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 `README.md` for high-level documentation
13 use self::SelectionCandidate::*;
14 use self::EvaluationResult::*;
16 use super::coherence::{self, Conflict};
17 use super::DerivedObligationCause;
18 use super::IntercrateMode;
20 use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey};
21 use super::{PredicateObligation, TraitObligation, ObligationCause};
22 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation};
23 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch};
24 use super::{ObjectCastObligation, Obligation};
25 use super::TraitNotObjectSafe;
27 use super::SelectionResult;
28 use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure, VtableGenerator,
29 VtableFnPointer, VtableObject, VtableAutoImpl};
30 use super::{VtableImplData, VtableObjectData, VtableBuiltinData, VtableGeneratorData,
31 VtableClosureData, VtableAutoImplData, VtableFnPointerData};
34 use dep_graph::{DepNodeIndex, DepKind};
35 use hir::def_id::DefId;
37 use infer::{InferCtxt, InferOk, TypeFreshener};
38 use ty::subst::{Kind, Subst, Substs};
39 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
41 use ty::relate::TypeRelation;
42 use middle::lang_items;
44 use rustc_data_structures::bitvec::BitVector;
45 use rustc_data_structures::snapshot_vec::{SnapshotVecDelegate, SnapshotVec};
47 use std::cell::RefCell;
50 use std::marker::PhantomData;
56 use util::nodemap::FxHashMap;
58 struct InferredObligationsSnapshotVecDelegate<'tcx> {
59 phantom: PhantomData<&'tcx i32>,
61 impl<'tcx> SnapshotVecDelegate for InferredObligationsSnapshotVecDelegate<'tcx> {
62 type Value = PredicateObligation<'tcx>;
64 fn reverse(_: &mut Vec<Self::Value>, _: Self::Undo) {}
67 pub struct SelectionContext<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
68 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
70 /// Freshener used specifically for skolemizing entries on the
71 /// obligation stack. This ensures that all entries on the stack
72 /// at one time will have the same set of skolemized entries,
73 /// which is important for checking for trait bounds that
74 /// recursively require themselves.
75 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
77 /// If true, indicates that the evaluation should be conservative
78 /// and consider the possibility of types outside this crate.
79 /// This comes up primarily when resolving ambiguity. Imagine
80 /// there is some trait reference `$0 : Bar` where `$0` is an
81 /// inference variable. If `intercrate` is true, then we can never
82 /// say for sure that this reference is not implemented, even if
83 /// there are *no impls at all for `Bar`*, because `$0` could be
84 /// bound to some type that in a downstream crate that implements
85 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
86 /// though, we set this to false, because we are only interested
87 /// in types that the user could actually have written --- in
88 /// other words, we consider `$0 : Bar` to be unimplemented if
89 /// there is no type that the user could *actually name* that
90 /// would satisfy it. This avoids crippling inference, basically.
91 intercrate: Option<IntercrateMode>,
93 inferred_obligations: SnapshotVec<InferredObligationsSnapshotVecDelegate<'tcx>>,
95 intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
99 pub enum IntercrateAmbiguityCause {
102 self_desc: Option<String>,
104 UpstreamCrateUpdate {
106 self_desc: Option<String>,
110 impl IntercrateAmbiguityCause {
111 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
112 /// See #23980 for details.
113 pub fn add_intercrate_ambiguity_hint<'a, 'tcx>(&self,
114 err: &mut ::errors::DiagnosticBuilder) {
115 err.note(&self.intercrate_ambiguity_hint());
118 pub fn intercrate_ambiguity_hint(&self) -> String {
120 &IntercrateAmbiguityCause::DownstreamCrate { ref trait_desc, ref self_desc } => {
121 let self_desc = if let &Some(ref ty) = self_desc {
122 format!(" for type `{}`", ty)
123 } else { "".to_string() };
124 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
126 &IntercrateAmbiguityCause::UpstreamCrateUpdate { ref trait_desc, ref self_desc } => {
127 let self_desc = if let &Some(ref ty) = self_desc {
128 format!(" for type `{}`", ty)
129 } else { "".to_string() };
130 format!("upstream crates may add new impl of trait `{}`{} \
132 trait_desc, self_desc)
138 // A stack that walks back up the stack frame.
139 struct TraitObligationStack<'prev, 'tcx: 'prev> {
140 obligation: &'prev TraitObligation<'tcx>,
142 /// Trait ref from `obligation` but skolemized with the
143 /// selection-context's freshener. Used to check for recursion.
144 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
146 previous: TraitObligationStackList<'prev, 'tcx>,
150 pub struct SelectionCache<'tcx> {
151 hashmap: RefCell<FxHashMap<ty::TraitRef<'tcx>,
152 WithDepNode<SelectionResult<'tcx, SelectionCandidate<'tcx>>>>>,
155 /// The selection process begins by considering all impls, where
156 /// clauses, and so forth that might resolve an obligation. Sometimes
157 /// we'll be able to say definitively that (e.g.) an impl does not
158 /// apply to the obligation: perhaps it is defined for `usize` but the
159 /// obligation is for `int`. In that case, we drop the impl out of the
160 /// list. But the other cases are considered *candidates*.
162 /// For selection to succeed, there must be exactly one matching
163 /// candidate. If the obligation is fully known, this is guaranteed
164 /// by coherence. However, if the obligation contains type parameters
165 /// or variables, there may be multiple such impls.
167 /// It is not a real problem if multiple matching impls exist because
168 /// of type variables - it just means the obligation isn't sufficiently
169 /// elaborated. In that case we report an ambiguity, and the caller can
170 /// try again after more type information has been gathered or report a
171 /// "type annotations required" error.
173 /// However, with type parameters, this can be a real problem - type
174 /// parameters don't unify with regular types, but they *can* unify
175 /// with variables from blanket impls, and (unless we know its bounds
176 /// will always be satisfied) picking the blanket impl will be wrong
177 /// for at least *some* substitutions. To make this concrete, if we have
179 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
180 /// impl<T: fmt::Debug> AsDebug for T {
182 /// fn debug(self) -> fmt::Debug { self }
184 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
186 /// we can't just use the impl to resolve the <T as AsDebug> obligation
187 /// - a type from another crate (that doesn't implement fmt::Debug) could
188 /// implement AsDebug.
190 /// Because where-clauses match the type exactly, multiple clauses can
191 /// only match if there are unresolved variables, and we can mostly just
192 /// report this ambiguity in that case. This is still a problem - we can't
193 /// *do anything* with ambiguities that involve only regions. This is issue
196 /// If a single where-clause matches and there are no inference
197 /// variables left, then it definitely matches and we can just select
200 /// In fact, we even select the where-clause when the obligation contains
201 /// inference variables. The can lead to inference making "leaps of logic",
202 /// for example in this situation:
204 /// pub trait Foo<T> { fn foo(&self) -> T; }
205 /// impl<T> Foo<()> for T { fn foo(&self) { } }
206 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
208 /// pub fn foo<T>(t: T) where T: Foo<bool> {
209 /// println!("{:?}", <T as Foo<_>>::foo(&t));
211 /// fn main() { foo(false); }
213 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
214 /// impl and the where-clause. We select the where-clause and unify $0=bool,
215 /// so the program prints "false". However, if the where-clause is omitted,
216 /// the blanket impl is selected, we unify $0=(), and the program prints
219 /// Exactly the same issues apply to projection and object candidates, except
220 /// that we can have both a projection candidate and a where-clause candidate
221 /// for the same obligation. In that case either would do (except that
222 /// different "leaps of logic" would occur if inference variables are
223 /// present), and we just pick the where-clause. This is, for example,
224 /// required for associated types to work in default impls, as the bounds
225 /// are visible both as projection bounds and as where-clauses from the
226 /// parameter environment.
227 #[derive(PartialEq,Eq,Debug,Clone)]
228 enum SelectionCandidate<'tcx> {
229 BuiltinCandidate { has_nested: bool },
230 ParamCandidate(ty::PolyTraitRef<'tcx>),
231 ImplCandidate(DefId),
232 AutoImplCandidate(DefId),
234 /// This is a trait matching with a projected type as `Self`, and
235 /// we found an applicable bound in the trait definition.
238 /// Implementation of a `Fn`-family trait by one of the anonymous types
239 /// generated for a `||` expression.
242 /// Implementation of a `Generator` trait by one of the anonymous types
243 /// generated for a generator.
246 /// Implementation of a `Fn`-family trait by one of the anonymous
247 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
252 BuiltinObjectCandidate,
254 BuiltinUnsizeCandidate,
257 impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> {
258 type Lifted = SelectionCandidate<'tcx>;
259 fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
261 BuiltinCandidate { has_nested } => {
266 ImplCandidate(def_id) => ImplCandidate(def_id),
267 AutoImplCandidate(def_id) => AutoImplCandidate(def_id),
268 ProjectionCandidate => ProjectionCandidate,
269 FnPointerCandidate => FnPointerCandidate,
270 ObjectCandidate => ObjectCandidate,
271 BuiltinObjectCandidate => BuiltinObjectCandidate,
272 BuiltinUnsizeCandidate => BuiltinUnsizeCandidate,
273 ClosureCandidate => ClosureCandidate,
274 GeneratorCandidate => GeneratorCandidate,
276 ParamCandidate(ref trait_ref) => {
277 return tcx.lift(trait_ref).map(ParamCandidate);
283 struct SelectionCandidateSet<'tcx> {
284 // a list of candidates that definitely apply to the current
285 // obligation (meaning: types unify).
286 vec: Vec<SelectionCandidate<'tcx>>,
288 // if this is true, then there were candidates that might or might
289 // not have applied, but we couldn't tell. This occurs when some
290 // of the input types are type variables, in which case there are
291 // various "builtin" rules that might or might not trigger.
295 #[derive(PartialEq,Eq,Debug,Clone)]
296 struct EvaluatedCandidate<'tcx> {
297 candidate: SelectionCandidate<'tcx>,
298 evaluation: EvaluationResult,
301 /// When does the builtin impl for `T: Trait` apply?
302 enum BuiltinImplConditions<'tcx> {
303 /// The impl is conditional on T1,T2,.. : Trait
304 Where(ty::Binder<Vec<Ty<'tcx>>>),
305 /// There is no built-in impl. There may be some other
306 /// candidate (a where-clause or user-defined impl).
308 /// There is *no* impl for this, builtin or not. Ignore
309 /// all where-clauses.
311 /// It is unknown whether there is an impl.
315 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
316 /// The result of trait evaluation. The order is important
317 /// here as the evaluation of a list is the maximum of the
320 /// The evaluation results are ordered:
321 /// - `EvaluatedToOk` implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
322 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
323 /// - the "union" of evaluation results is equal to their maximum -
324 /// all the "potential success" candidates can potentially succeed,
325 /// so they are no-ops when unioned with a definite error, and within
326 /// the categories it's easy to see that the unions are correct.
327 enum EvaluationResult {
328 /// Evaluation successful
330 /// Evaluation is known to be ambiguous - it *might* hold for some
331 /// assignment of inference variables, but it might not.
333 /// While this has the same meaning as `EvaluatedToUnknown` - we can't
334 /// know whether this obligation holds or not - it is the result we
335 /// would get with an empty stack, and therefore is cacheable.
337 /// Evaluation failed because of recursion involving inference
338 /// variables. We are somewhat imprecise there, so we don't actually
339 /// know the real result.
341 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
343 /// Evaluation failed because we encountered an obligation we are already
344 /// trying to prove on this branch.
346 /// We know this branch can't be a part of a minimal proof-tree for
347 /// the "root" of our cycle, because then we could cut out the recursion
348 /// and maintain a valid proof tree. However, this does not mean
349 /// that all the obligations on this branch do not hold - it's possible
350 /// that we entered this branch "speculatively", and that there
351 /// might be some other way to prove this obligation that does not
352 /// go through this cycle - so we can't cache this as a failure.
354 /// For example, suppose we have this:
356 /// ```rust,ignore (pseudo-Rust)
357 /// pub trait Trait { fn xyz(); }
358 /// // This impl is "useless", but we can still have
359 /// // an `impl Trait for SomeUnsizedType` somewhere.
360 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
362 /// pub fn foo<T: Trait + ?Sized>() {
363 /// <T as Trait>::xyz();
367 /// When checking `foo`, we have to prove `T: Trait`. This basically
368 /// translates into this:
370 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
372 /// When we try to prove it, we first go the first option, which
373 /// recurses. This shows us that the impl is "useless" - it won't
374 /// tell us that `T: Trait` unless it already implemented `Trait`
375 /// by some other means. However, that does not prevent `T: Trait`
376 /// does not hold, because of the bound (which can indeed be satisfied
377 /// by `SomeUnsizedType` from another crate).
379 /// FIXME: when an `EvaluatedToRecur` goes past its parent root, we
380 /// ought to convert it to an `EvaluatedToErr`, because we know
381 /// there definitely isn't a proof tree for that obligation. Not
382 /// doing so is still sound - there isn't any proof tree, so the
383 /// branch still can't be a part of a minimal one - but does not
384 /// re-enable caching.
386 /// Evaluation failed
390 impl EvaluationResult {
391 fn may_apply(self) -> bool {
395 EvaluatedToUnknown => true,
398 EvaluatedToRecur => false
402 fn is_stack_dependent(self) -> bool {
405 EvaluatedToRecur => true,
409 EvaluatedToErr => false,
415 pub struct EvaluationCache<'tcx> {
416 hashmap: RefCell<FxHashMap<ty::PolyTraitRef<'tcx>, WithDepNode<EvaluationResult>>>
419 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
420 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
423 freshener: infcx.freshener(),
425 inferred_obligations: SnapshotVec::new(),
426 intercrate_ambiguity_causes: Vec::new(),
430 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
431 mode: IntercrateMode) -> SelectionContext<'cx, 'gcx, 'tcx> {
432 debug!("intercrate({:?})", mode);
435 freshener: infcx.freshener(),
436 intercrate: Some(mode),
437 inferred_obligations: SnapshotVec::new(),
438 intercrate_ambiguity_causes: Vec::new(),
442 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
446 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
450 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
454 pub fn intercrate_ambiguity_causes(&self) -> &[IntercrateAmbiguityCause] {
455 &self.intercrate_ambiguity_causes
458 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
460 fn in_snapshot<R, F>(&mut self, f: F) -> R
461 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
463 // The irrefutable nature of the operation means we don't need to snapshot the
464 // inferred_obligations vector.
465 self.infcx.in_snapshot(|snapshot| f(self, snapshot))
468 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
470 fn probe<R, F>(&mut self, f: F) -> R
471 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
473 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
474 let result = self.infcx.probe(|snapshot| f(self, snapshot));
475 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
479 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
480 /// the transaction fails and s.t. old obligations are retained.
481 fn commit_if_ok<T, E, F>(&mut self, f: F) -> Result<T, E> where
482 F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> Result<T, E>
484 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
485 match self.infcx.commit_if_ok(|snapshot| f(self, snapshot)) {
487 self.inferred_obligations.commit(inferred_obligations_snapshot);
491 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
498 ///////////////////////////////////////////////////////////////////////////
501 // The selection phase tries to identify *how* an obligation will
502 // be resolved. For example, it will identify which impl or
503 // parameter bound is to be used. The process can be inconclusive
504 // if the self type in the obligation is not fully inferred. Selection
505 // can result in an error in one of two ways:
507 // 1. If no applicable impl or parameter bound can be found.
508 // 2. If the output type parameters in the obligation do not match
509 // those specified by the impl/bound. For example, if the obligation
510 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
511 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
513 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
514 /// type environment by performing unification.
515 pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
516 -> SelectionResult<'tcx, Selection<'tcx>> {
517 debug!("select({:?})", obligation);
518 assert!(!obligation.predicate.has_escaping_regions());
520 let tcx = self.tcx();
522 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
523 let ret = match self.candidate_from_obligation(&stack)? {
526 let mut candidate = self.confirm_candidate(obligation, candidate)?;
527 let inferred_obligations = (*self.inferred_obligations).into_iter().cloned();
528 candidate.nested_obligations_mut().extend(inferred_obligations);
533 // Test whether this is a `()` which was produced by defaulting a
534 // diverging type variable with `!` disabled. If so, we may need
535 // to raise a warning.
536 if obligation.predicate.skip_binder().self_ty().is_defaulted_unit() {
537 let mut raise_warning = true;
538 // Don't raise a warning if the trait is implemented for ! and only
539 // permits a trivial implementation for !. This stops us warning
540 // about (for example) `(): Clone` becoming `!: Clone` because such
541 // a switch can't cause code to stop compiling or execute
543 let mut never_obligation = obligation.clone();
544 let def_id = never_obligation.predicate.skip_binder().trait_ref.def_id;
545 never_obligation.predicate = never_obligation.predicate.map_bound(|mut trait_pred| {
546 // Swap out () with ! so we can check if the trait is impld for !
548 let trait_ref = &mut trait_pred.trait_ref;
549 let unit_substs = trait_ref.substs;
550 let mut never_substs = Vec::with_capacity(unit_substs.len());
551 never_substs.push(From::from(tcx.types.never));
552 never_substs.extend(&unit_substs[1..]);
553 trait_ref.substs = tcx.intern_substs(&never_substs);
557 if let Ok(Some(..)) = self.select(&never_obligation) {
558 if !tcx.trait_relevant_for_never(def_id) {
559 // The trait is also implemented for ! and the resulting
560 // implementation cannot actually be invoked in any way.
561 raise_warning = false;
566 tcx.lint_node(lint::builtin::RESOLVE_TRAIT_ON_DEFAULTED_UNIT,
567 obligation.cause.body_id,
568 obligation.cause.span,
569 &format!("code relies on type inference rules which are likely \
576 ///////////////////////////////////////////////////////////////////////////
579 // Tests whether an obligation can be selected or whether an impl
580 // can be applied to particular types. It skips the "confirmation"
581 // step and hence completely ignores output type parameters.
583 // The result is "true" if the obligation *may* hold and "false" if
584 // we can be sure it does not.
586 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
587 pub fn evaluate_obligation(&mut self,
588 obligation: &PredicateObligation<'tcx>)
591 debug!("evaluate_obligation({:?})",
594 self.probe(|this, _| {
595 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
600 /// Evaluates whether the obligation `obligation` can be satisfied,
601 /// and returns `false` if not certain. However, this is not entirely
602 /// accurate if inference variables are involved.
603 pub fn evaluate_obligation_conservatively(&mut self,
604 obligation: &PredicateObligation<'tcx>)
607 debug!("evaluate_obligation_conservatively({:?})",
610 self.probe(|this, _| {
611 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
616 /// Evaluates the predicates in `predicates` recursively. Note that
617 /// this applies projections in the predicates, and therefore
618 /// is run within an inference probe.
619 fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
620 stack: TraitObligationStackList<'o, 'tcx>,
623 where I : Iterator<Item=&'a PredicateObligation<'tcx>>, 'tcx:'a
625 let mut result = EvaluatedToOk;
626 for obligation in predicates {
627 let eval = self.evaluate_predicate_recursively(stack, obligation);
628 debug!("evaluate_predicate_recursively({:?}) = {:?}",
630 if let EvaluatedToErr = eval {
631 // fast-path - EvaluatedToErr is the top of the lattice,
632 // so we don't need to look on the other predicates.
633 return EvaluatedToErr;
635 result = cmp::max(result, eval);
641 fn evaluate_predicate_recursively<'o>(&mut self,
642 previous_stack: TraitObligationStackList<'o, 'tcx>,
643 obligation: &PredicateObligation<'tcx>)
646 debug!("evaluate_predicate_recursively({:?})",
649 match obligation.predicate {
650 ty::Predicate::Trait(ref t) => {
651 assert!(!t.has_escaping_regions());
652 let obligation = obligation.with(t.clone());
653 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
656 ty::Predicate::Equate(ref p) => {
657 // does this code ever run?
658 match self.infcx.equality_predicate(&obligation.cause, obligation.param_env, p) {
659 Ok(InferOk { obligations, .. }) => {
660 self.inferred_obligations.extend(obligations);
663 Err(_) => EvaluatedToErr
667 ty::Predicate::Subtype(ref p) => {
668 // does this code ever run?
669 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
670 Some(Ok(InferOk { obligations, .. })) => {
671 self.inferred_obligations.extend(obligations);
674 Some(Err(_)) => EvaluatedToErr,
675 None => EvaluatedToAmbig,
679 ty::Predicate::WellFormed(ty) => {
680 match ty::wf::obligations(self.infcx,
681 obligation.param_env,
682 obligation.cause.body_id,
683 ty, obligation.cause.span) {
685 self.evaluate_predicates_recursively(previous_stack, obligations.iter()),
691 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
692 // we do not consider region relationships when
693 // evaluating trait matches
697 ty::Predicate::ObjectSafe(trait_def_id) => {
698 if self.tcx().is_object_safe(trait_def_id) {
705 ty::Predicate::Projection(ref data) => {
706 let project_obligation = obligation.with(data.clone());
707 match project::poly_project_and_unify_type(self, &project_obligation) {
708 Ok(Some(subobligations)) => {
709 let result = self.evaluate_predicates_recursively(previous_stack,
710 subobligations.iter());
712 ProjectionCacheKey::from_poly_projection_predicate(self, data)
714 self.infcx.projection_cache.borrow_mut().complete(key);
727 ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
728 match self.infcx.closure_kind(closure_def_id, closure_substs) {
729 Some(closure_kind) => {
730 if closure_kind.extends(kind) {
742 ty::Predicate::ConstEvaluatable(def_id, substs) => {
743 match self.tcx().lift_to_global(&(obligation.param_env, substs)) {
744 Some((param_env, substs)) => {
745 match self.tcx().const_eval(param_env.and((def_id, substs))) {
746 Ok(_) => EvaluatedToOk,
747 Err(_) => EvaluatedToErr
751 // Inference variables still left in param_env or substs.
759 fn evaluate_trait_predicate_recursively<'o>(&mut self,
760 previous_stack: TraitObligationStackList<'o, 'tcx>,
761 mut obligation: TraitObligation<'tcx>)
764 debug!("evaluate_trait_predicate_recursively({:?})",
767 if !self.intercrate.is_some() && obligation.is_global() {
768 // If a param env is consistent, global obligations do not depend on its particular
769 // value in order to work, so we can clear out the param env and get better
770 // caching. (If the current param env is inconsistent, we don't care what happens).
771 debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation);
772 obligation.param_env = ty::ParamEnv::empty(obligation.param_env.reveal);
775 let stack = self.push_stack(previous_stack, &obligation);
776 let fresh_trait_ref = stack.fresh_trait_ref;
777 if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
778 debug!("CACHE HIT: EVAL({:?})={:?}",
784 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
786 debug!("CACHE MISS: EVAL({:?})={:?}",
789 self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
794 fn evaluate_stack<'o>(&mut self,
795 stack: &TraitObligationStack<'o, 'tcx>)
798 // In intercrate mode, whenever any of the types are unbound,
799 // there can always be an impl. Even if there are no impls in
800 // this crate, perhaps the type would be unified with
801 // something from another crate that does provide an impl.
803 // In intra mode, we must still be conservative. The reason is
804 // that we want to avoid cycles. Imagine an impl like:
806 // impl<T:Eq> Eq for Vec<T>
808 // and a trait reference like `$0 : Eq` where `$0` is an
809 // unbound variable. When we evaluate this trait-reference, we
810 // will unify `$0` with `Vec<$1>` (for some fresh variable
811 // `$1`), on the condition that `$1 : Eq`. We will then wind
812 // up with many candidates (since that are other `Eq` impls
813 // that apply) and try to winnow things down. This results in
814 // a recursive evaluation that `$1 : Eq` -- as you can
815 // imagine, this is just where we started. To avoid that, we
816 // check for unbound variables and return an ambiguous (hence possible)
817 // match if we've seen this trait before.
819 // This suffices to allow chains like `FnMut` implemented in
820 // terms of `Fn` etc, but we could probably make this more
822 let unbound_input_types = stack.fresh_trait_ref.input_types().any(|ty| ty.is_fresh());
823 // this check was an imperfect workaround for a bug n the old
824 // intercrate mode, it should be removed when that goes away.
825 if unbound_input_types &&
826 self.intercrate == Some(IntercrateMode::Issue43355)
828 debug!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
829 stack.fresh_trait_ref);
830 // Heuristics: show the diagnostics when there are no candidates in crate.
831 if let Ok(candidate_set) = self.assemble_candidates(stack) {
832 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
833 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
834 let self_ty = trait_ref.self_ty();
835 let cause = IntercrateAmbiguityCause::DownstreamCrate {
836 trait_desc: trait_ref.to_string(),
837 self_desc: if self_ty.has_concrete_skeleton() {
838 Some(self_ty.to_string())
843 self.intercrate_ambiguity_causes.push(cause);
846 return EvaluatedToAmbig;
848 if unbound_input_types &&
849 stack.iter().skip(1).any(
850 |prev| stack.obligation.param_env == prev.obligation.param_env &&
851 self.match_fresh_trait_refs(&stack.fresh_trait_ref,
852 &prev.fresh_trait_ref))
854 debug!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
855 stack.fresh_trait_ref);
856 return EvaluatedToUnknown;
859 // If there is any previous entry on the stack that precisely
860 // matches this obligation, then we can assume that the
861 // obligation is satisfied for now (still all other conditions
862 // must be met of course). One obvious case this comes up is
863 // marker traits like `Send`. Think of a linked list:
865 // struct List<T> { data: T, next: Option<Box<List<T>>> {
867 // `Box<List<T>>` will be `Send` if `T` is `Send` and
868 // `Option<Box<List<T>>>` is `Send`, and in turn
869 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
872 // Note that we do this comparison using the `fresh_trait_ref`
873 // fields. Because these have all been skolemized using
874 // `self.freshener`, we can be sure that (a) this will not
875 // affect the inferencer state and (b) that if we see two
876 // skolemized types with the same index, they refer to the
877 // same unbound type variable.
878 if let Some(rec_index) =
880 .skip(1) // skip top-most frame
881 .position(|prev| stack.obligation.param_env == prev.obligation.param_env &&
882 stack.fresh_trait_ref == prev.fresh_trait_ref)
884 debug!("evaluate_stack({:?}) --> recursive",
885 stack.fresh_trait_ref);
886 let cycle = stack.iter().skip(1).take(rec_index+1);
887 let cycle = cycle.map(|stack| ty::Predicate::Trait(stack.obligation.predicate));
888 if self.coinductive_match(cycle) {
889 debug!("evaluate_stack({:?}) --> recursive, coinductive",
890 stack.fresh_trait_ref);
891 return EvaluatedToOk;
893 debug!("evaluate_stack({:?}) --> recursive, inductive",
894 stack.fresh_trait_ref);
895 return EvaluatedToRecur;
899 match self.candidate_from_obligation(stack) {
900 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
901 Ok(None) => EvaluatedToAmbig,
902 Err(..) => EvaluatedToErr
906 /// For defaulted traits, we use a co-inductive strategy to solve, so
907 /// that recursion is ok. This routine returns true if the top of the
908 /// stack (`cycle[0]`):
909 /// - is a defaulted trait, and
910 /// - it also appears in the backtrace at some position `X`; and,
911 /// - all the predicates at positions `X..` between `X` an the top are
912 /// also defaulted traits.
913 pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
914 where I: Iterator<Item=ty::Predicate<'tcx>>
916 let mut cycle = cycle;
917 cycle.all(|predicate| self.coinductive_predicate(predicate))
920 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
921 let result = match predicate {
922 ty::Predicate::Trait(ref data) => {
923 self.tcx().trait_is_auto(data.def_id())
929 debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
933 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
934 /// obligations are met. Returns true if `candidate` remains viable after this further
936 fn evaluate_candidate<'o>(&mut self,
937 stack: &TraitObligationStack<'o, 'tcx>,
938 candidate: &SelectionCandidate<'tcx>)
941 debug!("evaluate_candidate: depth={} candidate={:?}",
942 stack.obligation.recursion_depth, candidate);
943 let result = self.probe(|this, _| {
944 let candidate = (*candidate).clone();
945 match this.confirm_candidate(stack.obligation, candidate) {
947 this.evaluate_predicates_recursively(
949 selection.nested_obligations().iter())
951 Err(..) => EvaluatedToErr
954 debug!("evaluate_candidate: depth={} result={:?}",
955 stack.obligation.recursion_depth, result);
959 fn check_evaluation_cache(&self,
960 param_env: ty::ParamEnv<'tcx>,
961 trait_ref: ty::PolyTraitRef<'tcx>)
962 -> Option<EvaluationResult>
964 let tcx = self.tcx();
965 if self.can_use_global_caches(param_env) {
966 let cache = tcx.evaluation_cache.hashmap.borrow();
967 if let Some(cached) = cache.get(&trait_ref) {
968 return Some(cached.get(tcx));
971 self.infcx.evaluation_cache.hashmap
977 fn insert_evaluation_cache(&mut self,
978 param_env: ty::ParamEnv<'tcx>,
979 trait_ref: ty::PolyTraitRef<'tcx>,
980 dep_node: DepNodeIndex,
981 result: EvaluationResult)
983 // Avoid caching results that depend on more than just the trait-ref
984 // - the stack can create recursion.
985 if result.is_stack_dependent() {
989 if self.can_use_global_caches(param_env) {
990 let mut cache = self.tcx().evaluation_cache.hashmap.borrow_mut();
991 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
992 cache.insert(trait_ref, WithDepNode::new(dep_node, result));
997 self.infcx.evaluation_cache.hashmap
999 .insert(trait_ref, WithDepNode::new(dep_node, result));
1002 ///////////////////////////////////////////////////////////////////////////
1003 // CANDIDATE ASSEMBLY
1005 // The selection process begins by examining all in-scope impls,
1006 // caller obligations, and so forth and assembling a list of
1007 // candidates. See `README.md` and the `Candidate` type for more
1010 fn candidate_from_obligation<'o>(&mut self,
1011 stack: &TraitObligationStack<'o, 'tcx>)
1012 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
1014 // Watch out for overflow. This intentionally bypasses (and does
1015 // not update) the cache.
1016 let recursion_limit = self.infcx.tcx.sess.recursion_limit.get();
1017 if stack.obligation.recursion_depth >= recursion_limit {
1018 self.infcx().report_overflow_error(&stack.obligation, true);
1021 // Check the cache. Note that we skolemize the trait-ref
1022 // separately rather than using `stack.fresh_trait_ref` -- this
1023 // is because we want the unbound variables to be replaced
1024 // with fresh skolemized types starting from index 0.
1025 let cache_fresh_trait_pred =
1026 self.infcx.freshen(stack.obligation.predicate.clone());
1027 debug!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
1028 cache_fresh_trait_pred,
1030 assert!(!stack.obligation.predicate.has_escaping_regions());
1032 if let Some(c) = self.check_candidate_cache(stack.obligation.param_env,
1033 &cache_fresh_trait_pred) {
1034 debug!("CACHE HIT: SELECT({:?})={:?}",
1035 cache_fresh_trait_pred,
1040 // If no match, compute result and insert into cache.
1041 let (candidate, dep_node) = self.in_task(|this| {
1042 this.candidate_from_obligation_no_cache(stack)
1045 debug!("CACHE MISS: SELECT({:?})={:?}",
1046 cache_fresh_trait_pred, candidate);
1047 self.insert_candidate_cache(stack.obligation.param_env,
1048 cache_fresh_trait_pred,
1054 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1055 where OP: FnOnce(&mut Self) -> R
1057 let (result, dep_node) = self.tcx().dep_graph.with_anon_task(DepKind::TraitSelect, || {
1060 self.tcx().dep_graph.read_index(dep_node);
1064 // Treat negative impls as unimplemented
1065 fn filter_negative_impls(&self, candidate: SelectionCandidate<'tcx>)
1066 -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1067 if let ImplCandidate(def_id) = candidate {
1068 if self.tcx().impl_polarity(def_id) == hir::ImplPolarity::Negative {
1069 return Err(Unimplemented)
1075 fn candidate_from_obligation_no_cache<'o>(&mut self,
1076 stack: &TraitObligationStack<'o, 'tcx>)
1077 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
1079 if stack.obligation.predicate.references_error() {
1080 // If we encounter a `TyError`, we generally prefer the
1081 // most "optimistic" result in response -- that is, the
1082 // one least likely to report downstream errors. But
1083 // because this routine is shared by coherence and by
1084 // trait selection, there isn't an obvious "right" choice
1085 // here in that respect, so we opt to just return
1086 // ambiguity and let the upstream clients sort it out.
1090 match self.is_knowable(stack) {
1093 debug!("coherence stage: not knowable");
1094 // Heuristics: show the diagnostics when there are no candidates in crate.
1095 let candidate_set = self.assemble_candidates(stack)?;
1096 if !candidate_set.ambiguous && candidate_set.vec.iter().all(|c| {
1097 !self.evaluate_candidate(stack, &c).may_apply()
1099 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
1100 let self_ty = trait_ref.self_ty();
1101 let trait_desc = trait_ref.to_string();
1102 let self_desc = if self_ty.has_concrete_skeleton() {
1103 Some(self_ty.to_string())
1107 let cause = if let Conflict::Upstream = conflict {
1108 IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc }
1110 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
1112 self.intercrate_ambiguity_causes.push(cause);
1118 let candidate_set = self.assemble_candidates(stack)?;
1120 if candidate_set.ambiguous {
1121 debug!("candidate set contains ambig");
1125 let mut candidates = candidate_set.vec;
1127 debug!("assembled {} candidates for {:?}: {:?}",
1132 // At this point, we know that each of the entries in the
1133 // candidate set is *individually* applicable. Now we have to
1134 // figure out if they contain mutual incompatibilities. This
1135 // frequently arises if we have an unconstrained input type --
1136 // for example, we are looking for $0:Eq where $0 is some
1137 // unconstrained type variable. In that case, we'll get a
1138 // candidate which assumes $0 == int, one that assumes $0 ==
1139 // usize, etc. This spells an ambiguity.
1141 // If there is more than one candidate, first winnow them down
1142 // by considering extra conditions (nested obligations and so
1143 // forth). We don't winnow if there is exactly one
1144 // candidate. This is a relatively minor distinction but it
1145 // can lead to better inference and error-reporting. An
1146 // example would be if there was an impl:
1148 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
1150 // and we were to see some code `foo.push_clone()` where `boo`
1151 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
1152 // we were to winnow, we'd wind up with zero candidates.
1153 // Instead, we select the right impl now but report `Bar does
1154 // not implement Clone`.
1155 if candidates.len() == 1 {
1156 return self.filter_negative_impls(candidates.pop().unwrap());
1159 // Winnow, but record the exact outcome of evaluation, which
1160 // is needed for specialization.
1161 let mut candidates: Vec<_> = candidates.into_iter().filter_map(|c| {
1162 let eval = self.evaluate_candidate(stack, &c);
1163 if eval.may_apply() {
1164 Some(EvaluatedCandidate {
1173 // If there are STILL multiple candidate, we can further
1174 // reduce the list by dropping duplicates -- including
1175 // resolving specializations.
1176 if candidates.len() > 1 {
1178 while i < candidates.len() {
1180 (0..candidates.len())
1181 .filter(|&j| i != j)
1182 .any(|j| self.candidate_should_be_dropped_in_favor_of(&candidates[i],
1185 debug!("Dropping candidate #{}/{}: {:?}",
1186 i, candidates.len(), candidates[i]);
1187 candidates.swap_remove(i);
1189 debug!("Retaining candidate #{}/{}: {:?}",
1190 i, candidates.len(), candidates[i]);
1193 // If there are *STILL* multiple candidates, give up
1194 // and report ambiguity.
1196 debug!("multiple matches, ambig");
1203 // If there are *NO* candidates, then there are no impls --
1204 // that we know of, anyway. Note that in the case where there
1205 // are unbound type variables within the obligation, it might
1206 // be the case that you could still satisfy the obligation
1207 // from another crate by instantiating the type variables with
1208 // a type from another crate that does have an impl. This case
1209 // is checked for in `evaluate_stack` (and hence users
1210 // who might care about this case, like coherence, should use
1212 if candidates.is_empty() {
1213 return Err(Unimplemented);
1216 // Just one candidate left.
1217 self.filter_negative_impls(candidates.pop().unwrap().candidate)
1220 fn is_knowable<'o>(&mut self,
1221 stack: &TraitObligationStack<'o, 'tcx>)
1224 debug!("is_knowable(intercrate={:?})", self.intercrate);
1226 if !self.intercrate.is_some() {
1230 let obligation = &stack.obligation;
1231 let predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1233 // ok to skip binder because of the nature of the
1234 // trait-ref-is-knowable check, which does not care about
1236 let trait_ref = predicate.skip_binder().trait_ref;
1238 let result = coherence::trait_ref_is_knowable(self.tcx(), trait_ref);
1239 if let (Some(Conflict::Downstream { used_to_be_broken: true }),
1240 Some(IntercrateMode::Issue43355)) = (result, self.intercrate) {
1241 debug!("is_knowable: IGNORING conflict to be bug-compatible with #43355");
1248 /// Returns true if the global caches can be used.
1249 /// Do note that if the type itself is not in the
1250 /// global tcx, the local caches will be used.
1251 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1252 // If there are any where-clauses in scope, then we always use
1253 // a cache local to this particular scope. Otherwise, we
1254 // switch to a global cache. We used to try and draw
1255 // finer-grained distinctions, but that led to a serious of
1256 // annoying and weird bugs like #22019 and #18290. This simple
1257 // rule seems to be pretty clearly safe and also still retains
1258 // a very high hit rate (~95% when compiling rustc).
1259 if !param_env.caller_bounds.is_empty() {
1263 // Avoid using the master cache during coherence and just rely
1264 // on the local cache. This effectively disables caching
1265 // during coherence. It is really just a simplification to
1266 // avoid us having to fear that coherence results "pollute"
1267 // the master cache. Since coherence executes pretty quickly,
1268 // it's not worth going to more trouble to increase the
1269 // hit-rate I don't think.
1270 if self.intercrate.is_some() {
1274 // Otherwise, we can use the global cache.
1278 fn check_candidate_cache(&mut self,
1279 param_env: ty::ParamEnv<'tcx>,
1280 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
1281 -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
1283 let tcx = self.tcx();
1284 let trait_ref = &cache_fresh_trait_pred.0.trait_ref;
1285 if self.can_use_global_caches(param_env) {
1286 let cache = tcx.selection_cache.hashmap.borrow();
1287 if let Some(cached) = cache.get(&trait_ref) {
1288 return Some(cached.get(tcx));
1291 self.infcx.selection_cache.hashmap
1294 .map(|v| v.get(tcx))
1297 fn insert_candidate_cache(&mut self,
1298 param_env: ty::ParamEnv<'tcx>,
1299 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1300 dep_node: DepNodeIndex,
1301 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
1303 let tcx = self.tcx();
1304 let trait_ref = cache_fresh_trait_pred.0.trait_ref;
1305 if self.can_use_global_caches(param_env) {
1306 let mut cache = tcx.selection_cache.hashmap.borrow_mut();
1307 if let Some(trait_ref) = tcx.lift_to_global(&trait_ref) {
1308 if let Some(candidate) = tcx.lift_to_global(&candidate) {
1309 cache.insert(trait_ref, WithDepNode::new(dep_node, candidate));
1315 self.infcx.selection_cache.hashmap
1317 .insert(trait_ref, WithDepNode::new(dep_node, candidate));
1320 fn assemble_candidates<'o>(&mut self,
1321 stack: &TraitObligationStack<'o, 'tcx>)
1322 -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
1324 let TraitObligationStack { obligation, .. } = *stack;
1325 let ref obligation = Obligation {
1326 param_env: obligation.param_env,
1327 cause: obligation.cause.clone(),
1328 recursion_depth: obligation.recursion_depth,
1329 predicate: self.infcx().resolve_type_vars_if_possible(&obligation.predicate)
1332 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1333 // Self is a type variable (e.g. `_: AsRef<str>`).
1335 // This is somewhat problematic, as the current scheme can't really
1336 // handle it turning to be a projection. This does end up as truly
1337 // ambiguous in most cases anyway.
1339 // Take the fast path out - this also improves
1340 // performance by preventing assemble_candidates_from_impls from
1341 // matching every impl for this trait.
1342 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
1345 let mut candidates = SelectionCandidateSet {
1350 // Other bounds. Consider both in-scope bounds from fn decl
1351 // and applicable impls. There is a certain set of precedence rules here.
1353 let def_id = obligation.predicate.def_id();
1354 let lang_items = self.tcx().lang_items();
1355 if lang_items.copy_trait() == Some(def_id) {
1356 debug!("obligation self ty is {:?}",
1357 obligation.predicate.0.self_ty());
1359 // User-defined copy impls are permitted, but only for
1360 // structs and enums.
1361 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1363 // For other types, we'll use the builtin rules.
1364 let copy_conditions = self.copy_clone_conditions(obligation);
1365 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1366 } else if lang_items.sized_trait() == Some(def_id) {
1367 // Sized is never implementable by end-users, it is
1368 // always automatically computed.
1369 let sized_conditions = self.sized_conditions(obligation);
1370 self.assemble_builtin_bound_candidates(sized_conditions,
1372 } else if lang_items.unsize_trait() == Some(def_id) {
1373 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1375 if lang_items.clone_trait() == Some(def_id) {
1376 // Same builtin conditions as `Copy`, i.e. every type which has builtin support
1377 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
1378 // types have builtin support for `Clone`.
1379 let clone_conditions = self.copy_clone_conditions(obligation);
1380 self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
1383 self.assemble_generator_candidates(obligation, &mut candidates)?;
1384 self.assemble_closure_candidates(obligation, &mut candidates)?;
1385 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1386 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1387 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1390 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1391 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1392 // Auto implementations have lower priority, so we only
1393 // consider triggering a default if there is no other impl that can apply.
1394 if candidates.vec.is_empty() {
1395 self.assemble_candidates_from_auto_impls(obligation, &mut candidates)?;
1397 debug!("candidate list size: {}", candidates.vec.len());
1401 fn assemble_candidates_from_projected_tys(&mut self,
1402 obligation: &TraitObligation<'tcx>,
1403 candidates: &mut SelectionCandidateSet<'tcx>)
1405 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1407 // before we go into the whole skolemization thing, just
1408 // quickly check if the self-type is a projection at all.
1409 match obligation.predicate.0.trait_ref.self_ty().sty {
1410 ty::TyProjection(_) | ty::TyAnon(..) => {}
1411 ty::TyInfer(ty::TyVar(_)) => {
1412 span_bug!(obligation.cause.span,
1413 "Self=_ should have been handled by assemble_candidates");
1418 let result = self.probe(|this, snapshot| {
1419 this.match_projection_obligation_against_definition_bounds(obligation,
1424 candidates.vec.push(ProjectionCandidate);
1428 fn match_projection_obligation_against_definition_bounds(
1430 obligation: &TraitObligation<'tcx>,
1431 snapshot: &infer::CombinedSnapshot)
1434 let poly_trait_predicate =
1435 self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1436 let (skol_trait_predicate, skol_map) =
1437 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot);
1438 debug!("match_projection_obligation_against_definition_bounds: \
1439 skol_trait_predicate={:?} skol_map={:?}",
1440 skol_trait_predicate,
1443 let (def_id, substs) = match skol_trait_predicate.trait_ref.self_ty().sty {
1444 ty::TyProjection(ref data) =>
1445 (data.trait_ref(self.tcx()).def_id, data.substs),
1446 ty::TyAnon(def_id, substs) => (def_id, substs),
1449 obligation.cause.span,
1450 "match_projection_obligation_against_definition_bounds() called \
1451 but self-ty not a projection: {:?}",
1452 skol_trait_predicate.trait_ref.self_ty());
1455 debug!("match_projection_obligation_against_definition_bounds: \
1456 def_id={:?}, substs={:?}",
1459 let predicates_of = self.tcx().predicates_of(def_id);
1460 let bounds = predicates_of.instantiate(self.tcx(), substs);
1461 debug!("match_projection_obligation_against_definition_bounds: \
1465 let matching_bound =
1466 util::elaborate_predicates(self.tcx(), bounds.predicates)
1470 |this, _| this.match_projection(obligation,
1472 skol_trait_predicate.trait_ref.clone(),
1476 debug!("match_projection_obligation_against_definition_bounds: \
1477 matching_bound={:?}",
1479 match matching_bound {
1482 // Repeat the successful match, if any, this time outside of a probe.
1483 let result = self.match_projection(obligation,
1485 skol_trait_predicate.trait_ref.clone(),
1489 self.infcx.pop_skolemized(skol_map, snapshot);
1497 fn match_projection(&mut self,
1498 obligation: &TraitObligation<'tcx>,
1499 trait_bound: ty::PolyTraitRef<'tcx>,
1500 skol_trait_ref: ty::TraitRef<'tcx>,
1501 skol_map: &infer::SkolemizationMap<'tcx>,
1502 snapshot: &infer::CombinedSnapshot)
1505 assert!(!skol_trait_ref.has_escaping_regions());
1506 match self.infcx.at(&obligation.cause, obligation.param_env)
1507 .sup(ty::Binder(skol_trait_ref), trait_bound) {
1508 Ok(InferOk { obligations, .. }) => {
1509 self.inferred_obligations.extend(obligations);
1511 Err(_) => { return false; }
1514 self.infcx.leak_check(false, obligation.cause.span, skol_map, snapshot).is_ok()
1517 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1518 /// supplied to find out whether it is listed among them.
1520 /// Never affects inference environment.
1521 fn assemble_candidates_from_caller_bounds<'o>(&mut self,
1522 stack: &TraitObligationStack<'o, 'tcx>,
1523 candidates: &mut SelectionCandidateSet<'tcx>)
1524 -> Result<(),SelectionError<'tcx>>
1526 debug!("assemble_candidates_from_caller_bounds({:?})",
1530 stack.obligation.param_env.caller_bounds
1532 .filter_map(|o| o.to_opt_poly_trait_ref());
1534 // micro-optimization: filter out predicates relating to different
1536 let matching_bounds =
1537 all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
1539 let matching_bounds =
1540 matching_bounds.filter(
1541 |bound| self.evaluate_where_clause(stack, bound.clone()).may_apply());
1543 let param_candidates =
1544 matching_bounds.map(|bound| ParamCandidate(bound));
1546 candidates.vec.extend(param_candidates);
1551 fn evaluate_where_clause<'o>(&mut self,
1552 stack: &TraitObligationStack<'o, 'tcx>,
1553 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
1556 self.probe(move |this, _| {
1557 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1558 Ok(obligations) => {
1559 this.evaluate_predicates_recursively(stack.list(), obligations.iter())
1561 Err(()) => EvaluatedToErr
1566 fn assemble_generator_candidates(&mut self,
1567 obligation: &TraitObligation<'tcx>,
1568 candidates: &mut SelectionCandidateSet<'tcx>)
1569 -> Result<(),SelectionError<'tcx>>
1571 if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
1575 // ok to skip binder because the substs on generator types never
1576 // touch bound regions, they just capture the in-scope
1577 // type/region parameters
1578 let self_ty = *obligation.self_ty().skip_binder();
1580 ty::TyGenerator(..) => {
1581 debug!("assemble_generator_candidates: self_ty={:?} obligation={:?}",
1585 candidates.vec.push(GeneratorCandidate);
1588 ty::TyInfer(ty::TyVar(_)) => {
1589 debug!("assemble_generator_candidates: ambiguous self-type");
1590 candidates.ambiguous = true;
1593 _ => { return Ok(()); }
1597 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1598 /// FnMut<..>` where `X` is a closure type.
1600 /// Note: the type parameters on a closure candidate are modeled as *output* type
1601 /// parameters and hence do not affect whether this trait is a match or not. They will be
1602 /// unified during the confirmation step.
1603 fn assemble_closure_candidates(&mut self,
1604 obligation: &TraitObligation<'tcx>,
1605 candidates: &mut SelectionCandidateSet<'tcx>)
1606 -> Result<(),SelectionError<'tcx>>
1608 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.0.def_id()) {
1610 None => { return Ok(()); }
1613 // ok to skip binder because the substs on closure types never
1614 // touch bound regions, they just capture the in-scope
1615 // type/region parameters
1616 match obligation.self_ty().skip_binder().sty {
1617 ty::TyClosure(closure_def_id, closure_substs) => {
1618 debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}",
1620 match self.infcx.closure_kind(closure_def_id, closure_substs) {
1621 Some(closure_kind) => {
1622 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
1623 if closure_kind.extends(kind) {
1624 candidates.vec.push(ClosureCandidate);
1628 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1629 candidates.vec.push(ClosureCandidate);
1634 ty::TyInfer(ty::TyVar(_)) => {
1635 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1636 candidates.ambiguous = true;
1639 _ => { return Ok(()); }
1643 /// Implement one of the `Fn()` family for a fn pointer.
1644 fn assemble_fn_pointer_candidates(&mut self,
1645 obligation: &TraitObligation<'tcx>,
1646 candidates: &mut SelectionCandidateSet<'tcx>)
1647 -> Result<(),SelectionError<'tcx>>
1649 // We provide impl of all fn traits for fn pointers.
1650 if self.tcx().lang_items().fn_trait_kind(obligation.predicate.def_id()).is_none() {
1654 // ok to skip binder because what we are inspecting doesn't involve bound regions
1655 let self_ty = *obligation.self_ty().skip_binder();
1657 ty::TyInfer(ty::TyVar(_)) => {
1658 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1659 candidates.ambiguous = true; // could wind up being a fn() type
1662 // provide an impl, but only for suitable `fn` pointers
1663 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
1664 if let ty::Binder(ty::FnSig {
1665 unsafety: hir::Unsafety::Normal,
1669 }) = self_ty.fn_sig(self.tcx()) {
1670 candidates.vec.push(FnPointerCandidate);
1680 /// Search for impls that might apply to `obligation`.
1681 fn assemble_candidates_from_impls(&mut self,
1682 obligation: &TraitObligation<'tcx>,
1683 candidates: &mut SelectionCandidateSet<'tcx>)
1684 -> Result<(), SelectionError<'tcx>>
1686 debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
1688 self.tcx().for_each_relevant_impl(
1689 obligation.predicate.def_id(),
1690 obligation.predicate.0.trait_ref.self_ty(),
1692 self.probe(|this, snapshot| { /* [1] */
1693 match this.match_impl(impl_def_id, obligation, snapshot) {
1695 candidates.vec.push(ImplCandidate(impl_def_id));
1697 // NB: we can safely drop the skol map
1698 // since we are in a probe [1]
1699 mem::drop(skol_map);
1710 fn assemble_candidates_from_auto_impls(&mut self,
1711 obligation: &TraitObligation<'tcx>,
1712 candidates: &mut SelectionCandidateSet<'tcx>)
1713 -> Result<(), SelectionError<'tcx>>
1715 // OK to skip binder here because the tests we do below do not involve bound regions
1716 let self_ty = *obligation.self_ty().skip_binder();
1717 debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
1719 let def_id = obligation.predicate.def_id();
1721 if self.tcx().trait_is_auto(def_id) {
1723 ty::TyDynamic(..) => {
1724 // For object types, we don't know what the closed
1725 // over types are. This means we conservatively
1726 // say nothing; a candidate may be added by
1727 // `assemble_candidates_from_object_ty`.
1729 ty::TyForeign(..) => {
1730 // Since the contents of foreign types is unknown,
1731 // we don't add any `..` impl. Default traits could
1732 // still be provided by a manual implementation for
1733 // this trait and type.
1736 ty::TyProjection(..) => {
1737 // In these cases, we don't know what the actual
1738 // type is. Therefore, we cannot break it down
1739 // into its constituent types. So we don't
1740 // consider the `..` impl but instead just add no
1741 // candidates: this means that typeck will only
1742 // succeed if there is another reason to believe
1743 // that this obligation holds. That could be a
1744 // where-clause or, in the case of an object type,
1745 // it could be that the object type lists the
1746 // trait (e.g. `Foo+Send : Send`). See
1747 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1748 // for an example of a test case that exercises
1751 ty::TyInfer(ty::TyVar(_)) => {
1752 // the auto impl might apply, we don't know
1753 candidates.ambiguous = true;
1756 candidates.vec.push(AutoImplCandidate(def_id.clone()))
1764 /// Search for impls that might apply to `obligation`.
1765 fn assemble_candidates_from_object_ty(&mut self,
1766 obligation: &TraitObligation<'tcx>,
1767 candidates: &mut SelectionCandidateSet<'tcx>)
1769 debug!("assemble_candidates_from_object_ty(self_ty={:?})",
1770 obligation.self_ty().skip_binder());
1772 // Object-safety candidates are only applicable to object-safe
1773 // traits. Including this check is useful because it helps
1774 // inference in cases of traits like `BorrowFrom`, which are
1775 // not object-safe, and which rely on being able to infer the
1776 // self-type from one of the other inputs. Without this check,
1777 // these cases wind up being considered ambiguous due to a
1778 // (spurious) ambiguity introduced here.
1779 let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
1780 if !self.tcx().is_object_safe(predicate_trait_ref.def_id()) {
1784 self.probe(|this, _snapshot| {
1785 // the code below doesn't care about regions, and the
1786 // self-ty here doesn't escape this probe, so just erase
1788 let self_ty = this.tcx().erase_late_bound_regions(&obligation.self_ty());
1789 let poly_trait_ref = match self_ty.sty {
1790 ty::TyDynamic(ref data, ..) => {
1791 if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
1792 debug!("assemble_candidates_from_object_ty: matched builtin bound, \
1793 pushing candidate");
1794 candidates.vec.push(BuiltinObjectCandidate);
1798 match data.principal() {
1799 Some(p) => p.with_self_ty(this.tcx(), self_ty),
1803 ty::TyInfer(ty::TyVar(_)) => {
1804 debug!("assemble_candidates_from_object_ty: ambiguous");
1805 candidates.ambiguous = true; // could wind up being an object type
1813 debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1816 // Count only those upcast versions that match the trait-ref
1817 // we are looking for. Specifically, do not only check for the
1818 // correct trait, but also the correct type parameters.
1819 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1820 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1821 let upcast_trait_refs =
1822 util::supertraits(this.tcx(), poly_trait_ref)
1823 .filter(|upcast_trait_ref| {
1824 this.probe(|this, _| {
1825 let upcast_trait_ref = upcast_trait_ref.clone();
1826 this.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok()
1831 if upcast_trait_refs > 1 {
1832 // can be upcast in many ways; need more type information
1833 candidates.ambiguous = true;
1834 } else if upcast_trait_refs == 1 {
1835 candidates.vec.push(ObjectCandidate);
1840 /// Search for unsizing that might apply to `obligation`.
1841 fn assemble_candidates_for_unsizing(&mut self,
1842 obligation: &TraitObligation<'tcx>,
1843 candidates: &mut SelectionCandidateSet<'tcx>) {
1844 // We currently never consider higher-ranked obligations e.g.
1845 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1846 // because they are a priori invalid, and we could potentially add support
1847 // for them later, it's just that there isn't really a strong need for it.
1848 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1849 // impl, and those are generally applied to concrete types.
1851 // That said, one might try to write a fn with a where clause like
1852 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1853 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1854 // Still, you'd be more likely to write that where clause as
1856 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1857 // obligation above. Should be possible to extend this in the future.
1858 let source = match obligation.self_ty().no_late_bound_regions() {
1861 // Don't add any candidates if there are bound regions.
1865 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
1867 debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1870 let may_apply = match (&source.sty, &target.sty) {
1871 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1872 (&ty::TyDynamic(ref data_a, ..), &ty::TyDynamic(ref data_b, ..)) => {
1873 // Upcasts permit two things:
1875 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1876 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1878 // Note that neither of these changes requires any
1879 // change at runtime. Eventually this will be
1882 // We always upcast when we can because of reason
1883 // #2 (region bounds).
1884 match (data_a.principal(), data_b.principal()) {
1885 (Some(a), Some(b)) => a.def_id() == b.def_id() &&
1886 data_b.auto_traits()
1887 // All of a's auto traits need to be in b's auto traits.
1888 .all(|b| data_a.auto_traits().any(|a| a == b)),
1894 (_, &ty::TyDynamic(..)) => true,
1896 // Ambiguous handling is below T -> Trait, because inference
1897 // variables can still implement Unsize<Trait> and nested
1898 // obligations will have the final say (likely deferred).
1899 (&ty::TyInfer(ty::TyVar(_)), _) |
1900 (_, &ty::TyInfer(ty::TyVar(_))) => {
1901 debug!("assemble_candidates_for_unsizing: ambiguous");
1902 candidates.ambiguous = true;
1907 (&ty::TyArray(..), &ty::TySlice(_)) => true,
1909 // Struct<T> -> Struct<U>.
1910 (&ty::TyAdt(def_id_a, _), &ty::TyAdt(def_id_b, _)) if def_id_a.is_struct() => {
1911 def_id_a == def_id_b
1914 // (.., T) -> (.., U).
1915 (&ty::TyTuple(tys_a, _), &ty::TyTuple(tys_b, _)) => {
1916 tys_a.len() == tys_b.len()
1923 candidates.vec.push(BuiltinUnsizeCandidate);
1927 ///////////////////////////////////////////////////////////////////////////
1930 // Winnowing is the process of attempting to resolve ambiguity by
1931 // probing further. During the winnowing process, we unify all
1932 // type variables (ignoring skolemization) and then we also
1933 // attempt to evaluate recursive bounds to see if they are
1936 /// Returns true if `candidate_i` should be dropped in favor of
1937 /// `candidate_j`. Generally speaking we will drop duplicate
1938 /// candidates and prefer where-clause candidates.
1939 /// Returns true if `victim` should be dropped in favor of
1940 /// `other`. Generally speaking we will drop duplicate
1941 /// candidates and prefer where-clause candidates.
1943 /// See the comment for "SelectionCandidate" for more details.
1944 fn candidate_should_be_dropped_in_favor_of<'o>(
1946 victim: &EvaluatedCandidate<'tcx>,
1947 other: &EvaluatedCandidate<'tcx>)
1950 if victim.candidate == other.candidate {
1954 match other.candidate {
1956 ParamCandidate(_) | ProjectionCandidate => match victim.candidate {
1957 AutoImplCandidate(..) => {
1959 "default implementations shouldn't be recorded \
1960 when there are other valid candidates");
1964 GeneratorCandidate |
1965 FnPointerCandidate |
1966 BuiltinObjectCandidate |
1967 BuiltinUnsizeCandidate |
1968 BuiltinCandidate { .. } => {
1969 // We have a where-clause so don't go around looking
1974 ProjectionCandidate => {
1975 // Arbitrarily give param candidates priority
1976 // over projection and object candidates.
1979 ParamCandidate(..) => false,
1981 ImplCandidate(other_def) => {
1982 // See if we can toss out `victim` based on specialization.
1983 // This requires us to know *for sure* that the `other` impl applies
1984 // i.e. EvaluatedToOk:
1985 if other.evaluation == EvaluatedToOk {
1986 if let ImplCandidate(victim_def) = victim.candidate {
1987 let tcx = self.tcx().global_tcx();
1988 return tcx.specializes((other_def, victim_def)) ||
1989 tcx.impls_are_allowed_to_overlap(other_def, victim_def);
1999 ///////////////////////////////////////////////////////////////////////////
2002 // These cover the traits that are built-in to the language
2003 // itself. This includes `Copy` and `Sized` for sure. For the
2004 // moment, it also includes `Send` / `Sync` and a few others, but
2005 // those will hopefully change to library-defined traits in the
2008 // HACK: if this returns an error, selection exits without considering
2010 fn assemble_builtin_bound_candidates<'o>(&mut self,
2011 conditions: BuiltinImplConditions<'tcx>,
2012 candidates: &mut SelectionCandidateSet<'tcx>)
2013 -> Result<(),SelectionError<'tcx>>
2016 BuiltinImplConditions::Where(nested) => {
2017 debug!("builtin_bound: nested={:?}", nested);
2018 candidates.vec.push(BuiltinCandidate {
2019 has_nested: nested.skip_binder().len() > 0
2023 BuiltinImplConditions::None => { Ok(()) }
2024 BuiltinImplConditions::Ambiguous => {
2025 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
2026 Ok(candidates.ambiguous = true)
2028 BuiltinImplConditions::Never => { Err(Unimplemented) }
2032 fn sized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2033 -> BuiltinImplConditions<'tcx>
2035 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
2037 // NOTE: binder moved to (*)
2038 let self_ty = self.infcx.shallow_resolve(
2039 obligation.predicate.skip_binder().self_ty());
2042 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2043 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2044 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyRawPtr(..) |
2045 ty::TyChar | ty::TyRef(..) | ty::TyGenerator(..) |
2046 ty::TyArray(..) | ty::TyClosure(..) | ty::TyNever |
2048 // safe for everything
2049 Where(ty::Binder(Vec::new()))
2052 ty::TyStr | ty::TySlice(_) | ty::TyDynamic(..) | ty::TyForeign(..) => Never,
2054 ty::TyTuple(tys, _) => {
2055 Where(ty::Binder(tys.last().into_iter().cloned().collect()))
2058 ty::TyAdt(def, substs) => {
2059 let sized_crit = def.sized_constraint(self.tcx());
2060 // (*) binder moved here
2062 sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect()
2066 ty::TyProjection(_) | ty::TyParam(_) | ty::TyAnon(..) => None,
2067 ty::TyInfer(ty::TyVar(_)) => Ambiguous,
2069 ty::TyInfer(ty::FreshTy(_))
2070 | ty::TyInfer(ty::FreshIntTy(_))
2071 | ty::TyInfer(ty::FreshFloatTy(_)) => {
2072 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2078 fn copy_clone_conditions(&mut self, obligation: &TraitObligation<'tcx>)
2079 -> BuiltinImplConditions<'tcx>
2081 // NOTE: binder moved to (*)
2082 let self_ty = self.infcx.shallow_resolve(
2083 obligation.predicate.skip_binder().self_ty());
2085 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
2088 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
2089 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
2090 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyChar |
2091 ty::TyRawPtr(..) | ty::TyError | ty::TyNever |
2092 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
2093 Where(ty::Binder(Vec::new()))
2096 ty::TyDynamic(..) | ty::TyStr | ty::TySlice(..) |
2097 ty::TyGenerator(..) | ty::TyForeign(..) |
2098 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
2102 ty::TyArray(element_ty, _) => {
2103 // (*) binder moved here
2104 Where(ty::Binder(vec![element_ty]))
2107 ty::TyTuple(tys, _) => {
2108 // (*) binder moved here
2109 Where(ty::Binder(tys.to_vec()))
2112 ty::TyClosure(def_id, substs) => {
2113 let trait_id = obligation.predicate.def_id();
2115 Some(trait_id) == self.tcx().lang_items().copy_trait() &&
2116 self.tcx().has_copy_closures(def_id.krate);
2117 let clone_closures =
2118 Some(trait_id) == self.tcx().lang_items().clone_trait() &&
2119 self.tcx().has_clone_closures(def_id.krate);
2121 if copy_closures || clone_closures {
2122 Where(ty::Binder(substs.upvar_tys(def_id, self.tcx()).collect()))
2128 ty::TyAdt(..) | ty::TyProjection(..) | ty::TyParam(..) | ty::TyAnon(..) => {
2129 // Fallback to whatever user-defined impls exist in this case.
2133 ty::TyInfer(ty::TyVar(_)) => {
2134 // Unbound type variable. Might or might not have
2135 // applicable impls and so forth, depending on what
2136 // those type variables wind up being bound to.
2140 ty::TyInfer(ty::FreshTy(_))
2141 | ty::TyInfer(ty::FreshIntTy(_))
2142 | ty::TyInfer(ty::FreshFloatTy(_)) => {
2143 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
2149 /// For default impls, we need to break apart a type into its
2150 /// "constituent types" -- meaning, the types that it contains.
2152 /// Here are some (simple) examples:
2155 /// (i32, u32) -> [i32, u32]
2156 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2157 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2158 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2160 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
2170 ty::TyInfer(ty::IntVar(_)) |
2171 ty::TyInfer(ty::FloatVar(_)) |
2180 ty::TyProjection(..) |
2181 ty::TyInfer(ty::TyVar(_)) |
2182 ty::TyInfer(ty::FreshTy(_)) |
2183 ty::TyInfer(ty::FreshIntTy(_)) |
2184 ty::TyInfer(ty::FreshFloatTy(_)) => {
2185 bug!("asked to assemble constituent types of unexpected type: {:?}",
2189 ty::TyRawPtr(ty::TypeAndMut { ty: element_ty, ..}) |
2190 ty::TyRef(_, ty::TypeAndMut { ty: element_ty, ..}) => {
2194 ty::TyArray(element_ty, _) | ty::TySlice(element_ty) => {
2198 ty::TyTuple(ref tys, _) => {
2199 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2203 ty::TyClosure(def_id, ref substs) => {
2204 substs.upvar_tys(def_id, self.tcx()).collect()
2207 ty::TyGenerator(def_id, ref substs, interior) => {
2208 let witness = iter::once(interior.witness);
2209 substs.upvar_tys(def_id, self.tcx()).chain(witness).collect()
2212 // for `PhantomData<T>`, we pass `T`
2213 ty::TyAdt(def, substs) if def.is_phantom_data() => {
2214 substs.types().collect()
2217 ty::TyAdt(def, substs) => {
2219 .map(|f| f.ty(self.tcx(), substs))
2223 ty::TyAnon(def_id, substs) => {
2224 // We can resolve the `impl Trait` to its concrete type,
2225 // which enforces a DAG between the functions requiring
2226 // the auto trait bounds in question.
2227 vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
2232 fn collect_predicates_for_types(&mut self,
2233 param_env: ty::ParamEnv<'tcx>,
2234 cause: ObligationCause<'tcx>,
2235 recursion_depth: usize,
2236 trait_def_id: DefId,
2237 types: ty::Binder<Vec<Ty<'tcx>>>)
2238 -> Vec<PredicateObligation<'tcx>>
2240 // Because the types were potentially derived from
2241 // higher-ranked obligations they may reference late-bound
2242 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
2243 // yield a type like `for<'a> &'a int`. In general, we
2244 // maintain the invariant that we never manipulate bound
2245 // regions, so we have to process these bound regions somehow.
2247 // The strategy is to:
2249 // 1. Instantiate those regions to skolemized regions (e.g.,
2250 // `for<'a> &'a int` becomes `&0 int`.
2251 // 2. Produce something like `&'0 int : Copy`
2252 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
2254 types.skip_binder().into_iter().flat_map(|ty| { // binder moved -\
2255 let ty: ty::Binder<Ty<'tcx>> = ty::Binder(ty); // <----------/
2257 self.in_snapshot(|this, snapshot| {
2258 let (skol_ty, skol_map) =
2259 this.infcx().skolemize_late_bound_regions(&ty, snapshot);
2260 let Normalized { value: normalized_ty, mut obligations } =
2261 project::normalize_with_depth(this,
2266 let skol_obligation =
2267 this.tcx().predicate_for_trait_def(param_env,
2273 obligations.push(skol_obligation);
2274 this.infcx().plug_leaks(skol_map, snapshot, obligations)
2279 ///////////////////////////////////////////////////////////////////////////
2282 // Confirmation unifies the output type parameters of the trait
2283 // with the values found in the obligation, possibly yielding a
2284 // type error. See `README.md` for more details.
2286 fn confirm_candidate(&mut self,
2287 obligation: &TraitObligation<'tcx>,
2288 candidate: SelectionCandidate<'tcx>)
2289 -> Result<Selection<'tcx>,SelectionError<'tcx>>
2291 debug!("confirm_candidate({:?}, {:?})",
2296 BuiltinCandidate { has_nested } => {
2297 let data = self.confirm_builtin_candidate(obligation, has_nested);
2298 Ok(VtableBuiltin(data))
2301 ParamCandidate(param) => {
2302 let obligations = self.confirm_param_candidate(obligation, param);
2303 Ok(VtableParam(obligations))
2306 AutoImplCandidate(trait_def_id) => {
2307 let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
2308 Ok(VtableAutoImpl(data))
2311 ImplCandidate(impl_def_id) => {
2312 Ok(VtableImpl(self.confirm_impl_candidate(obligation, impl_def_id)))
2315 ClosureCandidate => {
2316 let vtable_closure = self.confirm_closure_candidate(obligation)?;
2317 Ok(VtableClosure(vtable_closure))
2320 GeneratorCandidate => {
2321 let vtable_generator = self.confirm_generator_candidate(obligation)?;
2322 Ok(VtableGenerator(vtable_generator))
2325 BuiltinObjectCandidate => {
2326 // This indicates something like `(Trait+Send) :
2327 // Send`. In this case, we know that this holds
2328 // because that's what the object type is telling us,
2329 // and there's really no additional obligations to
2330 // prove and no types in particular to unify etc.
2331 Ok(VtableParam(Vec::new()))
2334 ObjectCandidate => {
2335 let data = self.confirm_object_candidate(obligation);
2336 Ok(VtableObject(data))
2339 FnPointerCandidate => {
2341 self.confirm_fn_pointer_candidate(obligation)?;
2342 Ok(VtableFnPointer(data))
2345 ProjectionCandidate => {
2346 self.confirm_projection_candidate(obligation);
2347 Ok(VtableParam(Vec::new()))
2350 BuiltinUnsizeCandidate => {
2351 let data = self.confirm_builtin_unsize_candidate(obligation)?;
2352 Ok(VtableBuiltin(data))
2357 fn confirm_projection_candidate(&mut self,
2358 obligation: &TraitObligation<'tcx>)
2360 self.in_snapshot(|this, snapshot| {
2362 this.match_projection_obligation_against_definition_bounds(obligation,
2368 fn confirm_param_candidate(&mut self,
2369 obligation: &TraitObligation<'tcx>,
2370 param: ty::PolyTraitRef<'tcx>)
2371 -> Vec<PredicateObligation<'tcx>>
2373 debug!("confirm_param_candidate({:?},{:?})",
2377 // During evaluation, we already checked that this
2378 // where-clause trait-ref could be unified with the obligation
2379 // trait-ref. Repeat that unification now without any
2380 // transactional boundary; it should not fail.
2381 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2382 Ok(obligations) => obligations,
2384 bug!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2391 fn confirm_builtin_candidate(&mut self,
2392 obligation: &TraitObligation<'tcx>,
2394 -> VtableBuiltinData<PredicateObligation<'tcx>>
2396 debug!("confirm_builtin_candidate({:?}, {:?})",
2397 obligation, has_nested);
2399 let lang_items = self.tcx().lang_items();
2400 let obligations = if has_nested {
2401 let trait_def = obligation.predicate.def_id();
2402 let conditions = match trait_def {
2403 _ if Some(trait_def) == lang_items.sized_trait() => {
2404 self.sized_conditions(obligation)
2406 _ if Some(trait_def) == lang_items.copy_trait() => {
2407 self.copy_clone_conditions(obligation)
2409 _ if Some(trait_def) == lang_items.clone_trait() => {
2410 self.copy_clone_conditions(obligation)
2412 _ => bug!("unexpected builtin trait {:?}", trait_def)
2414 let nested = match conditions {
2415 BuiltinImplConditions::Where(nested) => nested,
2416 _ => bug!("obligation {:?} had matched a builtin impl but now doesn't",
2420 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2421 self.collect_predicates_for_types(obligation.param_env,
2423 obligation.recursion_depth+1,
2430 debug!("confirm_builtin_candidate: obligations={:?}",
2433 VtableBuiltinData { nested: obligations }
2436 /// This handles the case where a `impl Foo for ..` impl is being used.
2437 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2439 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2440 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2441 fn confirm_auto_impl_candidate(&mut self,
2442 obligation: &TraitObligation<'tcx>,
2443 trait_def_id: DefId)
2444 -> VtableAutoImplData<PredicateObligation<'tcx>>
2446 debug!("confirm_auto_impl_candidate({:?}, {:?})",
2450 // binder is moved below
2451 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2452 let types = self.constituent_types_for_ty(self_ty);
2453 self.vtable_auto_impl(obligation, trait_def_id, ty::Binder(types))
2456 /// See `confirm_auto_impl_candidate`
2457 fn vtable_auto_impl(&mut self,
2458 obligation: &TraitObligation<'tcx>,
2459 trait_def_id: DefId,
2460 nested: ty::Binder<Vec<Ty<'tcx>>>)
2461 -> VtableAutoImplData<PredicateObligation<'tcx>>
2463 debug!("vtable_auto_impl: nested={:?}", nested);
2465 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2466 let mut obligations = self.collect_predicates_for_types(
2467 obligation.param_env,
2469 obligation.recursion_depth+1,
2473 let trait_obligations = self.in_snapshot(|this, snapshot| {
2474 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2475 let (trait_ref, skol_map) =
2476 this.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot);
2477 let cause = obligation.derived_cause(ImplDerivedObligation);
2478 this.impl_or_trait_obligations(cause,
2479 obligation.recursion_depth + 1,
2480 obligation.param_env,
2487 obligations.extend(trait_obligations);
2489 debug!("vtable_auto_impl: obligations={:?}", obligations);
2491 VtableAutoImplData {
2497 fn confirm_impl_candidate(&mut self,
2498 obligation: &TraitObligation<'tcx>,
2500 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2502 debug!("confirm_impl_candidate({:?},{:?})",
2506 // First, create the substitutions by matching the impl again,
2507 // this time not in a probe.
2508 self.in_snapshot(|this, snapshot| {
2509 let (substs, skol_map) =
2510 this.rematch_impl(impl_def_id, obligation,
2512 debug!("confirm_impl_candidate substs={:?}", substs);
2513 let cause = obligation.derived_cause(ImplDerivedObligation);
2514 this.vtable_impl(impl_def_id,
2517 obligation.recursion_depth + 1,
2518 obligation.param_env,
2524 fn vtable_impl(&mut self,
2526 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2527 cause: ObligationCause<'tcx>,
2528 recursion_depth: usize,
2529 param_env: ty::ParamEnv<'tcx>,
2530 skol_map: infer::SkolemizationMap<'tcx>,
2531 snapshot: &infer::CombinedSnapshot)
2532 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2534 debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2540 let mut impl_obligations =
2541 self.impl_or_trait_obligations(cause,
2549 debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2553 // Because of RFC447, the impl-trait-ref and obligations
2554 // are sufficient to determine the impl substs, without
2555 // relying on projections in the impl-trait-ref.
2557 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2558 impl_obligations.append(&mut substs.obligations);
2560 VtableImplData { impl_def_id,
2561 substs: substs.value,
2562 nested: impl_obligations }
2565 fn confirm_object_candidate(&mut self,
2566 obligation: &TraitObligation<'tcx>)
2567 -> VtableObjectData<'tcx, PredicateObligation<'tcx>>
2569 debug!("confirm_object_candidate({:?})",
2572 // FIXME skipping binder here seems wrong -- we should
2573 // probably flatten the binder from the obligation and the
2574 // binder from the object. Have to try to make a broken test
2575 // case that results. -nmatsakis
2576 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2577 let poly_trait_ref = match self_ty.sty {
2578 ty::TyDynamic(ref data, ..) => {
2579 data.principal().unwrap().with_self_ty(self.tcx(), self_ty)
2582 span_bug!(obligation.cause.span,
2583 "object candidate with non-object");
2587 let mut upcast_trait_ref = None;
2591 let tcx = self.tcx();
2593 // We want to find the first supertrait in the list of
2594 // supertraits that we can unify with, and do that
2595 // unification. We know that there is exactly one in the list
2596 // where we can unify because otherwise select would have
2597 // reported an ambiguity. (When we do find a match, also
2598 // record it for later.)
2600 util::supertraits(tcx, poly_trait_ref)
2604 |this, _| this.match_poly_trait_ref(obligation, t))
2606 Ok(_) => { upcast_trait_ref = Some(t); false }
2611 // Additionally, for each of the nonmatching predicates that
2612 // we pass over, we sum up the set of number of vtable
2613 // entries, so that we can compute the offset for the selected
2616 nonmatching.map(|t| tcx.count_own_vtable_entries(t))
2622 upcast_trait_ref: upcast_trait_ref.unwrap(),
2628 fn confirm_fn_pointer_candidate(&mut self, obligation: &TraitObligation<'tcx>)
2629 -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
2631 debug!("confirm_fn_pointer_candidate({:?})",
2634 // ok to skip binder; it is reintroduced below
2635 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2636 let sig = self_ty.fn_sig(self.tcx());
2638 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2641 util::TupleArgumentsFlag::Yes)
2642 .map_bound(|(trait_ref, _)| trait_ref);
2644 let Normalized { value: trait_ref, obligations } =
2645 project::normalize_with_depth(self,
2646 obligation.param_env,
2647 obligation.cause.clone(),
2648 obligation.recursion_depth + 1,
2651 self.confirm_poly_trait_refs(obligation.cause.clone(),
2652 obligation.param_env,
2653 obligation.predicate.to_poly_trait_ref(),
2655 Ok(VtableFnPointerData { fn_ty: self_ty, nested: obligations })
2658 fn confirm_generator_candidate(&mut self,
2659 obligation: &TraitObligation<'tcx>)
2660 -> Result<VtableGeneratorData<'tcx, PredicateObligation<'tcx>>,
2661 SelectionError<'tcx>>
2663 // ok to skip binder because the substs on generator types never
2664 // touch bound regions, they just capture the in-scope
2665 // type/region parameters
2666 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2667 let (closure_def_id, substs) = match self_ty.sty {
2668 ty::TyGenerator(id, substs, _) => (id, substs),
2669 _ => bug!("closure candidate for non-closure {:?}", obligation)
2672 debug!("confirm_generator_candidate({:?},{:?},{:?})",
2678 self.generator_trait_ref_unnormalized(obligation, closure_def_id, substs);
2682 } = normalize_with_depth(self,
2683 obligation.param_env,
2684 obligation.cause.clone(),
2685 obligation.recursion_depth+1,
2688 debug!("confirm_generator_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2693 self.confirm_poly_trait_refs(obligation.cause.clone(),
2694 obligation.param_env,
2695 obligation.predicate.to_poly_trait_ref(),
2698 Ok(VtableGeneratorData {
2699 closure_def_id: closure_def_id,
2700 substs: substs.clone(),
2705 fn confirm_closure_candidate(&mut self,
2706 obligation: &TraitObligation<'tcx>)
2707 -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>,
2708 SelectionError<'tcx>>
2710 debug!("confirm_closure_candidate({:?})", obligation);
2712 let kind = match self.tcx().lang_items().fn_trait_kind(obligation.predicate.0.def_id()) {
2714 None => bug!("closure candidate for non-fn trait {:?}", obligation)
2717 // ok to skip binder because the substs on closure types never
2718 // touch bound regions, they just capture the in-scope
2719 // type/region parameters
2720 let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
2721 let (closure_def_id, substs) = match self_ty.sty {
2722 ty::TyClosure(id, substs) => (id, substs),
2723 _ => bug!("closure candidate for non-closure {:?}", obligation)
2727 self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs);
2731 } = normalize_with_depth(self,
2732 obligation.param_env,
2733 obligation.cause.clone(),
2734 obligation.recursion_depth+1,
2737 debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2742 self.confirm_poly_trait_refs(obligation.cause.clone(),
2743 obligation.param_env,
2744 obligation.predicate.to_poly_trait_ref(),
2747 obligations.push(Obligation::new(
2748 obligation.cause.clone(),
2749 obligation.param_env,
2750 ty::Predicate::ClosureKind(closure_def_id, substs, kind)));
2752 Ok(VtableClosureData {
2754 substs: substs.clone(),
2759 /// In the case of closure types and fn pointers,
2760 /// we currently treat the input type parameters on the trait as
2761 /// outputs. This means that when we have a match we have only
2762 /// considered the self type, so we have to go back and make sure
2763 /// to relate the argument types too. This is kind of wrong, but
2764 /// since we control the full set of impls, also not that wrong,
2765 /// and it DOES yield better error messages (since we don't report
2766 /// errors as if there is no applicable impl, but rather report
2767 /// errors are about mismatched argument types.
2769 /// Here is an example. Imagine we have a closure expression
2770 /// and we desugared it so that the type of the expression is
2771 /// `Closure`, and `Closure` expects an int as argument. Then it
2772 /// is "as if" the compiler generated this impl:
2774 /// impl Fn(int) for Closure { ... }
2776 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2777 /// we have matched the self-type `Closure`. At this point we'll
2778 /// compare the `int` to `usize` and generate an error.
2780 /// Note that this checking occurs *after* the impl has selected,
2781 /// because these output type parameters should not affect the
2782 /// selection of the impl. Therefore, if there is a mismatch, we
2783 /// report an error to the user.
2784 fn confirm_poly_trait_refs(&mut self,
2785 obligation_cause: ObligationCause<'tcx>,
2786 obligation_param_env: ty::ParamEnv<'tcx>,
2787 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
2788 expected_trait_ref: ty::PolyTraitRef<'tcx>)
2789 -> Result<(), SelectionError<'tcx>>
2791 let obligation_trait_ref = obligation_trait_ref.clone();
2793 .at(&obligation_cause, obligation_param_env)
2794 .sup(obligation_trait_ref, expected_trait_ref)
2795 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
2796 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
2799 fn confirm_builtin_unsize_candidate(&mut self,
2800 obligation: &TraitObligation<'tcx>,)
2801 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>>
2803 let tcx = self.tcx();
2805 // assemble_candidates_for_unsizing should ensure there are no late bound
2806 // regions here. See the comment there for more details.
2807 let source = self.infcx.shallow_resolve(
2808 obligation.self_ty().no_late_bound_regions().unwrap());
2809 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
2810 let target = self.infcx.shallow_resolve(target);
2812 debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2815 let mut nested = vec![];
2816 match (&source.sty, &target.sty) {
2817 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2818 (&ty::TyDynamic(ref data_a, r_a), &ty::TyDynamic(ref data_b, r_b)) => {
2819 // See assemble_candidates_for_unsizing for more info.
2820 // Binders reintroduced below in call to mk_existential_predicates.
2821 let principal = data_a.skip_binder().principal();
2822 let iter = principal.into_iter().map(ty::ExistentialPredicate::Trait)
2823 .chain(data_a.skip_binder().projection_bounds()
2824 .map(|x| ty::ExistentialPredicate::Projection(x)))
2825 .chain(data_b.auto_traits().map(ty::ExistentialPredicate::AutoTrait));
2826 let new_trait = tcx.mk_dynamic(
2827 ty::Binder(tcx.mk_existential_predicates(iter)), r_b);
2828 let InferOk { obligations, .. } =
2829 self.infcx.at(&obligation.cause, obligation.param_env)
2830 .eq(target, new_trait)
2831 .map_err(|_| Unimplemented)?;
2832 self.inferred_obligations.extend(obligations);
2834 // Register one obligation for 'a: 'b.
2835 let cause = ObligationCause::new(obligation.cause.span,
2836 obligation.cause.body_id,
2837 ObjectCastObligation(target));
2838 let outlives = ty::OutlivesPredicate(r_a, r_b);
2839 nested.push(Obligation::with_depth(cause,
2840 obligation.recursion_depth + 1,
2841 obligation.param_env,
2842 ty::Binder(outlives).to_predicate()));
2846 (_, &ty::TyDynamic(ref data, r)) => {
2847 let mut object_dids =
2848 data.auto_traits().chain(data.principal().map(|p| p.def_id()));
2849 if let Some(did) = object_dids.find(|did| {
2850 !tcx.is_object_safe(*did)
2852 return Err(TraitNotObjectSafe(did))
2855 let cause = ObligationCause::new(obligation.cause.span,
2856 obligation.cause.body_id,
2857 ObjectCastObligation(target));
2858 let mut push = |predicate| {
2859 nested.push(Obligation::with_depth(cause.clone(),
2860 obligation.recursion_depth + 1,
2861 obligation.param_env,
2865 // Create obligations:
2866 // - Casting T to Trait
2867 // - For all the various builtin bounds attached to the object cast. (In other
2868 // words, if the object type is Foo+Send, this would create an obligation for the
2870 // - Projection predicates
2871 for predicate in data.iter() {
2872 push(predicate.with_self_ty(tcx, source));
2875 // We can only make objects from sized types.
2876 let tr = ty::TraitRef {
2877 def_id: tcx.require_lang_item(lang_items::SizedTraitLangItem),
2878 substs: tcx.mk_substs_trait(source, &[]),
2880 push(tr.to_predicate());
2882 // If the type is `Foo+'a`, ensures that the type
2883 // being cast to `Foo+'a` outlives `'a`:
2884 let outlives = ty::OutlivesPredicate(source, r);
2885 push(ty::Binder(outlives).to_predicate());
2889 (&ty::TyArray(a, _), &ty::TySlice(b)) => {
2890 let InferOk { obligations, .. } =
2891 self.infcx.at(&obligation.cause, obligation.param_env)
2893 .map_err(|_| Unimplemented)?;
2894 self.inferred_obligations.extend(obligations);
2897 // Struct<T> -> Struct<U>.
2898 (&ty::TyAdt(def, substs_a), &ty::TyAdt(_, substs_b)) => {
2901 .map(|f| tcx.type_of(f.did))
2902 .collect::<Vec<_>>();
2904 // The last field of the structure has to exist and contain type parameters.
2905 let field = if let Some(&field) = fields.last() {
2908 return Err(Unimplemented);
2910 let mut ty_params = BitVector::new(substs_a.types().count());
2911 let mut found = false;
2912 for ty in field.walk() {
2913 if let ty::TyParam(p) = ty.sty {
2914 ty_params.insert(p.idx as usize);
2919 return Err(Unimplemented);
2922 // Replace type parameters used in unsizing with
2923 // TyError and ensure they do not affect any other fields.
2924 // This could be checked after type collection for any struct
2925 // with a potentially unsized trailing field.
2926 let params = substs_a.iter().enumerate().map(|(i, &k)| {
2927 if ty_params.contains(i) {
2928 Kind::from(tcx.types.err)
2933 let substs = tcx.mk_substs(params);
2934 for &ty in fields.split_last().unwrap().1 {
2935 if ty.subst(tcx, substs).references_error() {
2936 return Err(Unimplemented);
2940 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2941 let inner_source = field.subst(tcx, substs_a);
2942 let inner_target = field.subst(tcx, substs_b);
2944 // Check that the source struct with the target's
2945 // unsized parameters is equal to the target.
2946 let params = substs_a.iter().enumerate().map(|(i, &k)| {
2947 if ty_params.contains(i) {
2948 Kind::from(substs_b.type_at(i))
2953 let new_struct = tcx.mk_adt(def, tcx.mk_substs(params));
2954 let InferOk { obligations, .. } =
2955 self.infcx.at(&obligation.cause, obligation.param_env)
2956 .eq(target, new_struct)
2957 .map_err(|_| Unimplemented)?;
2958 self.inferred_obligations.extend(obligations);
2960 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2961 nested.push(tcx.predicate_for_trait_def(
2962 obligation.param_env,
2963 obligation.cause.clone(),
2964 obligation.predicate.def_id(),
2965 obligation.recursion_depth + 1,
2970 // (.., T) -> (.., U).
2971 (&ty::TyTuple(tys_a, _), &ty::TyTuple(tys_b, _)) => {
2972 assert_eq!(tys_a.len(), tys_b.len());
2974 // The last field of the tuple has to exist.
2975 let (a_last, a_mid) = if let Some(x) = tys_a.split_last() {
2978 return Err(Unimplemented);
2980 let b_last = tys_b.last().unwrap();
2982 // Check that the source tuple with the target's
2983 // last element is equal to the target.
2984 let new_tuple = tcx.mk_tup(a_mid.iter().chain(Some(b_last)), false);
2985 let InferOk { obligations, .. } =
2986 self.infcx.at(&obligation.cause, obligation.param_env)
2987 .eq(target, new_tuple)
2988 .map_err(|_| Unimplemented)?;
2989 self.inferred_obligations.extend(obligations);
2991 // Construct the nested T: Unsize<U> predicate.
2992 nested.push(tcx.predicate_for_trait_def(
2993 obligation.param_env,
2994 obligation.cause.clone(),
2995 obligation.predicate.def_id(),
2996 obligation.recursion_depth + 1,
3004 Ok(VtableBuiltinData { nested: nested })
3007 ///////////////////////////////////////////////////////////////////////////
3010 // Matching is a common path used for both evaluation and
3011 // confirmation. It basically unifies types that appear in impls
3012 // and traits. This does affect the surrounding environment;
3013 // therefore, when used during evaluation, match routines must be
3014 // run inside of a `probe()` so that their side-effects are
3017 fn rematch_impl(&mut self,
3019 obligation: &TraitObligation<'tcx>,
3020 snapshot: &infer::CombinedSnapshot)
3021 -> (Normalized<'tcx, &'tcx Substs<'tcx>>,
3022 infer::SkolemizationMap<'tcx>)
3024 match self.match_impl(impl_def_id, obligation, snapshot) {
3025 Ok((substs, skol_map)) => (substs, skol_map),
3027 bug!("Impl {:?} was matchable against {:?} but now is not",
3034 fn match_impl(&mut self,
3036 obligation: &TraitObligation<'tcx>,
3037 snapshot: &infer::CombinedSnapshot)
3038 -> Result<(Normalized<'tcx, &'tcx Substs<'tcx>>,
3039 infer::SkolemizationMap<'tcx>), ()>
3041 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
3043 // Before we create the substitutions and everything, first
3044 // consider a "quick reject". This avoids creating more types
3045 // and so forth that we need to.
3046 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
3050 let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions(
3051 &obligation.predicate,
3053 let skol_obligation_trait_ref = skol_obligation.trait_ref;
3055 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span,
3058 let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
3061 let impl_trait_ref =
3062 project::normalize_with_depth(self,
3063 obligation.param_env,
3064 obligation.cause.clone(),
3065 obligation.recursion_depth + 1,
3068 debug!("match_impl(impl_def_id={:?}, obligation={:?}, \
3069 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
3073 skol_obligation_trait_ref);
3075 let InferOk { obligations, .. } =
3076 self.infcx.at(&obligation.cause, obligation.param_env)
3077 .eq(skol_obligation_trait_ref, impl_trait_ref.value)
3079 debug!("match_impl: failed eq_trait_refs due to `{}`", e);
3082 self.inferred_obligations.extend(obligations);
3084 if let Err(e) = self.infcx.leak_check(false,
3085 obligation.cause.span,
3088 debug!("match_impl: failed leak check due to `{}`", e);
3092 debug!("match_impl: success impl_substs={:?}", impl_substs);
3095 obligations: impl_trait_ref.obligations
3099 fn fast_reject_trait_refs(&mut self,
3100 obligation: &TraitObligation,
3101 impl_trait_ref: &ty::TraitRef)
3104 // We can avoid creating type variables and doing the full
3105 // substitution if we find that any of the input types, when
3106 // simplified, do not match.
3108 obligation.predicate.skip_binder().input_types()
3109 .zip(impl_trait_ref.input_types())
3110 .any(|(obligation_ty, impl_ty)| {
3111 let simplified_obligation_ty =
3112 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
3113 let simplified_impl_ty =
3114 fast_reject::simplify_type(self.tcx(), impl_ty, false);
3116 simplified_obligation_ty.is_some() &&
3117 simplified_impl_ty.is_some() &&
3118 simplified_obligation_ty != simplified_impl_ty
3122 /// Normalize `where_clause_trait_ref` and try to match it against
3123 /// `obligation`. If successful, return any predicates that
3124 /// result from the normalization. Normalization is necessary
3125 /// because where-clauses are stored in the parameter environment
3127 fn match_where_clause_trait_ref(&mut self,
3128 obligation: &TraitObligation<'tcx>,
3129 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
3130 -> Result<Vec<PredicateObligation<'tcx>>,()>
3132 self.match_poly_trait_ref(obligation, where_clause_trait_ref)?;
3136 /// Returns `Ok` if `poly_trait_ref` being true implies that the
3137 /// obligation is satisfied.
3138 fn match_poly_trait_ref(&mut self,
3139 obligation: &TraitObligation<'tcx>,
3140 poly_trait_ref: ty::PolyTraitRef<'tcx>)
3143 debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
3147 self.infcx.at(&obligation.cause, obligation.param_env)
3148 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
3149 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
3153 ///////////////////////////////////////////////////////////////////////////
3156 fn match_fresh_trait_refs(&self,
3157 previous: &ty::PolyTraitRef<'tcx>,
3158 current: &ty::PolyTraitRef<'tcx>)
3161 let mut matcher = ty::_match::Match::new(self.tcx());
3162 matcher.relate(previous, current).is_ok()
3165 fn push_stack<'o,'s:'o>(&mut self,
3166 previous_stack: TraitObligationStackList<'s, 'tcx>,
3167 obligation: &'o TraitObligation<'tcx>)
3168 -> TraitObligationStack<'o, 'tcx>
3170 let fresh_trait_ref =
3171 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
3173 TraitObligationStack {
3176 previous: previous_stack,
3180 fn closure_trait_ref_unnormalized(&mut self,
3181 obligation: &TraitObligation<'tcx>,
3182 closure_def_id: DefId,
3183 substs: ty::ClosureSubsts<'tcx>)
3184 -> ty::PolyTraitRef<'tcx>
3186 let closure_type = self.infcx.fn_sig(closure_def_id)
3187 .subst(self.tcx(), substs.substs);
3188 let ty::Binder((trait_ref, _)) =
3189 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
3190 obligation.predicate.0.self_ty(), // (1)
3192 util::TupleArgumentsFlag::No);
3193 // (1) Feels icky to skip the binder here, but OTOH we know
3194 // that the self-type is an unboxed closure type and hence is
3195 // in fact unparameterized (or at least does not reference any
3196 // regions bound in the obligation). Still probably some
3197 // refactoring could make this nicer.
3199 ty::Binder(trait_ref)
3202 fn generator_trait_ref_unnormalized(&mut self,
3203 obligation: &TraitObligation<'tcx>,
3204 closure_def_id: DefId,
3205 substs: ty::ClosureSubsts<'tcx>)
3206 -> ty::PolyTraitRef<'tcx>
3208 let gen_sig = substs.generator_poly_sig(closure_def_id, self.tcx());
3209 let ty::Binder((trait_ref, ..)) =
3210 self.tcx().generator_trait_ref_and_outputs(obligation.predicate.def_id(),
3211 obligation.predicate.0.self_ty(), // (1)
3213 // (1) Feels icky to skip the binder here, but OTOH we know
3214 // that the self-type is an generator type and hence is
3215 // in fact unparameterized (or at least does not reference any
3216 // regions bound in the obligation). Still probably some
3217 // refactoring could make this nicer.
3219 ty::Binder(trait_ref)
3222 /// Returns the obligations that are implied by instantiating an
3223 /// impl or trait. The obligations are substituted and fully
3224 /// normalized. This is used when confirming an impl or default
3226 fn impl_or_trait_obligations(&mut self,
3227 cause: ObligationCause<'tcx>,
3228 recursion_depth: usize,
3229 param_env: ty::ParamEnv<'tcx>,
3230 def_id: DefId, // of impl or trait
3231 substs: &Substs<'tcx>, // for impl or trait
3232 skol_map: infer::SkolemizationMap<'tcx>,
3233 snapshot: &infer::CombinedSnapshot)
3234 -> Vec<PredicateObligation<'tcx>>
3236 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
3237 let tcx = self.tcx();
3239 // To allow for one-pass evaluation of the nested obligation,
3240 // each predicate must be preceded by the obligations required
3242 // for example, if we have:
3243 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
3244 // the impl will have the following predicates:
3245 // <V as Iterator>::Item = U,
3246 // U: Iterator, U: Sized,
3247 // V: Iterator, V: Sized,
3248 // <U as Iterator>::Item: Copy
3249 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
3250 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
3251 // `$1: Copy`, so we must ensure the obligations are emitted in
3253 let predicates = tcx.predicates_of(def_id);
3254 assert_eq!(predicates.parent, None);
3255 let predicates = predicates.predicates.iter().flat_map(|predicate| {
3256 let predicate = normalize_with_depth(self, param_env, cause.clone(), recursion_depth,
3257 &predicate.subst(tcx, substs));
3258 predicate.obligations.into_iter().chain(
3260 cause: cause.clone(),
3263 predicate: predicate.value
3266 self.infcx().plug_leaks(skol_map, snapshot, predicates)
3270 impl<'tcx> TraitObligation<'tcx> {
3271 #[allow(unused_comparisons)]
3272 pub fn derived_cause(&self,
3273 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
3274 -> ObligationCause<'tcx>
3277 * Creates a cause for obligations that are derived from
3278 * `obligation` by a recursive search (e.g., for a builtin
3279 * bound, or eventually a `impl Foo for ..`). If `obligation`
3280 * is itself a derived obligation, this is just a clone, but
3281 * otherwise we create a "derived obligation" cause so as to
3282 * keep track of the original root obligation for error
3286 let obligation = self;
3288 // NOTE(flaper87): As of now, it keeps track of the whole error
3289 // chain. Ideally, we should have a way to configure this either
3290 // by using -Z verbose or just a CLI argument.
3291 if obligation.recursion_depth >= 0 {
3292 let derived_cause = DerivedObligationCause {
3293 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
3294 parent_code: Rc::new(obligation.cause.code.clone())
3296 let derived_code = variant(derived_cause);
3297 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
3299 obligation.cause.clone()
3304 impl<'tcx> SelectionCache<'tcx> {
3305 pub fn new() -> SelectionCache<'tcx> {
3307 hashmap: RefCell::new(FxHashMap())
3312 impl<'tcx> EvaluationCache<'tcx> {
3313 pub fn new() -> EvaluationCache<'tcx> {
3315 hashmap: RefCell::new(FxHashMap())
3320 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {
3321 fn list(&'o self) -> TraitObligationStackList<'o,'tcx> {
3322 TraitObligationStackList::with(self)
3325 fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> {
3330 #[derive(Copy, Clone)]
3331 struct TraitObligationStackList<'o,'tcx:'o> {
3332 head: Option<&'o TraitObligationStack<'o,'tcx>>
3335 impl<'o,'tcx> TraitObligationStackList<'o,'tcx> {
3336 fn empty() -> TraitObligationStackList<'o,'tcx> {
3337 TraitObligationStackList { head: None }
3340 fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> {
3341 TraitObligationStackList { head: Some(r) }
3345 impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{
3346 type Item = &'o TraitObligationStack<'o,'tcx>;
3348 fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> {
3359 impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> {
3360 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
3361 write!(f, "TraitObligationStack({:?})", self.obligation)
3366 pub struct WithDepNode<T> {
3367 dep_node: DepNodeIndex,
3371 impl<T: Clone> WithDepNode<T> {
3372 pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self {
3373 WithDepNode { dep_node, cached_value }
3376 pub fn get(&self, tcx: TyCtxt) -> T {
3377 tcx.dep_graph.read_index(self.dep_node);
3378 self.cached_value.clone()