1 //! Candidate selection. See the [rustc dev guide] for more information on how this works.
3 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
5 use self::EvaluationResult
::*;
6 use self::SelectionCandidate
::*;
8 use super::coherence
::{self, Conflict}
;
9 use super::const_evaluatable
;
11 use super::project
::normalize_with_depth_to
;
12 use super::project
::ProjectionTyObligation
;
14 use super::util
::{closure_trait_ref_and_return_type, predicate_for_trait_def}
;
16 use super::DerivedObligationCause
;
17 use super::Normalized
;
18 use super::Obligation
;
19 use super::ObligationCauseCode
;
21 use super::SelectionResult
;
22 use super::TraitQueryMode
;
23 use super::{ErrorReporting, Overflow, SelectionError}
;
24 use super::{ObligationCause, PredicateObligation, TraitObligation}
;
26 use crate::infer
::{InferCtxt, InferOk, TypeFreshener}
;
27 use crate::traits
::error_reporting
::InferCtxtExt
;
28 use rustc_data_structures
::fx
::{FxHashMap, FxHashSet}
;
29 use rustc_data_structures
::stack
::ensure_sufficient_stack
;
30 use rustc_data_structures
::sync
::Lrc
;
31 use rustc_errors
::ErrorReported
;
33 use rustc_hir
::def_id
::DefId
;
34 use rustc_infer
::infer
::LateBoundRegionConversionTime
;
35 use rustc_middle
::dep_graph
::{DepKind, DepNodeIndex}
;
36 use rustc_middle
::mir
::interpret
::ErrorHandled
;
37 use rustc_middle
::thir
::abstract_const
::NotConstEvaluatable
;
38 use rustc_middle
::ty
::fast_reject
;
39 use rustc_middle
::ty
::print
::with_no_trimmed_paths
;
40 use rustc_middle
::ty
::relate
::TypeRelation
;
41 use rustc_middle
::ty
::subst
::{GenericArgKind, Subst, SubstsRef}
;
42 use rustc_middle
::ty
::WithConstness
;
43 use rustc_middle
::ty
::{self, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate}
;
44 use rustc_middle
::ty
::{Ty, TyCtxt, TypeFoldable}
;
45 use rustc_span
::symbol
::sym
;
47 use std
::cell
::{Cell, RefCell}
;
49 use std
::fmt
::{self, Display}
;
52 pub use rustc_middle
::traits
::select
::*;
54 mod candidate_assembly
;
57 #[derive(Clone, Debug)]
58 pub enum IntercrateAmbiguityCause
{
59 DownstreamCrate { trait_desc: String, self_desc: Option<String> }
,
60 UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> }
,
61 ReservationImpl { message: String }
,
64 impl IntercrateAmbiguityCause
{
65 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
66 /// See #23980 for details.
67 pub fn add_intercrate_ambiguity_hint(&self, err
: &mut rustc_errors
::DiagnosticBuilder
<'_
>) {
68 err
.note(&self.intercrate_ambiguity_hint());
71 pub fn intercrate_ambiguity_hint(&self) -> String
{
73 IntercrateAmbiguityCause
::DownstreamCrate { trait_desc, self_desc }
=> {
74 let self_desc
= if let Some(ty
) = self_desc
{
75 format
!(" for type `{}`", ty
)
79 format
!("downstream crates may implement trait `{}`{}", trait_desc
, self_desc
)
81 IntercrateAmbiguityCause
::UpstreamCrateUpdate { trait_desc, self_desc }
=> {
82 let self_desc
= if let Some(ty
) = self_desc
{
83 format
!(" for type `{}`", ty
)
88 "upstream crates may add a new impl of trait `{}`{} \
93 IntercrateAmbiguityCause
::ReservationImpl { message }
=> message
.clone(),
98 pub struct SelectionContext
<'cx
, 'tcx
> {
99 infcx
: &'cx InferCtxt
<'cx
, 'tcx
>,
101 /// Freshener used specifically for entries on the obligation
102 /// stack. This ensures that all entries on the stack at one time
103 /// will have the same set of placeholder entries, which is
104 /// important for checking for trait bounds that recursively
105 /// require themselves.
106 freshener
: TypeFreshener
<'cx
, 'tcx
>,
108 /// If `true`, indicates that the evaluation should be conservative
109 /// and consider the possibility of types outside this crate.
110 /// This comes up primarily when resolving ambiguity. Imagine
111 /// there is some trait reference `$0: Bar` where `$0` is an
112 /// inference variable. If `intercrate` is true, then we can never
113 /// say for sure that this reference is not implemented, even if
114 /// there are *no impls at all for `Bar`*, because `$0` could be
115 /// bound to some type that in a downstream crate that implements
116 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
117 /// though, we set this to false, because we are only interested
118 /// in types that the user could actually have written --- in
119 /// other words, we consider `$0: Bar` to be unimplemented if
120 /// there is no type that the user could *actually name* that
121 /// would satisfy it. This avoids crippling inference, basically.
124 intercrate_ambiguity_causes
: Option
<Vec
<IntercrateAmbiguityCause
>>,
126 /// Controls whether or not to filter out negative impls when selecting.
127 /// This is used in librustdoc to distinguish between the lack of an impl
128 /// and a negative impl
129 allow_negative_impls
: bool
,
131 /// Are we in a const context that needs `~const` bounds to be const?
132 is_in_const_context
: bool
,
134 /// The mode that trait queries run in, which informs our error handling
135 /// policy. In essence, canonicalized queries need their errors propagated
136 /// rather than immediately reported because we do not have accurate spans.
137 query_mode
: TraitQueryMode
,
140 // A stack that walks back up the stack frame.
141 struct TraitObligationStack
<'prev
, 'tcx
> {
142 obligation
: &'prev TraitObligation
<'tcx
>,
144 /// The trait ref from `obligation` but "freshened" with the
145 /// selection-context's freshener. Used to check for recursion.
146 fresh_trait_ref
: ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>,
148 /// Starts out equal to `depth` -- if, during evaluation, we
149 /// encounter a cycle, then we will set this flag to the minimum
150 /// depth of that cycle for all participants in the cycle. These
151 /// participants will then forego caching their results. This is
152 /// not the most efficient solution, but it addresses #60010. The
153 /// problem we are trying to prevent:
155 /// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
156 /// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
157 /// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
159 /// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
160 /// is `EvaluatedToOk`; this is because they were only considered
161 /// ok on the premise that if `A: AutoTrait` held, but we indeed
162 /// encountered a problem (later on) with `A: AutoTrait. So we
163 /// currently set a flag on the stack node for `B: AutoTrait` (as
164 /// well as the second instance of `A: AutoTrait`) to suppress
167 /// This is a simple, targeted fix. A more-performant fix requires
168 /// deeper changes, but would permit more caching: we could
169 /// basically defer caching until we have fully evaluated the
170 /// tree, and then cache the entire tree at once. In any case, the
171 /// performance impact here shouldn't be so horrible: every time
172 /// this is hit, we do cache at least one trait, so we only
173 /// evaluate each member of a cycle up to N times, where N is the
174 /// length of the cycle. This means the performance impact is
175 /// bounded and we shouldn't have any terrible worst-cases.
176 reached_depth
: Cell
<usize>,
178 previous
: TraitObligationStackList
<'prev
, 'tcx
>,
180 /// The number of parent frames plus one (thus, the topmost frame has depth 1).
183 /// The depth-first number of this node in the search graph -- a
184 /// pre-order index. Basically, a freshly incremented counter.
188 struct SelectionCandidateSet
<'tcx
> {
189 // A list of candidates that definitely apply to the current
190 // obligation (meaning: types unify).
191 vec
: Vec
<SelectionCandidate
<'tcx
>>,
193 // If `true`, then there were candidates that might or might
194 // not have applied, but we couldn't tell. This occurs when some
195 // of the input types are type variables, in which case there are
196 // various "builtin" rules that might or might not trigger.
200 #[derive(PartialEq, Eq, Debug, Clone)]
201 struct EvaluatedCandidate
<'tcx
> {
202 candidate
: SelectionCandidate
<'tcx
>,
203 evaluation
: EvaluationResult
,
206 /// When does the builtin impl for `T: Trait` apply?
207 enum BuiltinImplConditions
<'tcx
> {
208 /// The impl is conditional on `T1, T2, ...: Trait`.
209 Where(ty
::Binder
<'tcx
, Vec
<Ty
<'tcx
>>>),
210 /// There is no built-in impl. There may be some other
211 /// candidate (a where-clause or user-defined impl).
213 /// It is unknown whether there is an impl.
217 impl<'cx
, 'tcx
> SelectionContext
<'cx
, 'tcx
> {
218 pub fn new(infcx
: &'cx InferCtxt
<'cx
, 'tcx
>) -> SelectionContext
<'cx
, 'tcx
> {
221 freshener
: infcx
.freshener_keep_static(),
223 intercrate_ambiguity_causes
: None
,
224 allow_negative_impls
: false,
225 is_in_const_context
: false,
226 query_mode
: TraitQueryMode
::Standard
,
230 pub fn intercrate(infcx
: &'cx InferCtxt
<'cx
, 'tcx
>) -> SelectionContext
<'cx
, 'tcx
> {
233 freshener
: infcx
.freshener_keep_static(),
235 intercrate_ambiguity_causes
: None
,
236 allow_negative_impls
: false,
237 is_in_const_context
: false,
238 query_mode
: TraitQueryMode
::Standard
,
242 pub fn with_negative(
243 infcx
: &'cx InferCtxt
<'cx
, 'tcx
>,
244 allow_negative_impls
: bool
,
245 ) -> SelectionContext
<'cx
, 'tcx
> {
246 debug
!(?allow_negative_impls
, "with_negative");
249 freshener
: infcx
.freshener_keep_static(),
251 intercrate_ambiguity_causes
: None
,
252 allow_negative_impls
,
253 is_in_const_context
: false,
254 query_mode
: TraitQueryMode
::Standard
,
258 pub fn with_query_mode(
259 infcx
: &'cx InferCtxt
<'cx
, 'tcx
>,
260 query_mode
: TraitQueryMode
,
261 ) -> SelectionContext
<'cx
, 'tcx
> {
262 debug
!(?query_mode
, "with_query_mode");
265 freshener
: infcx
.freshener_keep_static(),
267 intercrate_ambiguity_causes
: None
,
268 allow_negative_impls
: false,
269 is_in_const_context
: false,
274 pub fn with_constness(
275 infcx
: &'cx InferCtxt
<'cx
, 'tcx
>,
276 constness
: hir
::Constness
,
277 ) -> SelectionContext
<'cx
, 'tcx
> {
280 freshener
: infcx
.freshener_keep_static(),
282 intercrate_ambiguity_causes
: None
,
283 allow_negative_impls
: false,
284 is_in_const_context
: matches
!(constness
, hir
::Constness
::Const
),
285 query_mode
: TraitQueryMode
::Standard
,
289 /// Enables tracking of intercrate ambiguity causes. These are
290 /// used in coherence to give improved diagnostics. We don't do
291 /// this until we detect a coherence error because it can lead to
292 /// false overflow results (#47139) and because it costs
293 /// computation time.
294 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
295 assert
!(self.intercrate
);
296 assert
!(self.intercrate_ambiguity_causes
.is_none());
297 self.intercrate_ambiguity_causes
= Some(vec
![]);
298 debug
!("selcx: enable_tracking_intercrate_ambiguity_causes");
301 /// Gets the intercrate ambiguity causes collected since tracking
302 /// was enabled and disables tracking at the same time. If
303 /// tracking is not enabled, just returns an empty vector.
304 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec
<IntercrateAmbiguityCause
> {
305 assert
!(self.intercrate
);
306 self.intercrate_ambiguity_causes
.take().unwrap_or_default()
309 pub fn infcx(&self) -> &'cx InferCtxt
<'cx
, 'tcx
> {
313 pub fn tcx(&self) -> TyCtxt
<'tcx
> {
317 pub fn is_intercrate(&self) -> bool
{
321 /// Returns `true` if the trait predicate is considerd `const` to this selection context.
322 pub fn is_trait_predicate_const(&self, pred
: ty
::TraitPredicate
<'_
>) -> bool
{
323 matches
!(pred
.constness
, ty
::BoundConstness
::ConstIfConst
) && self.is_in_const_context
326 /// Returns `true` if the predicate is considered `const` to
327 /// this selection context.
328 pub fn is_predicate_const(&self, pred
: ty
::Predicate
<'_
>) -> bool
{
329 match pred
.kind().skip_binder() {
330 ty
::PredicateKind
::Trait(pred
) => self.is_trait_predicate_const(pred
),
335 ///////////////////////////////////////////////////////////////////////////
338 // The selection phase tries to identify *how* an obligation will
339 // be resolved. For example, it will identify which impl or
340 // parameter bound is to be used. The process can be inconclusive
341 // if the self type in the obligation is not fully inferred. Selection
342 // can result in an error in one of two ways:
344 // 1. If no applicable impl or parameter bound can be found.
345 // 2. If the output type parameters in the obligation do not match
346 // those specified by the impl/bound. For example, if the obligation
347 // is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
348 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
350 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
351 /// type environment by performing unification.
352 #[instrument(level = "debug", skip(self))]
355 obligation
: &TraitObligation
<'tcx
>,
356 ) -> SelectionResult
<'tcx
, Selection
<'tcx
>> {
357 let candidate
= match self.select_from_obligation(obligation
) {
358 Err(SelectionError
::Overflow
) => {
359 // In standard mode, overflow must have been caught and reported
361 assert
!(self.query_mode
== TraitQueryMode
::Canonical
);
362 return Err(SelectionError
::Overflow
);
364 Err(SelectionError
::Ambiguous(_
)) => {
373 Ok(Some(candidate
)) => candidate
,
376 match self.confirm_candidate(obligation
, candidate
) {
377 Err(SelectionError
::Overflow
) => {
378 assert
!(self.query_mode
== TraitQueryMode
::Canonical
);
379 Err(SelectionError
::Overflow
)
389 crate fn select_from_obligation(
391 obligation
: &TraitObligation
<'tcx
>,
392 ) -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>> {
393 debug_assert
!(!obligation
.predicate
.has_escaping_bound_vars());
395 let pec
= &ProvisionalEvaluationCache
::default();
396 let stack
= self.push_stack(TraitObligationStackList
::empty(pec
), obligation
);
398 self.candidate_from_obligation(&stack
)
401 ///////////////////////////////////////////////////////////////////////////
404 // Tests whether an obligation can be selected or whether an impl
405 // can be applied to particular types. It skips the "confirmation"
406 // step and hence completely ignores output type parameters.
408 // The result is "true" if the obligation *may* hold and "false" if
409 // we can be sure it does not.
411 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
412 pub fn predicate_may_hold_fatal(&mut self, obligation
: &PredicateObligation
<'tcx
>) -> bool
{
413 debug
!(?obligation
, "predicate_may_hold_fatal");
415 // This fatal query is a stopgap that should only be used in standard mode,
416 // where we do not expect overflow to be propagated.
417 assert
!(self.query_mode
== TraitQueryMode
::Standard
);
419 self.evaluate_root_obligation(obligation
)
420 .expect("Overflow should be caught earlier in standard query mode")
424 /// Evaluates whether the obligation `obligation` can be satisfied
425 /// and returns an `EvaluationResult`. This is meant for the
427 pub fn evaluate_root_obligation(
429 obligation
: &PredicateObligation
<'tcx
>,
430 ) -> Result
<EvaluationResult
, OverflowError
> {
431 self.evaluation_probe(|this
| {
432 this
.evaluate_predicate_recursively(
433 TraitObligationStackList
::empty(&ProvisionalEvaluationCache
::default()),
441 op
: impl FnOnce(&mut Self) -> Result
<EvaluationResult
, OverflowError
>,
442 ) -> Result
<EvaluationResult
, OverflowError
> {
443 self.infcx
.probe(|snapshot
| -> Result
<EvaluationResult
, OverflowError
> {
444 let result
= op(self)?
;
446 match self.infcx
.leak_check(true, snapshot
) {
448 Err(_
) => return Ok(EvaluatedToErr
),
451 match self.infcx
.region_constraints_added_in_snapshot(snapshot
) {
453 Some(_
) => Ok(result
.max(EvaluatedToOkModuloRegions
)),
458 /// Evaluates the predicates in `predicates` recursively. Note that
459 /// this applies projections in the predicates, and therefore
460 /// is run within an inference probe.
461 #[instrument(skip(self, stack), level = "debug")]
462 fn evaluate_predicates_recursively
<'o
, I
>(
464 stack
: TraitObligationStackList
<'o
, 'tcx
>,
466 ) -> Result
<EvaluationResult
, OverflowError
>
468 I
: IntoIterator
<Item
= PredicateObligation
<'tcx
>> + std
::fmt
::Debug
,
470 let mut result
= EvaluatedToOk
;
471 for obligation
in predicates
{
472 let eval
= self.evaluate_predicate_recursively(stack
, obligation
.clone())?
;
473 if let EvaluatedToErr
= eval
{
474 // fast-path - EvaluatedToErr is the top of the lattice,
475 // so we don't need to look on the other predicates.
476 return Ok(EvaluatedToErr
);
478 result
= cmp
::max(result
, eval
);
486 skip(self, previous_stack
),
487 fields(previous_stack
= ?previous_stack
.head())
489 fn evaluate_predicate_recursively
<'o
>(
491 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
492 obligation
: PredicateObligation
<'tcx
>,
493 ) -> Result
<EvaluationResult
, OverflowError
> {
494 // `previous_stack` stores a `TraitObligation`, while `obligation` is
495 // a `PredicateObligation`. These are distinct types, so we can't
496 // use any `Option` combinator method that would force them to be
498 match previous_stack
.head() {
499 Some(h
) => self.check_recursion_limit(&obligation
, h
.obligation
)?
,
500 None
=> self.check_recursion_limit(&obligation
, &obligation
)?
,
503 let result
= ensure_sufficient_stack(|| {
504 let bound_predicate
= obligation
.predicate
.kind();
505 match bound_predicate
.skip_binder() {
506 ty
::PredicateKind
::Trait(t
) => {
507 let t
= bound_predicate
.rebind(t
);
508 debug_assert
!(!t
.has_escaping_bound_vars());
509 let obligation
= obligation
.with(t
);
510 self.evaluate_trait_predicate_recursively(previous_stack
, obligation
)
513 ty
::PredicateKind
::Subtype(p
) => {
514 let p
= bound_predicate
.rebind(p
);
515 // Does this code ever run?
516 match self.infcx
.subtype_predicate(&obligation
.cause
, obligation
.param_env
, p
) {
517 Some(Ok(InferOk { mut obligations, .. }
)) => {
518 self.add_depth(obligations
.iter_mut(), obligation
.recursion_depth
);
519 self.evaluate_predicates_recursively(
521 obligations
.into_iter(),
524 Some(Err(_
)) => Ok(EvaluatedToErr
),
525 None
=> Ok(EvaluatedToAmbig
),
529 ty
::PredicateKind
::Coerce(p
) => {
530 let p
= bound_predicate
.rebind(p
);
531 // Does this code ever run?
532 match self.infcx
.coerce_predicate(&obligation
.cause
, obligation
.param_env
, p
) {
533 Some(Ok(InferOk { mut obligations, .. }
)) => {
534 self.add_depth(obligations
.iter_mut(), obligation
.recursion_depth
);
535 self.evaluate_predicates_recursively(
537 obligations
.into_iter(),
540 Some(Err(_
)) => Ok(EvaluatedToErr
),
541 None
=> Ok(EvaluatedToAmbig
),
545 ty
::PredicateKind
::WellFormed(arg
) => match wf
::obligations(
547 obligation
.param_env
,
548 obligation
.cause
.body_id
,
549 obligation
.recursion_depth
+ 1,
551 obligation
.cause
.span
,
553 Some(mut obligations
) => {
554 self.add_depth(obligations
.iter_mut(), obligation
.recursion_depth
);
555 self.evaluate_predicates_recursively(previous_stack
, obligations
)
557 None
=> Ok(EvaluatedToAmbig
),
560 ty
::PredicateKind
::TypeOutlives(pred
) => {
561 if pred
.0.is_known_global
() {
564 Ok(EvaluatedToOkModuloRegions
)
568 ty
::PredicateKind
::RegionOutlives(..) => {
569 // We do not consider region relationships when evaluating trait matches.
570 Ok(EvaluatedToOkModuloRegions
)
573 ty
::PredicateKind
::ObjectSafe(trait_def_id
) => {
574 if self.tcx().is_object_safe(trait_def_id
) {
581 ty
::PredicateKind
::Projection(data
) => {
582 let data
= bound_predicate
.rebind(data
);
583 let project_obligation
= obligation
.with(data
);
584 match project
::poly_project_and_unify_type(self, &project_obligation
) {
585 Ok(Ok(Some(mut subobligations
))) => {
586 self.add_depth(subobligations
.iter_mut(), obligation
.recursion_depth
);
587 self.evaluate_predicates_recursively(previous_stack
, subobligations
)
589 Ok(Ok(None
)) => Ok(EvaluatedToAmbig
),
590 Ok(Err(project
::InProgress
)) => Ok(EvaluatedToRecur
),
591 Err(_
) => Ok(EvaluatedToErr
),
595 ty
::PredicateKind
::ClosureKind(_
, closure_substs
, kind
) => {
596 match self.infcx
.closure_kind(closure_substs
) {
597 Some(closure_kind
) => {
598 if closure_kind
.extends(kind
) {
604 None
=> Ok(EvaluatedToAmbig
),
608 ty
::PredicateKind
::ConstEvaluatable(uv
) => {
609 match const_evaluatable
::is_const_evaluatable(
612 obligation
.param_env
,
613 obligation
.cause
.span
,
615 Ok(()) => Ok(EvaluatedToOk
),
616 Err(NotConstEvaluatable
::MentionsInfer
) => Ok(EvaluatedToAmbig
),
617 Err(NotConstEvaluatable
::MentionsParam
) => Ok(EvaluatedToErr
),
618 Err(_
) => Ok(EvaluatedToErr
),
622 ty
::PredicateKind
::ConstEquate(c1
, c2
) => {
623 debug
!(?c1
, ?c2
, "evaluate_predicate_recursively: equating consts");
625 if self.tcx().features().generic_const_exprs
{
626 // FIXME: we probably should only try to unify abstract constants
627 // if the constants depend on generic parameters.
629 // Let's just see where this breaks :shrug:
630 if let (ty
::ConstKind
::Unevaluated(a
), ty
::ConstKind
::Unevaluated(b
)) =
633 if self.infcx
.try_unify_abstract_consts(a
.shrink(), b
.shrink()) {
634 return Ok(EvaluatedToOk
);
639 let evaluate
= |c
: &'tcx ty
::Const
<'tcx
>| {
640 if let ty
::ConstKind
::Unevaluated(unevaluated
) = c
.val
{
643 obligation
.param_env
,
645 Some(obligation
.cause
.span
),
647 .map(|val
| ty
::Const
::from_value(self.tcx(), val
, c
.ty
))
653 match (evaluate(c1
), evaluate(c2
)) {
654 (Ok(c1
), Ok(c2
)) => {
657 .at(&obligation
.cause
, obligation
.param_env
)
660 Ok(_
) => Ok(EvaluatedToOk
),
661 Err(_
) => Ok(EvaluatedToErr
),
664 (Err(ErrorHandled
::Reported(ErrorReported
)), _
)
665 | (_
, Err(ErrorHandled
::Reported(ErrorReported
))) => Ok(EvaluatedToErr
),
666 (Err(ErrorHandled
::Linted
), _
) | (_
, Err(ErrorHandled
::Linted
)) => {
668 obligation
.cause
.span(self.tcx()),
669 "ConstEquate: const_eval_resolve returned an unexpected error"
672 (Err(ErrorHandled
::TooGeneric
), _
) | (_
, Err(ErrorHandled
::TooGeneric
)) => {
673 if c1
.has_infer_types_or_consts() || c2
.has_infer_types_or_consts() {
676 // Two different constants using generic parameters ~> error.
682 ty
::PredicateKind
::TypeWellFormedFromEnv(..) => {
683 bug
!("TypeWellFormedFromEnv is only used for chalk")
688 debug
!("finished: {:?} from {:?}", result
, obligation
);
693 #[instrument(skip(self, previous_stack), level = "debug")]
694 fn evaluate_trait_predicate_recursively
<'o
>(
696 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
697 mut obligation
: TraitObligation
<'tcx
>,
698 ) -> Result
<EvaluationResult
, OverflowError
> {
700 && obligation
.is_global(self.tcx())
705 .all(|bound
| bound
.definitely_needs_subst(self.tcx()))
707 // If a param env has no global bounds, global obligations do not
708 // depend on its particular value in order to work, so we can clear
709 // out the param env and get better caching.
711 obligation
.param_env
= obligation
.param_env
.without_caller_bounds();
714 let stack
= self.push_stack(previous_stack
, &obligation
);
715 let fresh_trait_ref
= stack
.fresh_trait_ref
;
717 debug
!(?fresh_trait_ref
);
719 if let Some(result
) = self.check_evaluation_cache(
720 obligation
.param_env
,
722 obligation
.polarity(),
724 debug
!(?result
, "CACHE HIT");
728 if let Some(result
) = stack
.cache().get_provisional(fresh_trait_ref
) {
729 debug
!(?result
, "PROVISIONAL CACHE HIT");
730 stack
.update_reached_depth(result
.reached_depth
);
731 return Ok(result
.result
);
734 // Check if this is a match for something already on the
735 // stack. If so, we don't want to insert the result into the
736 // main cache (it is cycle dependent) nor the provisional
737 // cache (which is meant for things that have completed but
738 // for a "backedge" -- this result *is* the backedge).
739 if let Some(cycle_result
) = self.check_evaluation_cycle(&stack
) {
740 return Ok(cycle_result
);
743 let (result
, dep_node
) = self.in_task(|this
| this
.evaluate_stack(&stack
));
744 let result
= result?
;
746 if !result
.must_apply_modulo_regions() {
747 stack
.cache().on_failure(stack
.dfn
);
750 let reached_depth
= stack
.reached_depth
.get();
751 if reached_depth
>= stack
.depth
{
752 debug
!(?result
, "CACHE MISS");
753 self.insert_evaluation_cache(
754 obligation
.param_env
,
756 obligation
.polarity(),
761 stack
.cache().on_completion(stack
.dfn
, |fresh_trait_ref
, provisional_result
| {
762 self.insert_evaluation_cache(
763 obligation
.param_env
,
765 obligation
.polarity(),
767 provisional_result
.max(result
),
771 debug
!(?result
, "PROVISIONAL");
773 "caching provisionally because {:?} \
774 is a cycle participant (at depth {}, reached depth {})",
775 fresh_trait_ref
, stack
.depth
, reached_depth
,
778 stack
.cache().insert_provisional(stack
.dfn
, reached_depth
, fresh_trait_ref
, result
);
784 /// If there is any previous entry on the stack that precisely
785 /// matches this obligation, then we can assume that the
786 /// obligation is satisfied for now (still all other conditions
787 /// must be met of course). One obvious case this comes up is
788 /// marker traits like `Send`. Think of a linked list:
790 /// struct List<T> { data: T, next: Option<Box<List<T>>> }
792 /// `Box<List<T>>` will be `Send` if `T` is `Send` and
793 /// `Option<Box<List<T>>>` is `Send`, and in turn
794 /// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
797 /// Note that we do this comparison using the `fresh_trait_ref`
798 /// fields. Because these have all been freshened using
799 /// `self.freshener`, we can be sure that (a) this will not
800 /// affect the inferencer state and (b) that if we see two
801 /// fresh regions with the same index, they refer to the same
802 /// unbound type variable.
803 fn check_evaluation_cycle(
805 stack
: &TraitObligationStack
<'_
, 'tcx
>,
806 ) -> Option
<EvaluationResult
> {
807 if let Some(cycle_depth
) = stack
809 .skip(1) // Skip top-most frame.
811 stack
.obligation
.param_env
== prev
.obligation
.param_env
812 && stack
.fresh_trait_ref
== prev
.fresh_trait_ref
814 .map(|stack
| stack
.depth
)
816 debug
!("evaluate_stack --> recursive at depth {}", cycle_depth
);
818 // If we have a stack like `A B C D E A`, where the top of
819 // the stack is the final `A`, then this will iterate over
820 // `A, E, D, C, B` -- i.e., all the participants apart
821 // from the cycle head. We mark them as participating in a
822 // cycle. This suppresses caching for those nodes. See
823 // `in_cycle` field for more details.
824 stack
.update_reached_depth(cycle_depth
);
826 // Subtle: when checking for a coinductive cycle, we do
827 // not compare using the "freshened trait refs" (which
828 // have erased regions) but rather the fully explicit
829 // trait refs. This is important because it's only a cycle
830 // if the regions match exactly.
831 let cycle
= stack
.iter().skip(1).take_while(|s
| s
.depth
>= cycle_depth
);
832 let tcx
= self.tcx();
833 let cycle
= cycle
.map(|stack
| stack
.obligation
.predicate
.to_predicate(tcx
));
834 if self.coinductive_match(cycle
) {
835 debug
!("evaluate_stack --> recursive, coinductive");
838 debug
!("evaluate_stack --> recursive, inductive");
839 Some(EvaluatedToRecur
)
846 fn evaluate_stack
<'o
>(
848 stack
: &TraitObligationStack
<'o
, 'tcx
>,
849 ) -> Result
<EvaluationResult
, OverflowError
> {
850 // In intercrate mode, whenever any of the generics are unbound,
851 // there can always be an impl. Even if there are no impls in
852 // this crate, perhaps the type would be unified with
853 // something from another crate that does provide an impl.
855 // In intra mode, we must still be conservative. The reason is
856 // that we want to avoid cycles. Imagine an impl like:
858 // impl<T:Eq> Eq for Vec<T>
860 // and a trait reference like `$0 : Eq` where `$0` is an
861 // unbound variable. When we evaluate this trait-reference, we
862 // will unify `$0` with `Vec<$1>` (for some fresh variable
863 // `$1`), on the condition that `$1 : Eq`. We will then wind
864 // up with many candidates (since that are other `Eq` impls
865 // that apply) and try to winnow things down. This results in
866 // a recursive evaluation that `$1 : Eq` -- as you can
867 // imagine, this is just where we started. To avoid that, we
868 // check for unbound variables and return an ambiguous (hence possible)
869 // match if we've seen this trait before.
871 // This suffices to allow chains like `FnMut` implemented in
872 // terms of `Fn` etc, but we could probably make this more
874 let unbound_input_types
=
875 stack
.fresh_trait_ref
.value
.skip_binder().substs
.types().any(|ty
| ty
.is_fresh());
877 if stack
.obligation
.polarity() != ty
::ImplPolarity
::Negative
{
878 // This check was an imperfect workaround for a bug in the old
879 // intercrate mode; it should be removed when that goes away.
880 if unbound_input_types
&& self.intercrate
{
881 debug
!("evaluate_stack --> unbound argument, intercrate --> ambiguous",);
882 // Heuristics: show the diagnostics when there are no candidates in crate.
883 if self.intercrate_ambiguity_causes
.is_some() {
884 debug
!("evaluate_stack: intercrate_ambiguity_causes is some");
885 if let Ok(candidate_set
) = self.assemble_candidates(stack
) {
886 if !candidate_set
.ambiguous
&& candidate_set
.vec
.is_empty() {
887 let trait_ref
= stack
.obligation
.predicate
.skip_binder().trait_ref
;
888 let self_ty
= trait_ref
.self_ty();
889 let cause
= with_no_trimmed_paths(|| {
890 IntercrateAmbiguityCause
::DownstreamCrate
{
891 trait_desc
: trait_ref
.print_only_trait_path().to_string(),
892 self_desc
: if self_ty
.has_concrete_skeleton() {
893 Some(self_ty
.to_string())
900 debug
!(?cause
, "evaluate_stack: pushing cause");
901 self.intercrate_ambiguity_causes
.as_mut().unwrap().push(cause
);
905 return Ok(EvaluatedToAmbig
);
909 if unbound_input_types
910 && stack
.iter().skip(1).any(|prev
| {
911 stack
.obligation
.param_env
== prev
.obligation
.param_env
912 && self.match_fresh_trait_refs(
913 stack
.fresh_trait_ref
,
914 prev
.fresh_trait_ref
,
915 prev
.obligation
.param_env
,
919 debug
!("evaluate_stack --> unbound argument, recursive --> giving up",);
920 return Ok(EvaluatedToUnknown
);
923 match self.candidate_from_obligation(stack
) {
924 Ok(Some(c
)) => self.evaluate_candidate(stack
, &c
),
925 Err(SelectionError
::Ambiguous(_
)) => Ok(EvaluatedToAmbig
),
926 Ok(None
) => Ok(EvaluatedToAmbig
),
927 Err(Overflow
) => Err(OverflowError
::Canonical
),
928 Err(ErrorReporting
) => Err(OverflowError
::ErrorReporting
),
929 Err(..) => Ok(EvaluatedToErr
),
933 /// For defaulted traits, we use a co-inductive strategy to solve, so
934 /// that recursion is ok. This routine returns `true` if the top of the
935 /// stack (`cycle[0]`):
937 /// - is a defaulted trait,
938 /// - it also appears in the backtrace at some position `X`,
939 /// - all the predicates at positions `X..` between `X` and the top are
940 /// also defaulted traits.
941 pub fn coinductive_match
<I
>(&mut self, mut cycle
: I
) -> bool
943 I
: Iterator
<Item
= ty
::Predicate
<'tcx
>>,
945 cycle
.all(|predicate
| self.coinductive_predicate(predicate
))
948 fn coinductive_predicate(&self, predicate
: ty
::Predicate
<'tcx
>) -> bool
{
949 let result
= match predicate
.kind().skip_binder() {
950 ty
::PredicateKind
::Trait(ref data
) => self.tcx().trait_is_auto(data
.def_id()),
953 debug
!(?predicate
, ?result
, "coinductive_predicate");
957 /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
958 /// obligations are met. Returns whether `candidate` remains viable after this further
963 fields(depth
= stack
.obligation
.recursion_depth
)
965 fn evaluate_candidate
<'o
>(
967 stack
: &TraitObligationStack
<'o
, 'tcx
>,
968 candidate
: &SelectionCandidate
<'tcx
>,
969 ) -> Result
<EvaluationResult
, OverflowError
> {
970 let mut result
= self.evaluation_probe(|this
| {
971 let candidate
= (*candidate
).clone();
972 match this
.confirm_candidate(stack
.obligation
, candidate
) {
975 this
.evaluate_predicates_recursively(
977 selection
.nested_obligations().into_iter(),
980 Err(..) => Ok(EvaluatedToErr
),
984 // If we erased any lifetimes, then we want to use
985 // `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
986 // as your final result. The result will be cached using
987 // the freshened trait predicate as a key, so we need
988 // our result to be correct by *any* choice of original lifetimes,
989 // not just the lifetime choice for this particular (non-erased)
992 if stack
.fresh_trait_ref
.has_erased_regions() {
993 result
= result
.max(EvaluatedToOkModuloRegions
);
1000 fn check_evaluation_cache(
1002 param_env
: ty
::ParamEnv
<'tcx
>,
1003 trait_ref
: ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>,
1004 polarity
: ty
::ImplPolarity
,
1005 ) -> Option
<EvaluationResult
> {
1006 // Neither the global nor local cache is aware of intercrate
1007 // mode, so don't do any caching. In particular, we might
1008 // re-use the same `InferCtxt` with both an intercrate
1009 // and non-intercrate `SelectionContext`
1010 if self.intercrate
{
1014 let tcx
= self.tcx();
1015 if self.can_use_global_caches(param_env
) {
1016 if let Some(res
) = tcx
.evaluation_cache
.get(&(param_env
.and(trait_ref
), polarity
), tcx
)
1021 self.infcx
.evaluation_cache
.get(&(param_env
.and(trait_ref
), polarity
), tcx
)
1024 fn insert_evaluation_cache(
1026 param_env
: ty
::ParamEnv
<'tcx
>,
1027 trait_ref
: ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>,
1028 polarity
: ty
::ImplPolarity
,
1029 dep_node
: DepNodeIndex
,
1030 result
: EvaluationResult
,
1032 // Avoid caching results that depend on more than just the trait-ref
1033 // - the stack can create recursion.
1034 if result
.is_stack_dependent() {
1038 // Neither the global nor local cache is aware of intercrate
1039 // mode, so don't do any caching. In particular, we might
1040 // re-use the same `InferCtxt` with both an intercrate
1041 // and non-intercrate `SelectionContext`
1042 if self.intercrate
{
1046 if self.can_use_global_caches(param_env
) {
1047 if !trait_ref
.needs_infer() {
1048 debug
!(?trait_ref
, ?result
, "insert_evaluation_cache global");
1049 // This may overwrite the cache with the same value
1050 // FIXME: Due to #50507 this overwrites the different values
1051 // This should be changed to use HashMapExt::insert_same
1052 // when that is fixed
1053 self.tcx().evaluation_cache
.insert(
1054 (param_env
.and(trait_ref
), polarity
),
1062 debug
!(?trait_ref
, ?result
, "insert_evaluation_cache");
1063 self.infcx
.evaluation_cache
.insert((param_env
.and(trait_ref
), polarity
), dep_node
, result
);
1066 /// For various reasons, it's possible for a subobligation
1067 /// to have a *lower* recursion_depth than the obligation used to create it.
1068 /// Projection sub-obligations may be returned from the projection cache,
1069 /// which results in obligations with an 'old' `recursion_depth`.
1070 /// Additionally, methods like `InferCtxt.subtype_predicate` produce
1071 /// subobligations without taking in a 'parent' depth, causing the
1072 /// generated subobligations to have a `recursion_depth` of `0`.
1074 /// To ensure that obligation_depth never decreases, we force all subobligations
1075 /// to have at least the depth of the original obligation.
1076 fn add_depth
<T
: 'cx
, I
: Iterator
<Item
= &'cx
mut Obligation
<'tcx
, T
>>>(
1081 it
.for_each(|o
| o
.recursion_depth
= cmp
::max(min_depth
, o
.recursion_depth
) + 1);
1084 fn check_recursion_depth
<T
: Display
+ TypeFoldable
<'tcx
>>(
1087 error_obligation
: &Obligation
<'tcx
, T
>,
1088 ) -> Result
<(), OverflowError
> {
1089 if !self.infcx
.tcx
.recursion_limit().value_within_limit(depth
) {
1090 match self.query_mode
{
1091 TraitQueryMode
::Standard
=> {
1092 if self.infcx
.is_tainted_by_errors() {
1093 return Err(OverflowError
::ErrorReporting
);
1095 self.infcx
.report_overflow_error(error_obligation
, true);
1097 TraitQueryMode
::Canonical
=> {
1098 return Err(OverflowError
::Canonical
);
1105 /// Checks that the recursion limit has not been exceeded.
1107 /// The weird return type of this function allows it to be used with the `try` (`?`)
1108 /// operator within certain functions.
1110 fn check_recursion_limit
<T
: Display
+ TypeFoldable
<'tcx
>, V
: Display
+ TypeFoldable
<'tcx
>>(
1112 obligation
: &Obligation
<'tcx
, T
>,
1113 error_obligation
: &Obligation
<'tcx
, V
>,
1114 ) -> Result
<(), OverflowError
> {
1115 self.check_recursion_depth(obligation
.recursion_depth
, error_obligation
)
1118 fn in_task
<OP
, R
>(&mut self, op
: OP
) -> (R
, DepNodeIndex
)
1120 OP
: FnOnce(&mut Self) -> R
,
1122 let (result
, dep_node
) =
1123 self.tcx().dep_graph
.with_anon_task(self.tcx(), DepKind
::TraitSelect
, || op(self));
1124 self.tcx().dep_graph
.read_index(dep_node
);
1128 /// filter_impls filters constant trait obligations and candidates that have a positive impl
1129 /// for a negative goal and a negative impl for a positive goal
1130 #[instrument(level = "debug", skip(self))]
1133 candidates
: Vec
<SelectionCandidate
<'tcx
>>,
1134 obligation
: &TraitObligation
<'tcx
>,
1135 ) -> Vec
<SelectionCandidate
<'tcx
>> {
1136 let tcx
= self.tcx();
1137 let mut result
= Vec
::with_capacity(candidates
.len());
1139 for candidate
in candidates
{
1140 // Respect const trait obligations
1141 if self.is_trait_predicate_const(obligation
.predicate
.skip_binder()) {
1144 ImplCandidate(def_id
)
1145 if tcx
.impl_constness(def_id
) == hir
::Constness
::Const
=> {}
1148 ty
::ConstnessAnd { constness: ty::BoundConstness::ConstIfConst, .. }
,
1152 AutoImplCandidate(..) => {}
1153 // generator, this will raise error in other places
1154 // or ignore error with const_async_blocks feature
1155 GeneratorCandidate
=> {}
1156 // FnDef where the function is const
1157 FnPointerCandidate { is_const: true }
=> {}
1158 ConstDropCandidate
=> {}
1160 // reject all other types of candidates
1166 if let ImplCandidate(def_id
) = candidate
{
1167 if ty
::ImplPolarity
::Reservation
== tcx
.impl_polarity(def_id
)
1168 || obligation
.polarity() == tcx
.impl_polarity(def_id
)
1169 || self.allow_negative_impls
1171 result
.push(candidate
);
1174 result
.push(candidate
);
1181 /// filter_reservation_impls filter reservation impl for any goal as ambiguous
1182 #[instrument(level = "debug", skip(self))]
1183 fn filter_reservation_impls(
1185 candidate
: SelectionCandidate
<'tcx
>,
1186 obligation
: &TraitObligation
<'tcx
>,
1187 ) -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>> {
1188 let tcx
= self.tcx();
1189 // Treat reservation impls as ambiguity.
1190 if let ImplCandidate(def_id
) = candidate
{
1191 if let ty
::ImplPolarity
::Reservation
= tcx
.impl_polarity(def_id
) {
1192 if let Some(intercrate_ambiguity_clauses
) = &mut self.intercrate_ambiguity_causes
{
1193 let attrs
= tcx
.get_attrs(def_id
);
1194 let attr
= tcx
.sess
.find_by_name(&attrs
, sym
::rustc_reservation_impl
);
1195 let value
= attr
.and_then(|a
| a
.value_str());
1196 if let Some(value
) = value
{
1198 "filter_reservation_impls: \
1199 reservation impl ambiguity on {:?}",
1202 intercrate_ambiguity_clauses
.push(
1203 IntercrateAmbiguityCause
::ReservationImpl
{
1204 message
: value
.to_string(),
1215 fn is_knowable
<'o
>(&mut self, stack
: &TraitObligationStack
<'o
, 'tcx
>) -> Option
<Conflict
> {
1216 debug
!("is_knowable(intercrate={:?})", self.intercrate
);
1218 if !self.intercrate
|| stack
.obligation
.polarity() == ty
::ImplPolarity
::Negative
{
1222 let obligation
= &stack
.obligation
;
1223 let predicate
= self.infcx().resolve_vars_if_possible(obligation
.predicate
);
1225 // Okay to skip binder because of the nature of the
1226 // trait-ref-is-knowable check, which does not care about
1228 let trait_ref
= predicate
.skip_binder().trait_ref
;
1230 coherence
::trait_ref_is_knowable(self.tcx(), trait_ref
)
1233 /// Returns `true` if the global caches can be used.
1234 fn can_use_global_caches(&self, param_env
: ty
::ParamEnv
<'tcx
>) -> bool
{
1235 // If there are any inference variables in the `ParamEnv`, then we
1236 // always use a cache local to this particular scope. Otherwise, we
1237 // switch to a global cache.
1238 if param_env
.needs_infer() {
1242 // Avoid using the master cache during coherence and just rely
1243 // on the local cache. This effectively disables caching
1244 // during coherence. It is really just a simplification to
1245 // avoid us having to fear that coherence results "pollute"
1246 // the master cache. Since coherence executes pretty quickly,
1247 // it's not worth going to more trouble to increase the
1248 // hit-rate, I don't think.
1249 if self.intercrate
{
1253 // Otherwise, we can use the global cache.
1257 fn check_candidate_cache(
1259 param_env
: ty
::ParamEnv
<'tcx
>,
1260 cache_fresh_trait_pred
: ty
::PolyTraitPredicate
<'tcx
>,
1261 ) -> Option
<SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>> {
1262 // Neither the global nor local cache is aware of intercrate
1263 // mode, so don't do any caching. In particular, we might
1264 // re-use the same `InferCtxt` with both an intercrate
1265 // and non-intercrate `SelectionContext`
1266 if self.intercrate
{
1269 let tcx
= self.tcx();
1270 let pred
= &cache_fresh_trait_pred
.skip_binder();
1271 let trait_ref
= pred
.trait_ref
;
1272 if self.can_use_global_caches(param_env
) {
1273 if let Some(res
) = tcx
1275 .get(&(param_env
.and(trait_ref
).with_constness(pred
.constness
), pred
.polarity
), tcx
)
1282 .get(&(param_env
.and(trait_ref
).with_constness(pred
.constness
), pred
.polarity
), tcx
)
1285 /// Determines whether can we safely cache the result
1286 /// of selecting an obligation. This is almost always `true`,
1287 /// except when dealing with certain `ParamCandidate`s.
1289 /// Ordinarily, a `ParamCandidate` will contain no inference variables,
1290 /// since it was usually produced directly from a `DefId`. However,
1291 /// certain cases (currently only librustdoc's blanket impl finder),
1292 /// a `ParamEnv` may be explicitly constructed with inference types.
1293 /// When this is the case, we do *not* want to cache the resulting selection
1294 /// candidate. This is due to the fact that it might not always be possible
1295 /// to equate the obligation's trait ref and the candidate's trait ref,
1296 /// if more constraints end up getting added to an inference variable.
1298 /// Because of this, we always want to re-run the full selection
1299 /// process for our obligation the next time we see it, since
1300 /// we might end up picking a different `SelectionCandidate` (or none at all).
1301 fn can_cache_candidate(
1303 result
: &SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>,
1305 // Neither the global nor local cache is aware of intercrate
1306 // mode, so don't do any caching. In particular, we might
1307 // re-use the same `InferCtxt` with both an intercrate
1308 // and non-intercrate `SelectionContext`
1309 if self.intercrate
{
1313 Ok(Some(SelectionCandidate
::ParamCandidate(trait_ref
))) => !trait_ref
.needs_infer(),
1318 fn insert_candidate_cache(
1320 param_env
: ty
::ParamEnv
<'tcx
>,
1321 cache_fresh_trait_pred
: ty
::PolyTraitPredicate
<'tcx
>,
1322 dep_node
: DepNodeIndex
,
1323 candidate
: SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>,
1325 let tcx
= self.tcx();
1326 let pred
= cache_fresh_trait_pred
.skip_binder();
1327 let trait_ref
= pred
.trait_ref
;
1329 if !self.can_cache_candidate(&candidate
) {
1330 debug
!(?trait_ref
, ?candidate
, "insert_candidate_cache - candidate is not cacheable");
1334 if self.can_use_global_caches(param_env
) {
1335 if let Err(Overflow
) = candidate
{
1336 // Don't cache overflow globally; we only produce this in certain modes.
1337 } else if !trait_ref
.needs_infer() {
1338 if !candidate
.needs_infer() {
1339 debug
!(?trait_ref
, ?candidate
, "insert_candidate_cache global");
1340 // This may overwrite the cache with the same value.
1341 tcx
.selection_cache
.insert(
1342 (param_env
.and(trait_ref
).with_constness(pred
.constness
), pred
.polarity
),
1351 debug
!(?trait_ref
, ?candidate
, "insert_candidate_cache local");
1352 self.infcx
.selection_cache
.insert(
1353 (param_env
.and(trait_ref
).with_constness(pred
.constness
), pred
.polarity
),
1359 /// Matches a predicate against the bounds of its self type.
1361 /// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
1362 /// a projection, look at the bounds of `T::Bar`, see if we can find a
1363 /// `Baz` bound. We return indexes into the list returned by
1364 /// `tcx.item_bounds` for any applicable bounds.
1365 fn match_projection_obligation_against_definition_bounds(
1367 obligation
: &TraitObligation
<'tcx
>,
1368 ) -> smallvec
::SmallVec
<[usize; 2]> {
1369 let poly_trait_predicate
= self.infcx().resolve_vars_if_possible(obligation
.predicate
);
1370 let placeholder_trait_predicate
=
1371 self.infcx().replace_bound_vars_with_placeholders(poly_trait_predicate
);
1373 ?placeholder_trait_predicate
,
1374 "match_projection_obligation_against_definition_bounds"
1377 let tcx
= self.infcx
.tcx
;
1378 let (def_id
, substs
) = match *placeholder_trait_predicate
.trait_ref
.self_ty().kind() {
1379 ty
::Projection(ref data
) => (data
.item_def_id
, data
.substs
),
1380 ty
::Opaque(def_id
, substs
) => (def_id
, substs
),
1383 obligation
.cause
.span
,
1384 "match_projection_obligation_against_definition_bounds() called \
1385 but self-ty is not a projection: {:?}",
1386 placeholder_trait_predicate
.trait_ref
.self_ty()
1390 let bounds
= tcx
.item_bounds(def_id
).subst(tcx
, substs
);
1392 // The bounds returned by `item_bounds` may contain duplicates after
1393 // normalization, so try to deduplicate when possible to avoid
1394 // unnecessary ambiguity.
1395 let mut distinct_normalized_bounds
= FxHashSet
::default();
1397 let matching_bounds
= bounds
1400 .filter_map(|(idx
, bound
)| {
1401 let bound_predicate
= bound
.kind();
1402 if let ty
::PredicateKind
::Trait(pred
) = bound_predicate
.skip_binder() {
1403 let bound
= bound_predicate
.rebind(pred
.trait_ref
);
1404 if self.infcx
.probe(|_
| {
1405 match self.match_normalize_trait_ref(
1408 placeholder_trait_predicate
.trait_ref
,
1411 Ok(Some(normalized_trait
))
1412 if distinct_normalized_bounds
.insert(normalized_trait
) =>
1426 debug
!(?matching_bounds
, "match_projection_obligation_against_definition_bounds");
1430 /// Equates the trait in `obligation` with trait bound. If the two traits
1431 /// can be equated and the normalized trait bound doesn't contain inference
1432 /// variables or placeholders, the normalized bound is returned.
1433 fn match_normalize_trait_ref(
1435 obligation
: &TraitObligation
<'tcx
>,
1436 trait_bound
: ty
::PolyTraitRef
<'tcx
>,
1437 placeholder_trait_ref
: ty
::TraitRef
<'tcx
>,
1438 ) -> Result
<Option
<ty
::PolyTraitRef
<'tcx
>>, ()> {
1439 debug_assert
!(!placeholder_trait_ref
.has_escaping_bound_vars());
1440 if placeholder_trait_ref
.def_id
!= trait_bound
.def_id() {
1441 // Avoid unnecessary normalization
1445 let Normalized { value: trait_bound, obligations: _ }
= ensure_sufficient_stack(|| {
1446 project
::normalize_with_depth(
1448 obligation
.param_env
,
1449 obligation
.cause
.clone(),
1450 obligation
.recursion_depth
+ 1,
1455 .at(&obligation
.cause
, obligation
.param_env
)
1456 .sup(ty
::Binder
::dummy(placeholder_trait_ref
), trait_bound
)
1457 .map(|InferOk { obligations: _, value: () }
| {
1458 // This method is called within a probe, so we can't have
1459 // inference variables and placeholders escape.
1460 if !trait_bound
.needs_infer() && !trait_bound
.has_placeholders() {
1469 fn evaluate_where_clause
<'o
>(
1471 stack
: &TraitObligationStack
<'o
, 'tcx
>,
1472 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
1473 ) -> Result
<EvaluationResult
, OverflowError
> {
1474 self.evaluation_probe(|this
| {
1475 match this
.match_where_clause_trait_ref(stack
.obligation
, where_clause_trait_ref
) {
1476 Ok(obligations
) => this
.evaluate_predicates_recursively(stack
.list(), obligations
),
1477 Err(()) => Ok(EvaluatedToErr
),
1482 pub(super) fn match_projection_projections(
1484 obligation
: &ProjectionTyObligation
<'tcx
>,
1485 env_predicate
: PolyProjectionPredicate
<'tcx
>,
1486 potentially_unnormalized_candidates
: bool
,
1488 let mut nested_obligations
= Vec
::new();
1489 let (infer_predicate
, _
) = self.infcx
.replace_bound_vars_with_fresh_vars(
1490 obligation
.cause
.span
,
1491 LateBoundRegionConversionTime
::HigherRankedType
,
1494 let infer_projection
= if potentially_unnormalized_candidates
{
1495 ensure_sufficient_stack(|| {
1496 project
::normalize_with_depth_to(
1498 obligation
.param_env
,
1499 obligation
.cause
.clone(),
1500 obligation
.recursion_depth
+ 1,
1501 infer_predicate
.projection_ty
,
1502 &mut nested_obligations
,
1506 infer_predicate
.projection_ty
1510 .at(&obligation
.cause
, obligation
.param_env
)
1511 .sup(obligation
.predicate
, infer_projection
)
1512 .map_or(false, |InferOk { obligations, value: () }
| {
1513 self.evaluate_predicates_recursively(
1514 TraitObligationStackList
::empty(&ProvisionalEvaluationCache
::default()),
1515 nested_obligations
.into_iter().chain(obligations
),
1517 .map_or(false, |res
| res
.may_apply())
1521 ///////////////////////////////////////////////////////////////////////////
1524 // Winnowing is the process of attempting to resolve ambiguity by
1525 // probing further. During the winnowing process, we unify all
1526 // type variables and then we also attempt to evaluate recursive
1527 // bounds to see if they are satisfied.
1529 /// Returns `true` if `victim` should be dropped in favor of
1530 /// `other`. Generally speaking we will drop duplicate
1531 /// candidates and prefer where-clause candidates.
1533 /// See the comment for "SelectionCandidate" for more details.
1534 fn candidate_should_be_dropped_in_favor_of(
1536 victim
: &EvaluatedCandidate
<'tcx
>,
1537 other
: &EvaluatedCandidate
<'tcx
>,
1540 if victim
.candidate
== other
.candidate
{
1544 // Check if a bound would previously have been removed when normalizing
1545 // the param_env so that it can be given the lowest priority. See
1546 // #50825 for the motivation for this.
1547 let is_global
= |cand
: &ty
::PolyTraitRef
<'tcx
>| {
1548 cand
.is_global(self.infcx
.tcx
) && !cand
.has_late_bound_regions()
1551 // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
1552 // and `DiscriminantKindCandidate` to anything else.
1554 // This is a fix for #53123 and prevents winnowing from accidentally extending the
1555 // lifetime of a variable.
1556 match (&other
.candidate
, &victim
.candidate
) {
1557 (_
, AutoImplCandidate(..)) | (AutoImplCandidate(..), _
) => {
1559 "default implementations shouldn't be recorded \
1560 when there are other valid candidates"
1566 BuiltinCandidate { has_nested: false }
1567 | DiscriminantKindCandidate
1569 | ConstDropCandidate
,
1574 BuiltinCandidate { has_nested: false }
1575 | DiscriminantKindCandidate
1577 | ConstDropCandidate
,
1581 ParamCandidate((other
, other_polarity
)),
1582 ParamCandidate((victim
, victim_polarity
)),
1584 let same_except_bound_vars
= other
.value
.skip_binder()
1585 == victim
.value
.skip_binder()
1586 && other
.constness
== victim
.constness
1587 && other_polarity
== victim_polarity
1588 && !other
.value
.skip_binder().has_escaping_bound_vars();
1589 if same_except_bound_vars
{
1590 // See issue #84398. In short, we can generate multiple ParamCandidates which are
1591 // the same except for unused bound vars. Just pick the one with the fewest bound vars
1592 // or the current one if tied (they should both evaluate to the same answer). This is
1593 // probably best characterized as a "hack", since we might prefer to just do our
1594 // best to *not* create essentially duplicate candidates in the first place.
1595 other
.value
.bound_vars().len() <= victim
.value
.bound_vars().len()
1596 } else if other
.value
== victim
.value
1597 && victim
.constness
== ty
::BoundConstness
::NotConst
1598 && other_polarity
== victim_polarity
1600 // Drop otherwise equivalent non-const candidates in favor of const candidates.
1607 // Drop otherwise equivalent non-const fn pointer candidates
1608 (FnPointerCandidate { .. }
, FnPointerCandidate { is_const: false }
) => true,
1610 // Global bounds from the where clause should be ignored
1611 // here (see issue #50825). Otherwise, we have a where
1612 // clause so don't go around looking for impls.
1613 // Arbitrarily give param candidates priority
1614 // over projection and object candidates.
1616 ParamCandidate(ref cand
),
1619 | GeneratorCandidate
1620 | FnPointerCandidate { .. }
1621 | BuiltinObjectCandidate
1622 | BuiltinUnsizeCandidate
1623 | TraitUpcastingUnsizeCandidate(_
)
1624 | BuiltinCandidate { .. }
1625 | TraitAliasCandidate(..)
1626 | ObjectCandidate(_
)
1627 | ProjectionCandidate(_
),
1628 ) => !is_global(&cand
.0.value
),
1629 (ObjectCandidate(_
) | ProjectionCandidate(_
), ParamCandidate(ref cand
)) => {
1630 // Prefer these to a global where-clause bound
1631 // (see issue #50825).
1632 is_global(&cand
.0.value
)
1637 | GeneratorCandidate
1638 | FnPointerCandidate { .. }
1639 | BuiltinObjectCandidate
1640 | BuiltinUnsizeCandidate
1641 | TraitUpcastingUnsizeCandidate(_
)
1642 | BuiltinCandidate { has_nested: true }
1643 | TraitAliasCandidate(..),
1644 ParamCandidate(ref cand
),
1646 // Prefer these to a global where-clause bound
1647 // (see issue #50825).
1648 is_global(&cand
.0.value
) && other
.evaluation
.must_apply_modulo_regions()
1651 (ProjectionCandidate(i
), ProjectionCandidate(j
))
1652 | (ObjectCandidate(i
), ObjectCandidate(j
)) => {
1653 // Arbitrarily pick the lower numbered candidate for backwards
1654 // compatibility reasons. Don't let this affect inference.
1655 i
< j
&& !needs_infer
1657 (ObjectCandidate(_
), ProjectionCandidate(_
))
1658 | (ProjectionCandidate(_
), ObjectCandidate(_
)) => {
1659 bug
!("Have both object and projection candidate")
1662 // Arbitrarily give projection and object candidates priority.
1664 ObjectCandidate(_
) | ProjectionCandidate(_
),
1667 | GeneratorCandidate
1668 | FnPointerCandidate { .. }
1669 | BuiltinObjectCandidate
1670 | BuiltinUnsizeCandidate
1671 | TraitUpcastingUnsizeCandidate(_
)
1672 | BuiltinCandidate { .. }
1673 | TraitAliasCandidate(..),
1679 | GeneratorCandidate
1680 | FnPointerCandidate { .. }
1681 | BuiltinObjectCandidate
1682 | BuiltinUnsizeCandidate
1683 | TraitUpcastingUnsizeCandidate(_
)
1684 | BuiltinCandidate { .. }
1685 | TraitAliasCandidate(..),
1686 ObjectCandidate(_
) | ProjectionCandidate(_
),
1689 (&ImplCandidate(other_def
), &ImplCandidate(victim_def
)) => {
1690 // See if we can toss out `victim` based on specialization.
1691 // This requires us to know *for sure* that the `other` impl applies
1692 // i.e., `EvaluatedToOk`.
1694 // FIXME(@lcnr): Using `modulo_regions` here seems kind of scary
1695 // to me but is required for `std` to compile, so I didn't change it
1697 let tcx
= self.tcx();
1698 if other
.evaluation
.must_apply_modulo_regions() {
1699 if tcx
.specializes((other_def
, victim_def
)) {
1704 if other
.evaluation
.must_apply_considering_regions() {
1705 match tcx
.impls_are_allowed_to_overlap(other_def
, victim_def
) {
1706 Some(ty
::ImplOverlapKind
::Permitted { marker: true }
) => {
1707 // Subtle: If the predicate we are evaluating has inference
1708 // variables, do *not* allow discarding candidates due to
1709 // marker trait impls.
1711 // Without this restriction, we could end up accidentally
1712 // constrainting inference variables based on an arbitrarily
1713 // chosen trait impl.
1715 // Imagine we have the following code:
1718 // #[marker] trait MyTrait {}
1719 // impl MyTrait for u8 {}
1720 // impl MyTrait for bool {}
1723 // And we are evaluating the predicate `<_#0t as MyTrait>`.
1725 // During selection, we will end up with one candidate for each
1726 // impl of `MyTrait`. If we were to discard one impl in favor
1727 // of the other, we would be left with one candidate, causing
1728 // us to "successfully" select the predicate, unifying
1729 // _#0t with (for example) `u8`.
1731 // However, we have no reason to believe that this unification
1732 // is correct - we've essentially just picked an arbitrary
1733 // *possibility* for _#0t, and required that this be the *only*
1736 // Eventually, we will either:
1737 // 1) Unify all inference variables in the predicate through
1738 // some other means (e.g. type-checking of a function). We will
1739 // then be in a position to drop marker trait candidates
1740 // without constraining inference variables (since there are
1741 // none left to constrin)
1742 // 2) Be left with some unconstrained inference variables. We
1743 // will then correctly report an inference error, since the
1744 // existence of multiple marker trait impls tells us nothing
1745 // about which one should actually apply.
1756 // Everything else is ambiguous
1760 | GeneratorCandidate
1761 | FnPointerCandidate { .. }
1762 | BuiltinObjectCandidate
1763 | BuiltinUnsizeCandidate
1764 | TraitUpcastingUnsizeCandidate(_
)
1765 | BuiltinCandidate { has_nested: true }
1766 | TraitAliasCandidate(..),
1769 | GeneratorCandidate
1770 | FnPointerCandidate { .. }
1771 | BuiltinObjectCandidate
1772 | BuiltinUnsizeCandidate
1773 | TraitUpcastingUnsizeCandidate(_
)
1774 | BuiltinCandidate { has_nested: true }
1775 | TraitAliasCandidate(..),
1780 fn sized_conditions(
1782 obligation
: &TraitObligation
<'tcx
>,
1783 ) -> BuiltinImplConditions
<'tcx
> {
1784 use self::BuiltinImplConditions
::{Ambiguous, None, Where}
;
1786 // NOTE: binder moved to (*)
1787 let self_ty
= self.infcx
.shallow_resolve(obligation
.predicate
.skip_binder().self_ty());
1789 match self_ty
.kind() {
1790 ty
::Infer(ty
::IntVar(_
) | ty
::FloatVar(_
))
1801 | ty
::GeneratorWitness(..)
1806 // safe for everything
1807 Where(ty
::Binder
::dummy(Vec
::new()))
1810 ty
::Str
| ty
::Slice(_
) | ty
::Dynamic(..) | ty
::Foreign(..) => None
,
1812 ty
::Tuple(tys
) => Where(
1815 .rebind(tys
.last().into_iter().map(|k
| k
.expect_ty()).collect()),
1818 ty
::Adt(def
, substs
) => {
1819 let sized_crit
= def
.sized_constraint(self.tcx());
1820 // (*) binder moved here
1822 obligation
.predicate
.rebind({
1823 sized_crit
.iter().map(|ty
| ty
.subst(self.tcx(), substs
)).collect()
1828 ty
::Projection(_
) | ty
::Param(_
) | ty
::Opaque(..) => None
,
1829 ty
::Infer(ty
::TyVar(_
)) => Ambiguous
,
1833 | ty
::Infer(ty
::FreshTy(_
) | ty
::FreshIntTy(_
) | ty
::FreshFloatTy(_
)) => {
1834 bug
!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty
);
1839 fn copy_clone_conditions(
1841 obligation
: &TraitObligation
<'tcx
>,
1842 ) -> BuiltinImplConditions
<'tcx
> {
1843 // NOTE: binder moved to (*)
1844 let self_ty
= self.infcx
.shallow_resolve(obligation
.predicate
.skip_binder().self_ty());
1846 use self::BuiltinImplConditions
::{Ambiguous, None, Where}
;
1848 match *self_ty
.kind() {
1849 ty
::Infer(ty
::IntVar(_
))
1850 | ty
::Infer(ty
::FloatVar(_
))
1853 | ty
::Error(_
) => Where(ty
::Binder
::dummy(Vec
::new())),
1862 | ty
::Ref(_
, _
, hir
::Mutability
::Not
)
1863 | ty
::Array(..) => {
1864 // Implementations provided in libcore
1872 | ty
::GeneratorWitness(..)
1874 | ty
::Ref(_
, _
, hir
::Mutability
::Mut
) => None
,
1877 // (*) binder moved here
1878 Where(obligation
.predicate
.rebind(tys
.iter().map(|k
| k
.expect_ty()).collect()))
1881 ty
::Closure(_
, substs
) => {
1882 // (*) binder moved here
1883 let ty
= self.infcx
.shallow_resolve(substs
.as_closure().tupled_upvars_ty());
1884 if let ty
::Infer(ty
::TyVar(_
)) = ty
.kind() {
1885 // Not yet resolved.
1888 Where(obligation
.predicate
.rebind(substs
.as_closure().upvar_tys().collect()))
1892 ty
::Adt(..) | ty
::Projection(..) | ty
::Param(..) | ty
::Opaque(..) => {
1893 // Fallback to whatever user-defined impls exist in this case.
1897 ty
::Infer(ty
::TyVar(_
)) => {
1898 // Unbound type variable. Might or might not have
1899 // applicable impls and so forth, depending on what
1900 // those type variables wind up being bound to.
1906 | ty
::Infer(ty
::FreshTy(_
) | ty
::FreshIntTy(_
) | ty
::FreshFloatTy(_
)) => {
1907 bug
!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty
);
1912 /// For default impls, we need to break apart a type into its
1913 /// "constituent types" -- meaning, the types that it contains.
1915 /// Here are some (simple) examples:
1918 /// (i32, u32) -> [i32, u32]
1919 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1920 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1921 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1923 fn constituent_types_for_ty(
1925 t
: ty
::Binder
<'tcx
, Ty
<'tcx
>>,
1926 ) -> ty
::Binder
<'tcx
, Vec
<Ty
<'tcx
>>> {
1927 match *t
.skip_binder().kind() {
1936 | ty
::Infer(ty
::IntVar(_
) | ty
::FloatVar(_
))
1938 | ty
::Char
=> ty
::Binder
::dummy(Vec
::new()),
1944 | ty
::Projection(..)
1946 | ty
::Infer(ty
::TyVar(_
) | ty
::FreshTy(_
) | ty
::FreshIntTy(_
) | ty
::FreshFloatTy(_
)) => {
1947 bug
!("asked to assemble constituent types of unexpected type: {:?}", t
);
1950 ty
::RawPtr(ty
::TypeAndMut { ty: element_ty, .. }
) | ty
::Ref(_
, element_ty
, _
) => {
1951 t
.rebind(vec
![element_ty
])
1954 ty
::Array(element_ty
, _
) | ty
::Slice(element_ty
) => t
.rebind(vec
![element_ty
]),
1956 ty
::Tuple(ref tys
) => {
1957 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1958 t
.rebind(tys
.iter().map(|k
| k
.expect_ty()).collect())
1961 ty
::Closure(_
, ref substs
) => {
1962 let ty
= self.infcx
.shallow_resolve(substs
.as_closure().tupled_upvars_ty());
1966 ty
::Generator(_
, ref substs
, _
) => {
1967 let ty
= self.infcx
.shallow_resolve(substs
.as_generator().tupled_upvars_ty());
1968 let witness
= substs
.as_generator().witness();
1969 t
.rebind(vec
![ty
].into_iter().chain(iter
::once(witness
)).collect())
1972 ty
::GeneratorWitness(types
) => {
1973 debug_assert
!(!types
.has_escaping_bound_vars());
1974 types
.map_bound(|types
| types
.to_vec())
1977 // For `PhantomData<T>`, we pass `T`.
1978 ty
::Adt(def
, substs
) if def
.is_phantom_data() => t
.rebind(substs
.types().collect()),
1980 ty
::Adt(def
, substs
) => {
1981 t
.rebind(def
.all_fields().map(|f
| f
.ty(self.tcx(), substs
)).collect())
1984 ty
::Opaque(def_id
, substs
) => {
1985 // We can resolve the `impl Trait` to its concrete type,
1986 // which enforces a DAG between the functions requiring
1987 // the auto trait bounds in question.
1988 t
.rebind(vec
![self.tcx().type_of(def_id
).subst(self.tcx(), substs
)])
1993 fn collect_predicates_for_types(
1995 param_env
: ty
::ParamEnv
<'tcx
>,
1996 cause
: ObligationCause
<'tcx
>,
1997 recursion_depth
: usize,
1998 trait_def_id
: DefId
,
1999 types
: ty
::Binder
<'tcx
, Vec
<Ty
<'tcx
>>>,
2000 ) -> Vec
<PredicateObligation
<'tcx
>> {
2001 // Because the types were potentially derived from
2002 // higher-ranked obligations they may reference late-bound
2003 // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
2004 // yield a type like `for<'a> &'a i32`. In general, we
2005 // maintain the invariant that we never manipulate bound
2006 // regions, so we have to process these bound regions somehow.
2008 // The strategy is to:
2010 // 1. Instantiate those regions to placeholder regions (e.g.,
2011 // `for<'a> &'a i32` becomes `&0 i32`.
2012 // 2. Produce something like `&'0 i32 : Copy`
2013 // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
2017 .skip_binder() // binder moved -\
2020 let ty
: ty
::Binder
<'tcx
, Ty
<'tcx
>> = types
.rebind(ty
); // <----/
2022 self.infcx
.commit_unconditionally(|_
| {
2023 let placeholder_ty
= self.infcx
.replace_bound_vars_with_placeholders(ty
);
2024 let Normalized { value: normalized_ty, mut obligations }
=
2025 ensure_sufficient_stack(|| {
2026 project
::normalize_with_depth(
2034 let placeholder_obligation
= predicate_for_trait_def(
2043 obligations
.push(placeholder_obligation
);
2050 ///////////////////////////////////////////////////////////////////////////
2053 // Matching is a common path used for both evaluation and
2054 // confirmation. It basically unifies types that appear in impls
2055 // and traits. This does affect the surrounding environment;
2056 // therefore, when used during evaluation, match routines must be
2057 // run inside of a `probe()` so that their side-effects are
2063 obligation
: &TraitObligation
<'tcx
>,
2064 ) -> Normalized
<'tcx
, SubstsRef
<'tcx
>> {
2065 match self.match_impl(impl_def_id
, obligation
) {
2066 Ok(substs
) => substs
,
2069 "Impl {:?} was matchable against {:?} but now is not",
2077 #[tracing::instrument(level = "debug", skip(self))]
2081 obligation
: &TraitObligation
<'tcx
>,
2082 ) -> Result
<Normalized
<'tcx
, SubstsRef
<'tcx
>>, ()> {
2083 let impl_trait_ref
= self.tcx().impl_trait_ref(impl_def_id
).unwrap();
2085 // Before we create the substitutions and everything, first
2086 // consider a "quick reject". This avoids creating more types
2087 // and so forth that we need to.
2088 if self.fast_reject_trait_refs(obligation
, &impl_trait_ref
) {
2092 let placeholder_obligation
=
2093 self.infcx().replace_bound_vars_with_placeholders(obligation
.predicate
);
2094 let placeholder_obligation_trait_ref
= placeholder_obligation
.trait_ref
;
2096 let impl_substs
= self.infcx
.fresh_substs_for_item(obligation
.cause
.span
, impl_def_id
);
2098 let impl_trait_ref
= impl_trait_ref
.subst(self.tcx(), impl_substs
);
2100 debug
!(?impl_trait_ref
);
2102 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations }
=
2103 ensure_sufficient_stack(|| {
2104 project
::normalize_with_depth(
2106 obligation
.param_env
,
2107 obligation
.cause
.clone(),
2108 obligation
.recursion_depth
+ 1,
2113 debug
!(?impl_trait_ref
, ?placeholder_obligation_trait_ref
);
2115 let cause
= ObligationCause
::new(
2116 obligation
.cause
.span
,
2117 obligation
.cause
.body_id
,
2118 ObligationCauseCode
::MatchImpl(obligation
.cause
.clone(), impl_def_id
),
2121 let InferOk { obligations, .. }
= self
2123 .at(&cause
, obligation
.param_env
)
2124 .eq(placeholder_obligation_trait_ref
, impl_trait_ref
)
2125 .map_err(|e
| debug
!("match_impl: failed eq_trait_refs due to `{}`", e
))?
;
2126 nested_obligations
.extend(obligations
);
2129 && self.tcx().impl_polarity(impl_def_id
) == ty
::ImplPolarity
::Reservation
2131 debug
!("match_impl: reservation impls only apply in intercrate mode");
2135 debug
!(?impl_substs
, ?nested_obligations
, "match_impl: success");
2136 Ok(Normalized { value: impl_substs, obligations: nested_obligations }
)
2139 fn fast_reject_trait_refs(
2141 obligation
: &TraitObligation
<'_
>,
2142 impl_trait_ref
: &ty
::TraitRef
<'_
>,
2144 // We can avoid creating type variables and doing the full
2145 // substitution if we find that any of the input types, when
2146 // simplified, do not match.
2148 iter
::zip(obligation
.predicate
.skip_binder().trait_ref
.substs
, impl_trait_ref
.substs
).any(
2149 |(obligation_arg
, impl_arg
)| {
2150 match (obligation_arg
.unpack(), impl_arg
.unpack()) {
2151 (GenericArgKind
::Type(obligation_ty
), GenericArgKind
::Type(impl_ty
)) => {
2152 let simplified_obligation_ty
=
2153 fast_reject
::simplify_type(self.tcx(), obligation_ty
, true);
2154 let simplified_impl_ty
=
2155 fast_reject
::simplify_type(self.tcx(), impl_ty
, false);
2157 simplified_obligation_ty
.is_some()
2158 && simplified_impl_ty
.is_some()
2159 && simplified_obligation_ty
!= simplified_impl_ty
2161 (GenericArgKind
::Lifetime(_
), GenericArgKind
::Lifetime(_
)) => {
2162 // Lifetimes can never cause a rejection.
2165 (GenericArgKind
::Const(_
), GenericArgKind
::Const(_
)) => {
2166 // Conservatively ignore consts (i.e. assume they might
2167 // unify later) until we have `fast_reject` support for
2168 // them (if we'll ever need it, even).
2171 _
=> unreachable
!(),
2177 /// Normalize `where_clause_trait_ref` and try to match it against
2178 /// `obligation`. If successful, return any predicates that
2179 /// result from the normalization.
2180 fn match_where_clause_trait_ref(
2182 obligation
: &TraitObligation
<'tcx
>,
2183 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
2184 ) -> Result
<Vec
<PredicateObligation
<'tcx
>>, ()> {
2185 self.match_poly_trait_ref(obligation
, where_clause_trait_ref
)
2188 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2189 /// obligation is satisfied.
2190 #[instrument(skip(self), level = "debug")]
2191 fn match_poly_trait_ref(
2193 obligation
: &TraitObligation
<'tcx
>,
2194 poly_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
2195 ) -> Result
<Vec
<PredicateObligation
<'tcx
>>, ()> {
2197 .at(&obligation
.cause
, obligation
.param_env
)
2198 .sup(obligation
.predicate
.to_poly_trait_ref(), poly_trait_ref
)
2199 .map(|InferOk { obligations, .. }
| obligations
)
2203 ///////////////////////////////////////////////////////////////////////////
2206 fn match_fresh_trait_refs(
2208 previous
: ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>,
2209 current
: ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>,
2210 param_env
: ty
::ParamEnv
<'tcx
>,
2212 let mut matcher
= ty
::_match
::Match
::new(self.tcx(), param_env
);
2213 matcher
.relate(previous
, current
).is_ok()
2218 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
2219 obligation
: &'o TraitObligation
<'tcx
>,
2220 ) -> TraitObligationStack
<'o
, 'tcx
> {
2221 let fresh_trait_ref
= obligation
2223 .to_poly_trait_ref()
2224 .fold_with(&mut self.freshener
)
2225 .with_constness(obligation
.predicate
.skip_binder().constness
);
2227 let dfn
= previous_stack
.cache
.next_dfn();
2228 let depth
= previous_stack
.depth() + 1;
2229 TraitObligationStack
{
2232 reached_depth
: Cell
::new(depth
),
2233 previous
: previous_stack
,
2239 #[instrument(skip(self), level = "debug")]
2240 fn closure_trait_ref_unnormalized(
2242 obligation
: &TraitObligation
<'tcx
>,
2243 substs
: SubstsRef
<'tcx
>,
2244 ) -> ty
::PolyTraitRef
<'tcx
> {
2245 let closure_sig
= substs
.as_closure().sig();
2247 debug
!(?closure_sig
);
2249 // (1) Feels icky to skip the binder here, but OTOH we know
2250 // that the self-type is an unboxed closure type and hence is
2251 // in fact unparameterized (or at least does not reference any
2252 // regions bound in the obligation). Still probably some
2253 // refactoring could make this nicer.
2254 closure_trait_ref_and_return_type(
2256 obligation
.predicate
.def_id(),
2257 obligation
.predicate
.skip_binder().self_ty(), // (1)
2259 util
::TupleArgumentsFlag
::No
,
2261 .map_bound(|(trait_ref
, _
)| trait_ref
)
2264 fn generator_trait_ref_unnormalized(
2266 obligation
: &TraitObligation
<'tcx
>,
2267 substs
: SubstsRef
<'tcx
>,
2268 ) -> ty
::PolyTraitRef
<'tcx
> {
2269 let gen_sig
= substs
.as_generator().poly_sig();
2271 // (1) Feels icky to skip the binder here, but OTOH we know
2272 // that the self-type is an generator type and hence is
2273 // in fact unparameterized (or at least does not reference any
2274 // regions bound in the obligation). Still probably some
2275 // refactoring could make this nicer.
2277 super::util
::generator_trait_ref_and_outputs(
2279 obligation
.predicate
.def_id(),
2280 obligation
.predicate
.skip_binder().self_ty(), // (1)
2283 .map_bound(|(trait_ref
, ..)| trait_ref
)
2286 /// Returns the obligations that are implied by instantiating an
2287 /// impl or trait. The obligations are substituted and fully
2288 /// normalized. This is used when confirming an impl or default
2290 #[tracing::instrument(level = "debug", skip(self, cause, param_env))]
2291 fn impl_or_trait_obligations(
2293 cause
: ObligationCause
<'tcx
>,
2294 recursion_depth
: usize,
2295 param_env
: ty
::ParamEnv
<'tcx
>,
2296 def_id
: DefId
, // of impl or trait
2297 substs
: SubstsRef
<'tcx
>, // for impl or trait
2298 ) -> Vec
<PredicateObligation
<'tcx
>> {
2299 let tcx
= self.tcx();
2301 // To allow for one-pass evaluation of the nested obligation,
2302 // each predicate must be preceded by the obligations required
2304 // for example, if we have:
2305 // impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
2306 // the impl will have the following predicates:
2307 // <V as Iterator>::Item = U,
2308 // U: Iterator, U: Sized,
2309 // V: Iterator, V: Sized,
2310 // <U as Iterator>::Item: Copy
2311 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2312 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2313 // `$1: Copy`, so we must ensure the obligations are emitted in
2315 let predicates
= tcx
.predicates_of(def_id
);
2316 debug
!(?predicates
);
2317 assert_eq
!(predicates
.parent
, None
);
2318 let mut obligations
= Vec
::with_capacity(predicates
.predicates
.len());
2319 for (predicate
, _
) in predicates
.predicates
{
2321 let predicate
= normalize_with_depth_to(
2326 predicate
.subst(tcx
, substs
),
2329 obligations
.push(Obligation
{
2330 cause
: cause
.clone(),
2337 // We are performing deduplication here to avoid exponential blowups
2338 // (#38528) from happening, but the real cause of the duplication is
2339 // unknown. What we know is that the deduplication avoids exponential
2340 // amount of predicates being propagated when processing deeply nested
2343 // This code is hot enough that it's worth avoiding the allocation
2344 // required for the FxHashSet when possible. Special-casing lengths 0,
2345 // 1 and 2 covers roughly 75-80% of the cases.
2346 if obligations
.len() <= 1 {
2347 // No possibility of duplicates.
2348 } else if obligations
.len() == 2 {
2349 // Only two elements. Drop the second if they are equal.
2350 if obligations
[0] == obligations
[1] {
2351 obligations
.truncate(1);
2354 // Three or more elements. Use a general deduplication process.
2355 let mut seen
= FxHashSet
::default();
2356 obligations
.retain(|i
| seen
.insert(i
.clone()));
2363 trait TraitObligationExt
<'tcx
> {
2366 variant
: fn(DerivedObligationCause
<'tcx
>) -> ObligationCauseCode
<'tcx
>,
2367 ) -> ObligationCause
<'tcx
>;
2370 impl<'tcx
> TraitObligationExt
<'tcx
> for TraitObligation
<'tcx
> {
2373 variant
: fn(DerivedObligationCause
<'tcx
>) -> ObligationCauseCode
<'tcx
>,
2374 ) -> ObligationCause
<'tcx
> {
2376 * Creates a cause for obligations that are derived from
2377 * `obligation` by a recursive search (e.g., for a builtin
2378 * bound, or eventually a `auto trait Foo`). If `obligation`
2379 * is itself a derived obligation, this is just a clone, but
2380 * otherwise we create a "derived obligation" cause so as to
2381 * keep track of the original root obligation for error
2385 let obligation
= self;
2387 // NOTE(flaper87): As of now, it keeps track of the whole error
2388 // chain. Ideally, we should have a way to configure this either
2389 // by using -Z verbose or just a CLI argument.
2390 let derived_cause
= DerivedObligationCause
{
2391 parent_trait_ref
: obligation
.predicate
.to_poly_trait_ref(),
2392 parent_code
: Lrc
::new(obligation
.cause
.code
.clone()),
2394 let derived_code
= variant(derived_cause
);
2395 ObligationCause
::new(obligation
.cause
.span
, obligation
.cause
.body_id
, derived_code
)
2399 impl<'o
, 'tcx
> TraitObligationStack
<'o
, 'tcx
> {
2400 fn list(&'o
self) -> TraitObligationStackList
<'o
, 'tcx
> {
2401 TraitObligationStackList
::with(self)
2404 fn cache(&self) -> &'o ProvisionalEvaluationCache
<'tcx
> {
2408 fn iter(&'o
self) -> TraitObligationStackList
<'o
, 'tcx
> {
2412 /// Indicates that attempting to evaluate this stack entry
2413 /// required accessing something from the stack at depth `reached_depth`.
2414 fn update_reached_depth(&self, reached_depth
: usize) {
2416 self.depth
>= reached_depth
,
2417 "invoked `update_reached_depth` with something under this stack: \
2418 self.depth={} reached_depth={}",
2422 debug
!(reached_depth
, "update_reached_depth");
2424 while reached_depth
< p
.depth
{
2425 debug
!(?p
.fresh_trait_ref
, "update_reached_depth: marking as cycle participant");
2426 p
.reached_depth
.set(p
.reached_depth
.get().min(reached_depth
));
2427 p
= p
.previous
.head
.unwrap();
2432 /// The "provisional evaluation cache" is used to store intermediate cache results
2433 /// when solving auto traits. Auto traits are unusual in that they can support
2434 /// cycles. So, for example, a "proof tree" like this would be ok:
2436 /// - `Foo<T>: Send` :-
2437 /// - `Bar<T>: Send` :-
2438 /// - `Foo<T>: Send` -- cycle, but ok
2439 /// - `Baz<T>: Send`
2441 /// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
2442 /// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
2443 /// For non-auto traits, this cycle would be an error, but for auto traits (because
2444 /// they are coinductive) it is considered ok.
2446 /// However, there is a complication: at the point where we have
2447 /// "proven" `Bar<T>: Send`, we have in fact only proven it
2448 /// *provisionally*. In particular, we proved that `Bar<T>: Send`
2449 /// *under the assumption* that `Foo<T>: Send`. But what if we later
2450 /// find out this assumption is wrong? Specifically, we could
2451 /// encounter some kind of error proving `Baz<T>: Send`. In that case,
2452 /// `Bar<T>: Send` didn't turn out to be true.
2454 /// In Issue #60010, we found a bug in rustc where it would cache
2455 /// these intermediate results. This was fixed in #60444 by disabling
2456 /// *all* caching for things involved in a cycle -- in our example,
2457 /// that would mean we don't cache that `Bar<T>: Send`. But this led
2458 /// to large slowdowns.
2460 /// Specifically, imagine this scenario, where proving `Baz<T>: Send`
2461 /// first requires proving `Bar<T>: Send` (which is true:
2463 /// - `Foo<T>: Send` :-
2464 /// - `Bar<T>: Send` :-
2465 /// - `Foo<T>: Send` -- cycle, but ok
2466 /// - `Baz<T>: Send`
2467 /// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
2468 /// - `*const T: Send` -- but what if we later encounter an error?
2470 /// The *provisional evaluation cache* resolves this issue. It stores
2471 /// cache results that we've proven but which were involved in a cycle
2472 /// in some way. We track the minimal stack depth (i.e., the
2473 /// farthest from the top of the stack) that we are dependent on.
2474 /// The idea is that the cache results within are all valid -- so long as
2475 /// none of the nodes in between the current node and the node at that minimum
2476 /// depth result in an error (in which case the cached results are just thrown away).
2478 /// During evaluation, we consult this provisional cache and rely on
2479 /// it. Accessing a cached value is considered equivalent to accessing
2480 /// a result at `reached_depth`, so it marks the *current* solution as
2481 /// provisional as well. If an error is encountered, we toss out any
2482 /// provisional results added from the subtree that encountered the
2483 /// error. When we pop the node at `reached_depth` from the stack, we
2484 /// can commit all the things that remain in the provisional cache.
2485 struct ProvisionalEvaluationCache
<'tcx
> {
2486 /// next "depth first number" to issue -- just a counter
2489 /// Map from cache key to the provisionally evaluated thing.
2490 /// The cache entries contain the result but also the DFN in which they
2491 /// were added. The DFN is used to clear out values on failure.
2493 /// Imagine we have a stack like:
2495 /// - `A B C` and we add a cache for the result of C (DFN 2)
2496 /// - Then we have a stack `A B D` where `D` has DFN 3
2497 /// - We try to solve D by evaluating E: `A B D E` (DFN 4)
2498 /// - `E` generates various cache entries which have cyclic dependices on `B`
2499 /// - `A B D E F` and so forth
2500 /// - the DFN of `F` for example would be 5
2501 /// - then we determine that `E` is in error -- we will then clear
2502 /// all cache values whose DFN is >= 4 -- in this case, that
2503 /// means the cached value for `F`.
2504 map
: RefCell
<FxHashMap
<ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>, ProvisionalEvaluation
>>,
2507 /// A cache value for the provisional cache: contains the depth-first
2508 /// number (DFN) and result.
2509 #[derive(Copy, Clone, Debug)]
2510 struct ProvisionalEvaluation
{
2512 reached_depth
: usize,
2513 result
: EvaluationResult
,
2516 impl<'tcx
> Default
for ProvisionalEvaluationCache
<'tcx
> {
2517 fn default() -> Self {
2518 Self { dfn: Cell::new(0), map: Default::default() }
2522 impl<'tcx
> ProvisionalEvaluationCache
<'tcx
> {
2523 /// Get the next DFN in sequence (basically a counter).
2524 fn next_dfn(&self) -> usize {
2525 let result
= self.dfn
.get();
2526 self.dfn
.set(result
+ 1);
2530 /// Check the provisional cache for any result for
2531 /// `fresh_trait_ref`. If there is a hit, then you must consider
2532 /// it an access to the stack slots at depth
2533 /// `reached_depth` (from the returned value).
2536 fresh_trait_ref
: ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>,
2537 ) -> Option
<ProvisionalEvaluation
> {
2540 "get_provisional = {:#?}",
2541 self.map
.borrow().get(&fresh_trait_ref
),
2543 Some(*self.map
.borrow().get(&fresh_trait_ref
)?
)
2546 /// Insert a provisional result into the cache. The result came
2547 /// from the node with the given DFN. It accessed a minimum depth
2548 /// of `reached_depth` to compute. It evaluated `fresh_trait_ref`
2549 /// and resulted in `result`.
2550 fn insert_provisional(
2553 reached_depth
: usize,
2554 fresh_trait_ref
: ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>,
2555 result
: EvaluationResult
,
2557 debug
!(?from_dfn
, ?fresh_trait_ref
, ?result
, "insert_provisional");
2559 let mut map
= self.map
.borrow_mut();
2561 // Subtle: when we complete working on the DFN `from_dfn`, anything
2562 // that remains in the provisional cache must be dependent on some older
2563 // stack entry than `from_dfn`. We have to update their depth with our transitive
2564 // depth in that case or else it would be referring to some popped note.
2567 // A (reached depth 0)
2569 // B // depth 1 -- reached depth = 0
2570 // C // depth 2 -- reached depth = 1 (should be 0)
2573 // D (reached depth 1)
2574 // C (cache -- reached depth = 2)
2575 for (_k
, v
) in &mut *map
{
2576 if v
.from_dfn
>= from_dfn
{
2577 v
.reached_depth
= reached_depth
.min(v
.reached_depth
);
2581 map
.insert(fresh_trait_ref
, ProvisionalEvaluation { from_dfn, reached_depth, result }
);
2584 /// Invoked when the node with dfn `dfn` does not get a successful
2585 /// result. This will clear out any provisional cache entries
2586 /// that were added since `dfn` was created. This is because the
2587 /// provisional entries are things which must assume that the
2588 /// things on the stack at the time of their creation succeeded --
2589 /// since the failing node is presently at the top of the stack,
2590 /// these provisional entries must either depend on it or some
2592 fn on_failure(&self, dfn
: usize) {
2593 debug
!(?dfn
, "on_failure");
2594 self.map
.borrow_mut().retain(|key
, eval
| {
2595 if !eval
.from_dfn
>= dfn
{
2596 debug
!("on_failure: removing {:?}", key
);
2604 /// Invoked when the node at depth `depth` completed without
2605 /// depending on anything higher in the stack (if that completion
2606 /// was a failure, then `on_failure` should have been invoked
2607 /// already). The callback `op` will be invoked for each
2608 /// provisional entry that we can now confirm.
2610 /// Note that we may still have provisional cache items remaining
2611 /// in the cache when this is done. For example, if there is a
2614 /// * A depends on...
2615 /// * B depends on A
2616 /// * C depends on...
2617 /// * D depends on C
2620 /// Then as we complete the C node we will have a provisional cache
2621 /// with results for A, B, C, and D. This method would clear out
2622 /// the C and D results, but leave A and B provisional.
2624 /// This is determined based on the DFN: we remove any provisional
2625 /// results created since `dfn` started (e.g., in our example, dfn
2626 /// would be 2, representing the C node, and hence we would
2627 /// remove the result for D, which has DFN 3, but not the results for
2628 /// A and B, which have DFNs 0 and 1 respectively).
2632 mut op
: impl FnMut(ty
::ConstnessAnd
<ty
::PolyTraitRef
<'tcx
>>, EvaluationResult
),
2634 debug
!(?dfn
, "on_completion");
2636 for (fresh_trait_ref
, eval
) in
2637 self.map
.borrow_mut().drain_filter(|_k
, eval
| eval
.from_dfn
>= dfn
)
2639 debug
!(?fresh_trait_ref
, ?eval
, "on_completion");
2641 op(fresh_trait_ref
, eval
.result
);
2646 #[derive(Copy, Clone)]
2647 struct TraitObligationStackList
<'o
, 'tcx
> {
2648 cache
: &'o ProvisionalEvaluationCache
<'tcx
>,
2649 head
: Option
<&'o TraitObligationStack
<'o
, 'tcx
>>,
2652 impl<'o
, 'tcx
> TraitObligationStackList
<'o
, 'tcx
> {
2653 fn empty(cache
: &'o ProvisionalEvaluationCache
<'tcx
>) -> TraitObligationStackList
<'o
, 'tcx
> {
2654 TraitObligationStackList { cache, head: None }
2657 fn with(r
: &'o TraitObligationStack
<'o
, 'tcx
>) -> TraitObligationStackList
<'o
, 'tcx
> {
2658 TraitObligationStackList { cache: r.cache(), head: Some(r) }
2661 fn head(&self) -> Option
<&'o TraitObligationStack
<'o
, 'tcx
>> {
2665 fn depth(&self) -> usize {
2666 if let Some(head
) = self.head { head.depth }
else { 0 }
2670 impl<'o
, 'tcx
> Iterator
for TraitObligationStackList
<'o
, 'tcx
> {
2671 type Item
= &'o TraitObligationStack
<'o
, 'tcx
>;
2673 fn next(&mut self) -> Option
<&'o TraitObligationStack
<'o
, 'tcx
>> {
2680 impl<'o
, 'tcx
> fmt
::Debug
for TraitObligationStack
<'o
, 'tcx
> {
2681 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
2682 write
!(f
, "TraitObligationStack({:?})", self.obligation
)