compiler/rustc_trait_selection/src/traits/select/mod.rs

   1 //! Candidate selection. See the [rustc dev guide] for more information on how this works.
   2 //!
   3 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
   4
   5 // FIXME: The `map` field in ProvisionalEvaluationCache should be changed to
   6 // a `FxIndexMap` to avoid query instability, but right now it causes a perf regression. This would be
   7 // fixed or at least lightened by the addition of the `drain_filter` method to `FxIndexMap`
   8 // Relevant: https://github.com/rust-lang/rust/pull/103723 and https://github.com/bluss/indexmap/issues/242
   9 #![allow(rustc::potential_query_instability)]
  10
  11 use self::EvaluationResult::*;
  12 use self::SelectionCandidate::*;
  13
  14 use super::coherence::{self, Conflict};
  15 use super::const_evaluatable;
  16 use super::project;
  17 use super::project::normalize_with_depth_to;
  18 use super::project::ProjectionTyObligation;
  19 use super::util;
  20 use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
  21 use super::wf;
  22 use super::{
  23     ErrorReporting, ImplDerivedObligation, ImplDerivedObligationCause, Normalized, Obligation,
  24     ObligationCause, ObligationCauseCode, Overflow, PredicateObligation, Selection, SelectionError,
  25     SelectionResult, TraitObligation, TraitQueryMode,
  26 };
  27
  28 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
  29 use crate::traits::error_reporting::TypeErrCtxtExt;
  30 use crate::traits::project::ProjectAndUnifyResult;
  31 use crate::traits::project::ProjectionCacheKeyExt;
  32 use crate::traits::ProjectionCacheKey;
  33 use crate::traits::Unimplemented;
  34 use rustc_data_structures::fx::FxHashMap;
  35 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
  36 use rustc_data_structures::stack::ensure_sufficient_stack;
  37 use rustc_errors::Diagnostic;
  38 use rustc_hir as hir;
  39 use rustc_hir::def_id::DefId;
  40 use rustc_infer::infer::LateBoundRegionConversionTime;
  41 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
  42 use rustc_middle::mir::interpret::ErrorHandled;
  43 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  44 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
  45 use rustc_middle::ty::fold::BottomUpFolder;
  46 use rustc_middle::ty::relate::TypeRelation;
  47 use rustc_middle::ty::SubstsRef;
  48 use rustc_middle::ty::{self, EarlyBinder, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
  49 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitable};
  50 use rustc_span::symbol::sym;
  51
  52 use std::cell::{Cell, RefCell};
  53 use std::cmp;
  54 use std::fmt::{self, Display};
  55 use std::iter;
  56
  57 pub use rustc_middle::traits::select::*;
  58 use rustc_middle::ty::print::with_no_trimmed_paths;
  59
  60 mod candidate_assembly;
  61 mod confirmation;
  62
  63 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
  64 pub enum IntercrateAmbiguityCause {
  65     DownstreamCrate { trait_desc: String, self_desc: Option<String> },
  66     UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> },
  67     ReservationImpl { message: String },
  68 }
  69
  70 impl IntercrateAmbiguityCause {
  71     /// Emits notes when the overlap is caused by complex intercrate ambiguities.
  72     /// See #23980 for details.
  73     pub fn add_intercrate_ambiguity_hint(&self, err: &mut Diagnostic) {
  74         err.note(&self.intercrate_ambiguity_hint());
  75     }
  76
  77     pub fn intercrate_ambiguity_hint(&self) -> String {
  78         match self {
  79             IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc } => {
  80                 let self_desc = if let Some(ty) = self_desc {
  81                     format!(" for type `{}`", ty)
  82                 } else {
  83                     String::new()
  84                 };
  85                 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
  86             }
  87             IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc } => {
  88                 let self_desc = if let Some(ty) = self_desc {
  89                     format!(" for type `{}`", ty)
  90                 } else {
  91                     String::new()
  92                 };
  93                 format!(
  94                     "upstream crates may add a new impl of trait `{}`{} \
  95                      in future versions",
  96                     trait_desc, self_desc
  97                 )
  98             }
  99             IntercrateAmbiguityCause::ReservationImpl { message } => message.clone(),
 100         }
 101     }
 102 }
 103
 104 pub struct SelectionContext<'cx, 'tcx> {
 105     pub infcx: &'cx InferCtxt<'tcx>,
 106
 107     /// Freshener used specifically for entries on the obligation
 108     /// stack. This ensures that all entries on the stack at one time
 109     /// will have the same set of placeholder entries, which is
 110     /// important for checking for trait bounds that recursively
 111     /// require themselves.
 112     freshener: TypeFreshener<'cx, 'tcx>,
 113
 114     /// If `intercrate` is set, we remember predicates which were
 115     /// considered ambiguous because of impls potentially added in other crates.
 116     /// This is used in coherence to give improved diagnostics.
 117     /// We don't do his until we detect a coherence error because it can
 118     /// lead to false overflow results (#47139) and because always
 119     /// computing it may negatively impact performance.
 120     intercrate_ambiguity_causes: Option<FxIndexSet<IntercrateAmbiguityCause>>,
 121
 122     /// The mode that trait queries run in, which informs our error handling
 123     /// policy. In essence, canonicalized queries need their errors propagated
 124     /// rather than immediately reported because we do not have accurate spans.
 125     query_mode: TraitQueryMode,
 126 }
 127
 128 // A stack that walks back up the stack frame.
 129 struct TraitObligationStack<'prev, 'tcx> {
 130     obligation: &'prev TraitObligation<'tcx>,
 131
 132     /// The trait predicate from `obligation` but "freshened" with the
 133     /// selection-context's freshener. Used to check for recursion.
 134     fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
 135
 136     /// Starts out equal to `depth` -- if, during evaluation, we
 137     /// encounter a cycle, then we will set this flag to the minimum
 138     /// depth of that cycle for all participants in the cycle. These
 139     /// participants will then forego caching their results. This is
 140     /// not the most efficient solution, but it addresses #60010. The
 141     /// problem we are trying to prevent:
 142     ///
 143     /// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
 144     /// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
 145     /// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
 146     ///
 147     /// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
 148     /// is `EvaluatedToOk`; this is because they were only considered
 149     /// ok on the premise that if `A: AutoTrait` held, but we indeed
 150     /// encountered a problem (later on) with `A: AutoTrait. So we
 151     /// currently set a flag on the stack node for `B: AutoTrait` (as
 152     /// well as the second instance of `A: AutoTrait`) to suppress
 153     /// caching.
 154     ///
 155     /// This is a simple, targeted fix. A more-performant fix requires
 156     /// deeper changes, but would permit more caching: we could
 157     /// basically defer caching until we have fully evaluated the
 158     /// tree, and then cache the entire tree at once. In any case, the
 159     /// performance impact here shouldn't be so horrible: every time
 160     /// this is hit, we do cache at least one trait, so we only
 161     /// evaluate each member of a cycle up to N times, where N is the
 162     /// length of the cycle. This means the performance impact is
 163     /// bounded and we shouldn't have any terrible worst-cases.
 164     reached_depth: Cell<usize>,
 165
 166     previous: TraitObligationStackList<'prev, 'tcx>,
 167
 168     /// The number of parent frames plus one (thus, the topmost frame has depth 1).
 169     depth: usize,
 170
 171     /// The depth-first number of this node in the search graph -- a
 172     /// pre-order index. Basically, a freshly incremented counter.
 173     dfn: usize,
 174 }
 175
 176 struct SelectionCandidateSet<'tcx> {
 177     // A list of candidates that definitely apply to the current
 178     // obligation (meaning: types unify).
 179     vec: Vec<SelectionCandidate<'tcx>>,
 180
 181     // If `true`, then there were candidates that might or might
 182     // not have applied, but we couldn't tell. This occurs when some
 183     // of the input types are type variables, in which case there are
 184     // various "builtin" rules that might or might not trigger.
 185     ambiguous: bool,
 186 }
 187
 188 #[derive(PartialEq, Eq, Debug, Clone)]
 189 struct EvaluatedCandidate<'tcx> {
 190     candidate: SelectionCandidate<'tcx>,
 191     evaluation: EvaluationResult,
 192 }
 193
 194 /// When does the builtin impl for `T: Trait` apply?
 195 #[derive(Debug)]
 196 enum BuiltinImplConditions<'tcx> {
 197     /// The impl is conditional on `T1, T2, ...: Trait`.
 198     Where(ty::Binder<'tcx, Vec<Ty<'tcx>>>),
 199     /// There is no built-in impl. There may be some other
 200     /// candidate (a where-clause or user-defined impl).
 201     None,
 202     /// It is unknown whether there is an impl.
 203     Ambiguous,
 204 }
 205
 206 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 207     pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
 208         SelectionContext {
 209             infcx,
 210             freshener: infcx.freshener_keep_static(),
 211             intercrate_ambiguity_causes: None,
 212             query_mode: TraitQueryMode::Standard,
 213         }
 214     }
 215
 216     pub fn with_query_mode(
 217         infcx: &'cx InferCtxt<'tcx>,
 218         query_mode: TraitQueryMode,
 219     ) -> SelectionContext<'cx, 'tcx> {
 220         debug!(?query_mode, "with_query_mode");
 221         SelectionContext { query_mode, ..SelectionContext::new(infcx) }
 222     }
 223
 224     /// Enables tracking of intercrate ambiguity causes. See
 225     /// the documentation of [`Self::intercrate_ambiguity_causes`] for more.
 226     pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
 227         assert!(self.is_intercrate());
 228         assert!(self.intercrate_ambiguity_causes.is_none());
 229         self.intercrate_ambiguity_causes = Some(FxIndexSet::default());
 230         debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
 231     }
 232
 233     /// Gets the intercrate ambiguity causes collected since tracking
 234     /// was enabled and disables tracking at the same time. If
 235     /// tracking is not enabled, just returns an empty vector.
 236     pub fn take_intercrate_ambiguity_causes(&mut self) -> FxIndexSet<IntercrateAmbiguityCause> {
 237         assert!(self.is_intercrate());
 238         self.intercrate_ambiguity_causes.take().unwrap_or_default()
 239     }
 240
 241     pub fn tcx(&self) -> TyCtxt<'tcx> {
 242         self.infcx.tcx
 243     }
 244
 245     pub fn is_intercrate(&self) -> bool {
 246         self.infcx.intercrate
 247     }
 248
 249     ///////////////////////////////////////////////////////////////////////////
 250     // Selection
 251     //
 252     // The selection phase tries to identify *how* an obligation will
 253     // be resolved. For example, it will identify which impl or
 254     // parameter bound is to be used. The process can be inconclusive
 255     // if the self type in the obligation is not fully inferred. Selection
 256     // can result in an error in one of two ways:
 257     //
 258     // 1. If no applicable impl or parameter bound can be found.
 259     // 2. If the output type parameters in the obligation do not match
 260     //    those specified by the impl/bound. For example, if the obligation
 261     //    is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
 262     //    `impl<T> Iterable<T> for Vec<T>`, than an error would result.
 263
 264     /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
 265     /// type environment by performing unification.
 266     #[instrument(level = "debug", skip(self), ret)]
 267     pub fn select(
 268         &mut self,
 269         obligation: &TraitObligation<'tcx>,
 270     ) -> SelectionResult<'tcx, Selection<'tcx>> {
 271         let candidate = match self.select_from_obligation(obligation) {
 272             Err(SelectionError::Overflow(OverflowError::Canonical)) => {
 273                 // In standard mode, overflow must have been caught and reported
 274                 // earlier.
 275                 assert!(self.query_mode == TraitQueryMode::Canonical);
 276                 return Err(SelectionError::Overflow(OverflowError::Canonical));
 277             }
 278             Err(e) => {
 279                 return Err(e);
 280             }
 281             Ok(None) => {
 282                 return Ok(None);
 283             }
 284             Ok(Some(candidate)) => candidate,
 285         };
 286
 287         match self.confirm_candidate(obligation, candidate) {
 288             Err(SelectionError::Overflow(OverflowError::Canonical)) => {
 289                 assert!(self.query_mode == TraitQueryMode::Canonical);
 290                 Err(SelectionError::Overflow(OverflowError::Canonical))
 291             }
 292             Err(e) => Err(e),
 293             Ok(candidate) => Ok(Some(candidate)),
 294         }
 295     }
 296
 297     pub(crate) fn select_from_obligation(
 298         &mut self,
 299         obligation: &TraitObligation<'tcx>,
 300     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
 301         debug_assert!(!obligation.predicate.has_escaping_bound_vars());
 302
 303         let pec = &ProvisionalEvaluationCache::default();
 304         let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
 305
 306         self.candidate_from_obligation(&stack)
 307     }
 308
 309     #[instrument(level = "debug", skip(self), ret)]
 310     fn candidate_from_obligation<'o>(
 311         &mut self,
 312         stack: &TraitObligationStack<'o, 'tcx>,
 313     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
 314         // Watch out for overflow. This intentionally bypasses (and does
 315         // not update) the cache.
 316         self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
 317
 318         // Check the cache. Note that we freshen the trait-ref
 319         // separately rather than using `stack.fresh_trait_ref` --
 320         // this is because we want the unbound variables to be
 321         // replaced with fresh types starting from index 0.
 322         let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
 323         debug!(?cache_fresh_trait_pred);
 324         debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
 325
 326         if let Some(c) =
 327             self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
 328         {
 329             debug!("CACHE HIT");
 330             return c;
 331         }
 332
 333         // If no match, compute result and insert into cache.
 334         //
 335         // FIXME(nikomatsakis) -- this cache is not taking into
 336         // account cycles that may have occurred in forming the
 337         // candidate. I don't know of any specific problems that
 338         // result but it seems awfully suspicious.
 339         let (candidate, dep_node) =
 340             self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
 341
 342         debug!("CACHE MISS");
 343         self.insert_candidate_cache(
 344             stack.obligation.param_env,
 345             cache_fresh_trait_pred,
 346             dep_node,
 347             candidate.clone(),
 348         );
 349         candidate
 350     }
 351
 352     fn candidate_from_obligation_no_cache<'o>(
 353         &mut self,
 354         stack: &TraitObligationStack<'o, 'tcx>,
 355     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
 356         if let Err(conflict) = self.is_knowable(stack) {
 357             debug!("coherence stage: not knowable");
 358             if self.intercrate_ambiguity_causes.is_some() {
 359                 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
 360                 // Heuristics: show the diagnostics when there are no candidates in crate.
 361                 if let Ok(candidate_set) = self.assemble_candidates(stack) {
 362                     let mut no_candidates_apply = true;
 363
 364                     for c in candidate_set.vec.iter() {
 365                         if self.evaluate_candidate(stack, &c)?.may_apply() {
 366                             no_candidates_apply = false;
 367                             break;
 368                         }
 369                     }
 370
 371                     if !candidate_set.ambiguous && no_candidates_apply {
 372                         let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
 373                         if !trait_ref.references_error() {
 374                             let self_ty = trait_ref.self_ty();
 375                             let (trait_desc, self_desc) = with_no_trimmed_paths!({
 376                                 let trait_desc = trait_ref.print_only_trait_path().to_string();
 377                                 let self_desc = if self_ty.has_concrete_skeleton() {
 378                                     Some(self_ty.to_string())
 379                                 } else {
 380                                     None
 381                                 };
 382                                 (trait_desc, self_desc)
 383                             });
 384                             let cause = if let Conflict::Upstream = conflict {
 385                                 IntercrateAmbiguityCause::UpstreamCrateUpdate {
 386                                     trait_desc,
 387                                     self_desc,
 388                                 }
 389                             } else {
 390                                 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
 391                             };
 392                             debug!(?cause, "evaluate_stack: pushing cause");
 393                             self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
 394                         }
 395                     }
 396                 }
 397             }
 398             return Ok(None);
 399         }
 400
 401         let candidate_set = self.assemble_candidates(stack)?;
 402
 403         if candidate_set.ambiguous {
 404             debug!("candidate set contains ambig");
 405             return Ok(None);
 406         }
 407
 408         let candidates = candidate_set.vec;
 409
 410         debug!(?stack, ?candidates, "assembled {} candidates", candidates.len());
 411
 412         // At this point, we know that each of the entries in the
 413         // candidate set is *individually* applicable. Now we have to
 414         // figure out if they contain mutual incompatibilities. This
 415         // frequently arises if we have an unconstrained input type --
 416         // for example, we are looking for `$0: Eq` where `$0` is some
 417         // unconstrained type variable. In that case, we'll get a
 418         // candidate which assumes $0 == int, one that assumes `$0 ==
 419         // usize`, etc. This spells an ambiguity.
 420
 421         let mut candidates = self.filter_impls(candidates, stack.obligation);
 422
 423         // If there is more than one candidate, first winnow them down
 424         // by considering extra conditions (nested obligations and so
 425         // forth). We don't winnow if there is exactly one
 426         // candidate. This is a relatively minor distinction but it
 427         // can lead to better inference and error-reporting. An
 428         // example would be if there was an impl:
 429         //
 430         //     impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
 431         //
 432         // and we were to see some code `foo.push_clone()` where `boo`
 433         // is a `Vec<Bar>` and `Bar` does not implement `Clone`.  If
 434         // we were to winnow, we'd wind up with zero candidates.
 435         // Instead, we select the right impl now but report "`Bar` does
 436         // not implement `Clone`".
 437         if candidates.len() == 1 {
 438             return self.filter_reservation_impls(candidates.pop().unwrap(), stack.obligation);
 439         }
 440
 441         // Winnow, but record the exact outcome of evaluation, which
 442         // is needed for specialization. Propagate overflow if it occurs.
 443         let mut candidates = candidates
 444             .into_iter()
 445             .map(|c| match self.evaluate_candidate(stack, &c) {
 446                 Ok(eval) if eval.may_apply() => {
 447                     Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
 448                 }
 449                 Ok(_) => Ok(None),
 450                 Err(OverflowError::Canonical) => Err(Overflow(OverflowError::Canonical)),
 451                 Err(OverflowError::ErrorReporting) => Err(ErrorReporting),
 452                 Err(OverflowError::Error(e)) => Err(Overflow(OverflowError::Error(e))),
 453             })
 454             .flat_map(Result::transpose)
 455             .collect::<Result<Vec<_>, _>>()?;
 456
 457         debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
 458
 459         let needs_infer = stack.obligation.predicate.has_non_region_infer();
 460
 461         // If there are STILL multiple candidates, we can further
 462         // reduce the list by dropping duplicates -- including
 463         // resolving specializations.
 464         if candidates.len() > 1 {
 465             let mut i = 0;
 466             while i < candidates.len() {
 467                 let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
 468                     self.candidate_should_be_dropped_in_favor_of(
 469                         &candidates[i],
 470                         &candidates[j],
 471                         needs_infer,
 472                     )
 473                 });
 474                 if is_dup {
 475                     debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
 476                     candidates.swap_remove(i);
 477                 } else {
 478                     debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
 479                     i += 1;
 480
 481                     // If there are *STILL* multiple candidates, give up
 482                     // and report ambiguity.
 483                     if i > 1 {
 484                         debug!("multiple matches, ambig");
 485                         return Ok(None);
 486                     }
 487                 }
 488             }
 489         }
 490
 491         // If there are *NO* candidates, then there are no impls --
 492         // that we know of, anyway. Note that in the case where there
 493         // are unbound type variables within the obligation, it might
 494         // be the case that you could still satisfy the obligation
 495         // from another crate by instantiating the type variables with
 496         // a type from another crate that does have an impl. This case
 497         // is checked for in `evaluate_stack` (and hence users
 498         // who might care about this case, like coherence, should use
 499         // that function).
 500         if candidates.is_empty() {
 501             // If there's an error type, 'downgrade' our result from
 502             // `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
 503             // emitting additional spurious errors, since we're guaranteed
 504             // to have emitted at least one.
 505             if stack.obligation.predicate.references_error() {
 506                 debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous");
 507                 return Ok(None);
 508             }
 509             return Err(Unimplemented);
 510         }
 511
 512         // Just one candidate left.
 513         self.filter_reservation_impls(candidates.pop().unwrap().candidate, stack.obligation)
 514     }
 515
 516     ///////////////////////////////////////////////////////////////////////////
 517     // EVALUATION
 518     //
 519     // Tests whether an obligation can be selected or whether an impl
 520     // can be applied to particular types. It skips the "confirmation"
 521     // step and hence completely ignores output type parameters.
 522     //
 523     // The result is "true" if the obligation *may* hold and "false" if
 524     // we can be sure it does not.
 525
 526     /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
 527     pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
 528         debug!(?obligation, "predicate_may_hold_fatal");
 529
 530         // This fatal query is a stopgap that should only be used in standard mode,
 531         // where we do not expect overflow to be propagated.
 532         assert!(self.query_mode == TraitQueryMode::Standard);
 533
 534         self.evaluate_root_obligation(obligation)
 535             .expect("Overflow should be caught earlier in standard query mode")
 536             .may_apply()
 537     }
 538
 539     /// Evaluates whether the obligation `obligation` can be satisfied
 540     /// and returns an `EvaluationResult`. This is meant for the
 541     /// *initial* call.
 542     pub fn evaluate_root_obligation(
 543         &mut self,
 544         obligation: &PredicateObligation<'tcx>,
 545     ) -> Result<EvaluationResult, OverflowError> {
 546         self.evaluation_probe(|this| {
 547             this.evaluate_predicate_recursively(
 548                 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
 549                 obligation.clone(),
 550             )
 551         })
 552     }
 553
 554     fn evaluation_probe(
 555         &mut self,
 556         op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
 557     ) -> Result<EvaluationResult, OverflowError> {
 558         self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
 559             let result = op(self)?;
 560
 561             match self.infcx.leak_check(true, snapshot) {
 562                 Ok(()) => {}
 563                 Err(_) => return Ok(EvaluatedToErr),
 564             }
 565
 566             if self.infcx.opaque_types_added_in_snapshot(snapshot) {
 567                 return Ok(result.max(EvaluatedToOkModuloOpaqueTypes));
 568             }
 569
 570             match self.infcx.region_constraints_added_in_snapshot(snapshot) {
 571                 None => Ok(result),
 572                 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
 573             }
 574         })
 575     }
 576
 577     /// Evaluates the predicates in `predicates` recursively. Note that
 578     /// this applies projections in the predicates, and therefore
 579     /// is run within an inference probe.
 580     #[instrument(skip(self, stack), level = "debug")]
 581     fn evaluate_predicates_recursively<'o, I>(
 582         &mut self,
 583         stack: TraitObligationStackList<'o, 'tcx>,
 584         predicates: I,
 585     ) -> Result<EvaluationResult, OverflowError>
 586     where
 587         I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
 588     {
 589         let mut result = EvaluatedToOk;
 590         for obligation in predicates {
 591             let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
 592             if let EvaluatedToErr = eval {
 593                 // fast-path - EvaluatedToErr is the top of the lattice,
 594                 // so we don't need to look on the other predicates.
 595                 return Ok(EvaluatedToErr);
 596             } else {
 597                 result = cmp::max(result, eval);
 598             }
 599         }
 600         Ok(result)
 601     }
 602
 603     #[instrument(
 604         level = "debug",
 605         skip(self, previous_stack),
 606         fields(previous_stack = ?previous_stack.head())
 607         ret,
 608     )]
 609     fn evaluate_predicate_recursively<'o>(
 610         &mut self,
 611         previous_stack: TraitObligationStackList<'o, 'tcx>,
 612         obligation: PredicateObligation<'tcx>,
 613     ) -> Result<EvaluationResult, OverflowError> {
 614         // `previous_stack` stores a `TraitObligation`, while `obligation` is
 615         // a `PredicateObligation`. These are distinct types, so we can't
 616         // use any `Option` combinator method that would force them to be
 617         // the same.
 618         match previous_stack.head() {
 619             Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
 620             None => self.check_recursion_limit(&obligation, &obligation)?,
 621         }
 622
 623         ensure_sufficient_stack(|| {
 624             let bound_predicate = obligation.predicate.kind();
 625             match bound_predicate.skip_binder() {
 626                 ty::PredicateKind::Clause(ty::Clause::Trait(t)) => {
 627                     let t = bound_predicate.rebind(t);
 628                     debug_assert!(!t.has_escaping_bound_vars());
 629                     let obligation = obligation.with(self.tcx(), t);
 630                     self.evaluate_trait_predicate_recursively(previous_stack, obligation)
 631                 }
 632
 633                 ty::PredicateKind::Subtype(p) => {
 634                     let p = bound_predicate.rebind(p);
 635                     // Does this code ever run?
 636                     match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
 637                         Ok(Ok(InferOk { mut obligations, .. })) => {
 638                             self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
 639                             self.evaluate_predicates_recursively(
 640                                 previous_stack,
 641                                 obligations.into_iter(),
 642                             )
 643                         }
 644                         Ok(Err(_)) => Ok(EvaluatedToErr),
 645                         Err(..) => Ok(EvaluatedToAmbig),
 646                     }
 647                 }
 648
 649                 ty::PredicateKind::Coerce(p) => {
 650                     let p = bound_predicate.rebind(p);
 651                     // Does this code ever run?
 652                     match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
 653                         Ok(Ok(InferOk { mut obligations, .. })) => {
 654                             self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
 655                             self.evaluate_predicates_recursively(
 656                                 previous_stack,
 657                                 obligations.into_iter(),
 658                             )
 659                         }
 660                         Ok(Err(_)) => Ok(EvaluatedToErr),
 661                         Err(..) => Ok(EvaluatedToAmbig),
 662                     }
 663                 }
 664
 665                 ty::PredicateKind::WellFormed(arg) => {
 666                     // So, there is a bit going on here. First, `WellFormed` predicates
 667                     // are coinductive, like trait predicates with auto traits.
 668                     // This means that we need to detect if we have recursively
 669                     // evaluated `WellFormed(X)`. Otherwise, we would run into
 670                     // a "natural" overflow error.
 671                     //
 672                     // Now, the next question is whether we need to do anything
 673                     // special with caching. Considering the following tree:
 674                     // - `WF(Foo<T>)`
 675                     //   - `Bar<T>: Send`
 676                     //     - `WF(Foo<T>)`
 677                     //   - `Foo<T>: Trait`
 678                     // In this case, the innermost `WF(Foo<T>)` should return
 679                     // `EvaluatedToOk`, since it's coinductive. Then if
 680                     // `Bar<T>: Send` is resolved to `EvaluatedToOk`, it can be
 681                     // inserted into a cache (because without thinking about `WF`
 682                     // goals, it isn't in a cycle). If `Foo<T>: Trait` later doesn't
 683                     // hold, then `Bar<T>: Send` shouldn't hold. Therefore, we
 684                     // *do* need to keep track of coinductive cycles.
 685
 686                     let cache = previous_stack.cache;
 687                     let dfn = cache.next_dfn();
 688
 689                     for stack_arg in previous_stack.cache.wf_args.borrow().iter().rev() {
 690                         if stack_arg.0 != arg {
 691                             continue;
 692                         }
 693                         debug!("WellFormed({:?}) on stack", arg);
 694                         if let Some(stack) = previous_stack.head {
 695                             // Okay, let's imagine we have two different stacks:
 696                             //   `T: NonAutoTrait -> WF(T) -> T: NonAutoTrait`
 697                             //   `WF(T) -> T: NonAutoTrait -> WF(T)`
 698                             // Because of this, we need to check that all
 699                             // predicates between the WF goals are coinductive.
 700                             // Otherwise, we can say that `T: NonAutoTrait` is
 701                             // true.
 702                             // Let's imagine we have a predicate stack like
 703                             //         `Foo: Bar -> WF(T) -> T: NonAutoTrait -> T: Auto
 704                             // depth   ^1                    ^2                 ^3
 705                             // and the current predicate is `WF(T)`. `wf_args`
 706                             // would contain `(T, 1)`. We want to check all
 707                             // trait predicates greater than `1`. The previous
 708                             // stack would be `T: Auto`.
 709                             let cycle = stack.iter().take_while(|s| s.depth > stack_arg.1);
 710                             let tcx = self.tcx();
 711                             let cycle =
 712                                 cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
 713                             if self.coinductive_match(cycle) {
 714                                 stack.update_reached_depth(stack_arg.1);
 715                                 return Ok(EvaluatedToOk);
 716                             } else {
 717                                 return Ok(EvaluatedToRecur);
 718                             }
 719                         }
 720                         return Ok(EvaluatedToOk);
 721                     }
 722
 723                     match wf::obligations(
 724                         self.infcx,
 725                         obligation.param_env,
 726                         obligation.cause.body_id,
 727                         obligation.recursion_depth + 1,
 728                         arg,
 729                         obligation.cause.span,
 730                     ) {
 731                         Some(mut obligations) => {
 732                             self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
 733
 734                             cache.wf_args.borrow_mut().push((arg, previous_stack.depth()));
 735                             let result =
 736                                 self.evaluate_predicates_recursively(previous_stack, obligations);
 737                             cache.wf_args.borrow_mut().pop();
 738
 739                             let result = result?;
 740
 741                             if !result.must_apply_modulo_regions() {
 742                                 cache.on_failure(dfn);
 743                             }
 744
 745                             cache.on_completion(dfn);
 746
 747                             Ok(result)
 748                         }
 749                         None => Ok(EvaluatedToAmbig),
 750                     }
 751                 }
 752
 753                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(pred)) => {
 754                     // A global type with no late-bound regions can only
 755                     // contain the "'static" lifetime (any other lifetime
 756                     // would either be late-bound or local), so it is guaranteed
 757                     // to outlive any other lifetime
 758                     if pred.0.is_global() && !pred.0.has_late_bound_regions() {
 759                         Ok(EvaluatedToOk)
 760                     } else {
 761                         Ok(EvaluatedToOkModuloRegions)
 762                     }
 763                 }
 764
 765                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..)) => {
 766                     // We do not consider region relationships when evaluating trait matches.
 767                     Ok(EvaluatedToOkModuloRegions)
 768                 }
 769
 770                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 771                     if self.tcx().is_object_safe(trait_def_id) {
 772                         Ok(EvaluatedToOk)
 773                     } else {
 774                         Ok(EvaluatedToErr)
 775                     }
 776                 }
 777
 778                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 779                     let data = bound_predicate.rebind(data);
 780                     let project_obligation = obligation.with(self.tcx(), data);
 781                     match project::poly_project_and_unify_type(self, &project_obligation) {
 782                         ProjectAndUnifyResult::Holds(mut subobligations) => {
 783                             'compute_res: {
 784                                 // If we've previously marked this projection as 'complete', then
 785                                 // use the final cached result (either `EvaluatedToOk` or
 786                                 // `EvaluatedToOkModuloRegions`), and skip re-evaluating the
 787                                 // sub-obligations.
 788                                 if let Some(key) =
 789                                     ProjectionCacheKey::from_poly_projection_predicate(self, data)
 790                                 {
 791                                     if let Some(cached_res) = self
 792                                         .infcx
 793                                         .inner
 794                                         .borrow_mut()
 795                                         .projection_cache()
 796                                         .is_complete(key)
 797                                     {
 798                                         break 'compute_res Ok(cached_res);
 799                                     }
 800                                 }
 801
 802                                 self.add_depth(
 803                                     subobligations.iter_mut(),
 804                                     obligation.recursion_depth,
 805                                 );
 806                                 let res = self.evaluate_predicates_recursively(
 807                                     previous_stack,
 808                                     subobligations,
 809                                 );
 810                                 if let Ok(eval_rslt) = res
 811                                     && (eval_rslt == EvaluatedToOk || eval_rslt == EvaluatedToOkModuloRegions)
 812                                     && let Some(key) =
 813                                         ProjectionCacheKey::from_poly_projection_predicate(
 814                                             self, data,
 815                                         )
 816                                 {
 817                                     // If the result is something that we can cache, then mark this
 818                                     // entry as 'complete'. This will allow us to skip evaluating the
 819                                     // subobligations at all the next time we evaluate the projection
 820                                     // predicate.
 821                                     self.infcx
 822                                         .inner
 823                                         .borrow_mut()
 824                                         .projection_cache()
 825                                         .complete(key, eval_rslt);
 826                                 }
 827                                 res
 828                             }
 829                         }
 830                         ProjectAndUnifyResult::FailedNormalization => Ok(EvaluatedToAmbig),
 831                         ProjectAndUnifyResult::Recursive => Ok(EvaluatedToRecur),
 832                         ProjectAndUnifyResult::MismatchedProjectionTypes(_) => Ok(EvaluatedToErr),
 833                     }
 834                 }
 835
 836                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 837                     match self.infcx.closure_kind(closure_substs) {
 838                         Some(closure_kind) => {
 839                             if closure_kind.extends(kind) {
 840                                 Ok(EvaluatedToOk)
 841                             } else {
 842                                 Ok(EvaluatedToErr)
 843                             }
 844                         }
 845                         None => Ok(EvaluatedToAmbig),
 846                     }
 847                 }
 848
 849                 ty::PredicateKind::ConstEvaluatable(uv) => {
 850                     match const_evaluatable::is_const_evaluatable(
 851                         self.infcx,
 852                         uv,
 853                         obligation.param_env,
 854                         obligation.cause.span,
 855                     ) {
 856                         Ok(()) => Ok(EvaluatedToOk),
 857                         Err(NotConstEvaluatable::MentionsInfer) => Ok(EvaluatedToAmbig),
 858                         Err(NotConstEvaluatable::MentionsParam) => Ok(EvaluatedToErr),
 859                         Err(_) => Ok(EvaluatedToErr),
 860                     }
 861                 }
 862
 863                 ty::PredicateKind::ConstEquate(c1, c2) => {
 864                     let tcx = self.tcx();
 865                     assert!(
 866                         tcx.features().generic_const_exprs,
 867                         "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
 868                     );
 869
 870                     {
 871                         let c1 = tcx.expand_abstract_consts(c1);
 872                         let c2 = tcx.expand_abstract_consts(c2);
 873                         debug!(
 874                             "evalaute_predicate_recursively: equating consts:\nc1= {:?}\nc2= {:?}",
 875                             c1, c2
 876                         );
 877
 878                         use rustc_hir::def::DefKind;
 879                         use ty::ConstKind::Unevaluated;
 880                         match (c1.kind(), c2.kind()) {
 881                             (Unevaluated(a), Unevaluated(b))
 882                                 if a.def.did == b.def.did
 883                                     && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
 884                             {
 885                                 if let Ok(new_obligations) = self
 886                                     .infcx
 887                                     .at(&obligation.cause, obligation.param_env)
 888                                     .trace(c1, c2)
 889                                     .eq(a.substs, b.substs)
 890                                 {
 891                                     let mut obligations = new_obligations.obligations;
 892                                     self.add_depth(
 893                                         obligations.iter_mut(),
 894                                         obligation.recursion_depth,
 895                                     );
 896                                     return self.evaluate_predicates_recursively(
 897                                         previous_stack,
 898                                         obligations.into_iter(),
 899                                     );
 900                                 }
 901                             }
 902                             (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
 903                             (_, _) => {
 904                                 if let Ok(new_obligations) = self
 905                                     .infcx
 906                                     .at(&obligation.cause, obligation.param_env)
 907                                     .eq(c1, c2)
 908                                 {
 909                                     let mut obligations = new_obligations.obligations;
 910                                     self.add_depth(
 911                                         obligations.iter_mut(),
 912                                         obligation.recursion_depth,
 913                                     );
 914                                     return self.evaluate_predicates_recursively(
 915                                         previous_stack,
 916                                         obligations.into_iter(),
 917                                     );
 918                                 }
 919                             }
 920                         }
 921                     }
 922
 923                     let evaluate = |c: ty::Const<'tcx>| {
 924                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 925                             match self.infcx.try_const_eval_resolve(
 926                                 obligation.param_env,
 927                                 unevaluated,
 928                                 c.ty(),
 929                                 Some(obligation.cause.span),
 930                             ) {
 931                                 Ok(val) => Ok(val),
 932                                 Err(e) => Err(e),
 933                             }
 934                         } else {
 935                             Ok(c)
 936                         }
 937                     };
 938
 939                     match (evaluate(c1), evaluate(c2)) {
 940                         (Ok(c1), Ok(c2)) => {
 941                             match self.infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
 942                             {
 943                                 Ok(inf_ok) => self.evaluate_predicates_recursively(
 944                                     previous_stack,
 945                                     inf_ok.into_obligations(),
 946                                 ),
 947                                 Err(_) => Ok(EvaluatedToErr),
 948                             }
 949                         }
 950                         (Err(ErrorHandled::Reported(_)), _)
 951                         | (_, Err(ErrorHandled::Reported(_))) => Ok(EvaluatedToErr),
 952                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 953                             if c1.has_non_region_infer() || c2.has_non_region_infer() {
 954                                 Ok(EvaluatedToAmbig)
 955                             } else {
 956                                 // Two different constants using generic parameters ~> error.
 957                                 Ok(EvaluatedToErr)
 958                             }
 959                         }
 960                     }
 961                 }
 962                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 963                     bug!("TypeWellFormedFromEnv is only used for chalk")
 964                 }
 965                 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
 966             }
 967         })
 968     }
 969
 970     #[instrument(skip(self, previous_stack), level = "debug", ret)]
 971     fn evaluate_trait_predicate_recursively<'o>(
 972         &mut self,
 973         previous_stack: TraitObligationStackList<'o, 'tcx>,
 974         mut obligation: TraitObligation<'tcx>,
 975     ) -> Result<EvaluationResult, OverflowError> {
 976         if !self.is_intercrate()
 977             && obligation.is_global()
 978             && obligation.param_env.caller_bounds().iter().all(|bound| bound.needs_subst())
 979         {
 980             // If a param env has no global bounds, global obligations do not
 981             // depend on its particular value in order to work, so we can clear
 982             // out the param env and get better caching.
 983             debug!("in global");
 984             obligation.param_env = obligation.param_env.without_caller_bounds();
 985         }
 986
 987         let stack = self.push_stack(previous_stack, &obligation);
 988         let mut fresh_trait_pred = stack.fresh_trait_pred;
 989         let mut param_env = obligation.param_env;
 990
 991         fresh_trait_pred = fresh_trait_pred.map_bound(|mut pred| {
 992             pred.remap_constness(&mut param_env);
 993             pred
 994         });
 995
 996         debug!(?fresh_trait_pred);
 997
 998         // If a trait predicate is in the (local or global) evaluation cache,
 999         // then we know it holds without cycles.
1000         if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
1001             debug!("CACHE HIT");
1002             return Ok(result);
1003         }
1004
1005         if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
1006             debug!("PROVISIONAL CACHE HIT");
1007             stack.update_reached_depth(result.reached_depth);
1008             return Ok(result.result);
1009         }
1010
1011         // Check if this is a match for something already on the
1012         // stack. If so, we don't want to insert the result into the
1013         // main cache (it is cycle dependent) nor the provisional
1014         // cache (which is meant for things that have completed but
1015         // for a "backedge" -- this result *is* the backedge).
1016         if let Some(cycle_result) = self.check_evaluation_cycle(&stack) {
1017             return Ok(cycle_result);
1018         }
1019
1020         let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
1021         let result = result?;
1022
1023         if !result.must_apply_modulo_regions() {
1024             stack.cache().on_failure(stack.dfn);
1025         }
1026
1027         let reached_depth = stack.reached_depth.get();
1028         if reached_depth >= stack.depth {
1029             debug!("CACHE MISS");
1030             self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
1031             stack.cache().on_completion(stack.dfn);
1032         } else {
1033             debug!("PROVISIONAL");
1034             debug!(
1035                 "caching provisionally because {:?} \
1036                  is a cycle participant (at depth {}, reached depth {})",
1037                 fresh_trait_pred, stack.depth, reached_depth,
1038             );
1039
1040             stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
1041         }
1042
1043         Ok(result)
1044     }
1045
1046     /// If there is any previous entry on the stack that precisely
1047     /// matches this obligation, then we can assume that the
1048     /// obligation is satisfied for now (still all other conditions
1049     /// must be met of course). One obvious case this comes up is
1050     /// marker traits like `Send`. Think of a linked list:
1051     ///
1052     ///     struct List<T> { data: T, next: Option<Box<List<T>>> }
1053     ///
1054     /// `Box<List<T>>` will be `Send` if `T` is `Send` and
1055     /// `Option<Box<List<T>>>` is `Send`, and in turn
1056     /// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
1057     /// `Send`.
1058     ///
1059     /// Note that we do this comparison using the `fresh_trait_ref`
1060     /// fields. Because these have all been freshened using
1061     /// `self.freshener`, we can be sure that (a) this will not
1062     /// affect the inferencer state and (b) that if we see two
1063     /// fresh regions with the same index, they refer to the same
1064     /// unbound type variable.
1065     fn check_evaluation_cycle(
1066         &mut self,
1067         stack: &TraitObligationStack<'_, 'tcx>,
1068     ) -> Option<EvaluationResult> {
1069         if let Some(cycle_depth) = stack
1070             .iter()
1071             .skip(1) // Skip top-most frame.
1072             .find(|prev| {
1073                 stack.obligation.param_env == prev.obligation.param_env
1074                     && stack.fresh_trait_pred == prev.fresh_trait_pred
1075             })
1076             .map(|stack| stack.depth)
1077         {
1078             debug!("evaluate_stack --> recursive at depth {}", cycle_depth);
1079
1080             // If we have a stack like `A B C D E A`, where the top of
1081             // the stack is the final `A`, then this will iterate over
1082             // `A, E, D, C, B` -- i.e., all the participants apart
1083             // from the cycle head. We mark them as participating in a
1084             // cycle. This suppresses caching for those nodes. See
1085             // `in_cycle` field for more details.
1086             stack.update_reached_depth(cycle_depth);
1087
1088             // Subtle: when checking for a coinductive cycle, we do
1089             // not compare using the "freshened trait refs" (which
1090             // have erased regions) but rather the fully explicit
1091             // trait refs. This is important because it's only a cycle
1092             // if the regions match exactly.
1093             let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
1094             let tcx = self.tcx();
1095             let cycle = cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
1096             if self.coinductive_match(cycle) {
1097                 debug!("evaluate_stack --> recursive, coinductive");
1098                 Some(EvaluatedToOk)
1099             } else {
1100                 debug!("evaluate_stack --> recursive, inductive");
1101                 Some(EvaluatedToRecur)
1102             }
1103         } else {
1104             None
1105         }
1106     }
1107
1108     fn evaluate_stack<'o>(
1109         &mut self,
1110         stack: &TraitObligationStack<'o, 'tcx>,
1111     ) -> Result<EvaluationResult, OverflowError> {
1112         // In intercrate mode, whenever any of the generics are unbound,
1113         // there can always be an impl. Even if there are no impls in
1114         // this crate, perhaps the type would be unified with
1115         // something from another crate that does provide an impl.
1116         //
1117         // In intra mode, we must still be conservative. The reason is
1118         // that we want to avoid cycles. Imagine an impl like:
1119         //
1120         //     impl<T:Eq> Eq for Vec<T>
1121         //
1122         // and a trait reference like `$0 : Eq` where `$0` is an
1123         // unbound variable. When we evaluate this trait-reference, we
1124         // will unify `$0` with `Vec<$1>` (for some fresh variable
1125         // `$1`), on the condition that `$1 : Eq`. We will then wind
1126         // up with many candidates (since that are other `Eq` impls
1127         // that apply) and try to winnow things down. This results in
1128         // a recursive evaluation that `$1 : Eq` -- as you can
1129         // imagine, this is just where we started. To avoid that, we
1130         // check for unbound variables and return an ambiguous (hence possible)
1131         // match if we've seen this trait before.
1132         //
1133         // This suffices to allow chains like `FnMut` implemented in
1134         // terms of `Fn` etc, but we could probably make this more
1135         // precise still.
1136         let unbound_input_types =
1137             stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
1138
1139         if unbound_input_types
1140             && stack.iter().skip(1).any(|prev| {
1141                 stack.obligation.param_env == prev.obligation.param_env
1142                     && self.match_fresh_trait_refs(
1143                         stack.fresh_trait_pred,
1144                         prev.fresh_trait_pred,
1145                         prev.obligation.param_env,
1146                     )
1147             })
1148         {
1149             debug!("evaluate_stack --> unbound argument, recursive --> giving up",);
1150             return Ok(EvaluatedToUnknown);
1151         }
1152
1153         match self.candidate_from_obligation(stack) {
1154             Ok(Some(c)) => self.evaluate_candidate(stack, &c),
1155             Ok(None) => Ok(EvaluatedToAmbig),
1156             Err(Overflow(OverflowError::Canonical)) => Err(OverflowError::Canonical),
1157             Err(ErrorReporting) => Err(OverflowError::ErrorReporting),
1158             Err(..) => Ok(EvaluatedToErr),
1159         }
1160     }
1161
1162     /// For defaulted traits, we use a co-inductive strategy to solve, so
1163     /// that recursion is ok. This routine returns `true` if the top of the
1164     /// stack (`cycle[0]`):
1165     ///
1166     /// - is a defaulted trait,
1167     /// - it also appears in the backtrace at some position `X`,
1168     /// - all the predicates at positions `X..` between `X` and the top are
1169     ///   also defaulted traits.
1170     pub(crate) fn coinductive_match<I>(&mut self, mut cycle: I) -> bool
1171     where
1172         I: Iterator<Item = ty::Predicate<'tcx>>,
1173     {
1174         cycle.all(|predicate| self.coinductive_predicate(predicate))
1175     }
1176
1177     fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
1178         let result = match predicate.kind().skip_binder() {
1179             ty::PredicateKind::Clause(ty::Clause::Trait(ref data)) => {
1180                 self.tcx().trait_is_coinductive(data.def_id())
1181             }
1182             ty::PredicateKind::WellFormed(_) => true,
1183             _ => false,
1184         };
1185         debug!(?predicate, ?result, "coinductive_predicate");
1186         result
1187     }
1188
1189     /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
1190     /// obligations are met. Returns whether `candidate` remains viable after this further
1191     /// scrutiny.
1192     #[instrument(
1193         level = "debug",
1194         skip(self, stack),
1195         fields(depth = stack.obligation.recursion_depth),
1196         ret
1197     )]
1198     fn evaluate_candidate<'o>(
1199         &mut self,
1200         stack: &TraitObligationStack<'o, 'tcx>,
1201         candidate: &SelectionCandidate<'tcx>,
1202     ) -> Result<EvaluationResult, OverflowError> {
1203         let mut result = self.evaluation_probe(|this| {
1204             let candidate = (*candidate).clone();
1205             match this.confirm_candidate(stack.obligation, candidate) {
1206                 Ok(selection) => {
1207                     debug!(?selection);
1208                     this.evaluate_predicates_recursively(
1209                         stack.list(),
1210                         selection.nested_obligations().into_iter(),
1211                     )
1212                 }
1213                 Err(..) => Ok(EvaluatedToErr),
1214             }
1215         })?;
1216
1217         // If we erased any lifetimes, then we want to use
1218         // `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
1219         // as your final result. The result will be cached using
1220         // the freshened trait predicate as a key, so we need
1221         // our result to be correct by *any* choice of original lifetimes,
1222         // not just the lifetime choice for this particular (non-erased)
1223         // predicate.
1224         // See issue #80691
1225         if stack.fresh_trait_pred.has_erased_regions() {
1226             result = result.max(EvaluatedToOkModuloRegions);
1227         }
1228
1229         Ok(result)
1230     }
1231
1232     fn check_evaluation_cache(
1233         &self,
1234         param_env: ty::ParamEnv<'tcx>,
1235         trait_pred: ty::PolyTraitPredicate<'tcx>,
1236     ) -> Option<EvaluationResult> {
1237         // Neither the global nor local cache is aware of intercrate
1238         // mode, so don't do any caching. In particular, we might
1239         // re-use the same `InferCtxt` with both an intercrate
1240         // and non-intercrate `SelectionContext`
1241         if self.is_intercrate() {
1242             return None;
1243         }
1244
1245         let tcx = self.tcx();
1246         if self.can_use_global_caches(param_env) {
1247             if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) {
1248                 return Some(res);
1249             }
1250         }
1251         self.infcx.evaluation_cache.get(&(param_env, trait_pred), tcx)
1252     }
1253
1254     fn insert_evaluation_cache(
1255         &mut self,
1256         param_env: ty::ParamEnv<'tcx>,
1257         trait_pred: ty::PolyTraitPredicate<'tcx>,
1258         dep_node: DepNodeIndex,
1259         result: EvaluationResult,
1260     ) {
1261         // Avoid caching results that depend on more than just the trait-ref
1262         // - the stack can create recursion.
1263         if result.is_stack_dependent() {
1264             return;
1265         }
1266
1267         // Neither the global nor local cache is aware of intercrate
1268         // mode, so don't do any caching. In particular, we might
1269         // re-use the same `InferCtxt` with both an intercrate
1270         // and non-intercrate `SelectionContext`
1271         if self.is_intercrate() {
1272             return;
1273         }
1274
1275         if self.can_use_global_caches(param_env) {
1276             if !trait_pred.needs_infer() {
1277                 debug!(?trait_pred, ?result, "insert_evaluation_cache global");
1278                 // This may overwrite the cache with the same value
1279                 // FIXME: Due to #50507 this overwrites the different values
1280                 // This should be changed to use HashMapExt::insert_same
1281                 // when that is fixed
1282                 self.tcx().evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1283                 return;
1284             }
1285         }
1286
1287         debug!(?trait_pred, ?result, "insert_evaluation_cache");
1288         self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1289     }
1290
1291     /// For various reasons, it's possible for a subobligation
1292     /// to have a *lower* recursion_depth than the obligation used to create it.
1293     /// Projection sub-obligations may be returned from the projection cache,
1294     /// which results in obligations with an 'old' `recursion_depth`.
1295     /// Additionally, methods like `InferCtxt.subtype_predicate` produce
1296     /// subobligations without taking in a 'parent' depth, causing the
1297     /// generated subobligations to have a `recursion_depth` of `0`.
1298     ///
1299     /// To ensure that obligation_depth never decreases, we force all subobligations
1300     /// to have at least the depth of the original obligation.
1301     fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
1302         &self,
1303         it: I,
1304         min_depth: usize,
1305     ) {
1306         it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
1307     }
1308
1309     fn check_recursion_depth<T>(
1310         &self,
1311         depth: usize,
1312         error_obligation: &Obligation<'tcx, T>,
1313     ) -> Result<(), OverflowError>
1314     where
1315         T: ToPredicate<'tcx> + Clone,
1316     {
1317         if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
1318             match self.query_mode {
1319                 TraitQueryMode::Standard => {
1320                     if let Some(e) = self.infcx.tainted_by_errors() {
1321                         return Err(OverflowError::Error(e));
1322                     }
1323                     self.infcx.err_ctxt().report_overflow_obligation(error_obligation, true);
1324                 }
1325                 TraitQueryMode::Canonical => {
1326                     return Err(OverflowError::Canonical);
1327                 }
1328             }
1329         }
1330         Ok(())
1331     }
1332
1333     /// Checks that the recursion limit has not been exceeded.
1334     ///
1335     /// The weird return type of this function allows it to be used with the `try` (`?`)
1336     /// operator within certain functions.
1337     #[inline(always)]
1338     fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V>(
1339         &self,
1340         obligation: &Obligation<'tcx, T>,
1341         error_obligation: &Obligation<'tcx, V>,
1342     ) -> Result<(), OverflowError>
1343     where
1344         V: ToPredicate<'tcx> + Clone,
1345     {
1346         self.check_recursion_depth(obligation.recursion_depth, error_obligation)
1347     }
1348
1349     fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1350     where
1351         OP: FnOnce(&mut Self) -> R,
1352     {
1353         let (result, dep_node) =
1354             self.tcx().dep_graph.with_anon_task(self.tcx(), DepKind::TraitSelect, || op(self));
1355         self.tcx().dep_graph.read_index(dep_node);
1356         (result, dep_node)
1357     }
1358
1359     /// filter_impls filters constant trait obligations and candidates that have a positive impl
1360     /// for a negative goal and a negative impl for a positive goal
1361     #[instrument(level = "debug", skip(self, candidates))]
1362     fn filter_impls(
1363         &mut self,
1364         candidates: Vec<SelectionCandidate<'tcx>>,
1365         obligation: &TraitObligation<'tcx>,
1366     ) -> Vec<SelectionCandidate<'tcx>> {
1367         trace!("{candidates:#?}");
1368         let tcx = self.tcx();
1369         let mut result = Vec::with_capacity(candidates.len());
1370
1371         for candidate in candidates {
1372             // Respect const trait obligations
1373             if obligation.is_const() {
1374                 match candidate {
1375                     // const impl
1376                     ImplCandidate(def_id) if tcx.constness(def_id) == hir::Constness::Const => {}
1377                     // const param
1378                     ParamCandidate(trait_pred) if trait_pred.is_const_if_const() => {}
1379                     // const projection
1380                     ProjectionCandidate(_, ty::BoundConstness::ConstIfConst) => {}
1381                     // auto trait impl
1382                     AutoImplCandidate => {}
1383                     // generator / future, this will raise error in other places
1384                     // or ignore error with const_async_blocks feature
1385                     GeneratorCandidate => {}
1386                     FutureCandidate => {}
1387                     // FnDef where the function is const
1388                     FnPointerCandidate { is_const: true } => {}
1389                     ConstDestructCandidate(_) => {}
1390                     _ => {
1391                         // reject all other types of candidates
1392                         continue;
1393                     }
1394                 }
1395             }
1396
1397             if let ImplCandidate(def_id) = candidate {
1398                 if ty::ImplPolarity::Reservation == tcx.impl_polarity(def_id)
1399                     || obligation.polarity() == tcx.impl_polarity(def_id)
1400                 {
1401                     result.push(candidate);
1402                 }
1403             } else {
1404                 result.push(candidate);
1405             }
1406         }
1407
1408         trace!("{result:#?}");
1409         result
1410     }
1411
1412     /// filter_reservation_impls filter reservation impl for any goal as ambiguous
1413     #[instrument(level = "debug", skip(self))]
1414     fn filter_reservation_impls(
1415         &mut self,
1416         candidate: SelectionCandidate<'tcx>,
1417         obligation: &TraitObligation<'tcx>,
1418     ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1419         let tcx = self.tcx();
1420         // Treat reservation impls as ambiguity.
1421         if let ImplCandidate(def_id) = candidate {
1422             if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
1423                 if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
1424                     let value = tcx
1425                         .get_attr(def_id, sym::rustc_reservation_impl)
1426                         .and_then(|a| a.value_str());
1427                     if let Some(value) = value {
1428                         debug!(
1429                             "filter_reservation_impls: \
1430                                  reservation impl ambiguity on {:?}",
1431                             def_id
1432                         );
1433                         intercrate_ambiguity_clauses.insert(
1434                             IntercrateAmbiguityCause::ReservationImpl {
1435                                 message: value.to_string(),
1436                             },
1437                         );
1438                     }
1439                 }
1440                 return Ok(None);
1441             }
1442         }
1443         Ok(Some(candidate))
1444     }
1445
1446     fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
1447         debug!("is_knowable(intercrate={:?})", self.is_intercrate());
1448
1449         if !self.is_intercrate() || stack.obligation.polarity() == ty::ImplPolarity::Negative {
1450             return Ok(());
1451         }
1452
1453         let obligation = &stack.obligation;
1454         let predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1455
1456         // Okay to skip binder because of the nature of the
1457         // trait-ref-is-knowable check, which does not care about
1458         // bound regions.
1459         let trait_ref = predicate.skip_binder().trait_ref;
1460
1461         coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
1462     }
1463
1464     /// Returns `true` if the global caches can be used.
1465     fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1466         // If there are any inference variables in the `ParamEnv`, then we
1467         // always use a cache local to this particular scope. Otherwise, we
1468         // switch to a global cache.
1469         if param_env.needs_infer() {
1470             return false;
1471         }
1472
1473         // Avoid using the master cache during coherence and just rely
1474         // on the local cache. This effectively disables caching
1475         // during coherence. It is really just a simplification to
1476         // avoid us having to fear that coherence results "pollute"
1477         // the master cache. Since coherence executes pretty quickly,
1478         // it's not worth going to more trouble to increase the
1479         // hit-rate, I don't think.
1480         if self.is_intercrate() {
1481             return false;
1482         }
1483
1484         // Otherwise, we can use the global cache.
1485         true
1486     }
1487
1488     fn check_candidate_cache(
1489         &mut self,
1490         mut param_env: ty::ParamEnv<'tcx>,
1491         cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1492     ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1493         // Neither the global nor local cache is aware of intercrate
1494         // mode, so don't do any caching. In particular, we might
1495         // re-use the same `InferCtxt` with both an intercrate
1496         // and non-intercrate `SelectionContext`
1497         if self.is_intercrate() {
1498             return None;
1499         }
1500         let tcx = self.tcx();
1501         let mut pred = cache_fresh_trait_pred.skip_binder();
1502         pred.remap_constness(&mut param_env);
1503
1504         if self.can_use_global_caches(param_env) {
1505             if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
1506                 return Some(res);
1507             }
1508         }
1509         self.infcx.selection_cache.get(&(param_env, pred), tcx)
1510     }
1511
1512     /// Determines whether can we safely cache the result
1513     /// of selecting an obligation. This is almost always `true`,
1514     /// except when dealing with certain `ParamCandidate`s.
1515     ///
1516     /// Ordinarily, a `ParamCandidate` will contain no inference variables,
1517     /// since it was usually produced directly from a `DefId`. However,
1518     /// certain cases (currently only librustdoc's blanket impl finder),
1519     /// a `ParamEnv` may be explicitly constructed with inference types.
1520     /// When this is the case, we do *not* want to cache the resulting selection
1521     /// candidate. This is due to the fact that it might not always be possible
1522     /// to equate the obligation's trait ref and the candidate's trait ref,
1523     /// if more constraints end up getting added to an inference variable.
1524     ///
1525     /// Because of this, we always want to re-run the full selection
1526     /// process for our obligation the next time we see it, since
1527     /// we might end up picking a different `SelectionCandidate` (or none at all).
1528     fn can_cache_candidate(
1529         &self,
1530         result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1531     ) -> bool {
1532         // Neither the global nor local cache is aware of intercrate
1533         // mode, so don't do any caching. In particular, we might
1534         // re-use the same `InferCtxt` with both an intercrate
1535         // and non-intercrate `SelectionContext`
1536         if self.is_intercrate() {
1537             return false;
1538         }
1539         match result {
1540             Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.needs_infer(),
1541             _ => true,
1542         }
1543     }
1544
1545     #[instrument(skip(self, param_env, cache_fresh_trait_pred, dep_node), level = "debug")]
1546     fn insert_candidate_cache(
1547         &mut self,
1548         mut param_env: ty::ParamEnv<'tcx>,
1549         cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1550         dep_node: DepNodeIndex,
1551         candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1552     ) {
1553         let tcx = self.tcx();
1554         let mut pred = cache_fresh_trait_pred.skip_binder();
1555
1556         pred.remap_constness(&mut param_env);
1557
1558         if !self.can_cache_candidate(&candidate) {
1559             debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
1560             return;
1561         }
1562
1563         if self.can_use_global_caches(param_env) {
1564             if let Err(Overflow(OverflowError::Canonical)) = candidate {
1565                 // Don't cache overflow globally; we only produce this in certain modes.
1566             } else if !pred.needs_infer() {
1567                 if !candidate.needs_infer() {
1568                     debug!(?pred, ?candidate, "insert_candidate_cache global");
1569                     // This may overwrite the cache with the same value.
1570                     tcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1571                     return;
1572                 }
1573             }
1574         }
1575
1576         debug!(?pred, ?candidate, "insert_candidate_cache local");
1577         self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1578     }
1579
1580     /// Matches a predicate against the bounds of its self type.
1581     ///
1582     /// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
1583     /// a projection, look at the bounds of `T::Bar`, see if we can find a
1584     /// `Baz` bound. We return indexes into the list returned by
1585     /// `tcx.item_bounds` for any applicable bounds.
1586     #[instrument(level = "debug", skip(self), ret)]
1587     fn match_projection_obligation_against_definition_bounds(
1588         &mut self,
1589         obligation: &TraitObligation<'tcx>,
1590     ) -> smallvec::SmallVec<[(usize, ty::BoundConstness); 2]> {
1591         let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1592         let placeholder_trait_predicate =
1593             self.infcx.replace_bound_vars_with_placeholders(poly_trait_predicate);
1594         debug!(?placeholder_trait_predicate);
1595
1596         let tcx = self.infcx.tcx;
1597         let (def_id, substs) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
1598             ty::Alias(_, ty::AliasTy { def_id, substs, .. }) => (def_id, substs),
1599             _ => {
1600                 span_bug!(
1601                     obligation.cause.span,
1602                     "match_projection_obligation_against_definition_bounds() called \
1603                      but self-ty is not a projection: {:?}",
1604                     placeholder_trait_predicate.trait_ref.self_ty()
1605                 );
1606             }
1607         };
1608         let bounds = tcx.bound_item_bounds(def_id).subst(tcx, substs);
1609
1610         // The bounds returned by `item_bounds` may contain duplicates after
1611         // normalization, so try to deduplicate when possible to avoid
1612         // unnecessary ambiguity.
1613         let mut distinct_normalized_bounds = FxHashSet::default();
1614
1615         bounds
1616             .iter()
1617             .enumerate()
1618             .filter_map(|(idx, bound)| {
1619                 let bound_predicate = bound.kind();
1620                 if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
1621                     bound_predicate.skip_binder()
1622                 {
1623                     let bound = bound_predicate.rebind(pred.trait_ref);
1624                     if self.infcx.probe(|_| {
1625                         match self.match_normalize_trait_ref(
1626                             obligation,
1627                             bound,
1628                             placeholder_trait_predicate.trait_ref,
1629                         ) {
1630                             Ok(None) => true,
1631                             Ok(Some(normalized_trait))
1632                                 if distinct_normalized_bounds.insert(normalized_trait) =>
1633                             {
1634                                 true
1635                             }
1636                             _ => false,
1637                         }
1638                     }) {
1639                         return Some((idx, pred.constness));
1640                     }
1641                 }
1642                 None
1643             })
1644             .collect()
1645     }
1646
1647     /// Equates the trait in `obligation` with trait bound. If the two traits
1648     /// can be equated and the normalized trait bound doesn't contain inference
1649     /// variables or placeholders, the normalized bound is returned.
1650     fn match_normalize_trait_ref(
1651         &mut self,
1652         obligation: &TraitObligation<'tcx>,
1653         trait_bound: ty::PolyTraitRef<'tcx>,
1654         placeholder_trait_ref: ty::TraitRef<'tcx>,
1655     ) -> Result<Option<ty::PolyTraitRef<'tcx>>, ()> {
1656         debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
1657         if placeholder_trait_ref.def_id != trait_bound.def_id() {
1658             // Avoid unnecessary normalization
1659             return Err(());
1660         }
1661
1662         let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
1663             project::normalize_with_depth(
1664                 self,
1665                 obligation.param_env,
1666                 obligation.cause.clone(),
1667                 obligation.recursion_depth + 1,
1668                 trait_bound,
1669             )
1670         });
1671         self.infcx
1672             .at(&obligation.cause, obligation.param_env)
1673             .define_opaque_types(false)
1674             .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
1675             .map(|InferOk { obligations: _, value: () }| {
1676                 // This method is called within a probe, so we can't have
1677                 // inference variables and placeholders escape.
1678                 if !trait_bound.needs_infer() && !trait_bound.has_placeholders() {
1679                     Some(trait_bound)
1680                 } else {
1681                     None
1682                 }
1683             })
1684             .map_err(|_| ())
1685     }
1686
1687     fn where_clause_may_apply<'o>(
1688         &mut self,
1689         stack: &TraitObligationStack<'o, 'tcx>,
1690         where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1691     ) -> Result<EvaluationResult, OverflowError> {
1692         self.evaluation_probe(|this| {
1693             match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1694                 Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
1695                 Err(()) => Ok(EvaluatedToErr),
1696             }
1697         })
1698     }
1699
1700     /// Return `Yes` if the obligation's predicate type applies to the env_predicate, and
1701     /// `No` if it does not. Return `Ambiguous` in the case that the projection type is a GAT,
1702     /// and applying this env_predicate constrains any of the obligation's GAT substitutions.
1703     ///
1704     /// This behavior is a somewhat of a hack to prevent over-constraining inference variables
1705     /// in cases like #91762.
1706     pub(super) fn match_projection_projections(
1707         &mut self,
1708         obligation: &ProjectionTyObligation<'tcx>,
1709         env_predicate: PolyProjectionPredicate<'tcx>,
1710         potentially_unnormalized_candidates: bool,
1711     ) -> ProjectionMatchesProjection {
1712         let mut nested_obligations = Vec::new();
1713         let infer_predicate = self.infcx.replace_bound_vars_with_fresh_vars(
1714             obligation.cause.span,
1715             LateBoundRegionConversionTime::HigherRankedType,
1716             env_predicate,
1717         );
1718         let infer_projection = if potentially_unnormalized_candidates {
1719             ensure_sufficient_stack(|| {
1720                 project::normalize_with_depth_to(
1721                     self,
1722                     obligation.param_env,
1723                     obligation.cause.clone(),
1724                     obligation.recursion_depth + 1,
1725                     infer_predicate.projection_ty,
1726                     &mut nested_obligations,
1727                 )
1728             })
1729         } else {
1730             infer_predicate.projection_ty
1731         };
1732
1733         let is_match = self
1734             .infcx
1735             .at(&obligation.cause, obligation.param_env)
1736             .define_opaque_types(false)
1737             .sup(obligation.predicate, infer_projection)
1738             .map_or(false, |InferOk { obligations, value: () }| {
1739                 self.evaluate_predicates_recursively(
1740                     TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
1741                     nested_obligations.into_iter().chain(obligations),
1742                 )
1743                 .map_or(false, |res| res.may_apply())
1744             });
1745
1746         if is_match {
1747             let generics = self.tcx().generics_of(obligation.predicate.def_id);
1748             // FIXME(generic-associated-types): Addresses aggressive inference in #92917.
1749             // If this type is a GAT, and of the GAT substs resolve to something new,
1750             // that means that we must have newly inferred something about the GAT.
1751             // We should give up in that case.
1752             if !generics.params.is_empty()
1753                 && obligation.predicate.substs[generics.parent_count..]
1754                     .iter()
1755                     .any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
1756             {
1757                 ProjectionMatchesProjection::Ambiguous
1758             } else {
1759                 ProjectionMatchesProjection::Yes
1760             }
1761         } else {
1762             ProjectionMatchesProjection::No
1763         }
1764     }
1765
1766     ///////////////////////////////////////////////////////////////////////////
1767     // WINNOW
1768     //
1769     // Winnowing is the process of attempting to resolve ambiguity by
1770     // probing further. During the winnowing process, we unify all
1771     // type variables and then we also attempt to evaluate recursive
1772     // bounds to see if they are satisfied.
1773
1774     /// Returns `true` if `victim` should be dropped in favor of
1775     /// `other`. Generally speaking we will drop duplicate
1776     /// candidates and prefer where-clause candidates.
1777     ///
1778     /// See the comment for "SelectionCandidate" for more details.
1779     fn candidate_should_be_dropped_in_favor_of(
1780         &mut self,
1781         victim: &EvaluatedCandidate<'tcx>,
1782         other: &EvaluatedCandidate<'tcx>,
1783         needs_infer: bool,
1784     ) -> bool {
1785         if victim.candidate == other.candidate {
1786             return true;
1787         }
1788
1789         // Check if a bound would previously have been removed when normalizing
1790         // the param_env so that it can be given the lowest priority. See
1791         // #50825 for the motivation for this.
1792         let is_global = |cand: &ty::PolyTraitPredicate<'tcx>| {
1793             cand.is_global() && !cand.has_late_bound_regions()
1794         };
1795
1796         // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
1797         // `DiscriminantKindCandidate`, `ConstDestructCandidate`
1798         // to anything else.
1799         //
1800         // This is a fix for #53123 and prevents winnowing from accidentally extending the
1801         // lifetime of a variable.
1802         match (&other.candidate, &victim.candidate) {
1803             (_, AutoImplCandidate) | (AutoImplCandidate, _) => {
1804                 bug!(
1805                     "default implementations shouldn't be recorded \
1806                     when there are other valid candidates"
1807                 );
1808             }
1809
1810             // FIXME(@jswrenn): this should probably be more sophisticated
1811             (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => false,
1812
1813             // (*)
1814             (BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => true,
1815             (_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => false,
1816
1817             (ParamCandidate(other), ParamCandidate(victim)) => {
1818                 let same_except_bound_vars = other.skip_binder().trait_ref
1819                     == victim.skip_binder().trait_ref
1820                     && other.skip_binder().constness == victim.skip_binder().constness
1821                     && other.skip_binder().polarity == victim.skip_binder().polarity
1822                     && !other.skip_binder().trait_ref.has_escaping_bound_vars();
1823                 if same_except_bound_vars {
1824                     // See issue #84398. In short, we can generate multiple ParamCandidates which are
1825                     // the same except for unused bound vars. Just pick the one with the fewest bound vars
1826                     // or the current one if tied (they should both evaluate to the same answer). This is
1827                     // probably best characterized as a "hack", since we might prefer to just do our
1828                     // best to *not* create essentially duplicate candidates in the first place.
1829                     other.bound_vars().len() <= victim.bound_vars().len()
1830                 } else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
1831                     && victim.skip_binder().constness == ty::BoundConstness::NotConst
1832                     && other.skip_binder().polarity == victim.skip_binder().polarity
1833                 {
1834                     // Drop otherwise equivalent non-const candidates in favor of const candidates.
1835                     true
1836                 } else {
1837                     false
1838                 }
1839             }
1840
1841             // Drop otherwise equivalent non-const fn pointer candidates
1842             (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
1843
1844             // Global bounds from the where clause should be ignored
1845             // here (see issue #50825). Otherwise, we have a where
1846             // clause so don't go around looking for impls.
1847             // Arbitrarily give param candidates priority
1848             // over projection and object candidates.
1849             (
1850                 ParamCandidate(ref cand),
1851                 ImplCandidate(..)
1852                 | ClosureCandidate
1853                 | GeneratorCandidate
1854                 | FutureCandidate
1855                 | FnPointerCandidate { .. }
1856                 | BuiltinObjectCandidate
1857                 | BuiltinUnsizeCandidate
1858                 | TraitUpcastingUnsizeCandidate(_)
1859                 | BuiltinCandidate { .. }
1860                 | TraitAliasCandidate
1861                 | ObjectCandidate(_)
1862                 | ProjectionCandidate(..),
1863             ) => !is_global(cand),
1864             (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
1865                 // Prefer these to a global where-clause bound
1866                 // (see issue #50825).
1867                 is_global(cand)
1868             }
1869             (
1870                 ImplCandidate(_)
1871                 | ClosureCandidate
1872                 | GeneratorCandidate
1873                 | FutureCandidate
1874                 | FnPointerCandidate { .. }
1875                 | BuiltinObjectCandidate
1876                 | BuiltinUnsizeCandidate
1877                 | TraitUpcastingUnsizeCandidate(_)
1878                 | BuiltinCandidate { has_nested: true }
1879                 | TraitAliasCandidate,
1880                 ParamCandidate(ref cand),
1881             ) => {
1882                 // Prefer these to a global where-clause bound
1883                 // (see issue #50825).
1884                 is_global(cand) && other.evaluation.must_apply_modulo_regions()
1885             }
1886
1887             (ProjectionCandidate(i, _), ProjectionCandidate(j, _))
1888             | (ObjectCandidate(i), ObjectCandidate(j)) => {
1889                 // Arbitrarily pick the lower numbered candidate for backwards
1890                 // compatibility reasons. Don't let this affect inference.
1891                 i < j && !needs_infer
1892             }
1893             (ObjectCandidate(_), ProjectionCandidate(..))
1894             | (ProjectionCandidate(..), ObjectCandidate(_)) => {
1895                 bug!("Have both object and projection candidate")
1896             }
1897
1898             // Arbitrarily give projection and object candidates priority.
1899             (
1900                 ObjectCandidate(_) | ProjectionCandidate(..),
1901                 ImplCandidate(..)
1902                 | ClosureCandidate
1903                 | GeneratorCandidate
1904                 | FutureCandidate
1905                 | FnPointerCandidate { .. }
1906                 | BuiltinObjectCandidate
1907                 | BuiltinUnsizeCandidate
1908                 | TraitUpcastingUnsizeCandidate(_)
1909                 | BuiltinCandidate { .. }
1910                 | TraitAliasCandidate,
1911             ) => true,
1912
1913             (
1914                 ImplCandidate(..)
1915                 | ClosureCandidate
1916                 | GeneratorCandidate
1917                 | FutureCandidate
1918                 | FnPointerCandidate { .. }
1919                 | BuiltinObjectCandidate
1920                 | BuiltinUnsizeCandidate
1921                 | TraitUpcastingUnsizeCandidate(_)
1922                 | BuiltinCandidate { .. }
1923                 | TraitAliasCandidate,
1924                 ObjectCandidate(_) | ProjectionCandidate(..),
1925             ) => false,
1926
1927             (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
1928                 // See if we can toss out `victim` based on specialization.
1929                 // While this requires us to know *for sure* that the `other` impl applies
1930                 // we still use modulo regions here.
1931                 //
1932                 // This is fine as specialization currently assumes that specializing
1933                 // impls have to be always applicable, meaning that the only allowed
1934                 // region constraints may be constraints also present on the default impl.
1935                 let tcx = self.tcx();
1936                 if other.evaluation.must_apply_modulo_regions() {
1937                     if tcx.specializes((other_def, victim_def)) {
1938                         return true;
1939                     }
1940                 }
1941
1942                 if other.evaluation.must_apply_considering_regions() {
1943                     match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
1944                         Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
1945                             // Subtle: If the predicate we are evaluating has inference
1946                             // variables, do *not* allow discarding candidates due to
1947                             // marker trait impls.
1948                             //
1949                             // Without this restriction, we could end up accidentally
1950                             // constraining inference variables based on an arbitrarily
1951                             // chosen trait impl.
1952                             //
1953                             // Imagine we have the following code:
1954                             //
1955                             // ```rust
1956                             // #[marker] trait MyTrait {}
1957                             // impl MyTrait for u8 {}
1958                             // impl MyTrait for bool {}
1959                             // ```
1960                             //
1961                             // And we are evaluating the predicate `<_#0t as MyTrait>`.
1962                             //
1963                             // During selection, we will end up with one candidate for each
1964                             // impl of `MyTrait`. If we were to discard one impl in favor
1965                             // of the other, we would be left with one candidate, causing
1966                             // us to "successfully" select the predicate, unifying
1967                             // _#0t with (for example) `u8`.
1968                             //
1969                             // However, we have no reason to believe that this unification
1970                             // is correct - we've essentially just picked an arbitrary
1971                             // *possibility* for _#0t, and required that this be the *only*
1972                             // possibility.
1973                             //
1974                             // Eventually, we will either:
1975                             // 1) Unify all inference variables in the predicate through
1976                             // some other means (e.g. type-checking of a function). We will
1977                             // then be in a position to drop marker trait candidates
1978                             // without constraining inference variables (since there are
1979                             // none left to constrain)
1980                             // 2) Be left with some unconstrained inference variables. We
1981                             // will then correctly report an inference error, since the
1982                             // existence of multiple marker trait impls tells us nothing
1983                             // about which one should actually apply.
1984                             !needs_infer
1985                         }
1986                         Some(_) => true,
1987                         None => false,
1988                     }
1989                 } else {
1990                     false
1991                 }
1992             }
1993
1994             // Everything else is ambiguous
1995             (
1996                 ImplCandidate(_)
1997                 | ClosureCandidate
1998                 | GeneratorCandidate
1999                 | FutureCandidate
2000                 | FnPointerCandidate { .. }
2001                 | BuiltinObjectCandidate
2002                 | BuiltinUnsizeCandidate
2003                 | TraitUpcastingUnsizeCandidate(_)
2004                 | BuiltinCandidate { has_nested: true }
2005                 | TraitAliasCandidate,
2006                 ImplCandidate(_)
2007                 | ClosureCandidate
2008                 | GeneratorCandidate
2009                 | FutureCandidate
2010                 | FnPointerCandidate { .. }
2011                 | BuiltinObjectCandidate
2012                 | BuiltinUnsizeCandidate
2013                 | TraitUpcastingUnsizeCandidate(_)
2014                 | BuiltinCandidate { has_nested: true }
2015                 | TraitAliasCandidate,
2016             ) => false,
2017         }
2018     }
2019
2020     fn sized_conditions(
2021         &mut self,
2022         obligation: &TraitObligation<'tcx>,
2023     ) -> BuiltinImplConditions<'tcx> {
2024         use self::BuiltinImplConditions::{Ambiguous, None, Where};
2025
2026         // NOTE: binder moved to (*)
2027         let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2028
2029         match self_ty.kind() {
2030             ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2031             | ty::Uint(_)
2032             | ty::Int(_)
2033             | ty::Bool
2034             | ty::Float(_)
2035             | ty::FnDef(..)
2036             | ty::FnPtr(_)
2037             | ty::RawPtr(..)
2038             | ty::Char
2039             | ty::Ref(..)
2040             | ty::Generator(..)
2041             | ty::GeneratorWitness(..)
2042             | ty::Array(..)
2043             | ty::Closure(..)
2044             | ty::Never
2045             | ty::Dynamic(_, _, ty::DynStar)
2046             | ty::Error(_) => {
2047                 // safe for everything
2048                 Where(ty::Binder::dummy(Vec::new()))
2049             }
2050
2051             ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
2052
2053             ty::Tuple(tys) => Where(
2054                 obligation.predicate.rebind(tys.last().map_or_else(Vec::new, |&last| vec![last])),
2055             ),
2056
2057             ty::Adt(def, substs) => {
2058                 let sized_crit = def.sized_constraint(self.tcx());
2059                 // (*) binder moved here
2060                 Where(obligation.predicate.rebind({
2061                     sized_crit
2062                         .0
2063                         .iter()
2064                         .map(|ty| sized_crit.rebind(*ty).subst(self.tcx(), substs))
2065                         .collect()
2066                 }))
2067             }
2068
2069             ty::Alias(..) | ty::Param(_) => None,
2070             ty::Infer(ty::TyVar(_)) => Ambiguous,
2071
2072             ty::Placeholder(..)
2073             | ty::Bound(..)
2074             | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2075                 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2076             }
2077         }
2078     }
2079
2080     fn copy_clone_conditions(
2081         &mut self,
2082         obligation: &TraitObligation<'tcx>,
2083     ) -> BuiltinImplConditions<'tcx> {
2084         // NOTE: binder moved to (*)
2085         let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2086
2087         use self::BuiltinImplConditions::{Ambiguous, None, Where};
2088
2089         match *self_ty.kind() {
2090             ty::Infer(ty::IntVar(_))
2091             | ty::Infer(ty::FloatVar(_))
2092             | ty::FnDef(..)
2093             | ty::FnPtr(_)
2094             | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
2095
2096             ty::Uint(_)
2097             | ty::Int(_)
2098             | ty::Bool
2099             | ty::Float(_)
2100             | ty::Char
2101             | ty::RawPtr(..)
2102             | ty::Never
2103             | ty::Ref(_, _, hir::Mutability::Not)
2104             | ty::Array(..) => {
2105                 // Implementations provided in libcore
2106                 None
2107             }
2108
2109             ty::Dynamic(..)
2110             | ty::Str
2111             | ty::Slice(..)
2112             | ty::Generator(_, _, hir::Movability::Static)
2113             | ty::Foreign(..)
2114             | ty::Ref(_, _, hir::Mutability::Mut) => None,
2115
2116             ty::Tuple(tys) => {
2117                 // (*) binder moved here
2118                 Where(obligation.predicate.rebind(tys.iter().collect()))
2119             }
2120
2121             ty::Generator(_, substs, hir::Movability::Movable) => {
2122                 if self.tcx().features().generator_clone {
2123                     let resolved_upvars =
2124                         self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2125                     let resolved_witness =
2126                         self.infcx.shallow_resolve(substs.as_generator().witness());
2127                     if resolved_upvars.is_ty_var() || resolved_witness.is_ty_var() {
2128                         // Not yet resolved.
2129                         Ambiguous
2130                     } else {
2131                         let all = substs
2132                             .as_generator()
2133                             .upvar_tys()
2134                             .chain(iter::once(substs.as_generator().witness()))
2135                             .collect::<Vec<_>>();
2136                         Where(obligation.predicate.rebind(all))
2137                     }
2138                 } else {
2139                     None
2140                 }
2141             }
2142
2143             ty::GeneratorWitness(binder) => {
2144                 let witness_tys = binder.skip_binder();
2145                 for witness_ty in witness_tys.iter() {
2146                     let resolved = self.infcx.shallow_resolve(witness_ty);
2147                     if resolved.is_ty_var() {
2148                         return Ambiguous;
2149                     }
2150                 }
2151                 // (*) binder moved here
2152                 let all_vars = self.tcx().mk_bound_variable_kinds(
2153                     obligation.predicate.bound_vars().iter().chain(binder.bound_vars().iter()),
2154                 );
2155                 Where(ty::Binder::bind_with_vars(witness_tys.to_vec(), all_vars))
2156             }
2157
2158             ty::Closure(_, substs) => {
2159                 // (*) binder moved here
2160                 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2161                 if let ty::Infer(ty::TyVar(_)) = ty.kind() {
2162                     // Not yet resolved.
2163                     Ambiguous
2164                 } else {
2165                     Where(obligation.predicate.rebind(substs.as_closure().upvar_tys().collect()))
2166                 }
2167             }
2168
2169             ty::Adt(..) | ty::Alias(..) | ty::Param(..) => {
2170                 // Fallback to whatever user-defined impls exist in this case.
2171                 None
2172             }
2173
2174             ty::Infer(ty::TyVar(_)) => {
2175                 // Unbound type variable. Might or might not have
2176                 // applicable impls and so forth, depending on what
2177                 // those type variables wind up being bound to.
2178                 Ambiguous
2179             }
2180
2181             ty::Placeholder(..)
2182             | ty::Bound(..)
2183             | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2184                 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2185             }
2186         }
2187     }
2188
2189     /// For default impls, we need to break apart a type into its
2190     /// "constituent types" -- meaning, the types that it contains.
2191     ///
2192     /// Here are some (simple) examples:
2193     ///
2194     /// ```ignore (illustrative)
2195     /// (i32, u32) -> [i32, u32]
2196     /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2197     /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2198     /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2199     /// ```
2200     #[instrument(level = "debug", skip(self), ret)]
2201     fn constituent_types_for_ty(
2202         &self,
2203         t: ty::Binder<'tcx, Ty<'tcx>>,
2204     ) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
2205         match *t.skip_binder().kind() {
2206             ty::Uint(_)
2207             | ty::Int(_)
2208             | ty::Bool
2209             | ty::Float(_)
2210             | ty::FnDef(..)
2211             | ty::FnPtr(_)
2212             | ty::Str
2213             | ty::Error(_)
2214             | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2215             | ty::Never
2216             | ty::Char => ty::Binder::dummy(Vec::new()),
2217
2218             ty::Placeholder(..)
2219             | ty::Dynamic(..)
2220             | ty::Param(..)
2221             | ty::Foreign(..)
2222             | ty::Alias(ty::Projection, ..)
2223             | ty::Bound(..)
2224             | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2225                 bug!("asked to assemble constituent types of unexpected type: {:?}", t);
2226             }
2227
2228             ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
2229                 t.rebind(vec![element_ty])
2230             }
2231
2232             ty::Array(element_ty, _) | ty::Slice(element_ty) => t.rebind(vec![element_ty]),
2233
2234             ty::Tuple(ref tys) => {
2235                 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2236                 t.rebind(tys.iter().collect())
2237             }
2238
2239             ty::Closure(_, ref substs) => {
2240                 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2241                 t.rebind(vec![ty])
2242             }
2243
2244             ty::Generator(_, ref substs, _) => {
2245                 let ty = self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2246                 let witness = substs.as_generator().witness();
2247                 t.rebind([ty].into_iter().chain(iter::once(witness)).collect())
2248             }
2249
2250             ty::GeneratorWitness(types) => {
2251                 debug_assert!(!types.has_escaping_bound_vars());
2252                 types.map_bound(|types| types.to_vec())
2253             }
2254
2255             // For `PhantomData<T>`, we pass `T`.
2256             ty::Adt(def, substs) if def.is_phantom_data() => t.rebind(substs.types().collect()),
2257
2258             ty::Adt(def, substs) => {
2259                 t.rebind(def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect())
2260             }
2261
2262             ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
2263                 // We can resolve the `impl Trait` to its concrete type,
2264                 // which enforces a DAG between the functions requiring
2265                 // the auto trait bounds in question.
2266                 t.rebind(vec![self.tcx().bound_type_of(def_id).subst(self.tcx(), substs)])
2267             }
2268         }
2269     }
2270
2271     fn collect_predicates_for_types(
2272         &mut self,
2273         param_env: ty::ParamEnv<'tcx>,
2274         cause: ObligationCause<'tcx>,
2275         recursion_depth: usize,
2276         trait_def_id: DefId,
2277         types: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
2278     ) -> Vec<PredicateObligation<'tcx>> {
2279         // Because the types were potentially derived from
2280         // higher-ranked obligations they may reference late-bound
2281         // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
2282         // yield a type like `for<'a> &'a i32`. In general, we
2283         // maintain the invariant that we never manipulate bound
2284         // regions, so we have to process these bound regions somehow.
2285         //
2286         // The strategy is to:
2287         //
2288         // 1. Instantiate those regions to placeholder regions (e.g.,
2289         //    `for<'a> &'a i32` becomes `&0 i32`.
2290         // 2. Produce something like `&'0 i32 : Copy`
2291         // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
2292
2293         types
2294             .as_ref()
2295             .skip_binder() // binder moved -\
2296             .iter()
2297             .flat_map(|ty| {
2298                 let ty: ty::Binder<'tcx, Ty<'tcx>> = types.rebind(*ty); // <----/
2299
2300                 let placeholder_ty = self.infcx.replace_bound_vars_with_placeholders(ty);
2301                 let Normalized { value: normalized_ty, mut obligations } =
2302                     ensure_sufficient_stack(|| {
2303                         project::normalize_with_depth(
2304                             self,
2305                             param_env,
2306                             cause.clone(),
2307                             recursion_depth,
2308                             placeholder_ty,
2309                         )
2310                     });
2311                 let placeholder_obligation = predicate_for_trait_def(
2312                     self.tcx(),
2313                     param_env,
2314                     cause.clone(),
2315                     trait_def_id,
2316                     recursion_depth,
2317                     [normalized_ty],
2318                 );
2319                 obligations.push(placeholder_obligation);
2320                 obligations
2321             })
2322             .collect()
2323     }
2324
2325     ///////////////////////////////////////////////////////////////////////////
2326     // Matching
2327     //
2328     // Matching is a common path used for both evaluation and
2329     // confirmation.  It basically unifies types that appear in impls
2330     // and traits. This does affect the surrounding environment;
2331     // therefore, when used during evaluation, match routines must be
2332     // run inside of a `probe()` so that their side-effects are
2333     // contained.
2334
2335     fn rematch_impl(
2336         &mut self,
2337         impl_def_id: DefId,
2338         obligation: &TraitObligation<'tcx>,
2339     ) -> Normalized<'tcx, SubstsRef<'tcx>> {
2340         let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
2341         match self.match_impl(impl_def_id, impl_trait_ref, obligation) {
2342             Ok(substs) => substs,
2343             Err(()) => {
2344                 // FIXME: A rematch may fail when a candidate cache hit occurs
2345                 // on thefreshened form of the trait predicate, but the match
2346                 // fails for some reason that is not captured in the freshened
2347                 // cache key. For example, equating an impl trait ref against
2348                 // the placeholder trait ref may fail due the Generalizer relation
2349                 // raising a CyclicalTy error due to a sub_root_var relation
2350                 // for a variable being generalized...
2351                 self.infcx.tcx.sess.delay_span_bug(
2352                     obligation.cause.span,
2353                     &format!(
2354                         "Impl {:?} was matchable against {:?} but now is not",
2355                         impl_def_id, obligation
2356                     ),
2357                 );
2358                 let value = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2359                 let err = self.tcx().ty_error();
2360                 let value = value.fold_with(&mut BottomUpFolder {
2361                     tcx: self.tcx(),
2362                     ty_op: |_| err,
2363                     lt_op: |l| l,
2364                     ct_op: |c| c,
2365                 });
2366                 Normalized { value, obligations: vec![] }
2367             }
2368         }
2369     }
2370
2371     #[instrument(level = "debug", skip(self), ret)]
2372     fn match_impl(
2373         &mut self,
2374         impl_def_id: DefId,
2375         impl_trait_ref: EarlyBinder<ty::TraitRef<'tcx>>,
2376         obligation: &TraitObligation<'tcx>,
2377     ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
2378         let placeholder_obligation =
2379             self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
2380         let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
2381
2382         let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2383
2384         let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
2385
2386         debug!(?impl_trait_ref);
2387
2388         let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
2389             ensure_sufficient_stack(|| {
2390                 project::normalize_with_depth(
2391                     self,
2392                     obligation.param_env,
2393                     obligation.cause.clone(),
2394                     obligation.recursion_depth + 1,
2395                     impl_trait_ref,
2396                 )
2397             });
2398
2399         debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
2400
2401         let cause = ObligationCause::new(
2402             obligation.cause.span,
2403             obligation.cause.body_id,
2404             ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
2405         );
2406
2407         let InferOk { obligations, .. } = self
2408             .infcx
2409             .at(&cause, obligation.param_env)
2410             .define_opaque_types(false)
2411             .eq(placeholder_obligation_trait_ref, impl_trait_ref)
2412             .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{e}`"))?;
2413         nested_obligations.extend(obligations);
2414
2415         if !self.is_intercrate()
2416             && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
2417         {
2418             debug!("reservation impls only apply in intercrate mode");
2419             return Err(());
2420         }
2421
2422         Ok(Normalized { value: impl_substs, obligations: nested_obligations })
2423     }
2424
2425     fn fast_reject_trait_refs(
2426         &mut self,
2427         obligation: &TraitObligation<'tcx>,
2428         impl_trait_ref: &ty::TraitRef<'tcx>,
2429     ) -> bool {
2430         // We can avoid creating type variables and doing the full
2431         // substitution if we find that any of the input types, when
2432         // simplified, do not match.
2433         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
2434         iter::zip(obligation.predicate.skip_binder().trait_ref.substs, impl_trait_ref.substs)
2435             .any(|(obl, imp)| !drcx.generic_args_may_unify(obl, imp))
2436     }
2437
2438     /// Normalize `where_clause_trait_ref` and try to match it against
2439     /// `obligation`. If successful, return any predicates that
2440     /// result from the normalization.
2441     fn match_where_clause_trait_ref(
2442         &mut self,
2443         obligation: &TraitObligation<'tcx>,
2444         where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
2445     ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2446         self.match_poly_trait_ref(obligation, where_clause_trait_ref)
2447     }
2448
2449     /// Returns `Ok` if `poly_trait_ref` being true implies that the
2450     /// obligation is satisfied.
2451     #[instrument(skip(self), level = "debug")]
2452     fn match_poly_trait_ref(
2453         &mut self,
2454         obligation: &TraitObligation<'tcx>,
2455         poly_trait_ref: ty::PolyTraitRef<'tcx>,
2456     ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2457         self.infcx
2458             .at(&obligation.cause, obligation.param_env)
2459             // We don't want predicates for opaque types to just match all other types,
2460             // if there is an obligation on the opaque type, then that obligation must be met
2461             // opaquely. Otherwise we'd match any obligation to the opaque type and then error
2462             // out later.
2463             .define_opaque_types(false)
2464             .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
2465             .map(|InferOk { obligations, .. }| obligations)
2466             .map_err(|_| ())
2467     }
2468
2469     ///////////////////////////////////////////////////////////////////////////
2470     // Miscellany
2471
2472     fn match_fresh_trait_refs(
2473         &self,
2474         previous: ty::PolyTraitPredicate<'tcx>,
2475         current: ty::PolyTraitPredicate<'tcx>,
2476         param_env: ty::ParamEnv<'tcx>,
2477     ) -> bool {
2478         let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
2479         matcher.relate(previous, current).is_ok()
2480     }
2481
2482     fn push_stack<'o>(
2483         &mut self,
2484         previous_stack: TraitObligationStackList<'o, 'tcx>,
2485         obligation: &'o TraitObligation<'tcx>,
2486     ) -> TraitObligationStack<'o, 'tcx> {
2487         let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
2488
2489         let dfn = previous_stack.cache.next_dfn();
2490         let depth = previous_stack.depth() + 1;
2491         TraitObligationStack {
2492             obligation,
2493             fresh_trait_pred,
2494             reached_depth: Cell::new(depth),
2495             previous: previous_stack,
2496             dfn,
2497             depth,
2498         }
2499     }
2500
2501     #[instrument(skip(self), level = "debug")]
2502     fn closure_trait_ref_unnormalized(
2503         &mut self,
2504         obligation: &TraitObligation<'tcx>,
2505         substs: SubstsRef<'tcx>,
2506     ) -> ty::PolyTraitRef<'tcx> {
2507         let closure_sig = substs.as_closure().sig();
2508
2509         debug!(?closure_sig);
2510
2511         // NOTE: The self-type is an unboxed closure type and hence is
2512         // in fact unparameterized (or at least does not reference any
2513         // regions bound in the obligation).
2514         let self_ty = obligation
2515             .predicate
2516             .self_ty()
2517             .no_bound_vars()
2518             .expect("unboxed closure type should not capture bound vars from the predicate");
2519
2520         closure_trait_ref_and_return_type(
2521             self.tcx(),
2522             obligation.predicate.def_id(),
2523             self_ty,
2524             closure_sig,
2525             util::TupleArgumentsFlag::No,
2526         )
2527         .map_bound(|(trait_ref, _)| trait_ref)
2528     }
2529
2530     /// Returns the obligations that are implied by instantiating an
2531     /// impl or trait. The obligations are substituted and fully
2532     /// normalized. This is used when confirming an impl or default
2533     /// impl.
2534     #[instrument(level = "debug", skip(self, cause, param_env))]
2535     fn impl_or_trait_obligations(
2536         &mut self,
2537         cause: &ObligationCause<'tcx>,
2538         recursion_depth: usize,
2539         param_env: ty::ParamEnv<'tcx>,
2540         def_id: DefId,           // of impl or trait
2541         substs: SubstsRef<'tcx>, // for impl or trait
2542         parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
2543     ) -> Vec<PredicateObligation<'tcx>> {
2544         let tcx = self.tcx();
2545
2546         // To allow for one-pass evaluation of the nested obligation,
2547         // each predicate must be preceded by the obligations required
2548         // to normalize it.
2549         // for example, if we have:
2550         //    impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
2551         // the impl will have the following predicates:
2552         //    <V as Iterator>::Item = U,
2553         //    U: Iterator, U: Sized,
2554         //    V: Iterator, V: Sized,
2555         //    <U as Iterator>::Item: Copy
2556         // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2557         // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2558         // `$1: Copy`, so we must ensure the obligations are emitted in
2559         // that order.
2560         let predicates = tcx.bound_predicates_of(def_id);
2561         debug!(?predicates);
2562         assert_eq!(predicates.0.parent, None);
2563         let mut obligations = Vec::with_capacity(predicates.0.predicates.len());
2564         for (predicate, span) in predicates.0.predicates {
2565             let span = *span;
2566             let cause = cause.clone().derived_cause(parent_trait_pred, |derived| {
2567                 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
2568                     derived,
2569                     impl_def_id: def_id,
2570                     span,
2571                 }))
2572             });
2573             let predicate = normalize_with_depth_to(
2574                 self,
2575                 param_env,
2576                 cause.clone(),
2577                 recursion_depth,
2578                 predicates.rebind(*predicate).subst(tcx, substs),
2579                 &mut obligations,
2580             );
2581             obligations.push(Obligation { cause, recursion_depth, param_env, predicate });
2582         }
2583
2584         obligations
2585     }
2586 }
2587
2588 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
2589     fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2590         TraitObligationStackList::with(self)
2591     }
2592
2593     fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
2594         self.previous.cache
2595     }
2596
2597     fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2598         self.list()
2599     }
2600
2601     /// Indicates that attempting to evaluate this stack entry
2602     /// required accessing something from the stack at depth `reached_depth`.
2603     fn update_reached_depth(&self, reached_depth: usize) {
2604         assert!(
2605             self.depth >= reached_depth,
2606             "invoked `update_reached_depth` with something under this stack: \
2607              self.depth={} reached_depth={}",
2608             self.depth,
2609             reached_depth,
2610         );
2611         debug!(reached_depth, "update_reached_depth");
2612         let mut p = self;
2613         while reached_depth < p.depth {
2614             debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
2615             p.reached_depth.set(p.reached_depth.get().min(reached_depth));
2616             p = p.previous.head.unwrap();
2617         }
2618     }
2619 }
2620
2621 /// The "provisional evaluation cache" is used to store intermediate cache results
2622 /// when solving auto traits. Auto traits are unusual in that they can support
2623 /// cycles. So, for example, a "proof tree" like this would be ok:
2624 ///
2625 /// - `Foo<T>: Send` :-
2626 ///   - `Bar<T>: Send` :-
2627 ///     - `Foo<T>: Send` -- cycle, but ok
2628 ///   - `Baz<T>: Send`
2629 ///
2630 /// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
2631 /// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
2632 /// For non-auto traits, this cycle would be an error, but for auto traits (because
2633 /// they are coinductive) it is considered ok.
2634 ///
2635 /// However, there is a complication: at the point where we have
2636 /// "proven" `Bar<T>: Send`, we have in fact only proven it
2637 /// *provisionally*. In particular, we proved that `Bar<T>: Send`
2638 /// *under the assumption* that `Foo<T>: Send`. But what if we later
2639 /// find out this assumption is wrong?  Specifically, we could
2640 /// encounter some kind of error proving `Baz<T>: Send`. In that case,
2641 /// `Bar<T>: Send` didn't turn out to be true.
2642 ///
2643 /// In Issue #60010, we found a bug in rustc where it would cache
2644 /// these intermediate results. This was fixed in #60444 by disabling
2645 /// *all* caching for things involved in a cycle -- in our example,
2646 /// that would mean we don't cache that `Bar<T>: Send`.  But this led
2647 /// to large slowdowns.
2648 ///
2649 /// Specifically, imagine this scenario, where proving `Baz<T>: Send`
2650 /// first requires proving `Bar<T>: Send` (which is true:
2651 ///
2652 /// - `Foo<T>: Send` :-
2653 ///   - `Bar<T>: Send` :-
2654 ///     - `Foo<T>: Send` -- cycle, but ok
2655 ///   - `Baz<T>: Send`
2656 ///     - `Bar<T>: Send` -- would be nice for this to be a cache hit!
2657 ///     - `*const T: Send` -- but what if we later encounter an error?
2658 ///
2659 /// The *provisional evaluation cache* resolves this issue. It stores
2660 /// cache results that we've proven but which were involved in a cycle
2661 /// in some way. We track the minimal stack depth (i.e., the
2662 /// farthest from the top of the stack) that we are dependent on.
2663 /// The idea is that the cache results within are all valid -- so long as
2664 /// none of the nodes in between the current node and the node at that minimum
2665 /// depth result in an error (in which case the cached results are just thrown away).
2666 ///
2667 /// During evaluation, we consult this provisional cache and rely on
2668 /// it. Accessing a cached value is considered equivalent to accessing
2669 /// a result at `reached_depth`, so it marks the *current* solution as
2670 /// provisional as well. If an error is encountered, we toss out any
2671 /// provisional results added from the subtree that encountered the
2672 /// error.  When we pop the node at `reached_depth` from the stack, we
2673 /// can commit all the things that remain in the provisional cache.
2674 struct ProvisionalEvaluationCache<'tcx> {
2675     /// next "depth first number" to issue -- just a counter
2676     dfn: Cell<usize>,
2677
2678     /// Map from cache key to the provisionally evaluated thing.
2679     /// The cache entries contain the result but also the DFN in which they
2680     /// were added. The DFN is used to clear out values on failure.
2681     ///
2682     /// Imagine we have a stack like:
2683     ///
2684     /// - `A B C` and we add a cache for the result of C (DFN 2)
2685     /// - Then we have a stack `A B D` where `D` has DFN 3
2686     /// - We try to solve D by evaluating E: `A B D E` (DFN 4)
2687     /// - `E` generates various cache entries which have cyclic dependencies on `B`
2688     ///   - `A B D E F` and so forth
2689     ///   - the DFN of `F` for example would be 5
2690     /// - then we determine that `E` is in error -- we will then clear
2691     ///   all cache values whose DFN is >= 4 -- in this case, that
2692     ///   means the cached value for `F`.
2693     map: RefCell<FxHashMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
2694
2695     /// The stack of args that we assume to be true because a `WF(arg)` predicate
2696     /// is on the stack above (and because of wellformedness is coinductive).
2697     /// In an "ideal" world, this would share a stack with trait predicates in
2698     /// `TraitObligationStack`. However, trait predicates are *much* hotter than
2699     /// `WellFormed` predicates, and it's very likely that the additional matches
2700     /// will have a perf effect. The value here is the well-formed `GenericArg`
2701     /// and the depth of the trait predicate *above* that well-formed predicate.
2702     wf_args: RefCell<Vec<(ty::GenericArg<'tcx>, usize)>>,
2703 }
2704
2705 /// A cache value for the provisional cache: contains the depth-first
2706 /// number (DFN) and result.
2707 #[derive(Copy, Clone, Debug)]
2708 struct ProvisionalEvaluation {
2709     from_dfn: usize,
2710     reached_depth: usize,
2711     result: EvaluationResult,
2712 }
2713
2714 impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
2715     fn default() -> Self {
2716         Self { dfn: Cell::new(0), map: Default::default(), wf_args: Default::default() }
2717     }
2718 }
2719
2720 impl<'tcx> ProvisionalEvaluationCache<'tcx> {
2721     /// Get the next DFN in sequence (basically a counter).
2722     fn next_dfn(&self) -> usize {
2723         let result = self.dfn.get();
2724         self.dfn.set(result + 1);
2725         result
2726     }
2727
2728     /// Check the provisional cache for any result for
2729     /// `fresh_trait_ref`. If there is a hit, then you must consider
2730     /// it an access to the stack slots at depth
2731     /// `reached_depth` (from the returned value).
2732     fn get_provisional(
2733         &self,
2734         fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2735     ) -> Option<ProvisionalEvaluation> {
2736         debug!(
2737             ?fresh_trait_pred,
2738             "get_provisional = {:#?}",
2739             self.map.borrow().get(&fresh_trait_pred),
2740         );
2741         Some(*self.map.borrow().get(&fresh_trait_pred)?)
2742     }
2743
2744     /// Insert a provisional result into the cache. The result came
2745     /// from the node with the given DFN. It accessed a minimum depth
2746     /// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
2747     /// and resulted in `result`.
2748     fn insert_provisional(
2749         &self,
2750         from_dfn: usize,
2751         reached_depth: usize,
2752         fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2753         result: EvaluationResult,
2754     ) {
2755         debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
2756
2757         let mut map = self.map.borrow_mut();
2758
2759         // Subtle: when we complete working on the DFN `from_dfn`, anything
2760         // that remains in the provisional cache must be dependent on some older
2761         // stack entry than `from_dfn`. We have to update their depth with our transitive
2762         // depth in that case or else it would be referring to some popped note.
2763         //
2764         // Example:
2765         // A (reached depth 0)
2766         //   ...
2767         //      B // depth 1 -- reached depth = 0
2768         //          C // depth 2 -- reached depth = 1 (should be 0)
2769         //              B
2770         //          A // depth 0
2771         //   D (reached depth 1)
2772         //      C (cache -- reached depth = 2)
2773         for (_k, v) in &mut *map {
2774             if v.from_dfn >= from_dfn {
2775                 v.reached_depth = reached_depth.min(v.reached_depth);
2776             }
2777         }
2778
2779         map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
2780     }
2781
2782     /// Invoked when the node with dfn `dfn` does not get a successful
2783     /// result.  This will clear out any provisional cache entries
2784     /// that were added since `dfn` was created. This is because the
2785     /// provisional entries are things which must assume that the
2786     /// things on the stack at the time of their creation succeeded --
2787     /// since the failing node is presently at the top of the stack,
2788     /// these provisional entries must either depend on it or some
2789     /// ancestor of it.
2790     fn on_failure(&self, dfn: usize) {
2791         debug!(?dfn, "on_failure");
2792         self.map.borrow_mut().retain(|key, eval| {
2793             if !eval.from_dfn >= dfn {
2794                 debug!("on_failure: removing {:?}", key);
2795                 false
2796             } else {
2797                 true
2798             }
2799         });
2800     }
2801
2802     /// Invoked when the node at depth `depth` completed without
2803     /// depending on anything higher in the stack (if that completion
2804     /// was a failure, then `on_failure` should have been invoked
2805     /// already).
2806     ///
2807     /// Note that we may still have provisional cache items remaining
2808     /// in the cache when this is done. For example, if there is a
2809     /// cycle:
2810     ///
2811     /// * A depends on...
2812     ///     * B depends on A
2813     ///     * C depends on...
2814     ///         * D depends on C
2815     ///     * ...
2816     ///
2817     /// Then as we complete the C node we will have a provisional cache
2818     /// with results for A, B, C, and D. This method would clear out
2819     /// the C and D results, but leave A and B provisional.
2820     ///
2821     /// This is determined based on the DFN: we remove any provisional
2822     /// results created since `dfn` started (e.g., in our example, dfn
2823     /// would be 2, representing the C node, and hence we would
2824     /// remove the result for D, which has DFN 3, but not the results for
2825     /// A and B, which have DFNs 0 and 1 respectively).
2826     ///
2827     /// Note that we *do not* attempt to cache these cycle participants
2828     /// in the evaluation cache. Doing so would require carefully computing
2829     /// the correct `DepNode` to store in the cache entry:
2830     /// cycle participants may implicitly depend on query results
2831     /// related to other participants in the cycle, due to our logic
2832     /// which examines the evaluation stack.
2833     ///
2834     /// We used to try to perform this caching,
2835     /// but it lead to multiple incremental compilation ICEs
2836     /// (see #92987 and #96319), and was very hard to understand.
2837     /// Fortunately, removing the caching didn't seem to
2838     /// have a performance impact in practice.
2839     fn on_completion(&self, dfn: usize) {
2840         debug!(?dfn, "on_completion");
2841
2842         for (fresh_trait_pred, eval) in
2843             self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
2844         {
2845             debug!(?fresh_trait_pred, ?eval, "on_completion");
2846         }
2847     }
2848 }
2849
2850 #[derive(Copy, Clone)]
2851 struct TraitObligationStackList<'o, 'tcx> {
2852     cache: &'o ProvisionalEvaluationCache<'tcx>,
2853     head: Option<&'o TraitObligationStack<'o, 'tcx>>,
2854 }
2855
2856 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
2857     fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2858         TraitObligationStackList { cache, head: None }
2859     }
2860
2861     fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2862         TraitObligationStackList { cache: r.cache(), head: Some(r) }
2863     }
2864
2865     fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2866         self.head
2867     }
2868
2869     fn depth(&self) -> usize {
2870         if let Some(head) = self.head { head.depth } else { 0 }
2871     }
2872 }
2873
2874 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
2875     type Item = &'o TraitObligationStack<'o, 'tcx>;
2876
2877     fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2878         let o = self.head?;
2879         *self = o.previous;
2880         Some(o)
2881     }
2882 }
2883
2884 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
2885     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2886         write!(f, "TraitObligationStack({:?})", self.obligation)
2887     }
2888 }
2889
2890 pub enum ProjectionMatchesProjection {
2891     Yes,
2892     Ambiguous,
2893     No,
2894 }