compiler/rustc_infer/src/infer/mod.rs

   1 pub use self::freshen::TypeFreshener;
   2 pub use self::lexical_region_resolve::RegionResolutionError;
   3 pub use self::LateBoundRegionConversionTime::*;
   4 pub use self::RegionVariableOrigin::*;
   5 pub use self::SubregionOrigin::*;
   6 pub use self::ValuePairs::*;
   7
   8 use self::opaque_types::OpaqueTypeStorage;
   9 pub(crate) use self::undo_log::{InferCtxtUndoLogs, Snapshot, UndoLog};
  10
  11 use crate::traits::{self, ObligationCause, PredicateObligations, TraitEngine, TraitEngineExt};
  12
  13 use rustc_data_structures::fx::FxIndexMap;
  14 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  15 use rustc_data_structures::sync::Lrc;
  16 use rustc_data_structures::undo_log::Rollback;
  17 use rustc_data_structures::unify as ut;
  18 use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
  19 use rustc_hir::def_id::{DefId, LocalDefId};
  20 use rustc_middle::infer::canonical::{Canonical, CanonicalVarValues};
  21 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
  22 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind, ToType};
  23 use rustc_middle::mir::interpret::{ErrorHandled, EvalToValTreeResult};
  24 use rustc_middle::mir::ConstraintCategory;
  25 use rustc_middle::traits::select;
  26 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  27 use rustc_middle::ty::fold::BoundVarReplacerDelegate;
  28 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
  29 use rustc_middle::ty::relate::RelateResult;
  30 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, SubstsRef};
  31 use rustc_middle::ty::visit::TypeVisitable;
  32 pub use rustc_middle::ty::IntVarValue;
  33 use rustc_middle::ty::{self, GenericParamDefKind, InferConst, Ty, TyCtxt};
  34 use rustc_middle::ty::{ConstVid, FloatVid, IntVid, TyVid};
  35 use rustc_span::symbol::Symbol;
  36 use rustc_span::Span;
  37
  38 use std::cell::{Cell, RefCell};
  39 use std::fmt;
  40
  41 use self::combine::CombineFields;
  42 use self::error_reporting::TypeErrCtxt;
  43 use self::free_regions::RegionRelations;
  44 use self::lexical_region_resolve::LexicalRegionResolutions;
  45 use self::outlives::env::OutlivesEnvironment;
  46 use self::region_constraints::{GenericKind, RegionConstraintData, VarInfos, VerifyBound};
  47 use self::region_constraints::{
  48     RegionConstraintCollector, RegionConstraintStorage, RegionSnapshot,
  49 };
  50 use self::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  51
  52 pub mod at;
  53 pub mod canonical;
  54 mod combine;
  55 mod equate;
  56 pub mod error_reporting;
  57 pub mod free_regions;
  58 mod freshen;
  59 mod fudge;
  60 mod glb;
  61 mod higher_ranked;
  62 pub mod lattice;
  63 mod lexical_region_resolve;
  64 mod lub;
  65 pub mod nll_relate;
  66 pub mod opaque_types;
  67 pub mod outlives;
  68 mod projection;
  69 pub mod region_constraints;
  70 pub mod resolve;
  71 mod sub;
  72 pub mod type_variable;
  73 mod undo_log;
  74
  75 #[must_use]
  76 #[derive(Debug)]
  77 pub struct InferOk<'tcx, T> {
  78     pub value: T,
  79     pub obligations: PredicateObligations<'tcx>,
  80 }
  81 pub type InferResult<'tcx, T> = Result<InferOk<'tcx, T>, TypeError<'tcx>>;
  82
  83 pub type UnitResult<'tcx> = RelateResult<'tcx, ()>; // "unify result"
  84 pub type FixupResult<'tcx, T> = Result<T, FixupError<'tcx>>; // "fixup result"
  85
  86 pub(crate) type UnificationTable<'a, 'tcx, T> = ut::UnificationTable<
  87     ut::InPlace<T, &'a mut ut::UnificationStorage<T>, &'a mut InferCtxtUndoLogs<'tcx>>,
  88 >;
  89
  90 /// This type contains all the things within `InferCtxt` that sit within a
  91 /// `RefCell` and are involved with taking/rolling back snapshots. Snapshot
  92 /// operations are hot enough that we want only one call to `borrow_mut` per
  93 /// call to `start_snapshot` and `rollback_to`.
  94 #[derive(Clone)]
  95 pub struct InferCtxtInner<'tcx> {
  96     /// Cache for projections. This cache is snapshotted along with the infcx.
  97     ///
  98     /// Public so that `traits::project` can use it.
  99     pub projection_cache: traits::ProjectionCacheStorage<'tcx>,
 100
 101     /// We instantiate `UnificationTable` with `bounds<Ty>` because the types
 102     /// that might instantiate a general type variable have an order,
 103     /// represented by its upper and lower bounds.
 104     type_variable_storage: type_variable::TypeVariableStorage<'tcx>,
 105
 106     /// Map from const parameter variable to the kind of const it represents.
 107     const_unification_storage: ut::UnificationTableStorage<ty::ConstVid<'tcx>>,
 108
 109     /// Map from integral variable to the kind of integer it represents.
 110     int_unification_storage: ut::UnificationTableStorage<ty::IntVid>,
 111
 112     /// Map from floating variable to the kind of float it represents.
 113     float_unification_storage: ut::UnificationTableStorage<ty::FloatVid>,
 114
 115     /// Tracks the set of region variables and the constraints between them.
 116     /// This is initially `Some(_)` but when
 117     /// `resolve_regions_and_report_errors` is invoked, this gets set to `None`
 118     /// -- further attempts to perform unification, etc., may fail if new
 119     /// region constraints would've been added.
 120     region_constraint_storage: Option<RegionConstraintStorage<'tcx>>,
 121
 122     /// A set of constraints that regionck must validate. Each
 123     /// constraint has the form `T:'a`, meaning "some type `T` must
 124     /// outlive the lifetime 'a". These constraints derive from
 125     /// instantiated type parameters. So if you had a struct defined
 126     /// like
 127     /// ```ignore (illustrative)
 128     ///     struct Foo<T:'static> { ... }
 129     /// ```
 130     /// then in some expression `let x = Foo { ... }` it will
 131     /// instantiate the type parameter `T` with a fresh type `$0`. At
 132     /// the same time, it will record a region obligation of
 133     /// `$0:'static`. This will get checked later by regionck. (We
 134     /// can't generally check these things right away because we have
 135     /// to wait until types are resolved.)
 136     ///
 137     /// These are stored in a map keyed to the id of the innermost
 138     /// enclosing fn body / static initializer expression. This is
 139     /// because the location where the obligation was incurred can be
 140     /// relevant with respect to which sublifetime assumptions are in
 141     /// place. The reason that we store under the fn-id, and not
 142     /// something more fine-grained, is so that it is easier for
 143     /// regionck to be sure that it has found *all* the region
 144     /// obligations (otherwise, it's easy to fail to walk to a
 145     /// particular node-id).
 146     ///
 147     /// Before running `resolve_regions_and_report_errors`, the creator
 148     /// of the inference context is expected to invoke
 149     /// [`InferCtxt::process_registered_region_obligations`]
 150     /// for each body-id in this map, which will process the
 151     /// obligations within. This is expected to be done 'late enough'
 152     /// that all type inference variables have been bound and so forth.
 153     region_obligations: Vec<RegionObligation<'tcx>>,
 154
 155     undo_log: InferCtxtUndoLogs<'tcx>,
 156
 157     /// Caches for opaque type inference.
 158     pub opaque_type_storage: OpaqueTypeStorage<'tcx>,
 159 }
 160
 161 impl<'tcx> InferCtxtInner<'tcx> {
 162     fn new() -> InferCtxtInner<'tcx> {
 163         InferCtxtInner {
 164             projection_cache: Default::default(),
 165             type_variable_storage: type_variable::TypeVariableStorage::new(),
 166             undo_log: InferCtxtUndoLogs::default(),
 167             const_unification_storage: ut::UnificationTableStorage::new(),
 168             int_unification_storage: ut::UnificationTableStorage::new(),
 169             float_unification_storage: ut::UnificationTableStorage::new(),
 170             region_constraint_storage: Some(RegionConstraintStorage::new()),
 171             region_obligations: vec![],
 172             opaque_type_storage: Default::default(),
 173         }
 174     }
 175
 176     #[inline]
 177     pub fn region_obligations(&self) -> &[RegionObligation<'tcx>] {
 178         &self.region_obligations
 179     }
 180
 181     #[inline]
 182     pub fn projection_cache(&mut self) -> traits::ProjectionCache<'_, 'tcx> {
 183         self.projection_cache.with_log(&mut self.undo_log)
 184     }
 185
 186     #[inline]
 187     fn type_variables(&mut self) -> type_variable::TypeVariableTable<'_, 'tcx> {
 188         self.type_variable_storage.with_log(&mut self.undo_log)
 189     }
 190
 191     #[inline]
 192     pub fn opaque_types(&mut self) -> opaque_types::OpaqueTypeTable<'_, 'tcx> {
 193         self.opaque_type_storage.with_log(&mut self.undo_log)
 194     }
 195
 196     #[inline]
 197     fn int_unification_table(
 198         &mut self,
 199     ) -> ut::UnificationTable<
 200         ut::InPlace<
 201             ty::IntVid,
 202             &mut ut::UnificationStorage<ty::IntVid>,
 203             &mut InferCtxtUndoLogs<'tcx>,
 204         >,
 205     > {
 206         self.int_unification_storage.with_log(&mut self.undo_log)
 207     }
 208
 209     #[inline]
 210     fn float_unification_table(
 211         &mut self,
 212     ) -> ut::UnificationTable<
 213         ut::InPlace<
 214             ty::FloatVid,
 215             &mut ut::UnificationStorage<ty::FloatVid>,
 216             &mut InferCtxtUndoLogs<'tcx>,
 217         >,
 218     > {
 219         self.float_unification_storage.with_log(&mut self.undo_log)
 220     }
 221
 222     #[inline]
 223     fn const_unification_table(
 224         &mut self,
 225     ) -> ut::UnificationTable<
 226         ut::InPlace<
 227             ty::ConstVid<'tcx>,
 228             &mut ut::UnificationStorage<ty::ConstVid<'tcx>>,
 229             &mut InferCtxtUndoLogs<'tcx>,
 230         >,
 231     > {
 232         self.const_unification_storage.with_log(&mut self.undo_log)
 233     }
 234
 235     #[inline]
 236     pub fn unwrap_region_constraints(&mut self) -> RegionConstraintCollector<'_, 'tcx> {
 237         self.region_constraint_storage
 238             .as_mut()
 239             .expect("region constraints already solved")
 240             .with_log(&mut self.undo_log)
 241     }
 242 }
 243
 244 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 245 pub enum DefiningAnchor {
 246     /// `DefId` of the item.
 247     Bind(LocalDefId),
 248     /// When opaque types are not resolved, we `Bubble` up, meaning
 249     /// return the opaque/hidden type pair from query, for caller of query to handle it.
 250     Bubble,
 251     /// Used to catch type mismatch errors when handling opaque types.
 252     Error,
 253 }
 254
 255 pub struct InferCtxt<'tcx> {
 256     pub tcx: TyCtxt<'tcx>,
 257
 258     /// The `DefId` of the item in whose context we are performing inference or typeck.
 259     /// It is used to check whether an opaque type use is a defining use.
 260     ///
 261     /// If it is `DefiningAnchor::Bubble`, we can't resolve opaque types here and need to bubble up
 262     /// the obligation. This frequently happens for
 263     /// short lived InferCtxt within queries. The opaque type obligations are forwarded
 264     /// to the outside until the end up in an `InferCtxt` for typeck or borrowck.
 265     ///
 266     /// It is default value is `DefiningAnchor::Error`, this way it is easier to catch errors that
 267     /// might come up during inference or typeck.
 268     pub defining_use_anchor: DefiningAnchor,
 269
 270     /// Whether this inference context should care about region obligations in
 271     /// the root universe. Most notably, this is used during hir typeck as region
 272     /// solving is left to borrowck instead.
 273     pub considering_regions: bool,
 274
 275     pub inner: RefCell<InferCtxtInner<'tcx>>,
 276
 277     /// If set, this flag causes us to skip the 'leak check' during
 278     /// higher-ranked subtyping operations. This flag is a temporary one used
 279     /// to manage the removal of the leak-check: for the time being, we still run the
 280     /// leak-check, but we issue warnings. This flag can only be set to true
 281     /// when entering a snapshot.
 282     skip_leak_check: Cell<bool>,
 283
 284     /// Once region inference is done, the values for each variable.
 285     lexical_region_resolutions: RefCell<Option<LexicalRegionResolutions<'tcx>>>,
 286
 287     /// Caches the results of trait selection. This cache is used
 288     /// for things that have to do with the parameters in scope.
 289     pub selection_cache: select::SelectionCache<'tcx>,
 290
 291     /// Caches the results of trait evaluation.
 292     pub evaluation_cache: select::EvaluationCache<'tcx>,
 293
 294     /// the set of predicates on which errors have been reported, to
 295     /// avoid reporting the same error twice.
 296     pub reported_trait_errors: RefCell<FxIndexMap<Span, Vec<ty::Predicate<'tcx>>>>,
 297
 298     pub reported_closure_mismatch: RefCell<FxHashSet<(Span, Option<Span>)>>,
 299
 300     /// When an error occurs, we want to avoid reporting "derived"
 301     /// errors that are due to this original failure. Normally, we
 302     /// handle this with the `err_count_on_creation` count, which
 303     /// basically just tracks how many errors were reported when we
 304     /// started type-checking a fn and checks to see if any new errors
 305     /// have been reported since then. Not great, but it works.
 306     ///
 307     /// However, when errors originated in other passes -- notably
 308     /// resolve -- this heuristic breaks down. Therefore, we have this
 309     /// auxiliary flag that one can set whenever one creates a
 310     /// type-error that is due to an error in a prior pass.
 311     ///
 312     /// Don't read this flag directly, call `is_tainted_by_errors()`
 313     /// and `set_tainted_by_errors()`.
 314     tainted_by_errors: Cell<Option<ErrorGuaranteed>>,
 315
 316     /// Track how many errors were reported when this infcx is created.
 317     /// If the number of errors increases, that's also a sign (line
 318     /// `tainted_by_errors`) to avoid reporting certain kinds of errors.
 319     // FIXME(matthewjasper) Merge into `tainted_by_errors`
 320     err_count_on_creation: usize,
 321
 322     /// This flag is true while there is an active snapshot.
 323     in_snapshot: Cell<bool>,
 324
 325     /// What is the innermost universe we have created? Starts out as
 326     /// `UniverseIndex::root()` but grows from there as we enter
 327     /// universal quantifiers.
 328     ///
 329     /// N.B., at present, we exclude the universal quantifiers on the
 330     /// item we are type-checking, and just consider those names as
 331     /// part of the root universe. So this would only get incremented
 332     /// when we enter into a higher-ranked (`for<..>`) type or trait
 333     /// bound.
 334     universe: Cell<ty::UniverseIndex>,
 335
 336     /// During coherence we have to assume that other crates may add
 337     /// additional impls which we currently don't know about.
 338     ///
 339     /// To deal with this evaluation should be conservative
 340     /// and consider the possibility of impls from outside this crate.
 341     /// This comes up primarily when resolving ambiguity. Imagine
 342     /// there is some trait reference `$0: Bar` where `$0` is an
 343     /// inference variable. If `intercrate` is true, then we can never
 344     /// say for sure that this reference is not implemented, even if
 345     /// there are *no impls at all for `Bar`*, because `$0` could be
 346     /// bound to some type that in a downstream crate that implements
 347     /// `Bar`.
 348     ///
 349     /// Outside of coherence we set this to false because we are only
 350     /// interested in types that the user could actually have written.
 351     /// In other words, we consider `$0: Bar` to be unimplemented if
 352     /// there is no type that the user could *actually name* that
 353     /// would satisfy it. This avoids crippling inference, basically.
 354     pub intercrate: bool,
 355 }
 356
 357 /// See the `error_reporting` module for more details.
 358 #[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable, TypeVisitable)]
 359 pub enum ValuePairs<'tcx> {
 360     Regions(ExpectedFound<ty::Region<'tcx>>),
 361     Terms(ExpectedFound<ty::Term<'tcx>>),
 362     TraitRefs(ExpectedFound<ty::TraitRef<'tcx>>),
 363     PolyTraitRefs(ExpectedFound<ty::PolyTraitRef<'tcx>>),
 364 }
 365
 366 impl<'tcx> ValuePairs<'tcx> {
 367     pub fn ty(&self) -> Option<(Ty<'tcx>, Ty<'tcx>)> {
 368         if let ValuePairs::Terms(ExpectedFound { expected, found }) = self
 369             && let Some(expected) = expected.ty()
 370             && let Some(found) = found.ty()
 371         {
 372             Some((expected, found))
 373         } else {
 374             None
 375         }
 376     }
 377 }
 378
 379 /// The trace designates the path through inference that we took to
 380 /// encounter an error or subtyping constraint.
 381 ///
 382 /// See the `error_reporting` module for more details.
 383 #[derive(Clone, Debug)]
 384 pub struct TypeTrace<'tcx> {
 385     pub cause: ObligationCause<'tcx>,
 386     pub values: ValuePairs<'tcx>,
 387 }
 388
 389 /// The origin of a `r1 <= r2` constraint.
 390 ///
 391 /// See `error_reporting` module for more details
 392 #[derive(Clone, Debug)]
 393 pub enum SubregionOrigin<'tcx> {
 394     /// Arose from a subtyping relation
 395     Subtype(Box<TypeTrace<'tcx>>),
 396
 397     /// When casting `&'a T` to an `&'b Trait` object,
 398     /// relating `'a` to `'b`
 399     RelateObjectBound(Span),
 400
 401     /// Some type parameter was instantiated with the given type,
 402     /// and that type must outlive some region.
 403     RelateParamBound(Span, Ty<'tcx>, Option<Span>),
 404
 405     /// The given region parameter was instantiated with a region
 406     /// that must outlive some other region.
 407     RelateRegionParamBound(Span),
 408
 409     /// Creating a pointer `b` to contents of another reference
 410     Reborrow(Span),
 411
 412     /// Creating a pointer `b` to contents of an upvar
 413     ReborrowUpvar(Span, ty::UpvarId),
 414
 415     /// Data with type `Ty<'tcx>` was borrowed
 416     DataBorrowed(Ty<'tcx>, Span),
 417
 418     /// (&'a &'b T) where a >= b
 419     ReferenceOutlivesReferent(Ty<'tcx>, Span),
 420
 421     /// Comparing the signature and requirements of an impl method against
 422     /// the containing trait.
 423     CompareImplItemObligation {
 424         span: Span,
 425         impl_item_def_id: LocalDefId,
 426         trait_item_def_id: DefId,
 427     },
 428
 429     /// Checking that the bounds of a trait's associated type hold for a given impl
 430     CheckAssociatedTypeBounds {
 431         parent: Box<SubregionOrigin<'tcx>>,
 432         impl_item_def_id: LocalDefId,
 433         trait_item_def_id: DefId,
 434     },
 435
 436     AscribeUserTypeProvePredicate(Span),
 437 }
 438
 439 // `SubregionOrigin` is used a lot. Make sure it doesn't unintentionally get bigger.
 440 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 441 static_assert_size!(SubregionOrigin<'_>, 32);
 442
 443 impl<'tcx> SubregionOrigin<'tcx> {
 444     pub fn to_constraint_category(&self) -> ConstraintCategory<'tcx> {
 445         match self {
 446             Self::Subtype(type_trace) => type_trace.cause.to_constraint_category(),
 447             Self::AscribeUserTypeProvePredicate(span) => ConstraintCategory::Predicate(*span),
 448             _ => ConstraintCategory::BoringNoLocation,
 449         }
 450     }
 451 }
 452
 453 /// Times when we replace late-bound regions with variables:
 454 #[derive(Clone, Copy, Debug)]
 455 pub enum LateBoundRegionConversionTime {
 456     /// when a fn is called
 457     FnCall,
 458
 459     /// when two higher-ranked types are compared
 460     HigherRankedType,
 461
 462     /// when projecting an associated type
 463     AssocTypeProjection(DefId),
 464 }
 465
 466 /// Reasons to create a region inference variable
 467 ///
 468 /// See `error_reporting` module for more details
 469 #[derive(Copy, Clone, Debug)]
 470 pub enum RegionVariableOrigin {
 471     /// Region variables created for ill-categorized reasons,
 472     /// mostly indicates places in need of refactoring
 473     MiscVariable(Span),
 474
 475     /// Regions created by a `&P` or `[...]` pattern
 476     PatternRegion(Span),
 477
 478     /// Regions created by `&` operator
 479     AddrOfRegion(Span),
 480
 481     /// Regions created as part of an autoref of a method receiver
 482     Autoref(Span),
 483
 484     /// Regions created as part of an automatic coercion
 485     Coercion(Span),
 486
 487     /// Region variables created as the values for early-bound regions
 488     EarlyBoundRegion(Span, Symbol),
 489
 490     /// Region variables created for bound regions
 491     /// in a function or method that is called
 492     LateBoundRegion(Span, ty::BoundRegionKind, LateBoundRegionConversionTime),
 493
 494     UpvarRegion(ty::UpvarId, Span),
 495
 496     /// This origin is used for the inference variables that we create
 497     /// during NLL region processing.
 498     Nll(NllRegionVariableOrigin),
 499 }
 500
 501 #[derive(Copy, Clone, Debug)]
 502 pub enum NllRegionVariableOrigin {
 503     /// During NLL region processing, we create variables for free
 504     /// regions that we encounter in the function signature and
 505     /// elsewhere. This origin indices we've got one of those.
 506     FreeRegion,
 507
 508     /// "Universal" instantiation of a higher-ranked region (e.g.,
 509     /// from a `for<'a> T` binder). Meant to represent "any region".
 510     Placeholder(ty::PlaceholderRegion),
 511
 512     Existential {
 513         /// If this is true, then this variable was created to represent a lifetime
 514         /// bound in a `for` binder. For example, it might have been created to
 515         /// represent the lifetime `'a` in a type like `for<'a> fn(&'a u32)`.
 516         /// Such variables are created when we are trying to figure out if there
 517         /// is any valid instantiation of `'a` that could fit into some scenario.
 518         ///
 519         /// This is used to inform error reporting: in the case that we are trying to
 520         /// determine whether there is any valid instantiation of a `'a` variable that meets
 521         /// some constraint C, we want to blame the "source" of that `for` type,
 522         /// rather than blaming the source of the constraint C.
 523         from_forall: bool,
 524     },
 525 }
 526
 527 // FIXME(eddyb) investigate overlap between this and `TyOrConstInferVar`.
 528 #[derive(Copy, Clone, Debug)]
 529 pub enum FixupError<'tcx> {
 530     UnresolvedIntTy(IntVid),
 531     UnresolvedFloatTy(FloatVid),
 532     UnresolvedTy(TyVid),
 533     UnresolvedConst(ConstVid<'tcx>),
 534 }
 535
 536 /// See the `region_obligations` field for more information.
 537 #[derive(Clone, Debug)]
 538 pub struct RegionObligation<'tcx> {
 539     pub sub_region: ty::Region<'tcx>,
 540     pub sup_type: Ty<'tcx>,
 541     pub origin: SubregionOrigin<'tcx>,
 542 }
 543
 544 impl<'tcx> fmt::Display for FixupError<'tcx> {
 545     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 546         use self::FixupError::*;
 547
 548         match *self {
 549             UnresolvedIntTy(_) => write!(
 550                 f,
 551                 "cannot determine the type of this integer; \
 552                  add a suffix to specify the type explicitly"
 553             ),
 554             UnresolvedFloatTy(_) => write!(
 555                 f,
 556                 "cannot determine the type of this number; \
 557                  add a suffix to specify the type explicitly"
 558             ),
 559             UnresolvedTy(_) => write!(f, "unconstrained type"),
 560             UnresolvedConst(_) => write!(f, "unconstrained const value"),
 561         }
 562     }
 563 }
 564
 565 /// Used to configure inference contexts before their creation
 566 pub struct InferCtxtBuilder<'tcx> {
 567     tcx: TyCtxt<'tcx>,
 568     defining_use_anchor: DefiningAnchor,
 569     considering_regions: bool,
 570     /// Whether we are in coherence mode.
 571     intercrate: bool,
 572 }
 573
 574 pub trait TyCtxtInferExt<'tcx> {
 575     fn infer_ctxt(self) -> InferCtxtBuilder<'tcx>;
 576 }
 577
 578 impl<'tcx> TyCtxtInferExt<'tcx> for TyCtxt<'tcx> {
 579     fn infer_ctxt(self) -> InferCtxtBuilder<'tcx> {
 580         InferCtxtBuilder {
 581             tcx: self,
 582             defining_use_anchor: DefiningAnchor::Error,
 583             considering_regions: true,
 584             intercrate: false,
 585         }
 586     }
 587 }
 588
 589 impl<'tcx> InferCtxtBuilder<'tcx> {
 590     /// Whenever the `InferCtxt` should be able to handle defining uses of opaque types,
 591     /// you need to call this function. Otherwise the opaque type will be treated opaquely.
 592     ///
 593     /// It is only meant to be called in two places, for typeck
 594     /// (via `Inherited::build`) and for the inference context used
 595     /// in mir borrowck.
 596     pub fn with_opaque_type_inference(mut self, defining_use_anchor: DefiningAnchor) -> Self {
 597         self.defining_use_anchor = defining_use_anchor;
 598         self
 599     }
 600
 601     pub fn intercrate(mut self) -> Self {
 602         self.intercrate = true;
 603         self
 604     }
 605
 606     pub fn ignoring_regions(mut self) -> Self {
 607         self.considering_regions = false;
 608         self
 609     }
 610
 611     /// Given a canonical value `C` as a starting point, create an
 612     /// inference context that contains each of the bound values
 613     /// within instantiated as a fresh variable. The `f` closure is
 614     /// invoked with the new infcx, along with the instantiated value
 615     /// `V` and a substitution `S`. This substitution `S` maps from
 616     /// the bound values in `C` to their instantiated values in `V`
 617     /// (in other words, `S(C) = V`).
 618     pub fn build_with_canonical<T>(
 619         &mut self,
 620         span: Span,
 621         canonical: &Canonical<'tcx, T>,
 622     ) -> (InferCtxt<'tcx>, T, CanonicalVarValues<'tcx>)
 623     where
 624         T: TypeFoldable<'tcx>,
 625     {
 626         let infcx = self.build();
 627         let (value, subst) = infcx.instantiate_canonical_with_fresh_inference_vars(span, canonical);
 628         (infcx, value, subst)
 629     }
 630
 631     pub fn build(&mut self) -> InferCtxt<'tcx> {
 632         let InferCtxtBuilder { tcx, defining_use_anchor, considering_regions, intercrate } = *self;
 633         InferCtxt {
 634             tcx,
 635             defining_use_anchor,
 636             considering_regions,
 637             inner: RefCell::new(InferCtxtInner::new()),
 638             lexical_region_resolutions: RefCell::new(None),
 639             selection_cache: Default::default(),
 640             evaluation_cache: Default::default(),
 641             reported_trait_errors: Default::default(),
 642             reported_closure_mismatch: Default::default(),
 643             tainted_by_errors: Cell::new(None),
 644             err_count_on_creation: tcx.sess.err_count(),
 645             in_snapshot: Cell::new(false),
 646             skip_leak_check: Cell::new(false),
 647             universe: Cell::new(ty::UniverseIndex::ROOT),
 648             intercrate,
 649         }
 650     }
 651 }
 652
 653 impl<'tcx, T> InferOk<'tcx, T> {
 654     pub fn unit(self) -> InferOk<'tcx, ()> {
 655         InferOk { value: (), obligations: self.obligations }
 656     }
 657
 658     /// Extracts `value`, registering any obligations into `fulfill_cx`.
 659     pub fn into_value_registering_obligations(
 660         self,
 661         infcx: &InferCtxt<'tcx>,
 662         fulfill_cx: &mut dyn TraitEngine<'tcx>,
 663     ) -> T {
 664         let InferOk { value, obligations } = self;
 665         fulfill_cx.register_predicate_obligations(infcx, obligations);
 666         value
 667     }
 668 }
 669
 670 impl<'tcx> InferOk<'tcx, ()> {
 671     pub fn into_obligations(self) -> PredicateObligations<'tcx> {
 672         self.obligations
 673     }
 674 }
 675
 676 #[must_use = "once you start a snapshot, you should always consume it"]
 677 pub struct CombinedSnapshot<'tcx> {
 678     undo_snapshot: Snapshot<'tcx>,
 679     region_constraints_snapshot: RegionSnapshot,
 680     universe: ty::UniverseIndex,
 681     was_in_snapshot: bool,
 682 }
 683
 684 impl<'tcx> InferCtxt<'tcx> {
 685     /// Creates a `TypeErrCtxt` for emitting various inference errors.
 686     /// During typeck, use `FnCtxt::err_ctxt` instead.
 687     pub fn err_ctxt(&self) -> TypeErrCtxt<'_, 'tcx> {
 688         TypeErrCtxt {
 689             infcx: self,
 690             typeck_results: None,
 691             fallback_has_occurred: false,
 692             normalize_fn_sig: Box::new(|fn_sig| fn_sig),
 693         }
 694     }
 695
 696     pub fn is_in_snapshot(&self) -> bool {
 697         self.in_snapshot.get()
 698     }
 699
 700     pub fn freshen<T: TypeFoldable<'tcx>>(&self, t: T) -> T {
 701         t.fold_with(&mut self.freshener())
 702     }
 703
 704     /// Returns the origin of the type variable identified by `vid`, or `None`
 705     /// if this is not a type variable.
 706     ///
 707     /// No attempt is made to resolve `ty`.
 708     pub fn type_var_origin(&self, ty: Ty<'tcx>) -> Option<TypeVariableOrigin> {
 709         match *ty.kind() {
 710             ty::Infer(ty::TyVar(vid)) => {
 711                 Some(*self.inner.borrow_mut().type_variables().var_origin(vid))
 712             }
 713             _ => None,
 714         }
 715     }
 716
 717     pub fn freshener<'b>(&'b self) -> TypeFreshener<'b, 'tcx> {
 718         freshen::TypeFreshener::new(self, false)
 719     }
 720
 721     /// Like `freshener`, but does not replace `'static` regions.
 722     pub fn freshener_keep_static<'b>(&'b self) -> TypeFreshener<'b, 'tcx> {
 723         freshen::TypeFreshener::new(self, true)
 724     }
 725
 726     pub fn unsolved_variables(&self) -> Vec<Ty<'tcx>> {
 727         let mut inner = self.inner.borrow_mut();
 728         let mut vars: Vec<Ty<'_>> = inner
 729             .type_variables()
 730             .unsolved_variables()
 731             .into_iter()
 732             .map(|t| self.tcx.mk_ty_var(t))
 733             .collect();
 734         vars.extend(
 735             (0..inner.int_unification_table().len())
 736                 .map(|i| ty::IntVid { index: i as u32 })
 737                 .filter(|&vid| inner.int_unification_table().probe_value(vid).is_none())
 738                 .map(|v| self.tcx.mk_int_var(v)),
 739         );
 740         vars.extend(
 741             (0..inner.float_unification_table().len())
 742                 .map(|i| ty::FloatVid { index: i as u32 })
 743                 .filter(|&vid| inner.float_unification_table().probe_value(vid).is_none())
 744                 .map(|v| self.tcx.mk_float_var(v)),
 745         );
 746         vars
 747     }
 748
 749     fn combine_fields<'a>(
 750         &'a self,
 751         trace: TypeTrace<'tcx>,
 752         param_env: ty::ParamEnv<'tcx>,
 753         define_opaque_types: bool,
 754     ) -> CombineFields<'a, 'tcx> {
 755         CombineFields {
 756             infcx: self,
 757             trace,
 758             cause: None,
 759             param_env,
 760             obligations: PredicateObligations::new(),
 761             define_opaque_types,
 762         }
 763     }
 764
 765     fn start_snapshot(&self) -> CombinedSnapshot<'tcx> {
 766         debug!("start_snapshot()");
 767
 768         let in_snapshot = self.in_snapshot.replace(true);
 769
 770         let mut inner = self.inner.borrow_mut();
 771
 772         CombinedSnapshot {
 773             undo_snapshot: inner.undo_log.start_snapshot(),
 774             region_constraints_snapshot: inner.unwrap_region_constraints().start_snapshot(),
 775             universe: self.universe(),
 776             was_in_snapshot: in_snapshot,
 777         }
 778     }
 779
 780     #[instrument(skip(self, snapshot), level = "debug")]
 781     fn rollback_to(&self, cause: &str, snapshot: CombinedSnapshot<'tcx>) {
 782         let CombinedSnapshot {
 783             undo_snapshot,
 784             region_constraints_snapshot,
 785             universe,
 786             was_in_snapshot,
 787         } = snapshot;
 788
 789         self.in_snapshot.set(was_in_snapshot);
 790         self.universe.set(universe);
 791
 792         let mut inner = self.inner.borrow_mut();
 793         inner.rollback_to(undo_snapshot);
 794         inner.unwrap_region_constraints().rollback_to(region_constraints_snapshot);
 795     }
 796
 797     #[instrument(skip(self, snapshot), level = "debug")]
 798     fn commit_from(&self, snapshot: CombinedSnapshot<'tcx>) {
 799         let CombinedSnapshot {
 800             undo_snapshot,
 801             region_constraints_snapshot: _,
 802             universe: _,
 803             was_in_snapshot,
 804         } = snapshot;
 805
 806         self.in_snapshot.set(was_in_snapshot);
 807
 808         self.inner.borrow_mut().commit(undo_snapshot);
 809     }
 810
 811     /// Execute `f` and commit the bindings if closure `f` returns `Ok(_)`.
 812     #[instrument(skip(self, f), level = "debug")]
 813     pub fn commit_if_ok<T, E, F>(&self, f: F) -> Result<T, E>
 814     where
 815         F: FnOnce(&CombinedSnapshot<'tcx>) -> Result<T, E>,
 816     {
 817         let snapshot = self.start_snapshot();
 818         let r = f(&snapshot);
 819         debug!("commit_if_ok() -- r.is_ok() = {}", r.is_ok());
 820         match r {
 821             Ok(_) => {
 822                 self.commit_from(snapshot);
 823             }
 824             Err(_) => {
 825                 self.rollback_to("commit_if_ok -- error", snapshot);
 826             }
 827         }
 828         r
 829     }
 830
 831     /// Execute `f` then unroll any bindings it creates.
 832     #[instrument(skip(self, f), level = "debug")]
 833     pub fn probe<R, F>(&self, f: F) -> R
 834     where
 835         F: FnOnce(&CombinedSnapshot<'tcx>) -> R,
 836     {
 837         let snapshot = self.start_snapshot();
 838         let r = f(&snapshot);
 839         self.rollback_to("probe", snapshot);
 840         r
 841     }
 842
 843     /// If `should_skip` is true, then execute `f` then unroll any bindings it creates.
 844     #[instrument(skip(self, f), level = "debug")]
 845     pub fn probe_maybe_skip_leak_check<R, F>(&self, should_skip: bool, f: F) -> R
 846     where
 847         F: FnOnce(&CombinedSnapshot<'tcx>) -> R,
 848     {
 849         let snapshot = self.start_snapshot();
 850         let was_skip_leak_check = self.skip_leak_check.get();
 851         if should_skip {
 852             self.skip_leak_check.set(true);
 853         }
 854         let r = f(&snapshot);
 855         self.rollback_to("probe", snapshot);
 856         self.skip_leak_check.set(was_skip_leak_check);
 857         r
 858     }
 859
 860     /// Scan the constraints produced since `snapshot` began and returns:
 861     ///
 862     /// - `None` -- if none of them involve "region outlives" constraints
 863     /// - `Some(true)` -- if there are `'a: 'b` constraints where `'a` or `'b` is a placeholder
 864     /// - `Some(false)` -- if there are `'a: 'b` constraints but none involve placeholders
 865     pub fn region_constraints_added_in_snapshot(
 866         &self,
 867         snapshot: &CombinedSnapshot<'tcx>,
 868     ) -> Option<bool> {
 869         self.inner
 870             .borrow_mut()
 871             .unwrap_region_constraints()
 872             .region_constraints_added_in_snapshot(&snapshot.undo_snapshot)
 873     }
 874
 875     pub fn opaque_types_added_in_snapshot(&self, snapshot: &CombinedSnapshot<'tcx>) -> bool {
 876         self.inner.borrow().undo_log.opaque_types_in_snapshot(&snapshot.undo_snapshot)
 877     }
 878
 879     pub fn add_given(&self, sub: ty::Region<'tcx>, sup: ty::RegionVid) {
 880         self.inner.borrow_mut().unwrap_region_constraints().add_given(sub, sup);
 881     }
 882
 883     pub fn can_sub<T>(&self, param_env: ty::ParamEnv<'tcx>, a: T, b: T) -> UnitResult<'tcx>
 884     where
 885         T: at::ToTrace<'tcx>,
 886     {
 887         let origin = &ObligationCause::dummy();
 888         self.probe(|_| {
 889             self.at(origin, param_env).sub(a, b).map(|InferOk { obligations: _, .. }| {
 890                 // Ignore obligations, since we are unrolling
 891                 // everything anyway.
 892             })
 893         })
 894     }
 895
 896     pub fn can_eq<T>(&self, param_env: ty::ParamEnv<'tcx>, a: T, b: T) -> UnitResult<'tcx>
 897     where
 898         T: at::ToTrace<'tcx>,
 899     {
 900         let origin = &ObligationCause::dummy();
 901         self.probe(|_| {
 902             self.at(origin, param_env).eq(a, b).map(|InferOk { obligations: _, .. }| {
 903                 // Ignore obligations, since we are unrolling
 904                 // everything anyway.
 905             })
 906         })
 907     }
 908
 909     #[instrument(skip(self), level = "debug")]
 910     pub fn sub_regions(
 911         &self,
 912         origin: SubregionOrigin<'tcx>,
 913         a: ty::Region<'tcx>,
 914         b: ty::Region<'tcx>,
 915     ) {
 916         self.inner.borrow_mut().unwrap_region_constraints().make_subregion(origin, a, b);
 917     }
 918
 919     /// Require that the region `r` be equal to one of the regions in
 920     /// the set `regions`.
 921     #[instrument(skip(self), level = "debug")]
 922     pub fn member_constraint(
 923         &self,
 924         key: ty::OpaqueTypeKey<'tcx>,
 925         definition_span: Span,
 926         hidden_ty: Ty<'tcx>,
 927         region: ty::Region<'tcx>,
 928         in_regions: &Lrc<Vec<ty::Region<'tcx>>>,
 929     ) {
 930         self.inner.borrow_mut().unwrap_region_constraints().member_constraint(
 931             key,
 932             definition_span,
 933             hidden_ty,
 934             region,
 935             in_regions,
 936         );
 937     }
 938
 939     /// Processes a `Coerce` predicate from the fulfillment context.
 940     /// This is NOT the preferred way to handle coercion, which is to
 941     /// invoke `FnCtxt::coerce` or a similar method (see `coercion.rs`).
 942     ///
 943     /// This method here is actually a fallback that winds up being
 944     /// invoked when `FnCtxt::coerce` encounters unresolved type variables
 945     /// and records a coercion predicate. Presently, this method is equivalent
 946     /// to `subtype_predicate` -- that is, "coercing" `a` to `b` winds up
 947     /// actually requiring `a <: b`. This is of course a valid coercion,
 948     /// but it's not as flexible as `FnCtxt::coerce` would be.
 949     ///
 950     /// (We may refactor this in the future, but there are a number of
 951     /// practical obstacles. Among other things, `FnCtxt::coerce` presently
 952     /// records adjustments that are required on the HIR in order to perform
 953     /// the coercion, and we don't currently have a way to manage that.)
 954     pub fn coerce_predicate(
 955         &self,
 956         cause: &ObligationCause<'tcx>,
 957         param_env: ty::ParamEnv<'tcx>,
 958         predicate: ty::PolyCoercePredicate<'tcx>,
 959     ) -> Result<InferResult<'tcx, ()>, (TyVid, TyVid)> {
 960         let subtype_predicate = predicate.map_bound(|p| ty::SubtypePredicate {
 961             a_is_expected: false, // when coercing from `a` to `b`, `b` is expected
 962             a: p.a,
 963             b: p.b,
 964         });
 965         self.subtype_predicate(cause, param_env, subtype_predicate)
 966     }
 967
 968     pub fn subtype_predicate(
 969         &self,
 970         cause: &ObligationCause<'tcx>,
 971         param_env: ty::ParamEnv<'tcx>,
 972         predicate: ty::PolySubtypePredicate<'tcx>,
 973     ) -> Result<InferResult<'tcx, ()>, (TyVid, TyVid)> {
 974         // Check for two unresolved inference variables, in which case we can
 975         // make no progress. This is partly a micro-optimization, but it's
 976         // also an opportunity to "sub-unify" the variables. This isn't
 977         // *necessary* to prevent cycles, because they would eventually be sub-unified
 978         // anyhow during generalization, but it helps with diagnostics (we can detect
 979         // earlier that they are sub-unified).
 980         //
 981         // Note that we can just skip the binders here because
 982         // type variables can't (at present, at
 983         // least) capture any of the things bound by this binder.
 984         //
 985         // Note that this sub here is not just for diagnostics - it has semantic
 986         // effects as well.
 987         let r_a = self.shallow_resolve(predicate.skip_binder().a);
 988         let r_b = self.shallow_resolve(predicate.skip_binder().b);
 989         match (r_a.kind(), r_b.kind()) {
 990             (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
 991                 self.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
 992                 return Err((a_vid, b_vid));
 993             }
 994             _ => {}
 995         }
 996
 997         Ok(self.commit_if_ok(|_snapshot| {
 998             let ty::SubtypePredicate { a_is_expected, a, b } =
 999                 self.replace_bound_vars_with_placeholders(predicate);
1000
1001             let ok = self.at(cause, param_env).sub_exp(a_is_expected, a, b)?;
1002
1003             Ok(ok.unit())
1004         }))
1005     }
1006
1007     pub fn region_outlives_predicate(
1008         &self,
1009         cause: &traits::ObligationCause<'tcx>,
1010         predicate: ty::PolyRegionOutlivesPredicate<'tcx>,
1011     ) {
1012         let ty::OutlivesPredicate(r_a, r_b) = self.replace_bound_vars_with_placeholders(predicate);
1013         let origin =
1014             SubregionOrigin::from_obligation_cause(cause, || RelateRegionParamBound(cause.span));
1015         self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
1016     }
1017
1018     /// Number of type variables created so far.
1019     pub fn num_ty_vars(&self) -> usize {
1020         self.inner.borrow_mut().type_variables().num_vars()
1021     }
1022
1023     pub fn next_ty_var_id(&self, origin: TypeVariableOrigin) -> TyVid {
1024         self.inner.borrow_mut().type_variables().new_var(self.universe(), origin)
1025     }
1026
1027     pub fn next_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
1028         self.tcx.mk_ty_var(self.next_ty_var_id(origin))
1029     }
1030
1031     pub fn next_ty_var_id_in_universe(
1032         &self,
1033         origin: TypeVariableOrigin,
1034         universe: ty::UniverseIndex,
1035     ) -> TyVid {
1036         self.inner.borrow_mut().type_variables().new_var(universe, origin)
1037     }
1038
1039     pub fn next_ty_var_in_universe(
1040         &self,
1041         origin: TypeVariableOrigin,
1042         universe: ty::UniverseIndex,
1043     ) -> Ty<'tcx> {
1044         let vid = self.next_ty_var_id_in_universe(origin, universe);
1045         self.tcx.mk_ty_var(vid)
1046     }
1047
1048     pub fn next_const_var(&self, ty: Ty<'tcx>, origin: ConstVariableOrigin) -> ty::Const<'tcx> {
1049         self.tcx.mk_const(self.next_const_var_id(origin), ty)
1050     }
1051
1052     pub fn next_const_var_in_universe(
1053         &self,
1054         ty: Ty<'tcx>,
1055         origin: ConstVariableOrigin,
1056         universe: ty::UniverseIndex,
1057     ) -> ty::Const<'tcx> {
1058         let vid = self
1059             .inner
1060             .borrow_mut()
1061             .const_unification_table()
1062             .new_key(ConstVarValue { origin, val: ConstVariableValue::Unknown { universe } });
1063         self.tcx.mk_const(vid, ty)
1064     }
1065
1066     pub fn next_const_var_id(&self, origin: ConstVariableOrigin) -> ConstVid<'tcx> {
1067         self.inner.borrow_mut().const_unification_table().new_key(ConstVarValue {
1068             origin,
1069             val: ConstVariableValue::Unknown { universe: self.universe() },
1070         })
1071     }
1072
1073     fn next_int_var_id(&self) -> IntVid {
1074         self.inner.borrow_mut().int_unification_table().new_key(None)
1075     }
1076
1077     pub fn next_int_var(&self) -> Ty<'tcx> {
1078         self.tcx.mk_int_var(self.next_int_var_id())
1079     }
1080
1081     fn next_float_var_id(&self) -> FloatVid {
1082         self.inner.borrow_mut().float_unification_table().new_key(None)
1083     }
1084
1085     pub fn next_float_var(&self) -> Ty<'tcx> {
1086         self.tcx.mk_float_var(self.next_float_var_id())
1087     }
1088
1089     /// Creates a fresh region variable with the next available index.
1090     /// The variable will be created in the maximum universe created
1091     /// thus far, allowing it to name any region created thus far.
1092     pub fn next_region_var(&self, origin: RegionVariableOrigin) -> ty::Region<'tcx> {
1093         self.next_region_var_in_universe(origin, self.universe())
1094     }
1095
1096     /// Creates a fresh region variable with the next available index
1097     /// in the given universe; typically, you can use
1098     /// `next_region_var` and just use the maximal universe.
1099     pub fn next_region_var_in_universe(
1100         &self,
1101         origin: RegionVariableOrigin,
1102         universe: ty::UniverseIndex,
1103     ) -> ty::Region<'tcx> {
1104         let region_var =
1105             self.inner.borrow_mut().unwrap_region_constraints().new_region_var(universe, origin);
1106         self.tcx.mk_region(ty::ReVar(region_var))
1107     }
1108
1109     /// Return the universe that the region `r` was created in.  For
1110     /// most regions (e.g., `'static`, named regions from the user,
1111     /// etc) this is the root universe U0. For inference variables or
1112     /// placeholders, however, it will return the universe which they
1113     /// are associated.
1114     pub fn universe_of_region(&self, r: ty::Region<'tcx>) -> ty::UniverseIndex {
1115         self.inner.borrow_mut().unwrap_region_constraints().universe(r)
1116     }
1117
1118     /// Number of region variables created so far.
1119     pub fn num_region_vars(&self) -> usize {
1120         self.inner.borrow_mut().unwrap_region_constraints().num_region_vars()
1121     }
1122
1123     /// Just a convenient wrapper of `next_region_var` for using during NLL.
1124     pub fn next_nll_region_var(&self, origin: NllRegionVariableOrigin) -> ty::Region<'tcx> {
1125         self.next_region_var(RegionVariableOrigin::Nll(origin))
1126     }
1127
1128     /// Just a convenient wrapper of `next_region_var` for using during NLL.
1129     pub fn next_nll_region_var_in_universe(
1130         &self,
1131         origin: NllRegionVariableOrigin,
1132         universe: ty::UniverseIndex,
1133     ) -> ty::Region<'tcx> {
1134         self.next_region_var_in_universe(RegionVariableOrigin::Nll(origin), universe)
1135     }
1136
1137     pub fn var_for_def(&self, span: Span, param: &ty::GenericParamDef) -> GenericArg<'tcx> {
1138         match param.kind {
1139             GenericParamDefKind::Lifetime => {
1140                 // Create a region inference variable for the given
1141                 // region parameter definition.
1142                 self.next_region_var(EarlyBoundRegion(span, param.name)).into()
1143             }
1144             GenericParamDefKind::Type { .. } => {
1145                 // Create a type inference variable for the given
1146                 // type parameter definition. The substitutions are
1147                 // for actual parameters that may be referred to by
1148                 // the default of this type parameter, if it exists.
1149                 // e.g., `struct Foo<A, B, C = (A, B)>(...);` when
1150                 // used in a path such as `Foo::<T, U>::new()` will
1151                 // use an inference variable for `C` with `[T, U]`
1152                 // as the substitutions for the default, `(T, U)`.
1153                 let ty_var_id = self.inner.borrow_mut().type_variables().new_var(
1154                     self.universe(),
1155                     TypeVariableOrigin {
1156                         kind: TypeVariableOriginKind::TypeParameterDefinition(
1157                             param.name,
1158                             Some(param.def_id),
1159                         ),
1160                         span,
1161                     },
1162                 );
1163
1164                 self.tcx.mk_ty_var(ty_var_id).into()
1165             }
1166             GenericParamDefKind::Const { .. } => {
1167                 let origin = ConstVariableOrigin {
1168                     kind: ConstVariableOriginKind::ConstParameterDefinition(
1169                         param.name,
1170                         param.def_id,
1171                     ),
1172                     span,
1173                 };
1174                 let const_var_id =
1175                     self.inner.borrow_mut().const_unification_table().new_key(ConstVarValue {
1176                         origin,
1177                         val: ConstVariableValue::Unknown { universe: self.universe() },
1178                     });
1179                 self.tcx.mk_const(const_var_id, self.tcx.type_of(param.def_id)).into()
1180             }
1181         }
1182     }
1183
1184     /// Given a set of generics defined on a type or impl, returns a substitution mapping each
1185     /// type/region parameter to a fresh inference variable.
1186     pub fn fresh_substs_for_item(&self, span: Span, def_id: DefId) -> SubstsRef<'tcx> {
1187         InternalSubsts::for_item(self.tcx, def_id, |param, _| self.var_for_def(span, param))
1188     }
1189
1190     /// Returns `true` if errors have been reported since this infcx was
1191     /// created. This is sometimes used as a heuristic to skip
1192     /// reporting errors that often occur as a result of earlier
1193     /// errors, but where it's hard to be 100% sure (e.g., unresolved
1194     /// inference variables, regionck errors).
1195     #[must_use = "this method does not have any side effects"]
1196     pub fn tainted_by_errors(&self) -> Option<ErrorGuaranteed> {
1197         debug!(
1198             "is_tainted_by_errors(err_count={}, err_count_on_creation={}, \
1199              tainted_by_errors={})",
1200             self.tcx.sess.err_count(),
1201             self.err_count_on_creation,
1202             self.tainted_by_errors.get().is_some()
1203         );
1204
1205         if let Some(e) = self.tainted_by_errors.get() {
1206             return Some(e);
1207         }
1208
1209         if self.tcx.sess.err_count() > self.err_count_on_creation {
1210             // errors reported since this infcx was made
1211             let e = self.tcx.sess.has_errors().unwrap();
1212             self.set_tainted_by_errors(e);
1213             return Some(e);
1214         }
1215
1216         None
1217     }
1218
1219     /// Set the "tainted by errors" flag to true. We call this when we
1220     /// observe an error from a prior pass.
1221     pub fn set_tainted_by_errors(&self, e: ErrorGuaranteed) {
1222         debug!("set_tainted_by_errors(ErrorGuaranteed)");
1223         self.tainted_by_errors.set(Some(e));
1224     }
1225
1226     pub fn skip_region_resolution(&self) {
1227         let (var_infos, _) = {
1228             let mut inner = self.inner.borrow_mut();
1229             let inner = &mut *inner;
1230             // Note: `inner.region_obligations` may not be empty, because we
1231             // didn't necessarily call `process_registered_region_obligations`.
1232             // This is okay, because that doesn't introduce new vars.
1233             inner
1234                 .region_constraint_storage
1235                 .take()
1236                 .expect("regions already resolved")
1237                 .with_log(&mut inner.undo_log)
1238                 .into_infos_and_data()
1239         };
1240
1241         let lexical_region_resolutions = LexicalRegionResolutions {
1242             values: rustc_index::vec::IndexVec::from_elem_n(
1243                 crate::infer::lexical_region_resolve::VarValue::Value(self.tcx.lifetimes.re_erased),
1244                 var_infos.len(),
1245             ),
1246         };
1247
1248         let old_value = self.lexical_region_resolutions.replace(Some(lexical_region_resolutions));
1249         assert!(old_value.is_none());
1250     }
1251
1252     /// Process the region constraints and return any errors that
1253     /// result. After this, no more unification operations should be
1254     /// done -- or the compiler will panic -- but it is legal to use
1255     /// `resolve_vars_if_possible` as well as `fully_resolve`.
1256     pub fn resolve_regions(
1257         &self,
1258         outlives_env: &OutlivesEnvironment<'tcx>,
1259     ) -> Vec<RegionResolutionError<'tcx>> {
1260         let (var_infos, data) = {
1261             let mut inner = self.inner.borrow_mut();
1262             let inner = &mut *inner;
1263             assert!(
1264                 self.tainted_by_errors().is_some() || inner.region_obligations.is_empty(),
1265                 "region_obligations not empty: {:#?}",
1266                 inner.region_obligations
1267             );
1268             inner
1269                 .region_constraint_storage
1270                 .take()
1271                 .expect("regions already resolved")
1272                 .with_log(&mut inner.undo_log)
1273                 .into_infos_and_data()
1274         };
1275
1276         let region_rels = &RegionRelations::new(self.tcx, outlives_env.free_region_map());
1277
1278         let (lexical_region_resolutions, errors) =
1279             lexical_region_resolve::resolve(outlives_env.param_env, region_rels, var_infos, data);
1280
1281         let old_value = self.lexical_region_resolutions.replace(Some(lexical_region_resolutions));
1282         assert!(old_value.is_none());
1283
1284         errors
1285     }
1286     /// Obtains (and clears) the current set of region
1287     /// constraints. The inference context is still usable: further
1288     /// unifications will simply add new constraints.
1289     ///
1290     /// This method is not meant to be used with normal lexical region
1291     /// resolution. Rather, it is used in the NLL mode as a kind of
1292     /// interim hack: basically we run normal type-check and generate
1293     /// region constraints as normal, but then we take them and
1294     /// translate them into the form that the NLL solver
1295     /// understands. See the NLL module for mode details.
1296     pub fn take_and_reset_region_constraints(&self) -> RegionConstraintData<'tcx> {
1297         assert!(
1298             self.inner.borrow().region_obligations.is_empty(),
1299             "region_obligations not empty: {:#?}",
1300             self.inner.borrow().region_obligations
1301         );
1302
1303         self.inner.borrow_mut().unwrap_region_constraints().take_and_reset_data()
1304     }
1305
1306     /// Gives temporary access to the region constraint data.
1307     pub fn with_region_constraints<R>(
1308         &self,
1309         op: impl FnOnce(&RegionConstraintData<'tcx>) -> R,
1310     ) -> R {
1311         let mut inner = self.inner.borrow_mut();
1312         op(inner.unwrap_region_constraints().data())
1313     }
1314
1315     pub fn region_var_origin(&self, vid: ty::RegionVid) -> RegionVariableOrigin {
1316         let mut inner = self.inner.borrow_mut();
1317         let inner = &mut *inner;
1318         inner
1319             .region_constraint_storage
1320             .as_mut()
1321             .expect("regions already resolved")
1322             .with_log(&mut inner.undo_log)
1323             .var_origin(vid)
1324     }
1325
1326     /// Takes ownership of the list of variable regions. This implies
1327     /// that all the region constraints have already been taken, and
1328     /// hence that `resolve_regions_and_report_errors` can never be
1329     /// called. This is used only during NLL processing to "hand off" ownership
1330     /// of the set of region variables into the NLL region context.
1331     pub fn take_region_var_origins(&self) -> VarInfos {
1332         let mut inner = self.inner.borrow_mut();
1333         let (var_infos, data) = inner
1334             .region_constraint_storage
1335             .take()
1336             .expect("regions already resolved")
1337             .with_log(&mut inner.undo_log)
1338             .into_infos_and_data();
1339         assert!(data.is_empty());
1340         var_infos
1341     }
1342
1343     pub fn ty_to_string(&self, t: Ty<'tcx>) -> String {
1344         self.resolve_vars_if_possible(t).to_string()
1345     }
1346
1347     /// If `TyVar(vid)` resolves to a type, return that type. Else, return the
1348     /// universe index of `TyVar(vid)`.
1349     pub fn probe_ty_var(&self, vid: TyVid) -> Result<Ty<'tcx>, ty::UniverseIndex> {
1350         use self::type_variable::TypeVariableValue;
1351
1352         match self.inner.borrow_mut().type_variables().probe(vid) {
1353             TypeVariableValue::Known { value } => Ok(value),
1354             TypeVariableValue::Unknown { universe } => Err(universe),
1355         }
1356     }
1357
1358     /// Resolve any type variables found in `value` -- but only one
1359     /// level.  So, if the variable `?X` is bound to some type
1360     /// `Foo<?Y>`, then this would return `Foo<?Y>` (but `?Y` may
1361     /// itself be bound to a type).
1362     ///
1363     /// Useful when you only need to inspect the outermost level of
1364     /// the type and don't care about nested types (or perhaps you
1365     /// will be resolving them as well, e.g. in a loop).
1366     pub fn shallow_resolve<T>(&self, value: T) -> T
1367     where
1368         T: TypeFoldable<'tcx>,
1369     {
1370         value.fold_with(&mut ShallowResolver { infcx: self })
1371     }
1372
1373     pub fn root_var(&self, var: ty::TyVid) -> ty::TyVid {
1374         self.inner.borrow_mut().type_variables().root_var(var)
1375     }
1376
1377     /// Where possible, replaces type/const variables in
1378     /// `value` with their final value. Note that region variables
1379     /// are unaffected. If a type/const variable has not been unified, it
1380     /// is left as is. This is an idempotent operation that does
1381     /// not affect inference state in any way and so you can do it
1382     /// at will.
1383     pub fn resolve_vars_if_possible<T>(&self, value: T) -> T
1384     where
1385         T: TypeFoldable<'tcx>,
1386     {
1387         if !value.needs_infer() {
1388             return value; // Avoid duplicated subst-folding.
1389         }
1390         let mut r = resolve::OpportunisticVarResolver::new(self);
1391         value.fold_with(&mut r)
1392     }
1393
1394     pub fn resolve_numeric_literals_with_default<T>(&self, value: T) -> T
1395     where
1396         T: TypeFoldable<'tcx>,
1397     {
1398         if !value.needs_infer() {
1399             return value; // Avoid duplicated subst-folding.
1400         }
1401         let mut r = InferenceLiteralEraser { tcx: self.tcx };
1402         value.fold_with(&mut r)
1403     }
1404
1405     /// Returns the first unresolved type or const variable contained in `T`.
1406     pub fn first_unresolved_const_or_ty_var<T>(
1407         &self,
1408         value: &T,
1409     ) -> Option<(ty::Term<'tcx>, Option<Span>)>
1410     where
1411         T: TypeVisitable<'tcx>,
1412     {
1413         value.visit_with(&mut resolve::UnresolvedTypeOrConstFinder::new(self)).break_value()
1414     }
1415
1416     pub fn probe_const_var(
1417         &self,
1418         vid: ty::ConstVid<'tcx>,
1419     ) -> Result<ty::Const<'tcx>, ty::UniverseIndex> {
1420         match self.inner.borrow_mut().const_unification_table().probe_value(vid).val {
1421             ConstVariableValue::Known { value } => Ok(value),
1422             ConstVariableValue::Unknown { universe } => Err(universe),
1423         }
1424     }
1425
1426     pub fn fully_resolve<T: TypeFoldable<'tcx>>(&self, value: T) -> FixupResult<'tcx, T> {
1427         /*!
1428          * Attempts to resolve all type/region/const variables in
1429          * `value`. Region inference must have been run already (e.g.,
1430          * by calling `resolve_regions_and_report_errors`). If some
1431          * variable was never unified, an `Err` results.
1432          *
1433          * This method is idempotent, but it not typically not invoked
1434          * except during the writeback phase.
1435          */
1436
1437         let value = resolve::fully_resolve(self, value);
1438         assert!(
1439             value.as_ref().map_or(true, |value| !value.needs_infer()),
1440             "`{value:?}` is not fully resolved"
1441         );
1442         value
1443     }
1444
1445     pub fn replace_bound_vars_with_fresh_vars<T>(
1446         &self,
1447         span: Span,
1448         lbrct: LateBoundRegionConversionTime,
1449         value: ty::Binder<'tcx, T>,
1450     ) -> T
1451     where
1452         T: TypeFoldable<'tcx> + Copy,
1453     {
1454         if let Some(inner) = value.no_bound_vars() {
1455             return inner;
1456         }
1457
1458         struct ToFreshVars<'a, 'tcx> {
1459             infcx: &'a InferCtxt<'tcx>,
1460             span: Span,
1461             lbrct: LateBoundRegionConversionTime,
1462             map: FxHashMap<ty::BoundVar, ty::GenericArg<'tcx>>,
1463         }
1464
1465         impl<'tcx> BoundVarReplacerDelegate<'tcx> for ToFreshVars<'_, 'tcx> {
1466             fn replace_region(&mut self, br: ty::BoundRegion) -> ty::Region<'tcx> {
1467                 self.map
1468                     .entry(br.var)
1469                     .or_insert_with(|| {
1470                         self.infcx
1471                             .next_region_var(LateBoundRegion(self.span, br.kind, self.lbrct))
1472                             .into()
1473                     })
1474                     .expect_region()
1475             }
1476             fn replace_ty(&mut self, bt: ty::BoundTy) -> Ty<'tcx> {
1477                 self.map
1478                     .entry(bt.var)
1479                     .or_insert_with(|| {
1480                         self.infcx
1481                             .next_ty_var(TypeVariableOrigin {
1482                                 kind: TypeVariableOriginKind::MiscVariable,
1483                                 span: self.span,
1484                             })
1485                             .into()
1486                     })
1487                     .expect_ty()
1488             }
1489             fn replace_const(&mut self, bv: ty::BoundVar, ty: Ty<'tcx>) -> ty::Const<'tcx> {
1490                 self.map
1491                     .entry(bv)
1492                     .or_insert_with(|| {
1493                         self.infcx
1494                             .next_const_var(
1495                                 ty,
1496                                 ConstVariableOrigin {
1497                                     kind: ConstVariableOriginKind::MiscVariable,
1498                                     span: self.span,
1499                                 },
1500                             )
1501                             .into()
1502                     })
1503                     .expect_const()
1504             }
1505         }
1506         let delegate = ToFreshVars { infcx: self, span, lbrct, map: Default::default() };
1507         self.tcx.replace_bound_vars_uncached(value, delegate)
1508     }
1509
1510     /// See the [`region_constraints::RegionConstraintCollector::verify_generic_bound`] method.
1511     pub fn verify_generic_bound(
1512         &self,
1513         origin: SubregionOrigin<'tcx>,
1514         kind: GenericKind<'tcx>,
1515         a: ty::Region<'tcx>,
1516         bound: VerifyBound<'tcx>,
1517     ) {
1518         debug!("verify_generic_bound({:?}, {:?} <: {:?})", kind, a, bound);
1519
1520         self.inner
1521             .borrow_mut()
1522             .unwrap_region_constraints()
1523             .verify_generic_bound(origin, kind, a, bound);
1524     }
1525
1526     /// Obtains the latest type of the given closure; this may be a
1527     /// closure in the current function, in which case its
1528     /// `ClosureKind` may not yet be known.
1529     pub fn closure_kind(&self, closure_substs: SubstsRef<'tcx>) -> Option<ty::ClosureKind> {
1530         let closure_kind_ty = closure_substs.as_closure().kind_ty();
1531         let closure_kind_ty = self.shallow_resolve(closure_kind_ty);
1532         closure_kind_ty.to_opt_closure_kind()
1533     }
1534
1535     /// Clears the selection, evaluation, and projection caches. This is useful when
1536     /// repeatedly attempting to select an `Obligation` while changing only
1537     /// its `ParamEnv`, since `FulfillmentContext` doesn't use probing.
1538     pub fn clear_caches(&self) {
1539         self.selection_cache.clear();
1540         self.evaluation_cache.clear();
1541         self.inner.borrow_mut().projection_cache().clear();
1542     }
1543
1544     pub fn universe(&self) -> ty::UniverseIndex {
1545         self.universe.get()
1546     }
1547
1548     /// Creates and return a fresh universe that extends all previous
1549     /// universes. Updates `self.universe` to that new universe.
1550     pub fn create_next_universe(&self) -> ty::UniverseIndex {
1551         let u = self.universe.get().next_universe();
1552         self.universe.set(u);
1553         u
1554     }
1555
1556     pub fn try_const_eval_resolve(
1557         &self,
1558         param_env: ty::ParamEnv<'tcx>,
1559         unevaluated: ty::UnevaluatedConst<'tcx>,
1560         ty: Ty<'tcx>,
1561         span: Option<Span>,
1562     ) -> Result<ty::Const<'tcx>, ErrorHandled> {
1563         match self.const_eval_resolve(param_env, unevaluated, span) {
1564             Ok(Some(val)) => Ok(self.tcx.mk_const(val, ty)),
1565             Ok(None) => {
1566                 let tcx = self.tcx;
1567                 let def_id = unevaluated.def.did;
1568                 span_bug!(
1569                     tcx.def_span(def_id),
1570                     "unable to construct a constant value for the unevaluated constant {:?}",
1571                     unevaluated
1572                 );
1573             }
1574             Err(err) => Err(err),
1575         }
1576     }
1577
1578     /// Resolves and evaluates a constant.
1579     ///
1580     /// The constant can be located on a trait like `<A as B>::C`, in which case the given
1581     /// substitutions and environment are used to resolve the constant. Alternatively if the
1582     /// constant has generic parameters in scope the substitutions are used to evaluate the value of
1583     /// the constant. For example in `fn foo<T>() { let _ = [0; bar::<T>()]; }` the repeat count
1584     /// constant `bar::<T>()` requires a substitution for `T`, if the substitution for `T` is still
1585     /// too generic for the constant to be evaluated then `Err(ErrorHandled::TooGeneric)` is
1586     /// returned.
1587     ///
1588     /// This handles inferences variables within both `param_env` and `substs` by
1589     /// performing the operation on their respective canonical forms.
1590     #[instrument(skip(self), level = "debug")]
1591     pub fn const_eval_resolve(
1592         &self,
1593         mut param_env: ty::ParamEnv<'tcx>,
1594         unevaluated: ty::UnevaluatedConst<'tcx>,
1595         span: Option<Span>,
1596     ) -> EvalToValTreeResult<'tcx> {
1597         let mut substs = self.resolve_vars_if_possible(unevaluated.substs);
1598         debug!(?substs);
1599
1600         // Postpone the evaluation of constants whose substs depend on inference
1601         // variables
1602         let tcx = self.tcx;
1603         if substs.has_non_region_infer() {
1604             if let Some(ct) = tcx.bound_abstract_const(unevaluated.def)? {
1605                 let ct = tcx.expand_abstract_consts(ct.subst(tcx, substs));
1606                 if let Err(e) = ct.error_reported() {
1607                     return Err(ErrorHandled::Reported(e));
1608                 } else if ct.has_non_region_infer() || ct.has_non_region_param() {
1609                     return Err(ErrorHandled::TooGeneric);
1610                 } else {
1611                     substs = replace_param_and_infer_substs_with_placeholder(tcx, substs);
1612                 }
1613             } else {
1614                 substs = InternalSubsts::identity_for_item(tcx, unevaluated.def.did);
1615                 param_env = tcx.param_env(unevaluated.def.did);
1616             }
1617         }
1618
1619         let param_env_erased = tcx.erase_regions(param_env);
1620         let substs_erased = tcx.erase_regions(substs);
1621         debug!(?param_env_erased);
1622         debug!(?substs_erased);
1623
1624         let unevaluated = ty::UnevaluatedConst { def: unevaluated.def, substs: substs_erased };
1625
1626         // The return value is the evaluated value which doesn't contain any reference to inference
1627         // variables, thus we don't need to substitute back the original values.
1628         tcx.const_eval_resolve_for_typeck(param_env_erased, unevaluated, span)
1629     }
1630
1631     /// `ty_or_const_infer_var_changed` is equivalent to one of these two:
1632     ///   * `shallow_resolve(ty) != ty` (where `ty.kind = ty::Infer(_)`)
1633     ///   * `shallow_resolve(ct) != ct` (where `ct.kind = ty::ConstKind::Infer(_)`)
1634     ///
1635     /// However, `ty_or_const_infer_var_changed` is more efficient. It's always
1636     /// inlined, despite being large, because it has only two call sites that
1637     /// are extremely hot (both in `traits::fulfill`'s checking of `stalled_on`
1638     /// inference variables), and it handles both `Ty` and `ty::Const` without
1639     /// having to resort to storing full `GenericArg`s in `stalled_on`.
1640     #[inline(always)]
1641     pub fn ty_or_const_infer_var_changed(&self, infer_var: TyOrConstInferVar<'tcx>) -> bool {
1642         match infer_var {
1643             TyOrConstInferVar::Ty(v) => {
1644                 use self::type_variable::TypeVariableValue;
1645
1646                 // If `inlined_probe` returns a `Known` value, it never equals
1647                 // `ty::Infer(ty::TyVar(v))`.
1648                 match self.inner.borrow_mut().type_variables().inlined_probe(v) {
1649                     TypeVariableValue::Unknown { .. } => false,
1650                     TypeVariableValue::Known { .. } => true,
1651                 }
1652             }
1653
1654             TyOrConstInferVar::TyInt(v) => {
1655                 // If `inlined_probe_value` returns a value it's always a
1656                 // `ty::Int(_)` or `ty::UInt(_)`, which never matches a
1657                 // `ty::Infer(_)`.
1658                 self.inner.borrow_mut().int_unification_table().inlined_probe_value(v).is_some()
1659             }
1660
1661             TyOrConstInferVar::TyFloat(v) => {
1662                 // If `probe_value` returns a value it's always a
1663                 // `ty::Float(_)`, which never matches a `ty::Infer(_)`.
1664                 //
1665                 // Not `inlined_probe_value(v)` because this call site is colder.
1666                 self.inner.borrow_mut().float_unification_table().probe_value(v).is_some()
1667             }
1668
1669             TyOrConstInferVar::Const(v) => {
1670                 // If `probe_value` returns a `Known` value, it never equals
1671                 // `ty::ConstKind::Infer(ty::InferConst::Var(v))`.
1672                 //
1673                 // Not `inlined_probe_value(v)` because this call site is colder.
1674                 match self.inner.borrow_mut().const_unification_table().probe_value(v).val {
1675                     ConstVariableValue::Unknown { .. } => false,
1676                     ConstVariableValue::Known { .. } => true,
1677                 }
1678             }
1679         }
1680     }
1681 }
1682
1683 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
1684     /// Process the region constraints and report any errors that
1685     /// result. After this, no more unification operations should be
1686     /// done -- or the compiler will panic -- but it is legal to use
1687     /// `resolve_vars_if_possible` as well as `fully_resolve`.
1688     ///
1689     /// Make sure to call [`InferCtxt::process_registered_region_obligations`]
1690     /// first, or preferably use [`InferCtxt::check_region_obligations_and_report_errors`]
1691     /// to do both of these operations together.
1692     pub fn resolve_regions_and_report_errors(
1693         &self,
1694         generic_param_scope: LocalDefId,
1695         outlives_env: &OutlivesEnvironment<'tcx>,
1696     ) -> Option<ErrorGuaranteed> {
1697         let errors = self.resolve_regions(outlives_env);
1698
1699         if let None = self.tainted_by_errors() {
1700             // As a heuristic, just skip reporting region errors
1701             // altogether if other errors have been reported while
1702             // this infcx was in use.  This is totally hokey but
1703             // otherwise we have a hard time separating legit region
1704             // errors from silly ones.
1705             self.report_region_errors(generic_param_scope, &errors);
1706         }
1707
1708         (!errors.is_empty()).then(|| {
1709             self.tcx.sess.delay_span_bug(rustc_span::DUMMY_SP, "error should have been emitted")
1710         })
1711     }
1712
1713     // [Note-Type-error-reporting]
1714     // An invariant is that anytime the expected or actual type is Error (the special
1715     // error type, meaning that an error occurred when typechecking this expression),
1716     // this is a derived error. The error cascaded from another error (that was already
1717     // reported), so it's not useful to display it to the user.
1718     // The following methods implement this logic.
1719     // They check if either the actual or expected type is Error, and don't print the error
1720     // in this case. The typechecker should only ever report type errors involving mismatched
1721     // types using one of these methods, and should not call span_err directly for such
1722     // errors.
1723
1724     pub fn type_error_struct_with_diag<M>(
1725         &self,
1726         sp: Span,
1727         mk_diag: M,
1728         actual_ty: Ty<'tcx>,
1729     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>
1730     where
1731         M: FnOnce(String) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>,
1732     {
1733         let actual_ty = self.resolve_vars_if_possible(actual_ty);
1734         debug!("type_error_struct_with_diag({:?}, {:?})", sp, actual_ty);
1735
1736         let mut err = mk_diag(self.ty_to_string(actual_ty));
1737
1738         // Don't report an error if actual type is `Error`.
1739         if actual_ty.references_error() {
1740             err.downgrade_to_delayed_bug();
1741         }
1742
1743         err
1744     }
1745
1746     pub fn report_mismatched_types(
1747         &self,
1748         cause: &ObligationCause<'tcx>,
1749         expected: Ty<'tcx>,
1750         actual: Ty<'tcx>,
1751         err: TypeError<'tcx>,
1752     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1753         self.report_and_explain_type_error(TypeTrace::types(cause, true, expected, actual), err)
1754     }
1755
1756     pub fn report_mismatched_consts(
1757         &self,
1758         cause: &ObligationCause<'tcx>,
1759         expected: ty::Const<'tcx>,
1760         actual: ty::Const<'tcx>,
1761         err: TypeError<'tcx>,
1762     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1763         self.report_and_explain_type_error(TypeTrace::consts(cause, true, expected, actual), err)
1764     }
1765 }
1766
1767 /// Helper for `ty_or_const_infer_var_changed` (see comment on that), currently
1768 /// used only for `traits::fulfill`'s list of `stalled_on` inference variables.
1769 #[derive(Copy, Clone, Debug)]
1770 pub enum TyOrConstInferVar<'tcx> {
1771     /// Equivalent to `ty::Infer(ty::TyVar(_))`.
1772     Ty(TyVid),
1773     /// Equivalent to `ty::Infer(ty::IntVar(_))`.
1774     TyInt(IntVid),
1775     /// Equivalent to `ty::Infer(ty::FloatVar(_))`.
1776     TyFloat(FloatVid),
1777
1778     /// Equivalent to `ty::ConstKind::Infer(ty::InferConst::Var(_))`.
1779     Const(ConstVid<'tcx>),
1780 }
1781
1782 impl<'tcx> TyOrConstInferVar<'tcx> {
1783     /// Tries to extract an inference variable from a type or a constant, returns `None`
1784     /// for types other than `ty::Infer(_)` (or `InferTy::Fresh*`) and
1785     /// for constants other than `ty::ConstKind::Infer(_)` (or `InferConst::Fresh`).
1786     pub fn maybe_from_generic_arg(arg: GenericArg<'tcx>) -> Option<Self> {
1787         match arg.unpack() {
1788             GenericArgKind::Type(ty) => Self::maybe_from_ty(ty),
1789             GenericArgKind::Const(ct) => Self::maybe_from_const(ct),
1790             GenericArgKind::Lifetime(_) => None,
1791         }
1792     }
1793
1794     /// Tries to extract an inference variable from a type, returns `None`
1795     /// for types other than `ty::Infer(_)` (or `InferTy::Fresh*`).
1796     fn maybe_from_ty(ty: Ty<'tcx>) -> Option<Self> {
1797         match *ty.kind() {
1798             ty::Infer(ty::TyVar(v)) => Some(TyOrConstInferVar::Ty(v)),
1799             ty::Infer(ty::IntVar(v)) => Some(TyOrConstInferVar::TyInt(v)),
1800             ty::Infer(ty::FloatVar(v)) => Some(TyOrConstInferVar::TyFloat(v)),
1801             _ => None,
1802         }
1803     }
1804
1805     /// Tries to extract an inference variable from a constant, returns `None`
1806     /// for constants other than `ty::ConstKind::Infer(_)` (or `InferConst::Fresh`).
1807     fn maybe_from_const(ct: ty::Const<'tcx>) -> Option<Self> {
1808         match ct.kind() {
1809             ty::ConstKind::Infer(InferConst::Var(v)) => Some(TyOrConstInferVar::Const(v)),
1810             _ => None,
1811         }
1812     }
1813 }
1814
1815 /// Replace `{integer}` with `i32` and `{float}` with `f64`.
1816 /// Used only for diagnostics.
1817 struct InferenceLiteralEraser<'tcx> {
1818     tcx: TyCtxt<'tcx>,
1819 }
1820
1821 impl<'tcx> TypeFolder<'tcx> for InferenceLiteralEraser<'tcx> {
1822     fn tcx(&self) -> TyCtxt<'tcx> {
1823         self.tcx
1824     }
1825
1826     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1827         match ty.kind() {
1828             ty::Infer(ty::IntVar(_) | ty::FreshIntTy(_)) => self.tcx.types.i32,
1829             ty::Infer(ty::FloatVar(_) | ty::FreshFloatTy(_)) => self.tcx.types.f64,
1830             _ => ty.super_fold_with(self),
1831         }
1832     }
1833 }
1834
1835 struct ShallowResolver<'a, 'tcx> {
1836     infcx: &'a InferCtxt<'tcx>,
1837 }
1838
1839 impl<'a, 'tcx> TypeFolder<'tcx> for ShallowResolver<'a, 'tcx> {
1840     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1841         self.infcx.tcx
1842     }
1843
1844     /// If `ty` is a type variable of some kind, resolve it one level
1845     /// (but do not resolve types found in the result). If `typ` is
1846     /// not a type variable, just return it unmodified.
1847     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1848         match *ty.kind() {
1849             ty::Infer(ty::TyVar(v)) => {
1850                 // Not entirely obvious: if `typ` is a type variable,
1851                 // it can be resolved to an int/float variable, which
1852                 // can then be recursively resolved, hence the
1853                 // recursion. Note though that we prevent type
1854                 // variables from unifying to other type variables
1855                 // directly (though they may be embedded
1856                 // structurally), and we prevent cycles in any case,
1857                 // so this recursion should always be of very limited
1858                 // depth.
1859                 //
1860                 // Note: if these two lines are combined into one we get
1861                 // dynamic borrow errors on `self.inner`.
1862                 let known = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
1863                 known.map_or(ty, |t| self.fold_ty(t))
1864             }
1865
1866             ty::Infer(ty::IntVar(v)) => self
1867                 .infcx
1868                 .inner
1869                 .borrow_mut()
1870                 .int_unification_table()
1871                 .probe_value(v)
1872                 .map_or(ty, |v| v.to_type(self.infcx.tcx)),
1873
1874             ty::Infer(ty::FloatVar(v)) => self
1875                 .infcx
1876                 .inner
1877                 .borrow_mut()
1878                 .float_unification_table()
1879                 .probe_value(v)
1880                 .map_or(ty, |v| v.to_type(self.infcx.tcx)),
1881
1882             _ => ty,
1883         }
1884     }
1885
1886     fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
1887         if let ty::ConstKind::Infer(InferConst::Var(vid)) = ct.kind() {
1888             self.infcx
1889                 .inner
1890                 .borrow_mut()
1891                 .const_unification_table()
1892                 .probe_value(vid)
1893                 .val
1894                 .known()
1895                 .unwrap_or(ct)
1896         } else {
1897             ct
1898         }
1899     }
1900 }
1901
1902 impl<'tcx> TypeTrace<'tcx> {
1903     pub fn span(&self) -> Span {
1904         self.cause.span
1905     }
1906
1907     pub fn types(
1908         cause: &ObligationCause<'tcx>,
1909         a_is_expected: bool,
1910         a: Ty<'tcx>,
1911         b: Ty<'tcx>,
1912     ) -> TypeTrace<'tcx> {
1913         TypeTrace {
1914             cause: cause.clone(),
1915             values: Terms(ExpectedFound::new(a_is_expected, a.into(), b.into())),
1916         }
1917     }
1918
1919     pub fn poly_trait_refs(
1920         cause: &ObligationCause<'tcx>,
1921         a_is_expected: bool,
1922         a: ty::PolyTraitRef<'tcx>,
1923         b: ty::PolyTraitRef<'tcx>,
1924     ) -> TypeTrace<'tcx> {
1925         TypeTrace {
1926             cause: cause.clone(),
1927             values: PolyTraitRefs(ExpectedFound::new(a_is_expected, a.into(), b.into())),
1928         }
1929     }
1930
1931     pub fn consts(
1932         cause: &ObligationCause<'tcx>,
1933         a_is_expected: bool,
1934         a: ty::Const<'tcx>,
1935         b: ty::Const<'tcx>,
1936     ) -> TypeTrace<'tcx> {
1937         TypeTrace {
1938             cause: cause.clone(),
1939             values: Terms(ExpectedFound::new(a_is_expected, a.into(), b.into())),
1940         }
1941     }
1942 }
1943
1944 impl<'tcx> SubregionOrigin<'tcx> {
1945     pub fn span(&self) -> Span {
1946         match *self {
1947             Subtype(ref a) => a.span(),
1948             RelateObjectBound(a) => a,
1949             RelateParamBound(a, ..) => a,
1950             RelateRegionParamBound(a) => a,
1951             Reborrow(a) => a,
1952             ReborrowUpvar(a, _) => a,
1953             DataBorrowed(_, a) => a,
1954             ReferenceOutlivesReferent(_, a) => a,
1955             CompareImplItemObligation { span, .. } => span,
1956             AscribeUserTypeProvePredicate(span) => span,
1957             CheckAssociatedTypeBounds { ref parent, .. } => parent.span(),
1958         }
1959     }
1960
1961     pub fn from_obligation_cause<F>(cause: &traits::ObligationCause<'tcx>, default: F) -> Self
1962     where
1963         F: FnOnce() -> Self,
1964     {
1965         match *cause.code() {
1966             traits::ObligationCauseCode::ReferenceOutlivesReferent(ref_type) => {
1967                 SubregionOrigin::ReferenceOutlivesReferent(ref_type, cause.span)
1968             }
1969
1970             traits::ObligationCauseCode::CompareImplItemObligation {
1971                 impl_item_def_id,
1972                 trait_item_def_id,
1973                 kind: _,
1974             } => SubregionOrigin::CompareImplItemObligation {
1975                 span: cause.span,
1976                 impl_item_def_id,
1977                 trait_item_def_id,
1978             },
1979
1980             traits::ObligationCauseCode::CheckAssociatedTypeBounds {
1981                 impl_item_def_id,
1982                 trait_item_def_id,
1983             } => SubregionOrigin::CheckAssociatedTypeBounds {
1984                 impl_item_def_id,
1985                 trait_item_def_id,
1986                 parent: Box::new(default()),
1987             },
1988
1989             traits::ObligationCauseCode::AscribeUserTypeProvePredicate(span) => {
1990                 SubregionOrigin::AscribeUserTypeProvePredicate(span)
1991             }
1992
1993             _ => default(),
1994         }
1995     }
1996 }
1997
1998 impl RegionVariableOrigin {
1999     pub fn span(&self) -> Span {
2000         match *self {
2001             MiscVariable(a)
2002             | PatternRegion(a)
2003             | AddrOfRegion(a)
2004             | Autoref(a)
2005             | Coercion(a)
2006             | EarlyBoundRegion(a, ..)
2007             | LateBoundRegion(a, ..)
2008             | UpvarRegion(_, a) => a,
2009             Nll(..) => bug!("NLL variable used with `span`"),
2010         }
2011     }
2012 }
2013
2014 /// Replaces substs that reference param or infer variables with suitable
2015 /// placeholders. This function is meant to remove these param and infer
2016 /// substs when they're not actually needed to evaluate a constant.
2017 fn replace_param_and_infer_substs_with_placeholder<'tcx>(
2018     tcx: TyCtxt<'tcx>,
2019     substs: SubstsRef<'tcx>,
2020 ) -> SubstsRef<'tcx> {
2021     tcx.mk_substs(substs.iter().enumerate().map(|(idx, arg)| {
2022         match arg.unpack() {
2023             GenericArgKind::Type(_) if arg.has_non_region_param() || arg.has_non_region_infer() => {
2024                 tcx.mk_ty(ty::Placeholder(ty::PlaceholderType {
2025                     universe: ty::UniverseIndex::ROOT,
2026                     name: ty::BoundVar::from_usize(idx),
2027                 }))
2028                 .into()
2029             }
2030             GenericArgKind::Const(ct) if ct.has_non_region_infer() || ct.has_non_region_param() => {
2031                 let ty = ct.ty();
2032                 // If the type references param or infer, replace that too...
2033                 if ty.has_non_region_param() || ty.has_non_region_infer() {
2034                     bug!("const `{ct}`'s type should not reference params or types");
2035                 }
2036                 tcx.mk_const(
2037                     ty::PlaceholderConst {
2038                         universe: ty::UniverseIndex::ROOT,
2039                         name: ty::BoundVar::from_usize(idx),
2040                     },
2041                     ty,
2042                 )
2043                 .into()
2044             }
2045             _ => arg,
2046         }
2047     }))
2048 }