compiler/rustc_middle/src/traits/mod.rs

   1 //! Trait Resolution. 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
   4
   5 mod chalk;
   6 pub mod query;
   7 pub mod select;
   8 pub mod specialization_graph;
   9 mod structural_impls;
  10 pub mod util;
  11
  12 use crate::infer::canonical::Canonical;
  13 use crate::mir::ConstraintCategory;
  14 use crate::ty::abstract_const::NotConstEvaluatable;
  15 use crate::ty::subst::SubstsRef;
  16 use crate::ty::{self, AdtKind, Ty, TyCtxt};
  17
  18 use rustc_data_structures::sync::Lrc;
  19 use rustc_errors::{Applicability, Diagnostic};
  20 use rustc_hir as hir;
  21 use rustc_hir::def_id::{DefId, LocalDefId};
  22 use rustc_span::symbol::Symbol;
  23 use rustc_span::{Span, DUMMY_SP};
  24 use smallvec::SmallVec;
  25
  26 use std::borrow::Cow;
  27 use std::hash::{Hash, Hasher};
  28
  29 pub use self::select::{EvaluationCache, EvaluationResult, OverflowError, SelectionCache};
  30
  31 pub type CanonicalChalkEnvironmentAndGoal<'tcx> = Canonical<'tcx, ChalkEnvironmentAndGoal<'tcx>>;
  32
  33 pub use self::ObligationCauseCode::*;
  34
  35 pub use self::chalk::{ChalkEnvironmentAndGoal, RustInterner as ChalkRustInterner};
  36
  37 /// Depending on the stage of compilation, we want projection to be
  38 /// more or less conservative.
  39 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
  40 pub enum Reveal {
  41     /// At type-checking time, we refuse to project any associated
  42     /// type that is marked `default`. Non-`default` ("final") types
  43     /// are always projected. This is necessary in general for
  44     /// soundness of specialization. However, we *could* allow
  45     /// projections in fully-monomorphic cases. We choose not to,
  46     /// because we prefer for `default type` to force the type
  47     /// definition to be treated abstractly by any consumers of the
  48     /// impl. Concretely, that means that the following example will
  49     /// fail to compile:
  50     ///
  51     /// ```compile_fail,E0308
  52     /// #![feature(specialization)]
  53     /// trait Assoc {
  54     ///     type Output;
  55     /// }
  56     ///
  57     /// impl<T> Assoc for T {
  58     ///     default type Output = bool;
  59     /// }
  60     ///
  61     /// fn main() {
  62     ///     let x: <() as Assoc>::Output = true;
  63     /// }
  64     /// ```
  65     ///
  66     /// We also do not reveal the hidden type of opaque types during
  67     /// type-checking.
  68     UserFacing,
  69
  70     /// At codegen time, all monomorphic projections will succeed.
  71     /// Also, `impl Trait` is normalized to the concrete type,
  72     /// which has to be already collected by type-checking.
  73     ///
  74     /// NOTE: as `impl Trait`'s concrete type should *never*
  75     /// be observable directly by the user, `Reveal::All`
  76     /// should not be used by checks which may expose
  77     /// type equality or type contents to the user.
  78     /// There are some exceptions, e.g., around auto traits and
  79     /// transmute-checking, which expose some details, but
  80     /// not the whole concrete type of the `impl Trait`.
  81     All,
  82 }
  83
  84 /// The reason why we incurred this obligation; used for error reporting.
  85 ///
  86 /// Non-misc `ObligationCauseCode`s are stored on the heap. This gives the
  87 /// best trade-off between keeping the type small (which makes copies cheaper)
  88 /// while not doing too many heap allocations.
  89 ///
  90 /// We do not want to intern this as there are a lot of obligation causes which
  91 /// only live for a short period of time.
  92 #[derive(Clone, Debug, PartialEq, Eq, Lift)]
  93 pub struct ObligationCause<'tcx> {
  94     pub span: Span,
  95
  96     /// The ID of the fn body that triggered this obligation. This is
  97     /// used for region obligations to determine the precise
  98     /// environment in which the region obligation should be evaluated
  99     /// (in particular, closures can add new assumptions). See the
 100     /// field `region_obligations` of the `FulfillmentContext` for more
 101     /// information.
 102     pub body_id: hir::HirId,
 103
 104     code: InternedObligationCauseCode<'tcx>,
 105 }
 106
 107 // This custom hash function speeds up hashing for `Obligation` deduplication
 108 // greatly by skipping the `code` field, which can be large and complex. That
 109 // shouldn't affect hash quality much since there are several other fields in
 110 // `Obligation` which should be unique enough, especially the predicate itself
 111 // which is hashed as an interned pointer. See #90996.
 112 impl Hash for ObligationCause<'_> {
 113     fn hash<H: Hasher>(&self, state: &mut H) {
 114         self.body_id.hash(state);
 115         self.span.hash(state);
 116     }
 117 }
 118
 119 impl<'tcx> ObligationCause<'tcx> {
 120     #[inline]
 121     pub fn new(
 122         span: Span,
 123         body_id: hir::HirId,
 124         code: ObligationCauseCode<'tcx>,
 125     ) -> ObligationCause<'tcx> {
 126         ObligationCause { span, body_id, code: code.into() }
 127     }
 128
 129     pub fn misc(span: Span, body_id: hir::HirId) -> ObligationCause<'tcx> {
 130         ObligationCause::new(span, body_id, MiscObligation)
 131     }
 132
 133     #[inline(always)]
 134     pub fn dummy() -> ObligationCause<'tcx> {
 135         ObligationCause::dummy_with_span(DUMMY_SP)
 136     }
 137
 138     #[inline(always)]
 139     pub fn dummy_with_span(span: Span) -> ObligationCause<'tcx> {
 140         ObligationCause { span, body_id: hir::CRATE_HIR_ID, code: Default::default() }
 141     }
 142
 143     pub fn span(&self) -> Span {
 144         match *self.code() {
 145             ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
 146                 arm_span,
 147                 ..
 148             }) => arm_span,
 149             _ => self.span,
 150         }
 151     }
 152
 153     #[inline]
 154     pub fn code(&self) -> &ObligationCauseCode<'tcx> {
 155         &self.code
 156     }
 157
 158     pub fn map_code(
 159         &mut self,
 160         f: impl FnOnce(InternedObligationCauseCode<'tcx>) -> ObligationCauseCode<'tcx>,
 161     ) {
 162         self.code = f(std::mem::take(&mut self.code)).into();
 163     }
 164
 165     pub fn derived_cause(
 166         mut self,
 167         parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
 168         variant: impl FnOnce(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
 169     ) -> ObligationCause<'tcx> {
 170         /*!
 171          * Creates a cause for obligations that are derived from
 172          * `obligation` by a recursive search (e.g., for a builtin
 173          * bound, or eventually a `auto trait Foo`). If `obligation`
 174          * is itself a derived obligation, this is just a clone, but
 175          * otherwise we create a "derived obligation" cause so as to
 176          * keep track of the original root obligation for error
 177          * reporting.
 178          */
 179
 180         // NOTE(flaper87): As of now, it keeps track of the whole error
 181         // chain. Ideally, we should have a way to configure this either
 182         // by using -Z verbose or just a CLI argument.
 183         self.code =
 184             variant(DerivedObligationCause { parent_trait_pred, parent_code: self.code }).into();
 185         self
 186     }
 187
 188     pub fn to_constraint_category(&self) -> ConstraintCategory<'tcx> {
 189         match self.code() {
 190             MatchImpl(cause, _) => cause.to_constraint_category(),
 191             AscribeUserTypeProvePredicate(predicate_span) => {
 192                 ConstraintCategory::Predicate(*predicate_span)
 193             }
 194             _ => ConstraintCategory::BoringNoLocation,
 195         }
 196     }
 197 }
 198
 199 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 200 pub struct UnifyReceiverContext<'tcx> {
 201     pub assoc_item: ty::AssocItem,
 202     pub param_env: ty::ParamEnv<'tcx>,
 203     pub substs: SubstsRef<'tcx>,
 204 }
 205
 206 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift, Default)]
 207 pub struct InternedObligationCauseCode<'tcx> {
 208     /// `None` for `ObligationCauseCode::MiscObligation` (a common case, occurs ~60% of
 209     /// the time). `Some` otherwise.
 210     code: Option<Lrc<ObligationCauseCode<'tcx>>>,
 211 }
 212
 213 impl<'tcx> ObligationCauseCode<'tcx> {
 214     #[inline(always)]
 215     fn into(self) -> InternedObligationCauseCode<'tcx> {
 216         InternedObligationCauseCode {
 217             code: if let ObligationCauseCode::MiscObligation = self {
 218                 None
 219             } else {
 220                 Some(Lrc::new(self))
 221             },
 222         }
 223     }
 224 }
 225
 226 impl<'tcx> std::ops::Deref for InternedObligationCauseCode<'tcx> {
 227     type Target = ObligationCauseCode<'tcx>;
 228
 229     fn deref(&self) -> &Self::Target {
 230         self.code.as_deref().unwrap_or(&ObligationCauseCode::MiscObligation)
 231     }
 232 }
 233
 234 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 235 pub enum ObligationCauseCode<'tcx> {
 236     /// Not well classified or should be obvious from the span.
 237     MiscObligation,
 238
 239     /// A slice or array is WF only if `T: Sized`.
 240     SliceOrArrayElem,
 241
 242     /// A tuple is WF only if its middle elements are `Sized`.
 243     TupleElem,
 244
 245     /// This is the trait reference from the given projection.
 246     ProjectionWf(ty::ProjectionTy<'tcx>),
 247
 248     /// Must satisfy all of the where-clause predicates of the
 249     /// given item.
 250     ItemObligation(DefId),
 251
 252     /// Like `ItemObligation`, but carries the span of the
 253     /// predicate when it can be identified.
 254     BindingObligation(DefId, Span),
 255
 256     /// Like `ItemObligation`, but carries the `HirId` of the
 257     /// expression that caused the obligation, and the `usize`
 258     /// indicates exactly which predicate it is in the list of
 259     /// instantiated predicates.
 260     ExprItemObligation(DefId, rustc_hir::HirId, usize),
 261
 262     /// Combines `ExprItemObligation` and `BindingObligation`.
 263     ExprBindingObligation(DefId, Span, rustc_hir::HirId, usize),
 264
 265     /// A type like `&'a T` is WF only if `T: 'a`.
 266     ReferenceOutlivesReferent(Ty<'tcx>),
 267
 268     /// A type like `Box<Foo<'a> + 'b>` is WF only if `'b: 'a`.
 269     ObjectTypeBound(Ty<'tcx>, ty::Region<'tcx>),
 270
 271     /// Obligation incurred due to an object cast.
 272     ObjectCastObligation(/* Concrete type */ Ty<'tcx>, /* Object type */ Ty<'tcx>),
 273
 274     /// Obligation incurred due to a coercion.
 275     Coercion {
 276         source: Ty<'tcx>,
 277         target: Ty<'tcx>,
 278     },
 279
 280     /// Various cases where expressions must be `Sized` / `Copy` / etc.
 281     /// `L = X` implies that `L` is `Sized`.
 282     AssignmentLhsSized,
 283     /// `(x1, .., xn)` must be `Sized`.
 284     TupleInitializerSized,
 285     /// `S { ... }` must be `Sized`.
 286     StructInitializerSized,
 287     /// Type of each variable must be `Sized`.
 288     VariableType(hir::HirId),
 289     /// Argument type must be `Sized`.
 290     SizedArgumentType(Option<Span>),
 291     /// Return type must be `Sized`.
 292     SizedReturnType,
 293     /// Yield type must be `Sized`.
 294     SizedYieldType,
 295     /// Box expression result type must be `Sized`.
 296     SizedBoxType,
 297     /// Inline asm operand type must be `Sized`.
 298     InlineAsmSized,
 299     /// `[expr; N]` requires `type_of(expr): Copy`.
 300     RepeatElementCopy {
 301         /// If element is a `const fn` we display a help message suggesting to move the
 302         /// function call to a new `const` item while saying that `T` doesn't implement `Copy`.
 303         is_const_fn: bool,
 304     },
 305
 306     /// Types of fields (other than the last, except for packed structs) in a struct must be sized.
 307     FieldSized {
 308         adt_kind: AdtKind,
 309         span: Span,
 310         last: bool,
 311     },
 312
 313     /// Constant expressions must be sized.
 314     ConstSized,
 315
 316     /// `static` items must have `Sync` type.
 317     SharedStatic,
 318
 319     BuiltinDerivedObligation(DerivedObligationCause<'tcx>),
 320
 321     ImplDerivedObligation(Box<ImplDerivedObligationCause<'tcx>>),
 322
 323     DerivedObligation(DerivedObligationCause<'tcx>),
 324
 325     FunctionArgumentObligation {
 326         /// The node of the relevant argument in the function call.
 327         arg_hir_id: hir::HirId,
 328         /// The node of the function call.
 329         call_hir_id: hir::HirId,
 330         /// The obligation introduced by this argument.
 331         parent_code: InternedObligationCauseCode<'tcx>,
 332     },
 333
 334     /// Error derived when matching traits/impls; see ObligationCause for more details
 335     CompareImplItemObligation {
 336         impl_item_def_id: LocalDefId,
 337         trait_item_def_id: DefId,
 338         kind: ty::AssocKind,
 339     },
 340
 341     /// Checking that the bounds of a trait's associated type hold for a given impl
 342     CheckAssociatedTypeBounds {
 343         impl_item_def_id: LocalDefId,
 344         trait_item_def_id: DefId,
 345     },
 346
 347     /// Checking that this expression can be assigned to its target.
 348     ExprAssignable,
 349
 350     /// Computing common supertype in the arms of a match expression
 351     MatchExpressionArm(Box<MatchExpressionArmCause<'tcx>>),
 352
 353     /// Type error arising from type checking a pattern against an expected type.
 354     Pattern {
 355         /// The span of the scrutinee or type expression which caused the `root_ty` type.
 356         span: Option<Span>,
 357         /// The root expected type induced by a scrutinee or type expression.
 358         root_ty: Ty<'tcx>,
 359         /// Whether the `Span` came from an expression or a type expression.
 360         origin_expr: bool,
 361     },
 362
 363     /// Constants in patterns must have `Structural` type.
 364     ConstPatternStructural,
 365
 366     /// Computing common supertype in an if expression
 367     IfExpression(Box<IfExpressionCause<'tcx>>),
 368
 369     /// Computing common supertype of an if expression with no else counter-part
 370     IfExpressionWithNoElse,
 371
 372     /// `main` has wrong type
 373     MainFunctionType,
 374
 375     /// `start` has wrong type
 376     StartFunctionType,
 377
 378     /// Intrinsic has wrong type
 379     IntrinsicType,
 380
 381     /// A let else block does not diverge
 382     LetElse,
 383
 384     /// Method receiver
 385     MethodReceiver,
 386
 387     UnifyReceiver(Box<UnifyReceiverContext<'tcx>>),
 388
 389     /// `return` with no expression
 390     ReturnNoExpression,
 391
 392     /// `return` with an expression
 393     ReturnValue(hir::HirId),
 394
 395     /// Return type of this function
 396     ReturnType,
 397
 398     /// Opaque return type of this function
 399     OpaqueReturnType(Option<(Ty<'tcx>, Span)>),
 400
 401     /// Block implicit return
 402     BlockTailExpression(hir::HirId),
 403
 404     /// #[feature(trivial_bounds)] is not enabled
 405     TrivialBound,
 406
 407     /// If `X` is the concrete type of an opaque type `impl Y`, then `X` must implement `Y`
 408     OpaqueType,
 409
 410     AwaitableExpr(Option<hir::HirId>),
 411
 412     ForLoopIterator,
 413
 414     QuestionMark,
 415
 416     /// Well-formed checking. If a `WellFormedLoc` is provided,
 417     /// then it will be used to perform HIR-based wf checking
 418     /// after an error occurs, in order to generate a more precise error span.
 419     /// This is purely for diagnostic purposes - it is always
 420     /// correct to use `MiscObligation` instead, or to specify
 421     /// `WellFormed(None)`
 422     WellFormed(Option<WellFormedLoc>),
 423
 424     /// From `match_impl`. The cause for us having to match an impl, and the DefId we are matching against.
 425     MatchImpl(ObligationCause<'tcx>, DefId),
 426
 427     BinOp {
 428         rhs_span: Option<Span>,
 429         is_lit: bool,
 430         output_ty: Option<Ty<'tcx>>,
 431     },
 432
 433     AscribeUserTypeProvePredicate(Span),
 434 }
 435
 436 /// The 'location' at which we try to perform HIR-based wf checking.
 437 /// This information is used to obtain an `hir::Ty`, which
 438 /// we can walk in order to obtain precise spans for any
 439 /// 'nested' types (e.g. `Foo` in `Option<Foo>`).
 440 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
 441 pub enum WellFormedLoc {
 442     /// Use the type of the provided definition.
 443     Ty(LocalDefId),
 444     /// Use the type of the parameter of the provided function.
 445     /// We cannot use `hir::Param`, since the function may
 446     /// not have a body (e.g. a trait method definition)
 447     Param {
 448         /// The function to lookup the parameter in
 449         function: LocalDefId,
 450         /// The index of the parameter to use.
 451         /// Parameters are indexed from 0, with the return type
 452         /// being the last 'parameter'
 453         param_idx: u16,
 454     },
 455 }
 456
 457 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 458 pub struct ImplDerivedObligationCause<'tcx> {
 459     pub derived: DerivedObligationCause<'tcx>,
 460     pub impl_def_id: DefId,
 461     pub span: Span,
 462 }
 463
 464 impl<'tcx> ObligationCauseCode<'tcx> {
 465     // Return the base obligation, ignoring derived obligations.
 466     pub fn peel_derives(&self) -> &Self {
 467         let mut base_cause = self;
 468         while let Some((parent_code, _)) = base_cause.parent() {
 469             base_cause = parent_code;
 470         }
 471         base_cause
 472     }
 473
 474     pub fn parent(&self) -> Option<(&Self, Option<ty::PolyTraitPredicate<'tcx>>)> {
 475         match self {
 476             FunctionArgumentObligation { parent_code, .. } => Some((parent_code, None)),
 477             BuiltinDerivedObligation(derived)
 478             | DerivedObligation(derived)
 479             | ImplDerivedObligation(box ImplDerivedObligationCause { derived, .. }) => {
 480                 Some((&derived.parent_code, Some(derived.parent_trait_pred)))
 481             }
 482             _ => None,
 483         }
 484     }
 485
 486     pub fn peel_match_impls(&self) -> &Self {
 487         match self {
 488             MatchImpl(cause, _) => cause.code(),
 489             _ => self,
 490         }
 491     }
 492 }
 493
 494 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
 495 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 496 static_assert_size!(ObligationCauseCode<'_>, 48);
 497
 498 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 499 pub enum StatementAsExpression {
 500     CorrectType,
 501     NeedsBoxing,
 502 }
 503
 504 impl<'tcx> ty::Lift<'tcx> for StatementAsExpression {
 505     type Lifted = StatementAsExpression;
 506     fn lift_to_tcx(self, _tcx: TyCtxt<'tcx>) -> Option<StatementAsExpression> {
 507         Some(self)
 508     }
 509 }
 510
 511 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 512 pub struct MatchExpressionArmCause<'tcx> {
 513     pub arm_block_id: Option<hir::HirId>,
 514     pub arm_ty: Ty<'tcx>,
 515     pub arm_span: Span,
 516     pub prior_arm_block_id: Option<hir::HirId>,
 517     pub prior_arm_ty: Ty<'tcx>,
 518     pub prior_arm_span: Span,
 519     pub scrut_span: Span,
 520     pub source: hir::MatchSource,
 521     pub prior_arms: Vec<Span>,
 522     pub scrut_hir_id: hir::HirId,
 523     pub opt_suggest_box_span: Option<Span>,
 524 }
 525
 526 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 527 #[derive(Lift, TypeFoldable, TypeVisitable)]
 528 pub struct IfExpressionCause<'tcx> {
 529     pub then_id: hir::HirId,
 530     pub else_id: hir::HirId,
 531     pub then_ty: Ty<'tcx>,
 532     pub else_ty: Ty<'tcx>,
 533     pub outer_span: Option<Span>,
 534     pub opt_suggest_box_span: Option<Span>,
 535 }
 536
 537 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 538 pub struct DerivedObligationCause<'tcx> {
 539     /// The trait predicate of the parent obligation that led to the
 540     /// current obligation. Note that only trait obligations lead to
 541     /// derived obligations, so we just store the trait predicate here
 542     /// directly.
 543     pub parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
 544
 545     /// The parent trait had this cause.
 546     pub parent_code: InternedObligationCauseCode<'tcx>,
 547 }
 548
 549 #[derive(Clone, Debug, TypeFoldable, TypeVisitable, Lift)]
 550 pub enum SelectionError<'tcx> {
 551     /// The trait is not implemented.
 552     Unimplemented,
 553     /// After a closure impl has selected, its "outputs" were evaluated
 554     /// (which for closures includes the "input" type params) and they
 555     /// didn't resolve. See `confirm_poly_trait_refs` for more.
 556     OutputTypeParameterMismatch(
 557         ty::PolyTraitRef<'tcx>,
 558         ty::PolyTraitRef<'tcx>,
 559         ty::error::TypeError<'tcx>,
 560     ),
 561     /// The trait pointed by `DefId` is not object safe.
 562     TraitNotObjectSafe(DefId),
 563     /// A given constant couldn't be evaluated.
 564     NotConstEvaluatable(NotConstEvaluatable),
 565     /// Exceeded the recursion depth during type projection.
 566     Overflow(OverflowError),
 567     /// Signaling that an error has already been emitted, to avoid
 568     /// multiple errors being shown.
 569     ErrorReporting,
 570     /// Multiple applicable `impl`s where found. The `DefId`s correspond to
 571     /// all the `impl`s' Items.
 572     Ambiguous(Vec<DefId>),
 573 }
 574
 575 /// When performing resolution, it is typically the case that there
 576 /// can be one of three outcomes:
 577 ///
 578 /// - `Ok(Some(r))`: success occurred with result `r`
 579 /// - `Ok(None)`: could not definitely determine anything, usually due
 580 ///   to inconclusive type inference.
 581 /// - `Err(e)`: error `e` occurred
 582 pub type SelectionResult<'tcx, T> = Result<Option<T>, SelectionError<'tcx>>;
 583
 584 /// Given the successful resolution of an obligation, the `ImplSource`
 585 /// indicates where the impl comes from.
 586 ///
 587 /// For example, the obligation may be satisfied by a specific impl (case A),
 588 /// or it may be relative to some bound that is in scope (case B).
 589 ///
 590 /// ```ignore (illustrative)
 591 /// impl<T:Clone> Clone<T> for Option<T> { ... } // Impl_1
 592 /// impl<T:Clone> Clone<T> for Box<T> { ... }    // Impl_2
 593 /// impl Clone for i32 { ... }                   // Impl_3
 594 ///
 595 /// fn foo<T: Clone>(concrete: Option<Box<i32>>, param: T, mixed: Option<T>) {
 596 ///     // Case A: ImplSource points at a specific impl. Only possible when
 597 ///     // type is concretely known. If the impl itself has bounded
 598 ///     // type parameters, ImplSource will carry resolutions for those as well:
 599 ///     concrete.clone(); // ImplSource(Impl_1, [ImplSource(Impl_2, [ImplSource(Impl_3)])])
 600 ///
 601 ///     // Case A: ImplSource points at a specific impl. Only possible when
 602 ///     // type is concretely known. If the impl itself has bounded
 603 ///     // type parameters, ImplSource will carry resolutions for those as well:
 604 ///     concrete.clone(); // ImplSource(Impl_1, [ImplSource(Impl_2, [ImplSource(Impl_3)])])
 605 ///
 606 ///     // Case B: ImplSource must be provided by caller. This applies when
 607 ///     // type is a type parameter.
 608 ///     param.clone();    // ImplSource::Param
 609 ///
 610 ///     // Case C: A mix of cases A and B.
 611 ///     mixed.clone();    // ImplSource(Impl_1, [ImplSource::Param])
 612 /// }
 613 /// ```
 614 ///
 615 /// ### The type parameter `N`
 616 ///
 617 /// See explanation on `ImplSourceUserDefinedData`.
 618 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 619 #[derive(TypeFoldable, TypeVisitable)]
 620 pub enum ImplSource<'tcx, N> {
 621     /// ImplSource identifying a particular impl.
 622     UserDefined(ImplSourceUserDefinedData<'tcx, N>),
 623
 624     /// ImplSource for auto trait implementations.
 625     /// This carries the information and nested obligations with regards
 626     /// to an auto implementation for a trait `Trait`. The nested obligations
 627     /// ensure the trait implementation holds for all the constituent types.
 628     AutoImpl(ImplSourceAutoImplData<N>),
 629
 630     /// Successful resolution to an obligation provided by the caller
 631     /// for some type parameter. The `Vec<N>` represents the
 632     /// obligations incurred from normalizing the where-clause (if
 633     /// any).
 634     Param(Vec<N>, ty::BoundConstness),
 635
 636     /// Virtual calls through an object.
 637     Object(ImplSourceObjectData<'tcx, N>),
 638
 639     /// Successful resolution for a builtin trait.
 640     Builtin(ImplSourceBuiltinData<N>),
 641
 642     /// ImplSource for trait upcasting coercion
 643     TraitUpcasting(ImplSourceTraitUpcastingData<'tcx, N>),
 644
 645     /// ImplSource automatically generated for a closure. The `DefId` is the ID
 646     /// of the closure expression. This is an `ImplSource::UserDefined` in spirit, but the
 647     /// impl is generated by the compiler and does not appear in the source.
 648     Closure(ImplSourceClosureData<'tcx, N>),
 649
 650     /// Same as above, but for a function pointer type with the given signature.
 651     FnPointer(ImplSourceFnPointerData<'tcx, N>),
 652
 653     /// ImplSource for a builtin `DeterminantKind` trait implementation.
 654     DiscriminantKind(ImplSourceDiscriminantKindData),
 655
 656     /// ImplSource for a builtin `Pointee` trait implementation.
 657     Pointee(ImplSourcePointeeData),
 658
 659     /// ImplSource automatically generated for a generator.
 660     Generator(ImplSourceGeneratorData<'tcx, N>),
 661
 662     /// ImplSource for a trait alias.
 663     TraitAlias(ImplSourceTraitAliasData<'tcx, N>),
 664
 665     /// ImplSource for a `const Drop` implementation.
 666     ConstDestruct(ImplSourceConstDestructData<N>),
 667
 668     /// ImplSource for a `std::marker::Tuple` implementation.
 669     /// This has no nested predicates ever, so no data.
 670     Tuple,
 671 }
 672
 673 impl<'tcx, N> ImplSource<'tcx, N> {
 674     pub fn nested_obligations(self) -> Vec<N> {
 675         match self {
 676             ImplSource::UserDefined(i) => i.nested,
 677             ImplSource::Param(n, _) => n,
 678             ImplSource::Builtin(i) => i.nested,
 679             ImplSource::AutoImpl(d) => d.nested,
 680             ImplSource::Closure(c) => c.nested,
 681             ImplSource::Generator(c) => c.nested,
 682             ImplSource::Object(d) => d.nested,
 683             ImplSource::FnPointer(d) => d.nested,
 684             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
 685             | ImplSource::Pointee(ImplSourcePointeeData)
 686             | ImplSource::Tuple => Vec::new(),
 687             ImplSource::TraitAlias(d) => d.nested,
 688             ImplSource::TraitUpcasting(d) => d.nested,
 689             ImplSource::ConstDestruct(i) => i.nested,
 690         }
 691     }
 692
 693     pub fn borrow_nested_obligations(&self) -> &[N] {
 694         match &self {
 695             ImplSource::UserDefined(i) => &i.nested[..],
 696             ImplSource::Param(n, _) => &n,
 697             ImplSource::Builtin(i) => &i.nested,
 698             ImplSource::AutoImpl(d) => &d.nested,
 699             ImplSource::Closure(c) => &c.nested,
 700             ImplSource::Generator(c) => &c.nested,
 701             ImplSource::Object(d) => &d.nested,
 702             ImplSource::FnPointer(d) => &d.nested,
 703             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
 704             | ImplSource::Pointee(ImplSourcePointeeData)
 705             | ImplSource::Tuple => &[],
 706             ImplSource::TraitAlias(d) => &d.nested,
 707             ImplSource::TraitUpcasting(d) => &d.nested,
 708             ImplSource::ConstDestruct(i) => &i.nested,
 709         }
 710     }
 711
 712     pub fn map<M, F>(self, f: F) -> ImplSource<'tcx, M>
 713     where
 714         F: FnMut(N) -> M,
 715     {
 716         match self {
 717             ImplSource::UserDefined(i) => ImplSource::UserDefined(ImplSourceUserDefinedData {
 718                 impl_def_id: i.impl_def_id,
 719                 substs: i.substs,
 720                 nested: i.nested.into_iter().map(f).collect(),
 721             }),
 722             ImplSource::Param(n, ct) => ImplSource::Param(n.into_iter().map(f).collect(), ct),
 723             ImplSource::Builtin(i) => ImplSource::Builtin(ImplSourceBuiltinData {
 724                 nested: i.nested.into_iter().map(f).collect(),
 725             }),
 726             ImplSource::Object(o) => ImplSource::Object(ImplSourceObjectData {
 727                 upcast_trait_ref: o.upcast_trait_ref,
 728                 vtable_base: o.vtable_base,
 729                 nested: o.nested.into_iter().map(f).collect(),
 730             }),
 731             ImplSource::AutoImpl(d) => ImplSource::AutoImpl(ImplSourceAutoImplData {
 732                 trait_def_id: d.trait_def_id,
 733                 nested: d.nested.into_iter().map(f).collect(),
 734             }),
 735             ImplSource::Closure(c) => ImplSource::Closure(ImplSourceClosureData {
 736                 closure_def_id: c.closure_def_id,
 737                 substs: c.substs,
 738                 nested: c.nested.into_iter().map(f).collect(),
 739             }),
 740             ImplSource::Generator(c) => ImplSource::Generator(ImplSourceGeneratorData {
 741                 generator_def_id: c.generator_def_id,
 742                 substs: c.substs,
 743                 nested: c.nested.into_iter().map(f).collect(),
 744             }),
 745             ImplSource::FnPointer(p) => ImplSource::FnPointer(ImplSourceFnPointerData {
 746                 fn_ty: p.fn_ty,
 747                 nested: p.nested.into_iter().map(f).collect(),
 748             }),
 749             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData) => {
 750                 ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
 751             }
 752             ImplSource::Pointee(ImplSourcePointeeData) => {
 753                 ImplSource::Pointee(ImplSourcePointeeData)
 754             }
 755             ImplSource::TraitAlias(d) => ImplSource::TraitAlias(ImplSourceTraitAliasData {
 756                 alias_def_id: d.alias_def_id,
 757                 substs: d.substs,
 758                 nested: d.nested.into_iter().map(f).collect(),
 759             }),
 760             ImplSource::TraitUpcasting(d) => {
 761                 ImplSource::TraitUpcasting(ImplSourceTraitUpcastingData {
 762                     upcast_trait_ref: d.upcast_trait_ref,
 763                     vtable_vptr_slot: d.vtable_vptr_slot,
 764                     nested: d.nested.into_iter().map(f).collect(),
 765                 })
 766             }
 767             ImplSource::ConstDestruct(i) => {
 768                 ImplSource::ConstDestruct(ImplSourceConstDestructData {
 769                     nested: i.nested.into_iter().map(f).collect(),
 770                 })
 771             }
 772             ImplSource::Tuple => ImplSource::Tuple,
 773         }
 774     }
 775 }
 776
 777 /// Identifies a particular impl in the source, along with a set of
 778 /// substitutions from the impl's type/lifetime parameters. The
 779 /// `nested` vector corresponds to the nested obligations attached to
 780 /// the impl's type parameters.
 781 ///
 782 /// The type parameter `N` indicates the type used for "nested
 783 /// obligations" that are required by the impl. During type-check, this
 784 /// is `Obligation`, as one might expect. During codegen, however, this
 785 /// is `()`, because codegen only requires a shallow resolution of an
 786 /// impl, and nested obligations are satisfied later.
 787 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 788 #[derive(TypeFoldable, TypeVisitable)]
 789 pub struct ImplSourceUserDefinedData<'tcx, N> {
 790     pub impl_def_id: DefId,
 791     pub substs: SubstsRef<'tcx>,
 792     pub nested: Vec<N>,
 793 }
 794
 795 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 796 #[derive(TypeFoldable, TypeVisitable)]
 797 pub struct ImplSourceGeneratorData<'tcx, N> {
 798     pub generator_def_id: DefId,
 799     pub substs: SubstsRef<'tcx>,
 800     /// Nested obligations. This can be non-empty if the generator
 801     /// signature contains associated types.
 802     pub nested: Vec<N>,
 803 }
 804
 805 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 806 #[derive(TypeFoldable, TypeVisitable)]
 807 pub struct ImplSourceClosureData<'tcx, N> {
 808     pub closure_def_id: DefId,
 809     pub substs: SubstsRef<'tcx>,
 810     /// Nested obligations. This can be non-empty if the closure
 811     /// signature contains associated types.
 812     pub nested: Vec<N>,
 813 }
 814
 815 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 816 #[derive(TypeFoldable, TypeVisitable)]
 817 pub struct ImplSourceAutoImplData<N> {
 818     pub trait_def_id: DefId,
 819     pub nested: Vec<N>,
 820 }
 821
 822 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 823 #[derive(TypeFoldable, TypeVisitable)]
 824 pub struct ImplSourceTraitUpcastingData<'tcx, N> {
 825     /// `Foo` upcast to the obligation trait. This will be some supertrait of `Foo`.
 826     pub upcast_trait_ref: ty::PolyTraitRef<'tcx>,
 827
 828     /// The vtable is formed by concatenating together the method lists of
 829     /// the base object trait and all supertraits, pointers to supertrait vtable will
 830     /// be provided when necessary; this is the position of `upcast_trait_ref`'s vtable
 831     /// within that vtable.
 832     pub vtable_vptr_slot: Option<usize>,
 833
 834     pub nested: Vec<N>,
 835 }
 836
 837 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 838 #[derive(TypeFoldable, TypeVisitable)]
 839 pub struct ImplSourceBuiltinData<N> {
 840     pub nested: Vec<N>,
 841 }
 842
 843 #[derive(PartialEq, Eq, Clone, TyEncodable, TyDecodable, HashStable, Lift)]
 844 #[derive(TypeFoldable, TypeVisitable)]
 845 pub struct ImplSourceObjectData<'tcx, N> {
 846     /// `Foo` upcast to the obligation trait. This will be some supertrait of `Foo`.
 847     pub upcast_trait_ref: ty::PolyTraitRef<'tcx>,
 848
 849     /// The vtable is formed by concatenating together the method lists of
 850     /// the base object trait and all supertraits, pointers to supertrait vtable will
 851     /// be provided when necessary; this is the start of `upcast_trait_ref`'s methods
 852     /// in that vtable.
 853     pub vtable_base: usize,
 854
 855     pub nested: Vec<N>,
 856 }
 857
 858 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 859 #[derive(TypeFoldable, TypeVisitable)]
 860 pub struct ImplSourceFnPointerData<'tcx, N> {
 861     pub fn_ty: Ty<'tcx>,
 862     pub nested: Vec<N>,
 863 }
 864
 865 // FIXME(@lcnr): This should be  refactored and merged with other builtin vtables.
 866 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 867 pub struct ImplSourceDiscriminantKindData;
 868
 869 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 870 pub struct ImplSourcePointeeData;
 871
 872 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 873 #[derive(TypeFoldable, TypeVisitable)]
 874 pub struct ImplSourceConstDestructData<N> {
 875     pub nested: Vec<N>,
 876 }
 877
 878 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, Lift)]
 879 #[derive(TypeFoldable, TypeVisitable)]
 880 pub struct ImplSourceTraitAliasData<'tcx, N> {
 881     pub alias_def_id: DefId,
 882     pub substs: SubstsRef<'tcx>,
 883     pub nested: Vec<N>,
 884 }
 885
 886 #[derive(Clone, Debug, PartialEq, Eq, Hash, HashStable, PartialOrd, Ord)]
 887 pub enum ObjectSafetyViolation {
 888     /// `Self: Sized` declared on the trait.
 889     SizedSelf(SmallVec<[Span; 1]>),
 890
 891     /// Supertrait reference references `Self` an in illegal location
 892     /// (e.g., `trait Foo : Bar<Self>`).
 893     SupertraitSelf(SmallVec<[Span; 1]>),
 894
 895     /// Method has something illegal.
 896     Method(Symbol, MethodViolationCode, Span),
 897
 898     /// Associated const.
 899     AssocConst(Symbol, Span),
 900
 901     /// GAT
 902     GAT(Symbol, Span),
 903 }
 904
 905 impl ObjectSafetyViolation {
 906     pub fn error_msg(&self) -> Cow<'static, str> {
 907         match self {
 908             ObjectSafetyViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
 909             ObjectSafetyViolation::SupertraitSelf(ref spans) => {
 910                 if spans.iter().any(|sp| *sp != DUMMY_SP) {
 911                     "it uses `Self` as a type parameter".into()
 912                 } else {
 913                     "it cannot use `Self` as a type parameter in a supertrait or `where`-clause"
 914                         .into()
 915                 }
 916             }
 917             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
 918                 format!("associated function `{}` has no `self` parameter", name).into()
 919             }
 920             ObjectSafetyViolation::Method(
 921                 name,
 922                 MethodViolationCode::ReferencesSelfInput(_),
 923                 DUMMY_SP,
 924             ) => format!("method `{}` references the `Self` type in its parameters", name).into(),
 925             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfInput(_), _) => {
 926                 format!("method `{}` references the `Self` type in this parameter", name).into()
 927             }
 928             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
 929                 format!("method `{}` references the `Self` type in its return type", name).into()
 930             }
 931             ObjectSafetyViolation::Method(
 932                 name,
 933                 MethodViolationCode::ReferencesImplTraitInTrait,
 934                 _,
 935             ) => format!("method `{}` references an `impl Trait` type in its return type", name)
 936                 .into(),
 937             ObjectSafetyViolation::Method(
 938                 name,
 939                 MethodViolationCode::WhereClauseReferencesSelf,
 940                 _,
 941             ) => {
 942                 format!("method `{}` references the `Self` type in its `where` clause", name).into()
 943             }
 944             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) => {
 945                 format!("method `{}` has generic type parameters", name).into()
 946             }
 947             ObjectSafetyViolation::Method(
 948                 name,
 949                 MethodViolationCode::UndispatchableReceiver(_),
 950                 _,
 951             ) => format!("method `{}`'s `self` parameter cannot be dispatched on", name).into(),
 952             ObjectSafetyViolation::AssocConst(name, DUMMY_SP) => {
 953                 format!("it contains associated `const` `{}`", name).into()
 954             }
 955             ObjectSafetyViolation::AssocConst(..) => "it contains this associated `const`".into(),
 956             ObjectSafetyViolation::GAT(name, _) => {
 957                 format!("it contains the generic associated type `{}`", name).into()
 958             }
 959         }
 960     }
 961
 962     pub fn solution(&self, err: &mut Diagnostic) {
 963         match self {
 964             ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf(_) => {}
 965             ObjectSafetyViolation::Method(
 966                 name,
 967                 MethodViolationCode::StaticMethod(Some((add_self_sugg, make_sized_sugg))),
 968                 _,
 969             ) => {
 970                 err.span_suggestion(
 971                     add_self_sugg.1,
 972                     format!(
 973                         "consider turning `{}` into a method by giving it a `&self` argument",
 974                         name
 975                     ),
 976                     add_self_sugg.0.to_string(),
 977                     Applicability::MaybeIncorrect,
 978                 );
 979                 err.span_suggestion(
 980                     make_sized_sugg.1,
 981                     format!(
 982                         "alternatively, consider constraining `{}` so it does not apply to \
 983                              trait objects",
 984                         name
 985                     ),
 986                     make_sized_sugg.0.to_string(),
 987                     Applicability::MaybeIncorrect,
 988                 );
 989             }
 990             ObjectSafetyViolation::Method(
 991                 name,
 992                 MethodViolationCode::UndispatchableReceiver(Some(span)),
 993                 _,
 994             ) => {
 995                 err.span_suggestion(
 996                     *span,
 997                     &format!(
 998                         "consider changing method `{}`'s `self` parameter to be `&self`",
 999                         name
1000                     ),
1001                     "&Self",
1002                     Applicability::MachineApplicable,
1003                 );
1004             }
1005             ObjectSafetyViolation::AssocConst(name, _)
1006             | ObjectSafetyViolation::GAT(name, _)
1007             | ObjectSafetyViolation::Method(name, ..) => {
1008                 err.help(&format!("consider moving `{}` to another trait", name));
1009             }
1010         }
1011     }
1012
1013     pub fn spans(&self) -> SmallVec<[Span; 1]> {
1014         // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
1015         // diagnostics use a `note` instead of a `span_label`.
1016         match self {
1017             ObjectSafetyViolation::SupertraitSelf(spans)
1018             | ObjectSafetyViolation::SizedSelf(spans) => spans.clone(),
1019             ObjectSafetyViolation::AssocConst(_, span)
1020             | ObjectSafetyViolation::GAT(_, span)
1021             | ObjectSafetyViolation::Method(_, _, span)
1022                 if *span != DUMMY_SP =>
1023             {
1024                 smallvec![*span]
1025             }
1026             _ => smallvec![],
1027         }
1028     }
1029 }
1030
1031 /// Reasons a method might not be object-safe.
1032 #[derive(Clone, Debug, PartialEq, Eq, Hash, HashStable, PartialOrd, Ord)]
1033 pub enum MethodViolationCode {
1034     /// e.g., `fn foo()`
1035     StaticMethod(Option<(/* add &self */ (String, Span), /* add Self: Sized */ (String, Span))>),
1036
1037     /// e.g., `fn foo(&self, x: Self)`
1038     ReferencesSelfInput(Option<Span>),
1039
1040     /// e.g., `fn foo(&self) -> Self`
1041     ReferencesSelfOutput,
1042
1043     /// e.g., `fn foo(&self) -> impl Sized`
1044     ReferencesImplTraitInTrait,
1045
1046     /// e.g., `fn foo(&self) where Self: Clone`
1047     WhereClauseReferencesSelf,
1048
1049     /// e.g., `fn foo<A>()`
1050     Generic,
1051
1052     /// the method's receiver (`self` argument) can't be dispatched on
1053     UndispatchableReceiver(Option<Span>),
1054 }
1055
1056 /// These are the error cases for `codegen_select_candidate`.
1057 #[derive(Copy, Clone, Debug, Hash, HashStable, Encodable, Decodable)]
1058 pub enum CodegenObligationError {
1059     /// Ambiguity can happen when monomorphizing during trans
1060     /// expands to some humongous type that never occurred
1061     /// statically -- this humongous type can then overflow,
1062     /// leading to an ambiguous result. So report this as an
1063     /// overflow bug, since I believe this is the only case
1064     /// where ambiguity can result.
1065     Ambiguity,
1066     /// This can trigger when we probe for the source of a `'static` lifetime requirement
1067     /// on a trait object: `impl Foo for dyn Trait {}` has an implicit `'static` bound.
1068     /// This can also trigger when we have a global bound that is not actually satisfied,
1069     /// but was included during typeck due to the trivial_bounds feature.
1070     Unimplemented,
1071     FulfillmentError,
1072 }