one - but does not
381 /// re-enable caching.
383 /// Evaluation failed
387 impl EvaluationResult {
388 fn may_apply(self) -> bool {
392 EvaluatedToUnknown => true,
395 EvaluatedToRecur => false
399 fn is_stack_dependent(self) -> bool {
402 EvaluatedToRecur => true,
406 EvaluatedToErr => false,
412 pub struct EvaluationCache<'tcx> {
413 hashmap: RefCell<FxHashMap<ty::PolyTraitRef<'tcx>, WithDepNode<EvaluationResult>>>
416 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
417 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
420 freshener: infcx.freshener(),
422 intercrate_ambiguity_causes: None,
423 allow_negative_impls: false,
427 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
428 mode: IntercrateMode) -> SelectionContext<'cx, 'gcx, 'tcx> {
429 debug!("intercrate({:?})", mode);
432 freshener: infcx.freshener(),
433 intercrate: Some(mode),
434 intercrate_ambiguity_causes: None,
435 allow_negative_impls: false,
439 pub fn with_negative(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
440 allow_negative_impls: bool) -> SelectionContext<'cx, 'gcx, 'tcx> {
441 debug!("with_negative({:?})", allow_negative_impls);
444 freshener: infcx.freshener(),
446 intercrate_ambiguity_causes: None,
447 allow_negative_impls,
451 /// Enables tracking of intercrate ambiguity causes. These are
452 /// used in coherence to give improved diagnostics. We don't do
453 /// this until we detect a coherence error because it can lead to
454 /// false overflow results (#47139) and because it costs
455 /// computation time.
456 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
457 assert!(self.intercrate.is_some());
458 assert!(self.intercrate_ambiguity_causes.is_none());
459 self.intercrate_ambiguity_causes = Some(vec![]);
460 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
463 /// Gets the intercrate ambiguity causes collected since tracking
464 /// was enabled and disables tracking at the same time. If
465 /// tracking is not enabled, just returns an empty vector.
466 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
467 assert!(self.intercrate.is_some());
468 self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
471 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
475 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
479 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
483 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
485 fn in_snapshot<R, F>(&mut self, f: F) -> R
486 where F: FnOnce(&mut Self, &infer::CombinedSnapshot<'cx, 'tcx>) -> R
488 self.infcx.in_snapshot(|snapshot| f(self, snapshot))
491 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
493 fn probe<R, F>(&mut self, f: F) -> R
494 where F: FnOnce(&mut Self, &infer::CombinedSnapshot<'cx, 'tcx>) -> R
496 self.infcx.probe(|snapshot| f(self, snapshot))
499 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
500 /// the transaction fails and s.t. old obligations are retained.
501 fn commit_if_ok<T, E, F>(&mut self, f: F) -> Result<T, E> where
502 F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> Result<T, E>
504 self.infcx.commit_if_ok(|snapshot| f(self, snapshot))
508 ///////////////////////////////////////////////////////////////////////////
511 // The selection phase tries to identify *how* an obligation will
512 // be resolved. For example, it will identify which impl or
513 // parameter bound is to be used. The process can be inconclusive
514 // if the self type in the obligation is not fully inferred. Selection
515 // can result in an error in one of two ways:
517 // 1. If no applicable impl or parameter bound can be found.
518 // 2. If the output type parameters in the obligation do not match
519 // those specified by the impl/bound. For example, if the obligation
520 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
521 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
523 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
524 /// type environment by performing unification.
525 pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
526 -> SelectionResult<'tcx, Selection<'tcx>> {
527 debug!("select({:?})", obligation);
528 assert!(!obligation.predicate.has_escaping_regions());
530 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
531 let ret = match self.candidate_from_obligation(&stack)? {
533 Some(candidate) => Some(self.confirm_candidate(obligation, candidate)?)
539 ///////////////////////////////////////////////////////////////////////////
542 // Tests whether an obligation can be selected or whether an impl
543 // can be applied to particular types. It skips the "confirmation"
544 // step and hence completely ignores output type parameters.
546 // The result is "true" if the obligation *may* hold and "false" if
547 // we can be sure it does not.
549 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
550 pub fn evaluate_obligation(&mut self,
551 obligation: &PredicateObligation<'tcx>)
554 debug!("evaluate_obligation({:?})",
557 self.probe(|this, _| {
558 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
563 /// Evaluates whether the obligation `obligation` can be satisfied,
564 /// and returns `false` if not certain. However, this is not entirely
565 /// accurate if inference variables are involved.
566 pub fn evaluate_obligation_conservatively(&mut self,
567 obligation: &PredicateObligation<'tcx>)
570 debug!("evaluate_obligation_conservatively({:?})",
573 self.probe(|this, _| {
574 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
579 /// Evaluates the predicates in `predicates` recursively. Note that
580 /// this applies projections in the predicates, and therefore
581 /// is run within an inference probe.
582 fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
583 stack: TraitObligationStackList<'o, 'tcx>,
586 where I : IntoIterator<Item=&'a PredicateObligation<'tcx>>, 'tcx:'a
588 let mut result = EvaluatedToOk;
589 for obligation in predicates {
590 let eval = self.evaluate_predicate_recursively(stack, obligation);
591 debug!("evaluate_predicate_recursively({:?}) = {:?}",
593 if let EvaluatedToErr = eval {
594 // fast-path - EvaluatedToErr is the top of the lattice,
595 // so we don't need to look on the other predicates.
596 return EvaluatedToErr;
598 result = cmp::max(result, eval);
604 fn evaluate_predicate_recursively<'o>(&mut self,
605 previous_stack: TraitObligationStackList<'o, 'tcx>,
606 obligation: &PredicateObligation<'tcx>)
609 debug!("evaluate_predicate_recursively({:?})",
612 match obligation.predicate {
613 ty::Predicate::Trait(ref t) => {
614 assert!(!t.has_escaping_regions());
615 let obligation = obligation.with(t.clone());
616 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
619 ty::Predicate::Subtype(ref p) => {
620 // does this code ever run?
621 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
622 Some(Ok(InferOk { obligations, .. })) => {
623 self.evaluate_predicates_recursively(previous_stack, &obligations);
626 Some(Err(_)) => EvaluatedToErr,
627 None => EvaluatedToAmbig,
631 ty::Predicate::WellFormed(ty) => {
632 match ty::wf::obligations(self.infcx,
633 obligation.param_env,
634 obligation.cause.body_id,
635 ty, obligation.cause.span) {
637 self.evaluate_predicates_recursively(previous_stack, obligations.iter()),
643 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
644 // we do not consider region relationships when
645 // evaluating trait matches
649 ty::Predicate::ObjectSafe(trait_def_id) => {
650 if self.tcx().is_object_safe(trait_def_id) {
657 ty::Predicate::Projection(ref data) => {
658 let project_obligation = obligation.with(data.clone());
659 match project::poly_project_and_unify_type(self, &project_obligation) {
660 Ok(Some(subobligations)) => {
661 let result = self.evaluate_predicates_recursively(previous_stack,
662 subobligations.iter());
664 ProjectionCacheKey::from_poly_projection_predicate(self, data)
666 self.infcx.projection_cache.borrow_mut().complete(key);
679 ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
680 match self.infcx.closure_kind(closure_def_id, closure_substs) {
681 Some(closure_kind) => {
682 if closure_kind.extends(kind) {
694 ty::Predicate::ConstEvaluatable(def_id, substs) => {
695 let tcx = self.tcx();
696 match tcx.lift_to_global(&(obligation.param_env, substs)) {
697 Some((param_env, substs)) => {
698 let instance = ty::Instance::resolve(
704 if let Some(instance) = instance {
709 match self.tcx().const_eval(param_env.and(cid)) {
710 Ok(_) => EvaluatedToOk,
711 Err(_) => EvaluatedToErr
718 // Inference variables still left in param_env or substs.
726 fn evaluate_trait_predicate_recursively<'o>(&mut self,
727 previous_stack: TraitObligationStackList<'o, 'tcx>,
728 mut obligation: TraitObligation<'tcx>)
731 debug!("evaluate_trait_predicate_recursively({:?})",
734 if !self.intercrate.is_some() && obligation.is_global() {
735 // If a param env is consistent, global obligations do not depend on its particular
736 // value in order to work, so we can clear out the param env and get better
737 // caching. (If the current param env is inconsistent, we don't care what happens).
738 debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation);
739 obligation.param_env = obligation.param_env.without_caller_bounds();
742 let stack = self.push_stack(previous_stack, &obligation);
743 let fresh_trait_ref = stack.fresh_trait_ref;
744 if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
745 debug!("CACHE HIT: EVAL({:?})={:?}",
751 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
753 debug!("CACHE MISS: EVAL({:?})={:?}",
756 self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
761 fn evaluate_stack<'o>(&mut self,
762 stack: &TraitObligationStack<'o, 'tcx>)
765 // In intercrate mode, whenever any of the types are unbound,
766 // there can always be an impl. Even if there are no impls in
767 // this crate, perhaps the type would be unified with
768 // something from another crate that does provide an impl.
770 // In intra mode, we must still be conservative. The reason is
771 // that we want to avoid cycles. Imagine an impl like:
773 // impl<T:Eq> Eq for Vec<T>
775 // and a trait reference like `$0 : Eq` where `$0` is an
776 // unbound variable. When we evaluate this trait-reference, we
777 // will unify `$0` with `Vec<$1>` (for some fresh variable
778 // `$1`), on the condition that `$1 : Eq`. We will then wind
779 // up with many candidates (since that are other `Eq` impls
780 // that apply) and try to winnow things down. This results in
781 // a recursive evaluation that `$1 : Eq` -- as you can
782 // imagine, this is just where we started. To avoid that, we
783 // check for unbound variables and return an ambiguous (hence possible)
784 // match if we've seen this trait before.
786 // This suffices to allow chains like `FnMut` implemented in
787 // terms of `Fn` etc, but we could probably make this more
789 let unbound_input_types = stack.fresh_trait_ref.input_types().any(|ty| ty.is_fresh());
790 // this check was an imperfect workaround for a bug n the old
791 // intercrate mode, it should be removed when that goes away.
792 if unbound_input_types &&
793 self.intercrate == Some(IntercrateMode::Issue43355)
795 debug!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
796 stack.fresh_trait_ref);
797 // Heuristics: show the diagnostics when there are no candidates in crate.
798 if self.intercrate_ambiguity_causes.is_some() {
799 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
800 if let Ok(candidate_set) = self.assemble_candidates(stack) {
801 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
802 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
803 let self_ty = trait_ref.self_ty();
804 let cause = IntercrateAmbiguityCause::DownstreamCrate {
805 trait_desc: trait_ref.to_string(),
806 self_desc: if self_ty.has_concrete_skeleton() {
807 Some(self_ty.to_string())
812 debug!("evaluate_stack: pushing cause = {:?}", cause);
813 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
817 return EvaluatedToAmbig;
819 if unbound_input_types &&
820 stack.iter().skip(1).any(
821 |prev| stack.obligation.param_env == prev.obligation.param_env &&
822 self.match_fresh_trait_refs(&stack.fresh_trait_ref,
823 &prev.fresh_trait_ref))
825 debug!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
826 stack.fresh_trait_ref);
827 return EvaluatedToUnknown;
830 // If there is any previous entry on the stack that precisely
831 // matches this obligation, then we can assume that the
832 // obligation is satisfied for now (still all other conditions
833 // must be met of course). One obvious case this comes up is
834 // marker traits like `Send`. Think of a linked list:
836 // struct List<T> { data: T, next: Option<Box<List<T>>> {
838 // `Box<List<T>>` will be `Send` if `T` is `Send` and
839 // `Option<Box<List<T>>>` is `Send`, and in turn
840 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
843 // Note that we do this comparison using the `fresh_trait_ref`
844 // fields. Because these have all been skolemized using
845 // `self.freshener`, we can be sure that (a) this will not
846 // affect the inferencer state and (b) that if we see two
847 // skolemized types with the same index, they refer to the
848 // same unbound type variable.
849 if let Some(rec_index) =
851 .skip(1) // skip top-most frame
852 .position(|prev| stack.obligation.param_env == prev.obligation.param_env &&
853 stack.fresh_trait_ref == prev.fresh_trait_ref)
855 debug!("evaluate_stack({:?}) --> recursive",
856 stack.fresh_trait_ref);
857 let cycle = stack.iter().skip(1).take(rec_index+1);
858 let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate));
859 if self.coinductive_match(cycle) {
860 debug!("evaluate_stack({:?}) --> recursive, coinductive",
861 stack.fresh_trait_ref);
862 return EvaluatedToOk;
864 debug!("evaluate_stack({:?}) --> recursive, inductive",
865 stack.fresh_trait_ref);
866 return EvaluatedToRecur;
870 match self.candidate_from_obligation(stack) {
871 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
872 Ok(None) => EvaluatedToAmbig,
873 Err(..) => EvaluatedToErr
877 /// For defaulted traits, we use a co-inductive strategy to solve, so
878 /// that recursion is ok. This routine returns true if the top of the
879 /// stack (`cycle[0]`):
881 /// - is a defaulted trait, and
882 /// - it also appears in the backtrace at some position `X`; and,
883 /// - all the predicates at positions `X..` between `X` an the top are
884 /// also defaulted traits.
885 pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
886 where I: Iterator<Item=ty::Predicate<'tcx>>
888 let mut cycle = cycle;
889 cycle.all(|predicate| self.coinductive_predicate(predicate))
892 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
893 let result = match predicate {
894 ty::Predicate::Trait(ref data) => {
895 self.tcx().trait_is_auto(data.def_id())
901 debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
905 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
906 /// obligations are met. Returns true if `candidate` remains viable after this further
908 fn evaluate_candidate<'o>(&mut self,
909 stack: &TraitObligationStack<'o, 'tcx>,
910 candidate: &SelectionCandidate<'tcx>)
913 debug!("evaluate_candidate: depth={} candidate={:?}",
914 stack.obligation.recursion_depth, candidate);
915 let result = self.probe(|this, _| {
916 let candidate = (*candidate).clone();
917 match this.confirm_candidate(stack.obligation, candidate) {
919 this.evaluate_predicates_recursively(
921 selection.nested_obligations().iter())
923 Err(..) => EvaluatedToErr
926 debug!("evaluate_candidate: depth={} result={:?}",
927 stack.obligation.recursion_depth, result);
931 fn check_evaluation_cache(&self,
932 param_env: ty::ParamEnv<'tcx>,
933 trait_ref: ty::PolyTraitRef<'tcx>)
934 -> Option<EvaluationResult>
936 let tcx = self.tcx();
937 if self.can_use_global_caches(param_env) {
938 let cache = tcx.evaluation_cache.hashmap.borrow();
939 if let Some(cached) = cache.get(&trait_ref) {
940 return Some(cached.get(tcx));
943 self.infcx.evaluation_cache.hashmap
949 fn insert_evaluation_cache(&mut self,
950 param_env: ty::ParamEnv<'tcx>,
951 trait_ref: ty::PolyTraitRef<'tcx>,
952 dep_node: DepNodeIndex,
953 result: EvaluationResult)
955 // Avoid caching results that depend on more than just the trait-ref
956 // - the stack can create recursion.
957 if result.is_stack_dependent() {
961 if self.can_use_global_caches(param_env) {
962 let mut cache = self.tcx().evaluation_cache.hashmap.borrow_mut();
963 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
964 cache.insert(trait_ref, WithDepNode::new(dep_node, result));
969 self.infcx.evaluation_cache.hashmap
971 .insert(trait_ref, WithDepNode::new(dep_node, result));
974 ///////////////////////////////////////////////////////////////////////////
975 // CANDIDATE ASSEMBLY
977 // The selection process begins by examining all in-scope impls,
978 // caller obligations, and so forth and assembling a list of
979 // candidates. See [rustc guide] for more details.
982 // https://rust-lang-nursery.github.io/rustc-guide/trait-resolution.html#candidate-assembly
984 fn candidate_from_obligation<'o>(&mut self,
985 stack: &TraitObligationStack<'o, 'tcx>)
986 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
988 // Watch out for overflow. This intentionally bypasses (and does
989 // not update) the cache.
990 let recursion_limit = self.infcx.tcx.sess.recursion_limit.get();
991 if stack.obligation.recursion_depth >= recursion_limit {
992 self.infcx().report_overflow_error(&stack.obligation, true);
995 // Check the cache. Note that we skolemize the trait-ref
996 // separately rather than using `stack.fresh_trait_ref` -- this
997 // is because we want the unbound variables to be replaced
998 // with fresh skolemized types starting from index 0.
999 let cache_fresh_trait_pred =
1000 self.infcx.freshen(stack.obligation.predicate.clone());
1001 debug!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
1002 cache_fresh_trait_pred,
1004 assert!(!stack.obligation.predicate.has_escaping_regions());
1006 if let Some(c) = self.check_candidate_cache(stack.obligation.param_env,
1007 &cache_fresh_trait_pred) {
1008 debug!("CACHE HIT: SELECT({:?})={:?}",
1009 cache_fresh_trait_pred,
1014 // If no match, compute result and insert into cache.
1015 let (candidate, dep_node) = self.in_task(|this| {
1016 this.candidate_from_obligation_no_cache(stack)
1019 debug!("CACHE MISS: SELECT({:?})={:?}",
1020 cache_fresh_trait_pred, candidate);
1021 self.insert_candidate_cache(stack.obligation.param_env,
1022 cache_fresh_trait_pred,
1028 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1029 where OP: FnOnce(&mut Self) -> R
1031 let (result, dep_node) = self.tcx().dep_graph.with_anon_task(DepKind::TraitSelect, || {
1034 self.tcx().dep_graph.read_index(dep_node);
1038 // Treat negative impls as unimplemented
1039 fn filter_negative_impls(&self, candidate: SelectionCandidate<'tcx>)
1040 -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1041 if let ImplCandidate(def_id) = candidate {
1042 if !self.allow_negative_impls &&
1043 self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative {
1044 return Err(Unimplemented)
1050 fn candidate_from_obligation_no_cache<'o>(&mut self,
1051 stack: &TraitObligationStack<'o, 'tcx>)
1052 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
1054 if stack.obligation.predicate.references_error() {
1055 // If we encounter a `TyError`, we generally prefer the
1056 // most "optimistic" result in response -- that is, the
1057 // one least likely to report downstream errors. But
1058 // because this routine is shared by coherence and by
1059 // trait selection, there isn't an obvious "right" choice
1060 // here in that respect, so we opt to just return
1061 // ambiguity and let the upstream clients sort it out.
1065 match self.is_knowable(stack) {
1068 debug!("coherence stage: not knowable");
1069 if self.intercrate_ambiguity_causes.is_some() {
1070 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
1071 // Heuristics: show the diagnostics when there are no candidates in crate.
1072 let candidate_set = self.assemble_candidates(stack)?;
1073 if !candidate_set.ambiguous && candidate_set.vec.iter().all(|c| {
1074 !self.evaluate_candidate(stack, &c).may_apply()
1076 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
1077 let self_ty = trait_ref.self_ty();
1078 let trait_desc = trait_ref.to_string();
1079 let self_desc = if self_ty.has_concrete_skeleton() {
1080 Some(self_ty.to_string())
1084 let cause = if let Conflict::Upstream = conflict {
1085 IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc }
1087 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
1089 debug!("evaluate_stack: pushing cause = {:?}", cause);
1090 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
1097 let candidate_set = self.assemble_candidates(stack)?;
1099 if candidate_set.ambiguous {
1100 debug!("candidate set contains ambig");
1104 let mut candidates = candidate_set.vec;
1106 debug!("assembled {} candidates for {:?}: {:?}",
1111 // At this point, we know that each of the entries in the
1112 // candidate set is *individually* applicable. Now we have to
1113 // figure out if they contain mutual incompatibilities. This
1114 // frequently arises if we have an unconstrained input type --
1115 // for example, we are looking for $0:Eq where $0 is some
1116 // unconstrained type variable. In that case, we'll get a
1117 // candidate which assumes $0 == int, one that assumes $0 ==
1118 // usize, etc. This spells an ambiguity.
1120 // If there is more than one candidate, first winnow them down
1121 // by considering extra conditions (nested obligations and so
1122 // forth). We don't winnow if there is exactly one
1123 // candidate. This is a relatively minor distinction but it
1124 // can lead to better inference and error-reporting. An
1125 // example would be if there was an impl:
1127 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
1129 // and we were to see some code `foo.push_clone()` where `boo`
1130 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
1131 // we were to winnow, we'd wind up with zero candidates.
1132 // Instead, we select the right impl now but report `Bar does
1133 // not implement Clone`.
1134 if candidates.len() == 1 {
1135 return self.filter_negative_impls(candidates.pop().unwrap());
1138 // Winnow, but record the exact outcome of evaluation, which
1139 // is needed for specialization.
1140 let mut candidates: Vec<_> = candidates.into_iter().filter_map(|c| {
1141 let eval = self.evaluate_candidate(stack, &c);
1142 if eval.may_apply() {
1143 Some(EvaluatedCandidate {
1152 // If there are STILL multiple candidate, we can further
1153 // reduce the list by dropping duplicates -- including
1154 // resolving specializations.
1155 if candidates.len() > 1 {
1157 while i < candidates.len() {
1159 (0..candidates.len())
1160 .filter(|&j| i != j)
1161 .any(|j| self.candidate_should_be_dropped_in_favor_of(&candidates[i],
1164 debug!("Dropping candidate #{}/{}: {:?}",
1165 i, candidates.len(), candidates[i]);
1166 candidates.swap_remove(i);
1168 debug!("Retaining candidate #{}/{}: {:?}",
1169 i, candidates.len(), candidates[i]);
1172 // If there are *STILL* multiple candidates, give up
1173 // and report ambiguity.
1175 debug!("multiple matches, ambig");
1182 // If there are *NO* candidates, then there are no impls --
1183 // that we know of, anyway. Note that in the case where there
1184 // are unbound type variables within the obligation, it might
1185 // be the case that you could still satisfy the obligation
1186 // from another crate by instantiating the type variables with
1187 // a type from another crate that does have an impl. This case
1188 // is checked for in `evaluate_stack` (and hence users
1189 // who might care about this case, like coherence, should use
1191 if candidates.is_empty() {
1192 return Err(Unimplemented);
1195 // Just one candidate left.
1196 self.filter_negative_impls(candidates.pop().unwrap().candidate)
1199 fn is_knowable<'o>(&mut self,
1200 stack: &TraitObligationStack<'o, 'tcx>)
1203 debug!("is_knowable(intercrate={:?})", self.intercrate);
1205 if !self.intercrate.is_some() {
1209 let obligation = &stack.obligation;
1210 let predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1212 // ok to skip binder because of the nature of the
1213 // trait-ref-is-knowable check, which does not care about
1215 let trait_ref = predicate.skip_binder().trait_ref;
1217 let result = coherence::trait_ref_is_knowable(self.tcx(), trait_ref);
1218 if let (Some(Conflict::Downstream { used_to_be_broken: true }),
1219 Some(IntercrateMode::Issue43355)) = (result, self.intercrate) {
1220 debug!("is_knowable: IGNORING conflict to be bug-compatible with #43355");
1227 /// Returns true if the global caches can be used.
1228 /// Do note that if the type itself is not in the
1229 /// global tcx, the local caches will be used.
1230 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1231 // If there are any where-clauses in scope, then we always use
1232 // a cache local to this particular scope. Otherwise, we
1233 // switch to a global cache. We used to try and draw
1234 // finer-grained distinctions, but that led to a serious of
1235 // annoying and weird bugs like #22019 and #18290. This simple
1236 // rule seems to be pretty clearly safe and also still retains
1237 // a very high hit rate (~95% when compiling rustc).
1238 if !param_env.caller_bounds.is_empty() {
1242 // Avoid using the master cache during coherence and just rely
1243 // on the local cache. This effectively disables caching
1244 // during coherence. It is really just a simplification to
1245 // avoid us having to fear that coherence results "pollute"
1246 // the master cache. Since coherence executes pretty quickly,
1247 // it's not worth going to more trouble to increase the
1248 // hit-rate I don't think.
1249 if self.intercrate.is_some() {
1253 // Otherwise, we can use the global cache.
1257 fn check_candidate_cache(&mut self,
1258 param_env: ty::ParamEnv<'tcx>,
1259 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
1260 -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
1262 let tcx = self.tcx();
1263 let trait_ref = &cache_fresh_trait_pred.0.trait_ref;
1264 if self.can_use_global_caches(param_env) {
1265 let cache = tcx.selection_cache.hashmap.borrow();
1266 if let Some(cached) = cache.get(&trait_ref) {
1267 return Some(cached.get(tcx));
1270 self.infcx.selection_cache.hashmap
1273 .map(|v| v.get(tcx))
1276 fn insert_candidate_cache(&mut self,
1277 param_env: ty::ParamEnv<'tcx>,
1278 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1279 dep_node: DepNodeIndex,
1280 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
1282 let tcx = self.tcx();
1283 let trait_ref = cache_fresh_trait_pred.0.trait_ref;
1284 if self.can_use_global_caches(param_env) {
1285 let mut cache = tcx.selection_cache.hashmap.borrow_mut();
1286 if let Some(trait_ref) = tcx.lift_to_global(&trait_ref) {
1287 if let Some(candidate) = tcx.lift_to_global(&candidate) {
1288 cache.insert(trait_ref, WithDepNode::new(dep_node, candidate));
1294 self.infcx.selection_cache.hashmap
1296 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1299 fn assemble_candidates<'o>(&mut self,
1300 stack: &TraitObligationStack<'o, 'tcx>)
1301 -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
1303 let TraitObligationStack { obligation, .. } = *stack;
1304 let ref obligation = Obligation {
1305 param_env: obligation.param_env,
1306 cause: obligation.cause.clone(),
1307 recursion_depth: obligation.recursion_depth,
1308 predicate: self.infcx().resolve_type_vars_if_possible(&obligation.predicate)
1311 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1312 // Self is a type variable (e.g. `_: AsRef<str>`).
1314 // This is somewhat problematic, as the current scheme can't really
1315 // handle it turning to be a projection. This does end up as truly
1316 // ambiguous in most cases anyway.
1318 // Take the fast path out - this also improves
1319 // performance by preventing assemble_candidates_from_impls from
1320 // matching every impl for this trait.
1321 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
1324 let mut candidates = SelectionCandidateSet {
1329 // Other bounds. Consider both in-scope bounds from fn decl
1330 // and applicable impls. There is a certain set of precedence rules here.
1332 let def_id = obligation.predicate.def_id();
1333 let lang_items = self.tcx().lang_items();
1334 if lang_items.copy_trait() == Some(def_id) {
1335 debug!("obligation self ty is {:?}",
1336 obligation.predicate.0.self_ty());
1338 // User-defined copy impls are permitted, but only for
1339 // structs and enums.
1340 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1342 // For other types, we'll use the builtin rules.
1343 let copy_conditions = self.copy_clone_conditions(obligation);
1344 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1345 } else if lang_items.sized_trait() == Some(def_id) {
1346 // Sized is never implementable by end-users, it is
1347 // always automatically computed.
1348 let sized_conditions = self.sized_conditions(obligation);
1349 self.assemble_builtin_bound_candidates(sized_conditions,
1351 } else if lang_items.unsize_trait() == Some(def_id) {
1352 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1354 if lang_items.clone_trait() == Some(def_id) {
1355 // Same builtin conditions as `Copy`, i.e. every type which has builtin support
1356 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
1357 // types have builtin support for `Clone`.
1358 let clone_conditions = self.copy_clone_conditions(obligation);
1359 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
1362 self.assemble_generator_candidates(obligation, &mut candidates)?;
1363 self.assemble_closure_candidates(obligation, &mut candidates)?;
1364 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1365 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1366 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1369 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1370 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1371 // Auto implementations have lower priority, so we only
1372 // consider triggering a default if there is no other impl that can apply.
1373 if candidates.vec.is_empty() {
1374 self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?;
1376 debug!("candidate list size: {}", candidates.vec.len());
1380 fn assemble_candidates_from_projected_tys(&mut self,
1381 obligation: &TraitObligation<'tcx>,
1382 candidates: &mut SelectionCandidateSet<'tcx>)
1384 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1386 // before we go into the whole skolemization thing, just
1387 // quickly check if the self-type is a projection at all.
1388 match obligation.predicate.0.trait_ref.self_ty().sty {
1389 ty::TyProjection(_) | ty::TyAnon(..) => {}
1390 ty::TyInfer(ty::TyVar(_)) => {
1391 span_bug!(obligation.cause.span,
1392 "Self=_ should have been handled by assemble_candidates");
1397 let result = self.probe(|this, snapshot| {
1398 this.match_projection_obligation_against_definition_bounds(obligation,
1403 candidates.vec.push(ProjectionCandidate);
1407 fn match_projection_obligation_against_definition_bounds(
1409 obligation: &TraitObligation<'tcx>,
1410 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
1413 let poly_trait_predicate =
1414 self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1415 let (skol_trait_predicate, skol_map) =
1416 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot);
1417 debug!("match_projection_obligation_against_definition_bounds: \
1418 skol_trait_predicate={:?} skol_map={:?}",
1419 skol_trait_predicate,
1422 let (def_id, substs) = match skol_trait_predicate.trait_ref.self_ty().sty {
1423 ty::TyProjection(ref data) =>
1424 (data.trait_ref(self.tcx()).def_id, data.substs),
1425 ty::TyAnon(def_id, substs) => (def_id, substs),
1428 obligation.cause.span,
1429 "match_projection_obligation_against_definition_bounds() called \
1430 but self-ty not a projection: {:?}",
1431 skol_trait_predicate.trait_ref.self_ty());
1434 debug!("match_projection_obligation_against_definition_bounds: \
1435 def_id={:?}, substs={:?}",
1438 let predicates_of = self.tcx().predicates_of(def_id);
1439 let bounds = predicates_of.instantiate(self.tcx(), substs);
1440 debug!("match_projection_obligation_against_definition_bounds: \
1444 let matching_bound =
1445 util::elaborate_predicates(self.tcx(), bounds.predicates)
1449 |this, _| this.match_projection(obligation,
1451 skol_trait_predicate.trait_ref.clone(),
1455 debug!("match_projection_obligation_against_definition_bounds: \
1456 matching_bound={:?}",
1458 match matching_bound {
1461 // Repeat the successful match, if any, this time outside of a probe.
1462 let result = self.match_projection(obligation,
1464 skol_trait_predicate.trait_ref.clone(),
1468 self.infcx.pop_skolemized(skol_map, snapshot);
1476 fn match_projection(&mut self,
1477 obligation: &TraitObligation<'tcx>,
1478 trait_bound: ty::PolyTraitRef<'tcx>,
1479 skol_trait_ref: ty::TraitRef<'tcx>,
1480 skol_map: &infer::SkolemizationMap<'tcx>,
1481 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
1484 assert!(!skol_trait_ref.has_escaping_regions());
1485 if let Err(_) = self.infcx.at(&obligation.cause, obligation.param_env)
1486 .sup(ty::Binder(skol_trait_ref), trait_bound) {
1490 self.infcx.leak_check(false, obligation.cause.span, skol_map, snapshot).is_ok()
1493 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1494 /// supplied to find out whether it is listed among them.
1496 /// Never affects inference environment.
1497 fn assemble_candidates_from_caller_bounds<'o>(&mut self,
1498 stack: &TraitObligationStack<'o, 'tcx>,
1499 candidates: &mut SelectionCandidateSet<'tcx>)
1500 -> Result<(),SelectionError<'tcx>>
1502 debug!("assemble_candidates_from_caller_bounds({:?})",
1506 stack.obligation.param_env.caller_bounds
1508 .filter_map(|o| o.to_opt_poly_trait_ref());
1510 // micro-optimization: filter out predicates relating to different
1512 let matching_bounds =
1513 all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
1515 let matching_bounds =
1516 matching_bounds.filter(
1517 |bound| self.evaluate_where_clause(stack, bound.clone()).may_apply());
1519 let param_candidates =
1520 matching_bounds.map(|bound| ParamCandidate(bound));
1522 candidates.vec.extend(param_candidates);
1527 fn evaluate_where_clause<'o>(&mut self,
1528 stack: &TraitObligationStack<'o, 'tcx>,
1529 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
1532 self.probe(move |this, _| {
1533 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1534 Ok(obligations) => {
1535 this.evaluate_predicates_recursively(stack.list(), obligations.iter())
1537 Err(()) => EvaluatedToErr
1542 fn assemble_generator_candidates(&mut self,
1543 obligation: &TraitObligation<'tcx>,
1544 candidates: &mut SelectionCandidateSet<'tcx>)
1545 -> Result<(),SelectionError<'tcx>>
1547 if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
1551 // ok to skip binder because the substs on generator types never
1552 // touch bound regions, they just capture the in-scope
1553 // type/region parameters
1554 let self_ty = *obligation.self_ty().skip_binder();
1556 ty::TyGenerator(..) => {
1557 debug!("assemble_generator_candidates: self_ty={:?} obligation={:?}",
1561 candidates.vec.push(GeneratorCandidate);
1564 ty::TyInfer(ty::TyVar(_)) => {
1565 debug!("assemble_generator_candidates: ambiguous self-type");
1566 candidates.ambiguous = true;
1569 _ => { return Ok(()); }
1573 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1574 /// FnMut<..>` where `X` is a closure type.
1576 /// Note: the type parameters on a closure candidate are modeled as *output* type
1577 /// parameters and hence do not affect whether this trait is a match or not. They will be
1578 /// unified during the confirmation step.
1579 fn assemble_closure_candidates(&mut self,
1580 obligation: &TraitObligation<'tcx>,
1581 candidates: &mut SelectionCandidateSet<'tcx>)
1582 -> Result<(),SelectionError<'tcx>>
1584 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.0.def_id()) {
1586 None => { return Ok(()); }
1589 // ok to skip binder because the substs on closure types never
1590 // touch bound regions, they just capture the in-scope
1591 // type/region parameters
1592 match obligation.self_ty().skip_binder().sty {
1593 ty::TyClosure(closure_def_id, closure_substs) => {
1594 debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}",
1596 match self.infcx.closure_kind(closure_def_id, closure_substs) {
1597 Some(closure_kind) => {
1598 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
1599 if closure_kind.extends(kind) {
1600 candidates.vec.push(ClosureCandidate);
1604 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1605 candidates.vec.push(ClosureCandidate);
1610 ty::TyInfer(ty::TyVar(_)) => {
1611 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1612 candidates.ambiguous = true;
1615 _ => { return Ok(()); }
1619 /// Implement one of the `Fn()` family for a fn pointer.
1620 fn assemble_fn_pointer_candidates(&mut self,
1621 obligation: &TraitObligation<'tcx>,
1622 candidates: &mut SelectionCandidateSet<'tcx>)
1623 -> Result<(),SelectionError<'tcx>>
1625 // We provide impl of all fn traits for fn pointers.
1626 if self.tcx().lang_items().fn_trait_kind(obligation.predicate.def_id()).is_none() {
1630 // ok to skip binder because what we are inspecting doesn't involve bound regions
1631 let self_ty = *obligation.self_ty().skip_binder();
1633 ty::TyInfer(ty::TyVar(_)) => {
1634 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1635 candidates.ambiguous = true; // could wind up being a fn() type
1638 // provide an impl, but only for suitable `fn` pointers
1639 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
1640 if let ty::Binder(ty::FnSig {
1641 unsafety: hir::Unsafety::Normal,
1645 }) = self_ty.fn_sig(self.tcx()) {
1646 candidates.vec.push(FnPointerCandidate);
1656 /// Search for impls that might apply to `obligation`.
1657 fn assemble_candidates_from_impls(&mut self,
1658 obligation: &TraitObligation<'tcx>,
1659 candidates: &mut SelectionCandidateSet<'tcx>)
1660 -> Result<(), SelectionError<'tcx>>
1662 debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
1664 self.tcx().for_each_relevant_impl(
1665 obligation.predicate.def_id(),
1666 obligation.predicate.0.trait_ref.self_ty(),
1668 self.probe(|this, snapshot| { /* [1] */
1669 match this.match_impl(impl_def_id, obligation, snapshot) {
1671 candidates.vec.push(ImplCandidate(impl_def_id));
1673 // NB: we can safely drop the skol map
1674 // since we are in a probe [1]
1675 mem::drop(skol_map);
1686 fn assemble_candidates_from_auto_impls(&mut self,
1687 obligation: &TraitObligation<'tcx>,
1688 candidates: &mut SelectionCandidateSet<'tcx>)
1689 -> Result<(), SelectionError<'tcx>>
1691 // OK to skip binder here because the tests we do below do not involve bound regions
1692 let self_ty = *obligation.self_ty().skip_binder();
1693 debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
1695 let def_id = obligation.predicate.def_id();
1697 if self.tcx().trait_is_auto(def_id) {
1699 ty::TyDynamic(..) => {
1700 // For object types, we don't know what the closed
1701 // over types are. This means we conservatively
1702 // say nothing; a candidate may be added by
1703 // `assemble_candidates_from_object_ty`.
1705 ty::TyForeign(..) => {
1706 // Since the contents of foreign types is unknown,
1707 // we don't add any `..` impl. Default traits could
1708 // still be provided by a manual implementation for
1709 // this trait and type.
1712 ty::TyProjection(..) => {
1713 // In these cases, we don't know what the actual
1714 // type is. Therefore, we cannot break it down
1715 // into its constituent types. So we don't
1716 // consider the `..` impl but instead just add no
1717 // candidates: this means that typeck will only
1718 // succeed if there is another reason to believe
1719 // that this obligation holds. That could be a
1720 // where-clause or, in the case of an object type,
1721 // it could be that the object type lists the
1722 // trait (e.g. `Foo+Send : Send`). See
1723 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1724 // for an example of a test case that exercises
1727 ty::TyInfer(ty::TyVar(_)) => {
1728 // the auto impl might apply, we don't know
1729 candidates.ambiguous = true;
1732 candidates.vec.push(AutoImplCandidate(def_id.clone()))
1740 /// Search for impls that might apply to `obligation`.
1741 fn assemble_candidates_from_object_ty(&mut self,
1742 obligation: &TraitObligation<'tcx>,
1743 candidates: &mut SelectionCandidateSet<'tcx>)
1745 debug!("assemble_candidates_from_object_ty(self_ty={:?})",
1746 obligation.self_ty().skip_binder());
1748 // Object-safety candidates are only applicable to object-safe
1749 // traits. Including this check is useful because it helps
1750 // inference in cases of traits like `BorrowFrom`, which are
1751 // not object-safe, and which rely on being able to infer the
1752 // self-type from one of the other inputs. Without this check,
1753 // these cases wind up being considered ambiguous due to a
1754 // (spurious) ambiguity introduced here.
1755 let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
1756 if !self.tcx().is_object_safe(predicate_trait_ref.def_id()) {
1760 self.probe(|this, _snapshot| {
1761 // the code below doesn't care about regions, and the
1762 // self-ty here doesn't escape this probe, so just erase
1764 let self_ty = this.tcx().erase_late_bound_regions(&obligation.self_ty());
1765 let poly_trait_ref = match self_ty.sty {
1766 ty::TyDynamic(ref data, ..) => {
1767 if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
1768 debug!("assemble_candidates_from_object_ty: matched builtin bound, \
1769 pushing candidate");
1770 candidates.vec.push(BuiltinObjectCandidate);
1774 match data.principal() {
1775 Some(p) => p.with_self_ty(this.tcx(), self_ty),
1779 ty::TyInfer(ty::TyVar(_)) => {
1780 debug!("assemble_candidates_from_object_ty: ambiguous");
1781 candidates.ambiguous = true; // could wind up being an object type
1789 debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1792 // Count only those upcast versions that match the trait-ref
1793 // we are looking for. Specifically, do not only check for the
1794 // correct trait, but also the correct type parameters.
1795 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1796 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1797 let upcast_trait_refs =
1798 util::supertraits(this.tcx(), poly_trait_ref)
1799 .filter(|upcast_trait_ref| {
1800 this.probe(|this, _| {
1801 let upcast_trait_ref = upcast_trait_ref.clone();
1802 this.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok()
1807 if upcast_trait_refs > 1 {
1808 // can be upcast in many ways; need more type information
1809 candidates.ambiguous = true;
1810 } else if upcast_trait_refs == 1 {
1811 candidates.vec.push(ObjectCandidate);
1816 /// Search for unsizing that might apply to `obligation`.
1817 fn assemble_candidates_for_unsizing(&mut self,
1818 obligation: &TraitObligation<'tcx>,
1819 candidates: &mut SelectionCandidateSet<'tcx>) {
1820 // We currently never consider higher-ranked obligations e.g.
1821 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1822 // because they are a priori invalid, and we could potentially add support
1823 // for them later, it's just that there isn't really a strong need for it.
1824 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1825 // impl, and those are generally applied to concrete types.
1827 // That said, one might try to write a fn with a where clause like
1828 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1829 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1830 // Still, you'd be more likely to write that where clause as
1832 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1833 // obligation above. Should be possible to extend this in the future.
1834 let source = match obligation.self_ty().no_late_bound_regions() {
1837 // Don't add any candidates if there are bound regions.
1841 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
1843 debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1846 let may_apply = match (&source.sty, &target.sty) {
1847 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1848 (&ty::TyDynamic(ref data_a, ..), &ty::TyDynamic(ref data_b, ..)) => {
1849 // Upcasts permit two things:
1851 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1852 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1854 // Note that neither of these changes requires any
1855 // change at runtime. Eventually this will be
1858 // We always upcast when we can because of reason
1859 // #2 (region bounds).
1860 match (data_a.principal(), data_b.principal()) {
1861 (Some(a), Some(b)) => a.def_id() == b.def_id() &&
1862 data_b.auto_traits()
1863 // All of a's auto traits need to be in b's auto traits.
1864 .all(|b| data_a.auto_traits().any(|a| a == b)),
1870 (_, &ty::TyDynamic(..)) => true,
1872 // Ambiguous handling is below T -> Trait, because inference
1873 // variables can still implement Unsize<Trait> and nested
1874 // obligations will have the final say (likely deferred).
1875 (&ty::TyInfer(ty::TyVar(_)), _) |
1876 (_, &ty::TyInfer(ty::TyVar(_))) => {
1877 debug!("assemble_candidates_for_unsizing: ambiguous");
1878 candidates.ambiguous = true;
1883 (&ty::TyArray(..), &ty::TySlice(_)) => true,
1885 // Struct<T> -> Struct<U>.
1886 (&ty::TyAdt(def_id_a, _), &ty::TyAdt(def_id_b, _)) if def_id_a.is_struct() => {
1887 def_id_a == def_id_b
1890 // (.., T) -> (.., U).
1891 (&ty::TyTuple(tys_a), &ty::TyTuple(tys_b)) => {
1892 tys_a.len() == tys_b.len()
1899 candidates.vec.push(BuiltinUnsizeCandidate);
1903 ///////////////////////////////////////////////////////////////////////////
1906 // Winnowing is the process of attempting to resolve ambiguity by
1907 // probing further. During the winnowing process, we unify all
1908 // type variables (ignoring skolemization) and then we also
1909 // attempt to evaluate recursive bounds to see if they are
1912 /// Returns true if `candidate_i` should be dropped in favor of
1913 /// `candidate_j`. Generally speaking we will drop duplicate
1914 /// candidates and prefer where-clause candidates.
1915 /// Returns true if `victim` should be dropped in favor of
1916 /// `other`. Generally speaking we will drop duplicate
1917 /// candidates and prefer where-clause candidates.
1919 /// See the comment for "SelectionCandidate" for more details.
1920 fn candidate_should_be_dropped_in_favor_of<'o>(
1922 victim: &EvaluatedCandidate<'tcx>,
1923 other: &EvaluatedCandidate<'tcx>)
1926 if victim.candidate == other.candidate {
1930 match other.candidate {
1932 ParamCandidate(_) | ProjectionCandidate => match victim.candidate {
1933 AutoImplCandidate(..) => {
1935 "default implementations shouldn't be recorded \
1936 when there are other valid candidates");
1940 GeneratorCandidate |
1941 FnPointerCandidate |
1942 BuiltinObjectCandidate |
1943 BuiltinUnsizeCandidate |
1944 BuiltinCandidate { .. } => {
1945 // We have a where-clause so don't go around looking
1950 ProjectionCandidate => {
1951 // Arbitrarily give param candidates priority
1952 // over projection and object candidates.
1955 ParamCandidate(..) => false,
1957 ImplCandidate(other_def) => {
1958 // See if we can toss out `victim` based on specialization.
1959 // This requires us to know *for sure* that the `other` impl applies
1960 // i.e. EvaluatedToOk:
1961 if other.evaluation == EvaluatedToOk {
1962 if let ImplCandidate(victim_def) = victim.candidate {
1963 let tcx = self.tcx().global_tcx();
1964 return tcx.specializes((other_def, victim_def)) ||
1965 tcx.impls_are_allowed_to_overlap(other_def, victim_def);
1975 ///////////////////////////////////////////////////////////////////////////
1978 // These cover the traits that are built-in to the language
1979 // itself. This includes `Copy` and `Sized` for sure. For the
1980 // moment, it also includes `Send` / `Sync` and a few others, but
1981 // those will hopefully change to library-defined traits in the
1984 // HACK: if this returns an error, selection exits without considering
1986 fn assemble_builtin_bound_candidates<'o>(&mut self,
1987 conditions: BuiltinImplConditions<'tcx>,
1988 candidates: &mut SelectionCandidateSet<'tcx>)
1989 -> Result<(),SelectionError<'tcx>>
1992 BuiltinImplConditions::Where(nested) => {
1993 debug!("builtin_bound: nested={:?}", nested);
1994 candidates.vec.push(BuiltinCandidate {
1995 has_nested: nested.skip_binder().len() > 0
1999 BuiltinImplConditions::None => { Ok(()) }
2000 BuiltinImplConditions::Ambiguous => {
2001 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
2002 Ok(candidates.ambiguous = true)
2004 BuiltinImplConditions::Never => { Err(Unimplemented) }
2008 fn sized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2009 -> BuiltinImplConditions<'tcx>
2011 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
2013 // NOTE: binder moved to (*)
2014 let self_ty = self.infcx.shallow_resolve(
2015 obligation.predicate.skip_binder().self_ty());
2018 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2019 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2020 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyRawPtr(..) |
2021 ty::TyChar | ty::TyRef(..) | ty::TyGenerator(..) |
2022 ty::TyGeneratorWitness(..) | ty::TyArray(..) | ty::TyClosure(..) |
2023 ty::TyNever | ty::TyError => {
2024 // safe for everything
2025 Where(ty::Binder(Vec::new()))
2028 ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) | ty::TyForeign(..) => Never,
2030 ty::TyTuple(tys) => {
2031 Where(ty::Binder(tys.last().into_iter().cloned().collect()))
2034 ty::TyAdt(def, substs) => {
2035 let sized_crit = def.sized_constraint(self.tcx());
2036 // (*) binder moved here
2038 sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect()
2042 ty::TyProjection(_) | ty::TyParam(_) | ty::TyAnon(..) => None,
2043 ty::TyInfer(ty::TyVar(_)) => Ambiguous,
2045 ty::TyInfer(ty::CanonicalTy(_)) |
2046 ty::TyInfer(ty::FreshTy(_)) |
2047 ty::TyInfer(ty::FreshIntTy(_)) |
2048 ty::TyInfer(ty::FreshFloatTy(_)) => {
2049 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2055 fn copy_clone_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2056 -> BuiltinImplConditions<'tcx>
2058 // NOTE: binder moved to (*)
2059 let self_ty = self.infcx.shallow_resolve(
2060 obligation.predicate.skip_binder().self_ty());
2062 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
2065 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2066 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyError => {
2067 Where(ty::Binder(Vec::new()))
2070 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2071 ty::TyChar | ty::TyRawPtr(..) | ty::TyNever |
2072 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
2073 // Implementations provided in libcore
2077 ty::TyDynamic(..) | ty::TyStr | ty::TySlice(..) |
2078 ty::TyGenerator(..) | ty::TyGeneratorWitness(..) | ty::TyForeign(..) |
2079 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
2083 ty::TyArray(element_ty, _) => {
2084 // (*) binder moved here
2085 Where(ty::Binder(vec![element_ty]))
2088 ty::TyTuple(tys) => {
2089 // (*) binder moved here
2090 Where(ty::Binder(tys.to_vec()))
2093 ty::TyClosure(def_id, substs) => {
2094 let trait_id = obligation.predicate.def_id();
2095 let is_copy_trait = Some(trait_id) == self.tcx().lang_items().copy_trait();
2096 let is_clone_trait = Some(trait_id) == self.tcx().lang_items().clone_trait();
2097 if is_copy_trait || is_clone_trait {
2098 Where(ty::Binder(substs.upvar_tys(def_id, self.tcx()).collect()))
2104 ty::TyAdt(..) | ty::TyProjection(..) | ty::TyParam(..) | ty::TyAnon(..) => {
2105 // Fallback to whatever user-defined impls exist in this case.
2109 ty::TyInfer(ty::TyVar(_)) => {
2110 // Unbound type variable. Might or might not have
2111 // applicable impls and so forth, depending on what
2112 // those type variables wind up being bound to.
2116 ty::TyInfer(ty::CanonicalTy(_)) |
2117 ty::TyInfer(ty::FreshTy(_)) |
2118 ty::TyInfer(ty::FreshIntTy(_)) |
2119 ty::TyInfer(ty::FreshFloatTy(_)) => {
2120 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2126 /// For default impls, we need to break apart a type into its
2127 /// "constituent types" -- meaning, the types that it contains.
2129 /// Here are some (simple) examples:
2132 /// (i32, u32) -> [i32, u32]
2133 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2134 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2135 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2137 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
2147 ty::TyInfer(ty::IntVar(_)) |
2148 ty::TyInfer(ty::FloatVar(_)) |
2157 ty::TyProjection(..) |
2158 ty::TyInfer(ty::CanonicalTy(_)) |
2159 ty::TyInfer(ty::TyVar(_)) |
2160 ty::TyInfer(ty::FreshTy(_)) |
2161 ty::TyInfer(ty::FreshIntTy(_)) |
2162 ty::TyInfer(ty::FreshFloatTy(_)) => {
2163 bug!("asked to assemble constituent types of unexpected type: {:?}",
2167 ty::TyRawPtr(ty::TypeAndMut { ty: element_ty, ..}) |
2168 ty::TyRef(_, ty::TypeAndMut { ty: element_ty, ..}) => {
2172 ty::TyArray(element_ty, _) | ty::TySlice(element_ty) => {
2176 ty::TyTuple(ref tys) => {
2177 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2181 ty::TyClosure(def_id, ref substs) => {
2182 substs.upvar_tys(def_id, self.tcx()).collect()
2185 ty::TyGenerator(def_id, ref substs, interior) => {
2186 substs.upvar_tys(def_id, self.tcx()).chain(iter::once(interior.witness)).collect()
2189 ty::TyGeneratorWitness(types) => {
2190 // This is sound because no regions in the witness can refer to
2191 // the binder outside the witness. So we'll effectivly reuse
2192 // the implicit binder around the witness.
2193 types.skip_binder().to_vec()
2196 // for `PhantomData<T>`, we pass `T`
2197 ty::TyAdt(def, substs) if def.is_phantom_data() => {
2198 substs.types().collect()
2201 ty::TyAdt(def, substs) => {
2203 .map(|f| f.ty(self.tcx(), substs))
2207 ty::TyAnon(def_id, substs) => {
2208 // We can resolve the `impl Trait` to its concrete type,
2209 // which enforces a DAG between the functions requiring
2210 // the auto trait bounds in question.
2211 vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
2216 fn collect_predicates_for_types(&mut self,
2217 param_env: ty::ParamEnv<'tcx>,
2218 cause: ObligationCause<'tcx>,
2219 recursion_depth: usize,
2220 trait_def_id: DefId,
2221 types: ty::Binder<Vec<Ty<'tcx>>>)
2222 -> Vec<PredicateObligation<'tcx>>
2224 // Because the types were potentially derived from
2225 // higher-ranked obligations they may reference late-bound
2226 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
2227 // yield a type like `for<'a> &'a int`. In general, we
2228 // maintain the invariant that we never manipulate bound
2229 // regions, so we have to process these bound regions somehow.
2231 // The strategy is to:
2233 // 1. Instantiate those regions to skolemized regions (e.g.,
2234 // `for<'a> &'a int` becomes `&0 int`.
2235 // 2. Produce something like `&'0 int : Copy`
2236 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
2238 types.skip_binder().into_iter().flat_map(|ty| { // binder moved -\
2239 let ty: ty::Binder<Ty<'tcx>> = ty::Binder(ty); // <----------/
2241 self.in_snapshot(|this, snapshot| {
2242 let (skol_ty, skol_map) =
2243 this.infcx().skolemize_late_bound_regions(&ty, snapshot);
2244 let Normalized { value: normalized_ty, mut obligations } =
2245 project::normalize_with_depth(this,
2250 let skol_obligation =
2251 this.tcx().predicate_for_trait_def(param_env,
2257 obligations.push(skol_obligation);
2258 this.infcx().plug_leaks(skol_map, snapshot, obligations)
2263 ///////////////////////////////////////////////////////////////////////////
2266 // Confirmation unifies the output type parameters of the trait
2267 // with the values found in the obligation, possibly yielding a
2268 // type error. See [rustc guide] for more details.
2271 // https://rust-lang-nursery.github.io/rustc-guide/trait-resolution.html#confirmation
2273 fn confirm_candidate(&mut self,
2274 obligation: &TraitObligation<'tcx>,
2275 candidate: SelectionCandidate<'tcx>)
2276 -> Result<Selection<'tcx>,SelectionError<'tcx>>
2278 debug!("confirm_candidate({:?}, {:?})",
2283 BuiltinCandidate { has_nested } => {
2284 let data = self.confirm_builtin_candidate(obligation, has_nested);
2285 Ok(VtableBuiltin(data))
2288 ParamCandidate(param) => {
2289 let obligations = self.confirm_param_candidate(obligation, param);
2290 Ok(VtableParam(obligations))
2293 AutoImplCandidate(trait_def_id) => {
2294 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
2295 Ok(VtableAutoImpl(data))
2298 ImplCandidate(impl_def_id) => {
2299 Ok(VtableImpl(self.confirm_impl_candidate(obligation, impl_def_id)))
2302 ClosureCandidate => {
2303 let vtable_closure = self.confirm_closure_candidate(obligation)?;
2304 Ok(VtableClosure(vtable_closure))
2307 GeneratorCandidate => {
2308 let vtable_generator = self.confirm_generator_candidate(obligation)?;
2309 Ok(VtableGenerator(vtable_generator))
2312 BuiltinObjectCandidate => {
2313 // This indicates something like `(Trait+Send) :
2314 // Send`. In this case, we know that this holds
2315 // because that's what the object type is telling us,
2316 // and there's really no additional obligations to
2317 // prove and no types in particular to unify etc.
2318 Ok(VtableParam(Vec::new()))
2321 ObjectCandidate => {
2322 let data = self.confirm_object_candidate(obligation);
2323 Ok(VtableObject(data))
2326 FnPointerCandidate => {
2328 self.confirm_fn_pointer_candidate(obligation)?;
2329 Ok(VtableFnPointer(data))
2332 ProjectionCandidate => {
2333 self.confirm_projection_candidate(obligation);
2334 Ok(VtableParam(Vec::new()))
2337 BuiltinUnsizeCandidate => {
2338 let data = self.confirm_builtin_unsize_candidate(obligation)?;
2339 Ok(VtableBuiltin(data))
2344 fn confirm_projection_candidate(&mut self,
2345 obligation: &TraitObligation<'tcx>)
2347 self.in_snapshot(|this, snapshot| {
2349 this.match_projection_obligation_against_definition_bounds(obligation,
2355 fn confirm_param_candidate(&mut self,
2356 obligation: &TraitObligation<'tcx>,
2357 param: ty::PolyTraitRef<'tcx>)
2358 -> Vec<PredicateObligation<'tcx>>
2360 debug!("confirm_param_candidate({:?},{:?})",
2364 // During evaluation, we already checked that this
2365 // where-clause trait-ref could be unified with the obligation
2366 // trait-ref. Repeat that unification now without any
2367 // transactional boundary; it should not fail.
2368 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2369 Ok(obligations) => obligations,
2371 bug!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2378 fn confirm_builtin_candidate(&mut self,
2379 obligation: &TraitObligation<'tcx>,
2381 -> VtableBuiltinData<PredicateObligation<'tcx>>
2383 debug!("confirm_builtin_candidate({:?}, {:?})",
2384 obligation, has_nested);
2386 let lang_items = self.tcx().lang_items();
2387 let obligations = if has_nested {
2388 let trait_def = obligation.predicate.def_id();
2389 let conditions = match trait_def {
2390 _ if Some(trait_def) == lang_items.sized_trait() => {
2391 self.sized_conditions(obligation)
2393 _ if Some(trait_def) == lang_items.copy_trait() => {
2394 self.copy_clone_conditions(obligation)
2396 _ if Some(trait_def) == lang_items.clone_trait() => {
2397 self.copy_clone_conditions(obligation)
2399 _ => bug!("unexpected builtin trait {:?}", trait_def)
2401 let nested = match conditions {
2402 BuiltinImplConditions::Where(nested) => nested,
2403 _ => bug!("obligation {:?} had matched a builtin impl but now doesn't",
2407 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2408 self.collect_predicates_for_types(obligation.param_env,
2410 obligation.recursion_depth+1,
2417 debug!("confirm_builtin_candidate: obligations={:?}",
2420 VtableBuiltinData { nested: obligations }
2423 /// This handles the case where a `auto trait Foo` impl is being used.
2424 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2426 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2427 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2428 fn confirm_auto_impl_candidate(&mut self,
2429 obligation: &TraitObligation<'tcx>,
2430 trait_def_id: DefId)
2431 -> VtableAutoImplData<PredicateObligation<'tcx>>
2433 debug!("confirm_auto_impl_candidate({:?}, {:?})",
2437 // binder is moved below
2438 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2439 let types = self.constituent_types_for_ty(self_ty);
2440 self.vtable_auto_impl(obligation, trait_def_id, ty::Binder(types))
2443 /// See `confirm_auto_impl_candidate`
2444 fn vtable_auto_impl(&mut self,
2445 obligation: &TraitObligation<'tcx>,
2446 trait_def_id: DefId,
2447 nested: ty::Binder<Vec<Ty<'tcx>>>)
2448 -> VtableAutoImplData<PredicateObligation<'tcx>>
2450 debug!("vtable_auto_impl: nested={:?}", nested);
2452 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2453 let mut obligations = self.collect_predicates_for_types(
2454 obligation.param_env,
2456 obligation.recursion_depth+1,
2460 let trait_obligations = self.in_snapshot(|this, snapshot| {
2461 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2462 let (trait_ref, skol_map) =
2463 this.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot);
2464 let cause = obligation.derived_cause(ImplDerivedObligation);
2465 this.impl_or_trait_obligations(cause,
2466 obligation.recursion_depth + 1,
2467 obligation.param_env,
2474 obligations.extend(trait_obligations);
2476 debug!("vtable_auto_impl: obligations={:?}", obligations);
2478 VtableAutoImplData {
2484 fn confirm_impl_candidate(&mut self,
2485 obligation: &TraitObligation<'tcx>,
2487 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2489 debug!("confirm_impl_candidate({:?},{:?})",
2493 // First, create the substitutions by matching the impl again,
2494 // this time not in a probe.
2495 self.in_snapshot(|this, snapshot| {
2496 let (substs, skol_map) =
2497 this.rematch_impl(impl_def_id, obligation,
2499 debug!("confirm_impl_candidate substs={:?}", substs);
2500 let cause = obligation.derived_cause(ImplDerivedObligation);
2501 this.vtable_impl(impl_def_id,
2504 obligation.recursion_depth + 1,
2505 obligation.param_env,
2511 fn vtable_impl(&mut self,
2513 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2514 cause: ObligationCause<'tcx>,
2515 recursion_depth: usize,
2516 param_env: ty::ParamEnv<'tcx>,
2517 skol_map: infer::SkolemizationMap<'tcx>,
2518 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
2519 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2521 debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2527 let mut impl_obligations =
2528 self.impl_or_trait_obligations(cause,
2536 debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2540 // Because of RFC447, the impl-trait-ref and obligations
2541 // are sufficient to determine the impl substs, without
2542 // relying on projections in the impl-trait-ref.
2544 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2545 impl_obligations.append(&mut substs.obligations);
2547 VtableImplData { impl_def_id,
2548 substs: substs.value,
2549 nested: impl_obligations }
2552 fn confirm_object_candidate(&mut self,
2553 obligation: &TraitObligation<'tcx>)
2554 -> VtableObjectData<'tcx, PredicateObligation<'tcx>>
2556 debug!("confirm_object_candidate({:?})",
2559 // FIXME skipping binder here seems wrong -- we should
2560 // probably flatten the binder from the obligation and the
2561 // binder from the object. Have to try to make a broken test
2562 // case that results. -nmatsakis
2563 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2564 let poly_trait_ref = match self_ty.sty {
2565 ty::TyDynamic(ref data, ..) => {
2566 data.principal().unwrap().with_self_ty(self.tcx(), self_ty)
2569 span_bug!(obligation.cause.span,
2570 "object candidate with non-object");
2574 let mut upcast_trait_ref = None;
2575 let mut nested = vec![];
2579 let tcx = self.tcx();
2581 // We want to find the first supertrait in the list of
2582 // supertraits that we can unify with, and do that
2583 // unification. We know that there is exactly one in the list
2584 // where we can unify because otherwise select would have
2585 // reported an ambiguity. (When we do find a match, also
2586 // record it for later.)
2588 util::supertraits(tcx, poly_trait_ref)
2592 |this, _| this.match_poly_trait_ref(obligation, t))
2594 Ok(obligations) => {
2595 upcast_trait_ref = Some(t);
2596 nested.extend(obligations);
2603 // Additionally, for each of the nonmatching predicates that
2604 // we pass over, we sum up the set of number of vtable
2605 // entries, so that we can compute the offset for the selected
2608 nonmatching.map(|t| tcx.count_own_vtable_entries(t))
2614 upcast_trait_ref: upcast_trait_ref.unwrap(),
2620 fn confirm_fn_pointer_candidate(&mut self, obligation: &TraitObligation<'tcx>)
2621 -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
2623 debug!("confirm_fn_pointer_candidate({:?})",
2626 // ok to skip binder; it is reintroduced below
2627 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2628 let sig = self_ty.fn_sig(self.tcx());
2630 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2633 util::TupleArgumentsFlag::Yes)
2634 .map_bound(|(trait_ref, _)| trait_ref);
2636 let Normalized { value: trait_ref, obligations } =
2637 project::normalize_with_depth(self,
2638 obligation.param_env,
2639 obligation.cause.clone(),
2640 obligation.recursion_depth + 1,
2643 self.confirm_poly_trait_refs(obligation.cause.clone(),
2644 obligation.param_env,
2645 obligation.predicate.to_poly_trait_ref(),
2647 Ok(VtableFnPointerData { fn_ty: self_ty, nested: obligations })
2650 fn confirm_generator_candidate(&mut self,
2651 obligation: &TraitObligation<'tcx>)
2652 -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>,
2653 SelectionError<'tcx>>
2655 // ok to skip binder because the substs on generator types never
2656 // touch bound regions, they just capture the in-scope
2657 // type/region parameters
2658 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2659 let (closure_def_id, substs) = match self_ty.sty {
2660 ty::TyGenerator(id, substs, _) => (id, substs),
2661 _ => bug!("closure candidate for non-closure {:?}", obligation)
2664 debug!("confirm_generator_candidate({:?},{:?},{:?})",
2670 self.generator_trait_ref_unnormalized(obligation, closure_def_id, substs);
2674 } = normalize_with_depth(self,
2675 obligation.param_env,
2676 obligation.cause.clone(),
2677 obligation.recursion_depth+1,
2680 debug!("confirm_generator_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2686 self.confirm_poly_trait_refs(obligation.cause.clone(),
2687 obligation.param_env,
2688 obligation.predicate.to_poly_trait_ref(),
2691 Ok(VtableGeneratorData {
2692 closure_def_id: closure_def_id,
2693 substs: substs.clone(),
2698 fn confirm_closure_candidate(&mut self,
2699 obligation: &TraitObligation<'tcx>)
2700 -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>,
2701 SelectionError<'tcx>>
2703 debug!("confirm_closure_candidate({:?})", obligation);
2705 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.0.def_id()) {
2707 None => bug!("closure candidate for non-fn trait {:?}", obligation)
2710 // ok to skip binder because the substs on closure types never
2711 // touch bound regions, they just capture the in-scope
2712 // type/region parameters
2713 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2714 let (closure_def_id, substs) = match self_ty.sty {
2715 ty::TyClosure(id, substs) => (id, substs),
2716 _ => bug!("closure candidate for non-closure {:?}", obligation)
2720 self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs);
2724 } = normalize_with_depth(self,
2725 obligation.param_env,
2726 obligation.cause.clone(),
2727 obligation.recursion_depth+1,
2730 debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2736 self.confirm_poly_trait_refs(obligation.cause.clone(),
2737 obligation.param_env,
2738 obligation.predicate.to_poly_trait_ref(),
2741 obligations.push(Obligation::new(
2742 obligation.cause.clone(),
2743 obligation.param_env,
2744 ty::Predicate::ClosureKind(closure_def_id, substs, kind)));
2746 Ok(VtableClosureData {
2748 substs: substs.clone(),
2753 /// In the case of closure types and fn pointers,
2754 /// we currently treat the input type parameters on the trait as
2755 /// outputs. This means that when we have a match we have only
2756 /// considered the self type, so we have to go back and make sure
2757 /// to relate the argument types too. This is kind of wrong, but
2758 /// since we control the full set of impls, also not that wrong,
2759 /// and it DOES yield better error messages (since we don't report
2760 /// errors as if there is no applicable impl, but rather report
2761 /// errors are about mismatched argument types.
2763 /// Here is an example. Imagine we have a closure expression
2764 /// and we desugared it so that the type of the expression is
2765 /// `Closure`, and `Closure` expects an int as argument. Then it
2766 /// is "as if" the compiler generated this impl:
2768 /// impl Fn(int) for Closure { ... }
2770 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2771 /// we have matched the self-type `Closure`. At this point we'll
2772 /// compare the `int` to `usize` and generate an error.
2774 /// Note that this checking occurs *after* the impl has selected,
2775 /// because these output type parameters should not affect the
2776 /// selection of the impl. Therefore, if there is a mismatch, we
2777 /// report an error to the user.
2778 fn confirm_poly_trait_refs(&mut self,
2779 obligation_cause: ObligationCause<'tcx>,
2780 obligation_param_env: ty::ParamEnv<'tcx>,
2781 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
2782 expected_trait_ref: ty::PolyTraitRef<'tcx>)
2783 -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>>
2785 let obligation_trait_ref = obligation_trait_ref.clone();
2787 .at(&obligation_cause, obligation_param_env)
2788 .sup(obligation_trait_ref, expected_trait_ref)
2789 .map(|InferOk { obligations, .. }| obligations)
2790 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
2793 fn confirm_builtin_unsize_candidate(&mut self,
2794 obligation: &TraitObligation<'tcx>,)
2795 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>>
2797 let tcx = self.tcx();
2799 // assemble_candidates_for_unsizing should ensure there are no late bound
2800 // regions here. See the comment there for more details.
2801 let source = self.infcx.shallow_resolve(
2802 obligation.self_ty().no_late_bound_regions().unwrap());
2803 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
2804 let target = self.infcx.shallow_resolve(target);
2806 debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2809 let mut nested = vec![];
2810 match (&source.sty, &target.sty) {
2811 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2812 (&ty::TyDynamic(ref data_a, r_a), &ty::TyDynamic(ref data_b, r_b)) => {
2813 // See assemble_candidates_for_unsizing for more info.
2814 // Binders reintroduced below in call to mk_existential_predicates.
2815 let principal = data_a.skip_binder().principal();
2816 let iter = principal.into_iter().map(ty::ExistentialPredicate::Trait)
2817 .chain(data_a.skip_binder().projection_bounds()
2818 .map(|x| ty::ExistentialPredicate::Projection(x)))
2819 .chain(data_b.auto_traits().map(ty::ExistentialPredicate::AutoTrait));
2820 let new_trait = tcx.mk_dynamic(
2821 ty::Binder(tcx.mk_existential_predicates(iter)), r_b);
2822 let InferOk { obligations, .. } =
2823 self.infcx.at(&obligation.cause, obligation.param_env)
2824 .eq(target, new_trait)
2825 .map_err(|_| Unimplemented)?;
2826 nested.extend(obligations);
2828 // Register one obligation for 'a: 'b.
2829 let cause = ObligationCause::new(obligation.cause.span,
2830 obligation.cause.body_id,
2831 ObjectCastObligation(target));
2832 let outlives = ty::OutlivesPredicate(r_a, r_b);
2833 nested.push(Obligation::with_depth(cause,
2834 obligation.recursion_depth + 1,
2835 obligation.param_env,
2836 ty::Binder(outlives).to_predicate()));
2840 (_, &ty::TyDynamic(ref data, r)) => {
2841 let mut object_dids =
2842 data.auto_traits().chain(data.principal().map(|p| p.def_id()));
2843 if let Some(did) = object_dids.find(|did| {
2844 !tcx.is_object_safe(*did)
2846 return Err(TraitNotObjectSafe(did))
2849 let cause = ObligationCause::new(obligation.cause.span,
2850 obligation.cause.body_id,
2851 ObjectCastObligation(target));
2852 let mut push = |predicate| {
2853 nested.push(Obligation::with_depth(cause.clone(),
2854 obligation.recursion_depth + 1,
2855 obligation.param_env,
2859 // Create obligations:
2860 // - Casting T to Trait
2861 // - For all the various builtin bounds attached to the object cast. (In other
2862 // words, if the object type is Foo+Send, this would create an obligation for the
2864 // - Projection predicates
2865 for predicate in data.iter() {
2866 push(predicate.with_self_ty(tcx, source));
2869 // We can only make objects from sized types.
2870 let tr = ty::TraitRef {
2871 def_id: tcx.require_lang_item(lang_items::SizedTraitLangItem),
2872 substs: tcx.mk_substs_trait(source, &[]),
2874 push(tr.to_predicate());
2876 // If the type is `Foo+'a`, ensures that the type
2877 // being cast to `Foo+'a` outlives `'a`:
2878 let outlives = ty::OutlivesPredicate(source, r);
2879 push(ty::Binder(outlives).to_predicate());
2883 (&ty::TyArray(a, _), &ty::TySlice(b)) => {
2884 let InferOk { obligations, .. } =
2885 self.infcx.at(&obligation.cause, obligation.param_env)
2887 .map_err(|_| Unimplemented)?;
2888 nested.extend(obligations);
2891 // Struct<T> -> Struct<U>.
2892 (&ty::TyAdt(def, substs_a), &ty::TyAdt(_, substs_b)) => {
2895 .map(|f| tcx.type_of(f.did))
2896 .collect::<Vec<_>>();
2898 // The last field of the structure has to exist and contain type parameters.
2899 let field = if let Some(&field) = fields.last() {
2902 return Err(Unimplemented);
2904 let mut ty_params = BitVector::new(substs_a.types().count());
2905 let mut found = false;
2906 for ty in field.walk() {
2907 if let ty::TyParam(p) = ty.sty {
2908 ty_params.insert(p.idx as usize);
2913 return Err(Unimplemented);
2916 // Replace type parameters used in unsizing with
2917 // TyError and ensure they do not affect any other fields.
2918 // This could be checked after type collection for any struct
2919 // with a potentially unsized trailing field.
2920 let params = substs_a.iter().enumerate().map(|(i, &k)| {
2921 if ty_params.contains(i) {
2922 Kind::from(tcx.types.err)
2927 let substs = tcx.mk_substs(params);
2928 for &ty in fields.split_last().unwrap().1 {
2929 if ty.subst(tcx, substs).references_error() {
2930 return Err(Unimplemented);
2934 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2935 let inner_source = field.subst(tcx, substs_a);
2936 let inner_target = field.subst(tcx, substs_b);
2938 // Check that the source struct with the target's
2939 // unsized parameters is equal to the target.
2940 let params = substs_a.iter().enumerate().map(|(i, &k)| {
2941 if ty_params.contains(i) {
2942 substs_b.type_at(i).into()
2947 let new_struct = tcx.mk_adt(def, tcx.mk_substs(params));
2948 let InferOk { obligations, .. } =
2949 self.infcx.at(&obligation.cause, obligation.param_env)
2950 .eq(target, new_struct)
2951 .map_err(|_| Unimplemented)?;
2952 nested.extend(obligations);
2954 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2955 nested.push(tcx.predicate_for_trait_def(
2956 obligation.param_env,
2957 obligation.cause.clone(),
2958 obligation.predicate.def_id(),
2959 obligation.recursion_depth + 1,
2964 // (.., T) -> (.., U).
2965 (&ty::TyTuple(tys_a), &ty::TyTuple(tys_b)) => {
2966 assert_eq!(tys_a.len(), tys_b.len());
2968 // The last field of the tuple has to exist.
2969 let (a_last, a_mid) = if let Some(x) = tys_a.split_last() {
2972 return Err(Unimplemented);
2974 let b_last = tys_b.last().unwrap();
2976 // Check that the source tuple with the target's
2977 // last element is equal to the target.
2978 let new_tuple = tcx.mk_tup(a_mid.iter().chain(Some(b_last)));
2979 let InferOk { obligations, .. } =
2980 self.infcx.at(&obligation.cause, obligation.param_env)
2981 .eq(target, new_tuple)
2982 .map_err(|_| Unimplemented)?;
2983 nested.extend(obligations);
2985 // Construct the nested T: Unsize<U> predicate.
2986 nested.push(tcx.predicate_for_trait_def(
2987 obligation.param_env,
2988 obligation.cause.clone(),
2989 obligation.predicate.def_id(),
2990 obligation.recursion_depth + 1,
2998 Ok(VtableBuiltinData { nested: nested })
3001 ///////////////////////////////////////////////////////////////////////////
3004 // Matching is a common path used for both evaluation and
3005 // confirmation. It basically unifies types that appear in impls
3006 // and traits. This does affect the surrounding environment;
3007 // therefore, when used during evaluation, match routines must be
3008 // run inside of a `probe()` so that their side-effects are
3011 fn rematch_impl(&mut self,
3013 obligation: &TraitObligation<'tcx>,
3014 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
3015 -> (Normalized<'tcx, &'tcx Substs<'tcx>>,
3016 infer::SkolemizationMap<'tcx>)
3018 match self.match_impl(impl_def_id, obligation, snapshot) {
3019 Ok((substs, skol_map)) => (substs, skol_map),
3021 bug!("Impl {:?} was matchable against {:?} but now is not",
3028 fn match_impl(&mut self,
3030 obligation: &TraitObligation<'tcx>,
3031 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
3032 -> Result<(Normalized<'tcx, &'tcx Substs<'tcx>>,
3033 infer::SkolemizationMap<'tcx>), ()>
3035 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
3037 // Before we create the substitutions and everything, first
3038 // consider a "quick reject". This avoids creating more types
3039 // and so forth that we need to.
3040 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
3044 let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions(
3045 &obligation.predicate,
3047 let skol_obligation_trait_ref = skol_obligation.trait_ref;
3049 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span,
3052 let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
3055 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
3056 project::normalize_with_depth(self,
3057 obligation.param_env,
3058 obligation.cause.clone(),
3059 obligation.recursion_depth + 1,
3062 debug!("match_impl(impl_def_id={:?}, obligation={:?}, \
3063 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
3067 skol_obligation_trait_ref);
3069 let InferOk { obligations, .. } =
3070 self.infcx.at(&obligation.cause, obligation.param_env)
3071 .eq(skol_obligation_trait_ref, impl_trait_ref)
3073 debug!("match_impl: failed eq_trait_refs due to `{}`", e);
3076 nested_obligations.extend(obligations);
3078 if let Err(e) = self.infcx.leak_check(false,
3079 obligation.cause.span,
3082 debug!("match_impl: failed leak check due to `{}`", e);
3086 debug!("match_impl: success impl_substs={:?}", impl_substs);
3089 obligations: nested_obligations
3093 fn fast_reject_trait_refs(&mut self,
3094 obligation: &TraitObligation,
3095 impl_trait_ref: &ty::TraitRef)
3098 // We can avoid creating type variables and doing the full
3099 // substitution if we find that any of the input types, when
3100 // simplified, do not match.
3102 obligation.predicate.skip_binder().input_types()
3103 .zip(impl_trait_ref.input_types())
3104 .any(|(obligation_ty, impl_ty)| {
3105 let simplified_obligation_ty =
3106 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
3107 let simplified_impl_ty =
3108 fast_reject::simplify_type(self.tcx(), impl_ty, false);
3110 simplified_obligation_ty.is_some() &&
3111 simplified_impl_ty.is_some() &&
3112 simplified_obligation_ty != simplified_impl_ty
3116 /// Normalize `where_clause_trait_ref` and try to match it against
3117 /// `obligation`. If successful, return any predicates that
3118 /// result from the normalization. Normalization is necessary
3119 /// because where-clauses are stored in the parameter environment
3121 fn match_where_clause_trait_ref(&mut self,
3122 obligation: &TraitObligation<'tcx>,
3123 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
3124 -> Result<Vec<PredicateObligation<'tcx>>,()>
3126 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
3129 /// Returns `Ok` if `poly_trait_ref` being true implies that the
3130 /// obligation is satisfied.
3131 fn match_poly_trait_ref(&mut self,
3132 obligation: &TraitObligation<'tcx>,
3133 poly_trait_ref: ty::PolyTraitRef<'tcx>)
3134 -> Result<Vec<PredicateObligation<'tcx>>,()>
3136 debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
3140 self.infcx.at(&obligation.cause, obligation.param_env)
3141 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
3142 .map(|InferOk { obligations, .. }| obligations)
3146 ///////////////////////////////////////////////////////////////////////////
3149 fn match_fresh_trait_refs(&self,
3150 previous: &ty::PolyTraitRef<'tcx>,
3151 current: &ty::PolyTraitRef<'tcx>)
3154 let mut matcher = ty::_match::Match::new(self.tcx());
3155 matcher.relate(previous, current).is_ok()
3158 fn push_stack<'o,'s:'o>(&mut self,
3159 previous_stack: TraitObligationStackList<'s, 'tcx>,
3160 obligation: &'o TraitObligation<'tcx>)
3161 -> TraitObligationStack<'o, 'tcx>
3163 let fresh_trait_ref =
3164 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
3166 TraitObligationStack {
3169 previous: previous_stack,
3173 fn closure_trait_ref_unnormalized(&mut self,
3174 obligation: &TraitObligation<'tcx>,
3175 closure_def_id: DefId,
3176 substs: ty::ClosureSubsts<'tcx>)
3177 -> ty::PolyTraitRef<'tcx>
3179 let closure_type = self.infcx.closure_sig(closure_def_id, substs);
3180 let ty::Binder((trait_ref, _)) =
3181 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
3182 obligation.predicate.0.self_ty(), // (1)
3184 util::TupleArgumentsFlag::No);
3185 // (1) Feels icky to skip the binder here, but OTOH we know
3186 // that the self-type is an unboxed closure type and hence is
3187 // in fact unparameterized (or at least does not reference any
3188 // regions bound in the obligation). Still probably some
3189 // refactoring could make this nicer.
3191 ty::Binder(trait_ref)
3194 fn generator_trait_ref_unnormalized(&mut self,
3195 obligation: &TraitObligation<'tcx>,
3196 closure_def_id: DefId,
3197 substs: ty::ClosureSubsts<'tcx>)
3198 -> ty::PolyTraitRef<'tcx>
3200 let gen_sig = substs.generator_poly_sig(closure_def_id, self.tcx());
3201 let ty::Binder((trait_ref, ..)) =
3202 self.tcx().generator_trait_ref_and_outputs(obligation.predicate.def_id(),
3203 obligation.predicate.0.self_ty(), // (1)
3205 // (1) Feels icky to skip the binder here, but OTOH we know
3206 // that the self-type is an generator type and hence is
3207 // in fact unparameterized (or at least does not reference any
3208 // regions bound in the obligation). Still probably some
3209 // refactoring could make this nicer.
3211 ty::Binder(trait_ref)
3214 /// Returns the obligations that are implied by instantiating an
3215 /// impl or trait. The obligations are substituted and fully
3216 /// normalized. This is used when confirming an impl or default
3218 fn impl_or_trait_obligations(&mut self,
3219 cause: ObligationCause<'tcx>,
3220 recursion_depth: usize,
3221 param_env: ty::ParamEnv<'tcx>,
3222 def_id: DefId, // of impl or trait
3223 substs: &Substs<'tcx>, // for impl or trait
3224 skol_map: infer::SkolemizationMap<'tcx>,
3225 snapshot: &infer::CombinedSnapshot<'cx, 'tcx>)
3226 -> Vec<PredicateObligation<'tcx>>
3228 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
3229 let tcx = self.tcx();
3231 // To allow for one-pass evaluation of the nested obligation,
3232 // each predicate must be preceded by the obligations required
3234 // for example, if we have:
3235 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
3236 // the impl will have the following predicates:
3237 // <V as Iterator>::Item = U,
3238 // U: Iterator, U: Sized,
3239 // V: Iterator, V: Sized,
3240 // <U as Iterator>::Item: Copy
3241 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
3242 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
3243 // `$1: Copy`, so we must ensure the obligations are emitted in
3245 let predicates = tcx.predicates_of(def_id);
3246 assert_eq!(predicates.parent, None);
3247 let mut predicates: Vec<_> = predicates.predicates.iter().flat_map(|predicate| {
3248 let predicate = normalize_with_depth(self, param_env, cause.clone(), recursion_depth,
3249 &predicate.subst(tcx, substs));
3250 predicate.obligations.into_iter().chain(
3252 cause: cause.clone(),
3255 predicate: predicate.value
3258 // We are performing deduplication here to avoid exponential blowups
3259 // (#38528) from happening, but the real cause of the duplication is
3260 // unknown. What we know is that the deduplication avoids exponential
3261 // amount of predicates being propogated when processing deeply nested
3263 let mut seen = FxHashSet();
3264 predicates.retain(|i| seen.insert(i.clone()));
3265 self.infcx().plug_leaks(skol_map, snapshot, predicates)
3269 impl<'tcx> TraitObligation<'tcx> {
3270 #[allow(unused_comparisons)]
3271 pub fn derived_cause(&self,
3272 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
3273 -> ObligationCause<'tcx>
3276 * Creates a cause for obligations that are derived from
3277 * `obligation` by a recursive search (e.g., for a builtin
3278 * bound, or eventually a `auto trait Foo`). If `obligation`
3279 * is itself a derived obligation, this is just a clone, but
3280 * otherwise we create a "derived obligation" cause so as to
3281 * keep track of the original root obligation for error
3285 let obligation = self;
3287 // NOTE(flaper87): As of now, it keeps track of the whole error
3288 // chain. Ideally, we should have a way to configure this either
3289 // by using -Z verbose or just a CLI argument.
3290 if obligation.recursion_depth >= 0 {
3291 let derived_cause = DerivedObligationCause {
3292 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
3293 parent_code: Rc::new(obligation.cause.code.clone())
3295 let derived_code = variant(derived_cause);
3296 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
3298 obligation.cause.clone()
3303 impl<'tcx> SelectionCache<'tcx> {
3304 pub fn new() -> SelectionCache<'tcx> {
3306 hashmap: RefCell::new(FxHashMap())
3310 pub fn clear(&self) {
3311 *self.hashmap.borrow_mut() = FxHashMap()
3315 impl<'tcx> EvaluationCache<'tcx> {
3316 pub fn new() -> EvaluationCache<'tcx> {
3318 hashmap: RefCell::new(FxHashMap())
3322 pub fn clear(&self) {
3323 *self.hashmap.borrow_mut() = FxHashMap()
3327 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {
3328 fn list(&'o self) -> TraitObligationStackList<'o,'tcx> {
3329 TraitObligationStackList::with(self)
3332 fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> {
3337 #[derive(Copy, Clone)]
3338 struct TraitObligationStackList<'o,'tcx:'o> {
3339 head: Option<&'o TraitObligationStack<'o,'tcx>>
3342 impl<'o,'tcx> TraitObligationStackList<'o,'tcx> {
3343 fn empty() -> TraitObligationStackList<'o,'tcx> {
3344 TraitObligationStackList { head: None }
3347 fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> {
3348 TraitObligationStackList { head: Some(r) }
3352 impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{
3353 type Item = &'o TraitObligationStack<'o,'tcx>;
3355 fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> {
3366 impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> {
3367 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
3368 write!(f, "TraitObligationStack({:?})", self.obligation)
3373 pub struct WithDepNode<T> {
3374 dep_node: DepNodeIndex,
3378 impl<T: Clone> WithDepNode<T> {
3379 pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self {
3380 WithDepNode { dep_node, cached_value }
3383 pub fn get(&self, tcx: TyCtxt) -> T {
3384 tcx.dep_graph.read_index(self.dep_node);
3385 self.cached_value.clone()