src/librustc_middle/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
  11 use crate::infer::canonical::Canonical;
  12 use crate::mir::interpret::ErrorHandled;
  13 use crate::ty::subst::SubstsRef;
  14 use crate::ty::{self, AdtKind, Ty, TyCtxt};
  15
  16 use rustc_hir as hir;
  17 use rustc_hir::def_id::DefId;
  18 use rustc_span::symbol::Symbol;
  19 use rustc_span::{Span, DUMMY_SP};
  20 use smallvec::SmallVec;
  21
  22 use std::borrow::Cow;
  23 use std::fmt;
  24 use std::ops::Deref;
  25 use std::rc::Rc;
  26
  27 pub use self::select::{EvaluationCache, EvaluationResult, OverflowError, SelectionCache};
  28
  29 pub type ChalkCanonicalGoal<'tcx> = Canonical<'tcx, ChalkEnvironmentAndGoal<'tcx>>;
  30
  31 pub use self::ImplSource::*;
  32 pub use self::ObligationCauseCode::*;
  33
  34 pub use self::chalk::{
  35     ChalkEnvironmentAndGoal, ChalkEnvironmentClause, RustInterner as ChalkRustInterner,
  36 };
  37
  38 /// Depending on the stage of compilation, we want projection to be
  39 /// more or less conservative.
  40 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
  41 pub enum Reveal {
  42     /// At type-checking time, we refuse to project any associated
  43     /// type that is marked `default`. Non-`default` ("final") types
  44     /// are always projected. This is necessary in general for
  45     /// soundness of specialization. However, we *could* allow
  46     /// projections in fully-monomorphic cases. We choose not to,
  47     /// because we prefer for `default type` to force the type
  48     /// definition to be treated abstractly by any consumers of the
  49     /// impl. Concretely, that means that the following example will
  50     /// fail to compile:
  51     ///
  52     /// ```
  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 <() as Assoc>::Output = true;
  63     /// }
  64     /// ```
  65     UserFacing,
  66
  67     /// At codegen time, all monomorphic projections will succeed.
  68     /// Also, `impl Trait` is normalized to the concrete type,
  69     /// which has to be already collected by type-checking.
  70     ///
  71     /// NOTE: as `impl Trait`'s concrete type should *never*
  72     /// be observable directly by the user, `Reveal::All`
  73     /// should not be used by checks which may expose
  74     /// type equality or type contents to the user.
  75     /// There are some exceptions, e.g., around OIBITS and
  76     /// transmute-checking, which expose some details, but
  77     /// not the whole concrete type of the `impl Trait`.
  78     All,
  79 }
  80
  81 /// The reason why we incurred this obligation; used for error reporting.
  82 ///
  83 /// As the happy path does not care about this struct, storing this on the heap
  84 /// ends up increasing performance.
  85 ///
  86 /// We do not want to intern this as there are a lot of obligation causes which
  87 /// only live for a short period of time.
  88 #[derive(Clone, PartialEq, Eq, Hash, Lift)]
  89 pub struct ObligationCause<'tcx> {
  90     /// `None` for `ObligationCause::dummy`, `Some` otherwise.
  91     data: Option<Rc<ObligationCauseData<'tcx>>>,
  92 }
  93
  94 const DUMMY_OBLIGATION_CAUSE_DATA: ObligationCauseData<'static> =
  95     ObligationCauseData { span: DUMMY_SP, body_id: hir::CRATE_HIR_ID, code: MiscObligation };
  96
  97 // Correctly format `ObligationCause::dummy`.
  98 impl<'tcx> fmt::Debug for ObligationCause<'tcx> {
  99     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 100         ObligationCauseData::fmt(self, f)
 101     }
 102 }
 103
 104 impl Deref for ObligationCause<'tcx> {
 105     type Target = ObligationCauseData<'tcx>;
 106
 107     #[inline(always)]
 108     fn deref(&self) -> &Self::Target {
 109         self.data.as_deref().unwrap_or(&DUMMY_OBLIGATION_CAUSE_DATA)
 110     }
 111 }
 112
 113 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 114 pub struct ObligationCauseData<'tcx> {
 115     pub span: Span,
 116
 117     /// The ID of the fn body that triggered this obligation. This is
 118     /// used for region obligations to determine the precise
 119     /// environment in which the region obligation should be evaluated
 120     /// (in particular, closures can add new assumptions). See the
 121     /// field `region_obligations` of the `FulfillmentContext` for more
 122     /// information.
 123     pub body_id: hir::HirId,
 124
 125     pub code: ObligationCauseCode<'tcx>,
 126 }
 127
 128 impl<'tcx> ObligationCause<'tcx> {
 129     #[inline]
 130     pub fn new(
 131         span: Span,
 132         body_id: hir::HirId,
 133         code: ObligationCauseCode<'tcx>,
 134     ) -> ObligationCause<'tcx> {
 135         ObligationCause { data: Some(Rc::new(ObligationCauseData { span, body_id, code })) }
 136     }
 137
 138     pub fn misc(span: Span, body_id: hir::HirId) -> ObligationCause<'tcx> {
 139         ObligationCause::new(span, body_id, MiscObligation)
 140     }
 141
 142     pub fn dummy_with_span(span: Span) -> ObligationCause<'tcx> {
 143         ObligationCause::new(span, hir::CRATE_HIR_ID, MiscObligation)
 144     }
 145
 146     #[inline(always)]
 147     pub fn dummy() -> ObligationCause<'tcx> {
 148         ObligationCause { data: None }
 149     }
 150
 151     pub fn make_mut(&mut self) -> &mut ObligationCauseData<'tcx> {
 152         Rc::make_mut(self.data.get_or_insert_with(|| Rc::new(DUMMY_OBLIGATION_CAUSE_DATA)))
 153     }
 154
 155     pub fn span(&self, tcx: TyCtxt<'tcx>) -> Span {
 156         match self.code {
 157             ObligationCauseCode::CompareImplMethodObligation { .. }
 158             | ObligationCauseCode::MainFunctionType
 159             | ObligationCauseCode::StartFunctionType => {
 160                 tcx.sess.source_map().guess_head_span(self.span)
 161             }
 162             ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
 163                 arm_span,
 164                 ..
 165             }) => arm_span,
 166             _ => self.span,
 167         }
 168     }
 169 }
 170
 171 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 172 pub struct UnifyReceiverContext<'tcx> {
 173     pub assoc_item: ty::AssocItem,
 174     pub param_env: ty::ParamEnv<'tcx>,
 175     pub substs: SubstsRef<'tcx>,
 176 }
 177
 178 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 179 pub enum ObligationCauseCode<'tcx> {
 180     /// Not well classified or should be obvious from the span.
 181     MiscObligation,
 182
 183     /// A slice or array is WF only if `T: Sized`.
 184     SliceOrArrayElem,
 185
 186     /// A tuple is WF only if its middle elements are `Sized`.
 187     TupleElem,
 188
 189     /// This is the trait reference from the given projection.
 190     ProjectionWf(ty::ProjectionTy<'tcx>),
 191
 192     /// In an impl of trait `X` for type `Y`, type `Y` must
 193     /// also implement all supertraits of `X`.
 194     ItemObligation(DefId),
 195
 196     /// Like `ItemObligation`, but with extra detail on the source of the obligation.
 197     BindingObligation(DefId, Span),
 198
 199     /// A type like `&'a T` is WF only if `T: 'a`.
 200     ReferenceOutlivesReferent(Ty<'tcx>),
 201
 202     /// A type like `Box<Foo<'a> + 'b>` is WF only if `'b: 'a`.
 203     ObjectTypeBound(Ty<'tcx>, ty::Region<'tcx>),
 204
 205     /// Obligation incurred due to an object cast.
 206     ObjectCastObligation(/* Object type */ Ty<'tcx>),
 207
 208     /// Obligation incurred due to a coercion.
 209     Coercion {
 210         source: Ty<'tcx>,
 211         target: Ty<'tcx>,
 212     },
 213
 214     /// Various cases where expressions must be `Sized` / `Copy` / etc.
 215     /// `L = X` implies that `L` is `Sized`.
 216     AssignmentLhsSized,
 217     /// `(x1, .., xn)` must be `Sized`.
 218     TupleInitializerSized,
 219     /// `S { ... }` must be `Sized`.
 220     StructInitializerSized,
 221     /// Type of each variable must be `Sized`.
 222     VariableType(hir::HirId),
 223     /// Argument type must be `Sized`.
 224     SizedArgumentType(Option<Span>),
 225     /// Return type must be `Sized`.
 226     SizedReturnType,
 227     /// Yield type must be `Sized`.
 228     SizedYieldType,
 229     /// Inline asm operand type must be `Sized`.
 230     InlineAsmSized,
 231     /// `[T, ..n]` implies that `T` must be `Copy`.
 232     /// If `true`, suggest `const_in_array_repeat_expressions` feature flag.
 233     RepeatVec(bool),
 234
 235     /// Types of fields (other than the last, except for packed structs) in a struct must be sized.
 236     FieldSized {
 237         adt_kind: AdtKind,
 238         span: Span,
 239         last: bool,
 240     },
 241
 242     /// Constant expressions must be sized.
 243     ConstSized,
 244
 245     /// `static` items must have `Sync` type.
 246     SharedStatic,
 247
 248     BuiltinDerivedObligation(DerivedObligationCause<'tcx>),
 249
 250     ImplDerivedObligation(DerivedObligationCause<'tcx>),
 251
 252     DerivedObligation(DerivedObligationCause<'tcx>),
 253
 254     /// Error derived when matching traits/impls; see ObligationCause for more details
 255     CompareImplConstObligation,
 256
 257     /// Error derived when matching traits/impls; see ObligationCause for more details
 258     CompareImplMethodObligation {
 259         item_name: Symbol,
 260         impl_item_def_id: DefId,
 261         trait_item_def_id: DefId,
 262     },
 263
 264     /// Error derived when matching traits/impls; see ObligationCause for more details
 265     CompareImplTypeObligation {
 266         item_name: Symbol,
 267         impl_item_def_id: DefId,
 268         trait_item_def_id: DefId,
 269     },
 270
 271     /// Checking that this expression can be assigned where it needs to be
 272     // FIXME(eddyb) #11161 is the original Expr required?
 273     ExprAssignable,
 274
 275     /// Computing common supertype in the arms of a match expression
 276     MatchExpressionArm(Box<MatchExpressionArmCause<'tcx>>),
 277
 278     /// Type error arising from type checking a pattern against an expected type.
 279     Pattern {
 280         /// The span of the scrutinee or type expression which caused the `root_ty` type.
 281         span: Option<Span>,
 282         /// The root expected type induced by a scrutinee or type expression.
 283         root_ty: Ty<'tcx>,
 284         /// Whether the `Span` came from an expression or a type expression.
 285         origin_expr: bool,
 286     },
 287
 288     /// Constants in patterns must have `Structural` type.
 289     ConstPatternStructural,
 290
 291     /// Computing common supertype in an if expression
 292     IfExpression(Box<IfExpressionCause>),
 293
 294     /// Computing common supertype of an if expression with no else counter-part
 295     IfExpressionWithNoElse,
 296
 297     /// `main` has wrong type
 298     MainFunctionType,
 299
 300     /// `start` has wrong type
 301     StartFunctionType,
 302
 303     /// Intrinsic has wrong type
 304     IntrinsicType,
 305
 306     /// Method receiver
 307     MethodReceiver,
 308
 309     UnifyReceiver(Box<UnifyReceiverContext<'tcx>>),
 310
 311     /// `return` with no expression
 312     ReturnNoExpression,
 313
 314     /// `return` with an expression
 315     ReturnValue(hir::HirId),
 316
 317     /// Return type of this function
 318     ReturnType,
 319
 320     /// Block implicit return
 321     BlockTailExpression(hir::HirId),
 322
 323     /// #[feature(trivial_bounds)] is not enabled
 324     TrivialBound,
 325 }
 326
 327 impl ObligationCauseCode<'_> {
 328     // Return the base obligation, ignoring derived obligations.
 329     pub fn peel_derives(&self) -> &Self {
 330         let mut base_cause = self;
 331         while let BuiltinDerivedObligation(cause)
 332         | ImplDerivedObligation(cause)
 333         | DerivedObligation(cause) = base_cause
 334         {
 335             base_cause = &cause.parent_code;
 336         }
 337         base_cause
 338     }
 339 }
 340
 341 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
 342 #[cfg(target_arch = "x86_64")]
 343 static_assert_size!(ObligationCauseCode<'_>, 32);
 344
 345 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 346 pub struct MatchExpressionArmCause<'tcx> {
 347     pub arm_span: Span,
 348     pub semi_span: Option<Span>,
 349     pub source: hir::MatchSource,
 350     pub prior_arms: Vec<Span>,
 351     pub last_ty: Ty<'tcx>,
 352     pub scrut_hir_id: hir::HirId,
 353 }
 354
 355 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 356 pub struct IfExpressionCause {
 357     pub then: Span,
 358     pub outer: Option<Span>,
 359     pub semicolon: Option<Span>,
 360 }
 361
 362 #[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
 363 pub struct DerivedObligationCause<'tcx> {
 364     /// The trait reference of the parent obligation that led to the
 365     /// current obligation. Note that only trait obligations lead to
 366     /// derived obligations, so we just store the trait reference here
 367     /// directly.
 368     pub parent_trait_ref: ty::PolyTraitRef<'tcx>,
 369
 370     /// The parent trait had this cause.
 371     pub parent_code: Rc<ObligationCauseCode<'tcx>>,
 372 }
 373
 374 #[derive(Clone, Debug, TypeFoldable, Lift)]
 375 pub enum SelectionError<'tcx> {
 376     Unimplemented,
 377     OutputTypeParameterMismatch(
 378         ty::PolyTraitRef<'tcx>,
 379         ty::PolyTraitRef<'tcx>,
 380         ty::error::TypeError<'tcx>,
 381     ),
 382     TraitNotObjectSafe(DefId),
 383     ConstEvalFailure(ErrorHandled),
 384     Overflow,
 385 }
 386
 387 /// When performing resolution, it is typically the case that there
 388 /// can be one of three outcomes:
 389 ///
 390 /// - `Ok(Some(r))`: success occurred with result `r`
 391 /// - `Ok(None)`: could not definitely determine anything, usually due
 392 ///   to inconclusive type inference.
 393 /// - `Err(e)`: error `e` occurred
 394 pub type SelectionResult<'tcx, T> = Result<Option<T>, SelectionError<'tcx>>;
 395
 396 /// Given the successful resolution of an obligation, the `ImplSource`
 397 /// indicates where the impl comes from.
 398 ///
 399 /// For example, the obligation may be satisfied by a specific impl (case A),
 400 /// or it may be relative to some bound that is in scope (case B).
 401 ///
 402 /// ```
 403 /// impl<T:Clone> Clone<T> for Option<T> { ... } // Impl_1
 404 /// impl<T:Clone> Clone<T> for Box<T> { ... }    // Impl_2
 405 /// impl Clone for i32 { ... }                   // Impl_3
 406 ///
 407 /// fn foo<T: Clone>(concrete: Option<Box<i32>>, param: T, mixed: Option<T>) {
 408 ///     // Case A: Vtable points at a specific impl. Only possible when
 409 ///     // type is concretely known. If the impl itself has bounded
 410 ///     // type parameters, Vtable will carry resolutions for those as well:
 411 ///     concrete.clone(); // Vtable(Impl_1, [Vtable(Impl_2, [Vtable(Impl_3)])])
 412 ///
 413 ///     // Case A: ImplSource points at a specific impl. Only possible when
 414 ///     // type is concretely known. If the impl itself has bounded
 415 ///     // type parameters, ImplSource will carry resolutions for those as well:
 416 ///     concrete.clone(); // ImplSource(Impl_1, [ImplSource(Impl_2, [ImplSource(Impl_3)])])
 417 ///
 418 ///     // Case B: ImplSource must be provided by caller. This applies when
 419 ///     // type is a type parameter.
 420 ///     param.clone();    // ImplSourceParam
 421 ///
 422 ///     // Case C: A mix of cases A and B.
 423 ///     mixed.clone();    // ImplSource(Impl_1, [ImplSourceParam])
 424 /// }
 425 /// ```
 426 ///
 427 /// ### The type parameter `N`
 428 ///
 429 /// See explanation on `ImplSourceUserDefinedData`.
 430 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 431 pub enum ImplSource<'tcx, N> {
 432     /// ImplSource identifying a particular impl.
 433     ImplSourceUserDefined(ImplSourceUserDefinedData<'tcx, N>),
 434
 435     /// ImplSource for auto trait implementations.
 436     /// This carries the information and nested obligations with regards
 437     /// to an auto implementation for a trait `Trait`. The nested obligations
 438     /// ensure the trait implementation holds for all the constituent types.
 439     ImplSourceAutoImpl(ImplSourceAutoImplData<N>),
 440
 441     /// Successful resolution to an obligation provided by the caller
 442     /// for some type parameter. The `Vec<N>` represents the
 443     /// obligations incurred from normalizing the where-clause (if
 444     /// any).
 445     ImplSourceParam(Vec<N>),
 446
 447     /// Virtual calls through an object.
 448     ImplSourceObject(ImplSourceObjectData<'tcx, N>),
 449
 450     /// Successful resolution for a builtin trait.
 451     ImplSourceBuiltin(ImplSourceBuiltinData<N>),
 452
 453     /// ImplSource automatically generated for a closure. The `DefId` is the ID
 454     /// of the closure expression. This is a `ImplSourceUserDefined` in spirit, but the
 455     /// impl is generated by the compiler and does not appear in the source.
 456     ImplSourceClosure(ImplSourceClosureData<'tcx, N>),
 457
 458     /// Same as above, but for a function pointer type with the given signature.
 459     ImplSourceFnPointer(ImplSourceFnPointerData<'tcx, N>),
 460
 461     /// ImplSource for a builtin `DeterminantKind` trait implementation.
 462     ImplSourceDiscriminantKind(ImplSourceDiscriminantKindData),
 463
 464     /// ImplSource automatically generated for a generator.
 465     ImplSourceGenerator(ImplSourceGeneratorData<'tcx, N>),
 466
 467     /// ImplSource for a trait alias.
 468     ImplSourceTraitAlias(ImplSourceTraitAliasData<'tcx, N>),
 469 }
 470
 471 impl<'tcx, N> ImplSource<'tcx, N> {
 472     pub fn nested_obligations(self) -> Vec<N> {
 473         match self {
 474             ImplSourceUserDefined(i) => i.nested,
 475             ImplSourceParam(n) => n,
 476             ImplSourceBuiltin(i) => i.nested,
 477             ImplSourceAutoImpl(d) => d.nested,
 478             ImplSourceClosure(c) => c.nested,
 479             ImplSourceGenerator(c) => c.nested,
 480             ImplSourceObject(d) => d.nested,
 481             ImplSourceFnPointer(d) => d.nested,
 482             ImplSourceDiscriminantKind(ImplSourceDiscriminantKindData) => Vec::new(),
 483             ImplSourceTraitAlias(d) => d.nested,
 484         }
 485     }
 486
 487     pub fn borrow_nested_obligations(&self) -> &[N] {
 488         match &self {
 489             ImplSourceUserDefined(i) => &i.nested[..],
 490             ImplSourceParam(n) => &n[..],
 491             ImplSourceBuiltin(i) => &i.nested[..],
 492             ImplSourceAutoImpl(d) => &d.nested[..],
 493             ImplSourceClosure(c) => &c.nested[..],
 494             ImplSourceGenerator(c) => &c.nested[..],
 495             ImplSourceObject(d) => &d.nested[..],
 496             ImplSourceFnPointer(d) => &d.nested[..],
 497             ImplSourceDiscriminantKind(ImplSourceDiscriminantKindData) => &[],
 498             ImplSourceTraitAlias(d) => &d.nested[..],
 499         }
 500     }
 501
 502     pub fn map<M, F>(self, f: F) -> ImplSource<'tcx, M>
 503     where
 504         F: FnMut(N) -> M,
 505     {
 506         match self {
 507             ImplSourceUserDefined(i) => ImplSourceUserDefined(ImplSourceUserDefinedData {
 508                 impl_def_id: i.impl_def_id,
 509                 substs: i.substs,
 510                 nested: i.nested.into_iter().map(f).collect(),
 511             }),
 512             ImplSourceParam(n) => ImplSourceParam(n.into_iter().map(f).collect()),
 513             ImplSourceBuiltin(i) => ImplSourceBuiltin(ImplSourceBuiltinData {
 514                 nested: i.nested.into_iter().map(f).collect(),
 515             }),
 516             ImplSourceObject(o) => ImplSourceObject(ImplSourceObjectData {
 517                 upcast_trait_ref: o.upcast_trait_ref,
 518                 vtable_base: o.vtable_base,
 519                 nested: o.nested.into_iter().map(f).collect(),
 520             }),
 521             ImplSourceAutoImpl(d) => ImplSourceAutoImpl(ImplSourceAutoImplData {
 522                 trait_def_id: d.trait_def_id,
 523                 nested: d.nested.into_iter().map(f).collect(),
 524             }),
 525             ImplSourceClosure(c) => ImplSourceClosure(ImplSourceClosureData {
 526                 closure_def_id: c.closure_def_id,
 527                 substs: c.substs,
 528                 nested: c.nested.into_iter().map(f).collect(),
 529             }),
 530             ImplSourceGenerator(c) => ImplSourceGenerator(ImplSourceGeneratorData {
 531                 generator_def_id: c.generator_def_id,
 532                 substs: c.substs,
 533                 nested: c.nested.into_iter().map(f).collect(),
 534             }),
 535             ImplSourceFnPointer(p) => ImplSourceFnPointer(ImplSourceFnPointerData {
 536                 fn_ty: p.fn_ty,
 537                 nested: p.nested.into_iter().map(f).collect(),
 538             }),
 539             ImplSourceDiscriminantKind(ImplSourceDiscriminantKindData) => {
 540                 ImplSourceDiscriminantKind(ImplSourceDiscriminantKindData)
 541             }
 542             ImplSourceTraitAlias(d) => ImplSourceTraitAlias(ImplSourceTraitAliasData {
 543                 alias_def_id: d.alias_def_id,
 544                 substs: d.substs,
 545                 nested: d.nested.into_iter().map(f).collect(),
 546             }),
 547         }
 548     }
 549 }
 550
 551 /// Identifies a particular impl in the source, along with a set of
 552 /// substitutions from the impl's type/lifetime parameters. The
 553 /// `nested` vector corresponds to the nested obligations attached to
 554 /// the impl's type parameters.
 555 ///
 556 /// The type parameter `N` indicates the type used for "nested
 557 /// obligations" that are required by the impl. During type-check, this
 558 /// is `Obligation`, as one might expect. During codegen, however, this
 559 /// is `()`, because codegen only requires a shallow resolution of an
 560 /// impl, and nested obligations are satisfied later.
 561 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 562 pub struct ImplSourceUserDefinedData<'tcx, N> {
 563     pub impl_def_id: DefId,
 564     pub substs: SubstsRef<'tcx>,
 565     pub nested: Vec<N>,
 566 }
 567
 568 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 569 pub struct ImplSourceGeneratorData<'tcx, N> {
 570     pub generator_def_id: DefId,
 571     pub substs: SubstsRef<'tcx>,
 572     /// Nested obligations. This can be non-empty if the generator
 573     /// signature contains associated types.
 574     pub nested: Vec<N>,
 575 }
 576
 577 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 578 pub struct ImplSourceClosureData<'tcx, N> {
 579     pub closure_def_id: DefId,
 580     pub substs: SubstsRef<'tcx>,
 581     /// Nested obligations. This can be non-empty if the closure
 582     /// signature contains associated types.
 583     pub nested: Vec<N>,
 584 }
 585
 586 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 587 pub struct ImplSourceAutoImplData<N> {
 588     pub trait_def_id: DefId,
 589     pub nested: Vec<N>,
 590 }
 591
 592 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 593 pub struct ImplSourceBuiltinData<N> {
 594     pub nested: Vec<N>,
 595 }
 596
 597 #[derive(PartialEq, Eq, Clone, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 598 pub struct ImplSourceObjectData<'tcx, N> {
 599     /// `Foo` upcast to the obligation trait. This will be some supertrait of `Foo`.
 600     pub upcast_trait_ref: ty::PolyTraitRef<'tcx>,
 601
 602     /// The vtable is formed by concatenating together the method lists of
 603     /// the base object trait and all supertraits; this is the start of
 604     /// `upcast_trait_ref`'s methods in that vtable.
 605     pub vtable_base: usize,
 606
 607     pub nested: Vec<N>,
 608 }
 609
 610 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 611 pub struct ImplSourceFnPointerData<'tcx, N> {
 612     pub fn_ty: Ty<'tcx>,
 613     pub nested: Vec<N>,
 614 }
 615
 616 // FIXME(@lcnr): This should be  refactored and merged with other builtin vtables.
 617 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 618 pub struct ImplSourceDiscriminantKindData;
 619
 620 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 621 pub struct ImplSourceTraitAliasData<'tcx, N> {
 622     pub alias_def_id: DefId,
 623     pub substs: SubstsRef<'tcx>,
 624     pub nested: Vec<N>,
 625 }
 626
 627 #[derive(Clone, Debug, PartialEq, Eq, Hash, HashStable)]
 628 pub enum ObjectSafetyViolation {
 629     /// `Self: Sized` declared on the trait.
 630     SizedSelf(SmallVec<[Span; 1]>),
 631
 632     /// Supertrait reference references `Self` an in illegal location
 633     /// (e.g., `trait Foo : Bar<Self>`).
 634     SupertraitSelf(SmallVec<[Span; 1]>),
 635
 636     /// Method has something illegal.
 637     Method(Symbol, MethodViolationCode, Span),
 638
 639     /// Associated const.
 640     AssocConst(Symbol, Span),
 641 }
 642
 643 impl ObjectSafetyViolation {
 644     pub fn error_msg(&self) -> Cow<'static, str> {
 645         match *self {
 646             ObjectSafetyViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
 647             ObjectSafetyViolation::SupertraitSelf(ref spans) => {
 648                 if spans.iter().any(|sp| *sp != DUMMY_SP) {
 649                     "it uses `Self` as a type parameter in this".into()
 650                 } else {
 651                     "it cannot use `Self` as a type parameter in a supertrait or `where`-clause"
 652                         .into()
 653                 }
 654             }
 655             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
 656                 format!("associated function `{}` has no `self` parameter", name).into()
 657             }
 658             ObjectSafetyViolation::Method(
 659                 name,
 660                 MethodViolationCode::ReferencesSelfInput(_),
 661                 DUMMY_SP,
 662             ) => format!("method `{}` references the `Self` type in its parameters", name).into(),
 663             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfInput(_), _) => {
 664                 format!("method `{}` references the `Self` type in this parameter", name).into()
 665             }
 666             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
 667                 format!("method `{}` references the `Self` type in its return type", name).into()
 668             }
 669             ObjectSafetyViolation::Method(
 670                 name,
 671                 MethodViolationCode::WhereClauseReferencesSelf,
 672                 _,
 673             ) => {
 674                 format!("method `{}` references the `Self` type in its `where` clause", name).into()
 675             }
 676             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) => {
 677                 format!("method `{}` has generic type parameters", name).into()
 678             }
 679             ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) => {
 680                 format!("method `{}`'s `self` parameter cannot be dispatched on", name).into()
 681             }
 682             ObjectSafetyViolation::AssocConst(name, DUMMY_SP) => {
 683                 format!("it contains associated `const` `{}`", name).into()
 684             }
 685             ObjectSafetyViolation::AssocConst(..) => "it contains this associated `const`".into(),
 686         }
 687     }
 688
 689     pub fn solution(&self) -> Option<(String, Option<(String, Span)>)> {
 690         Some(match *self {
 691             ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf(_) => {
 692                 return None;
 693             }
 694             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(sugg), _) => (
 695                 format!(
 696                     "consider turning `{}` into a method by giving it a `&self` argument or \
 697                      constraining it so it does not apply to trait objects",
 698                     name
 699                 ),
 700                 sugg.map(|(sugg, sp)| (sugg.to_string(), sp)),
 701             ),
 702             ObjectSafetyViolation::Method(
 703                 name,
 704                 MethodViolationCode::UndispatchableReceiver,
 705                 span,
 706             ) => (
 707                 format!("consider changing method `{}`'s `self` parameter to be `&self`", name),
 708                 Some(("&Self".to_string(), span)),
 709             ),
 710             ObjectSafetyViolation::AssocConst(name, _)
 711             | ObjectSafetyViolation::Method(name, ..) => {
 712                 (format!("consider moving `{}` to another trait", name), None)
 713             }
 714         })
 715     }
 716
 717     pub fn spans(&self) -> SmallVec<[Span; 1]> {
 718         // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
 719         // diagnostics use a `note` instead of a `span_label`.
 720         match self {
 721             ObjectSafetyViolation::SupertraitSelf(spans)
 722             | ObjectSafetyViolation::SizedSelf(spans) => spans.clone(),
 723             ObjectSafetyViolation::AssocConst(_, span)
 724             | ObjectSafetyViolation::Method(_, _, span)
 725                 if *span != DUMMY_SP =>
 726             {
 727                 smallvec![*span]
 728             }
 729             _ => smallvec![],
 730         }
 731     }
 732 }
 733
 734 /// Reasons a method might not be object-safe.
 735 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
 736 pub enum MethodViolationCode {
 737     /// e.g., `fn foo()`
 738     StaticMethod(Option<(&'static str, Span)>),
 739
 740     /// e.g., `fn foo(&self, x: Self)`
 741     ReferencesSelfInput(usize),
 742
 743     /// e.g., `fn foo(&self) -> Self`
 744     ReferencesSelfOutput,
 745
 746     /// e.g., `fn foo(&self) where Self: Clone`
 747     WhereClauseReferencesSelf,
 748
 749     /// e.g., `fn foo<A>()`
 750     Generic,
 751
 752     /// the method's receiver (`self` argument) can't be dispatched on
 753     UndispatchableReceiver,
 754 }