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 pub mod query;
   6 pub mod select;
   7 pub mod specialization_graph;
   8 mod structural_impls;
   9
  10 use crate::mir::interpret::ErrorHandled;
  11 use crate::ty::subst::SubstsRef;
  12 use crate::ty::{self, AdtKind, List, Ty, TyCtxt};
  13
  14 use rustc_ast::ast;
  15 use rustc_hir as hir;
  16 use rustc_hir::def_id::DefId;
  17 use rustc_span::{Span, DUMMY_SP};
  18 use smallvec::SmallVec;
  19
  20 use std::borrow::Cow;
  21 use std::fmt::Debug;
  22 use std::rc::Rc;
  23
  24 pub use self::select::{EvaluationCache, EvaluationResult, OverflowError, SelectionCache};
  25
  26 pub use self::ObligationCauseCode::*;
  27 pub use self::SelectionError::*;
  28 pub use self::Vtable::*;
  29
  30 /// Depending on the stage of compilation, we want projection to be
  31 /// more or less conservative.
  32 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
  33 pub enum Reveal {
  34     /// At type-checking time, we refuse to project any associated
  35     /// type that is marked `default`. Non-`default` ("final") types
  36     /// are always projected. This is necessary in general for
  37     /// soundness of specialization. However, we *could* allow
  38     /// projections in fully-monomorphic cases. We choose not to,
  39     /// because we prefer for `default type` to force the type
  40     /// definition to be treated abstractly by any consumers of the
  41     /// impl. Concretely, that means that the following example will
  42     /// fail to compile:
  43     ///
  44     /// ```
  45     /// trait Assoc {
  46     ///     type Output;
  47     /// }
  48     ///
  49     /// impl<T> Assoc for T {
  50     ///     default type Output = bool;
  51     /// }
  52     ///
  53     /// fn main() {
  54     ///     let <() as Assoc>::Output = true;
  55     /// }
  56     /// ```
  57     UserFacing,
  58
  59     /// At codegen time, all monomorphic projections will succeed.
  60     /// Also, `impl Trait` is normalized to the concrete type,
  61     /// which has to be already collected by type-checking.
  62     ///
  63     /// NOTE: as `impl Trait`'s concrete type should *never*
  64     /// be observable directly by the user, `Reveal::All`
  65     /// should not be used by checks which may expose
  66     /// type equality or type contents to the user.
  67     /// There are some exceptions, e.g., around OIBITS and
  68     /// transmute-checking, which expose some details, but
  69     /// not the whole concrete type of the `impl Trait`.
  70     All,
  71 }
  72
  73 /// The reason why we incurred this obligation; used for error reporting.
  74 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
  75 pub struct ObligationCause<'tcx> {
  76     pub span: Span,
  77
  78     /// The ID of the fn body that triggered this obligation. This is
  79     /// used for region obligations to determine the precise
  80     /// environment in which the region obligation should be evaluated
  81     /// (in particular, closures can add new assumptions). See the
  82     /// field `region_obligations` of the `FulfillmentContext` for more
  83     /// information.
  84     pub body_id: hir::HirId,
  85
  86     pub code: ObligationCauseCode<'tcx>,
  87 }
  88
  89 impl<'tcx> ObligationCause<'tcx> {
  90     #[inline]
  91     pub fn new(
  92         span: Span,
  93         body_id: hir::HirId,
  94         code: ObligationCauseCode<'tcx>,
  95     ) -> ObligationCause<'tcx> {
  96         ObligationCause { span, body_id, code }
  97     }
  98
  99     pub fn misc(span: Span, body_id: hir::HirId) -> ObligationCause<'tcx> {
 100         ObligationCause { span, body_id, code: MiscObligation }
 101     }
 102
 103     pub fn dummy() -> ObligationCause<'tcx> {
 104         ObligationCause { span: DUMMY_SP, body_id: hir::CRATE_HIR_ID, code: MiscObligation }
 105     }
 106
 107     pub fn span(&self, tcx: TyCtxt<'tcx>) -> Span {
 108         match self.code {
 109             ObligationCauseCode::CompareImplMethodObligation { .. }
 110             | ObligationCauseCode::MainFunctionType
 111             | ObligationCauseCode::StartFunctionType => {
 112                 tcx.sess.source_map().guess_head_span(self.span)
 113             }
 114             ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
 115                 arm_span,
 116                 ..
 117             }) => arm_span,
 118             _ => self.span,
 119         }
 120     }
 121 }
 122
 123 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 124 pub enum ObligationCauseCode<'tcx> {
 125     /// Not well classified or should be obvious from the span.
 126     MiscObligation,
 127
 128     /// A slice or array is WF only if `T: Sized`.
 129     SliceOrArrayElem,
 130
 131     /// A tuple is WF only if its middle elements are `Sized`.
 132     TupleElem,
 133
 134     /// This is the trait reference from the given projection.
 135     ProjectionWf(ty::ProjectionTy<'tcx>),
 136
 137     /// In an impl of trait `X` for type `Y`, type `Y` must
 138     /// also implement all supertraits of `X`.
 139     ItemObligation(DefId),
 140
 141     /// Like `ItemObligation`, but with extra detail on the source of the obligation.
 142     BindingObligation(DefId, Span),
 143
 144     /// A type like `&'a T` is WF only if `T: 'a`.
 145     ReferenceOutlivesReferent(Ty<'tcx>),
 146
 147     /// A type like `Box<Foo<'a> + 'b>` is WF only if `'b: 'a`.
 148     ObjectTypeBound(Ty<'tcx>, ty::Region<'tcx>),
 149
 150     /// Obligation incurred due to an object cast.
 151     ObjectCastObligation(/* Object type */ Ty<'tcx>),
 152
 153     /// Obligation incurred due to a coercion.
 154     Coercion {
 155         source: Ty<'tcx>,
 156         target: Ty<'tcx>,
 157     },
 158
 159     /// Various cases where expressions must be `Sized` / `Copy` / etc.
 160     /// `L = X` implies that `L` is `Sized`.
 161     AssignmentLhsSized,
 162     /// `(x1, .., xn)` must be `Sized`.
 163     TupleInitializerSized,
 164     /// `S { ... }` must be `Sized`.
 165     StructInitializerSized,
 166     /// Type of each variable must be `Sized`.
 167     VariableType(hir::HirId),
 168     /// Argument type must be `Sized`.
 169     SizedArgumentType,
 170     /// Return type must be `Sized`.
 171     SizedReturnType,
 172     /// Yield type must be `Sized`.
 173     SizedYieldType,
 174     /// `[T, ..n]` implies that `T` must be `Copy`.
 175     /// If `true`, suggest `const_in_array_repeat_expressions` feature flag.
 176     RepeatVec(bool),
 177
 178     /// Types of fields (other than the last, except for packed structs) in a struct must be sized.
 179     FieldSized {
 180         adt_kind: AdtKind,
 181         last: bool,
 182     },
 183
 184     /// Constant expressions must be sized.
 185     ConstSized,
 186
 187     /// `static` items must have `Sync` type.
 188     SharedStatic,
 189
 190     BuiltinDerivedObligation(DerivedObligationCause<'tcx>),
 191
 192     ImplDerivedObligation(DerivedObligationCause<'tcx>),
 193
 194     DerivedObligation(DerivedObligationCause<'tcx>),
 195
 196     /// Error derived when matching traits/impls; see ObligationCause for more details
 197     CompareImplMethodObligation {
 198         item_name: ast::Name,
 199         impl_item_def_id: DefId,
 200         trait_item_def_id: DefId,
 201     },
 202
 203     /// Error derived when matching traits/impls; see ObligationCause for more details
 204     CompareImplTypeObligation {
 205         item_name: ast::Name,
 206         impl_item_def_id: DefId,
 207         trait_item_def_id: DefId,
 208     },
 209
 210     /// Checking that this expression can be assigned where it needs to be
 211     // FIXME(eddyb) #11161 is the original Expr required?
 212     ExprAssignable,
 213
 214     /// Computing common supertype in the arms of a match expression
 215     MatchExpressionArm(Box<MatchExpressionArmCause<'tcx>>),
 216
 217     /// Type error arising from type checking a pattern against an expected type.
 218     Pattern {
 219         /// The span of the scrutinee or type expression which caused the `root_ty` type.
 220         span: Option<Span>,
 221         /// The root expected type induced by a scrutinee or type expression.
 222         root_ty: Ty<'tcx>,
 223         /// Whether the `Span` came from an expression or a type expression.
 224         origin_expr: bool,
 225     },
 226
 227     /// Constants in patterns must have `Structural` type.
 228     ConstPatternStructural,
 229
 230     /// Computing common supertype in an if expression
 231     IfExpression(Box<IfExpressionCause>),
 232
 233     /// Computing common supertype of an if expression with no else counter-part
 234     IfExpressionWithNoElse,
 235
 236     /// `main` has wrong type
 237     MainFunctionType,
 238
 239     /// `start` has wrong type
 240     StartFunctionType,
 241
 242     /// Intrinsic has wrong type
 243     IntrinsicType,
 244
 245     /// Method receiver
 246     MethodReceiver,
 247
 248     /// `return` with no expression
 249     ReturnNoExpression,
 250
 251     /// `return` with an expression
 252     ReturnValue(hir::HirId),
 253
 254     /// Return type of this function
 255     ReturnType,
 256
 257     /// Block implicit return
 258     BlockTailExpression(hir::HirId),
 259
 260     /// #[feature(trivial_bounds)] is not enabled
 261     TrivialBound,
 262 }
 263
 264 impl ObligationCauseCode<'_> {
 265     // Return the base obligation, ignoring derived obligations.
 266     pub fn peel_derives(&self) -> &Self {
 267         let mut base_cause = self;
 268         while let BuiltinDerivedObligation(cause)
 269         | ImplDerivedObligation(cause)
 270         | DerivedObligation(cause) = base_cause
 271         {
 272             base_cause = &cause.parent_code;
 273         }
 274         base_cause
 275     }
 276 }
 277
 278 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
 279 #[cfg(target_arch = "x86_64")]
 280 static_assert_size!(ObligationCauseCode<'_>, 32);
 281
 282 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 283 pub struct MatchExpressionArmCause<'tcx> {
 284     pub arm_span: Span,
 285     pub source: hir::MatchSource,
 286     pub prior_arms: Vec<Span>,
 287     pub last_ty: Ty<'tcx>,
 288     pub scrut_hir_id: hir::HirId,
 289 }
 290
 291 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 292 pub struct IfExpressionCause {
 293     pub then: Span,
 294     pub outer: Option<Span>,
 295     pub semicolon: Option<Span>,
 296 }
 297
 298 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 299 pub struct DerivedObligationCause<'tcx> {
 300     /// The trait reference of the parent obligation that led to the
 301     /// current obligation. Note that only trait obligations lead to
 302     /// derived obligations, so we just store the trait reference here
 303     /// directly.
 304     pub parent_trait_ref: ty::PolyTraitRef<'tcx>,
 305
 306     /// The parent trait had this cause.
 307     pub parent_code: Rc<ObligationCauseCode<'tcx>>,
 308 }
 309
 310 /// The following types:
 311 /// * `WhereClause`,
 312 /// * `WellFormed`,
 313 /// * `FromEnv`,
 314 /// * `DomainGoal`,
 315 /// * `Goal`,
 316 /// * `Clause`,
 317 /// * `Environment`,
 318 /// * `InEnvironment`,
 319 /// are used for representing the trait system in the form of
 320 /// logic programming clauses. They are part of the interface
 321 /// for the chalk SLG solver.
 322 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable, Lift)]
 323 pub enum WhereClause<'tcx> {
 324     Implemented(ty::TraitPredicate<'tcx>),
 325     ProjectionEq(ty::ProjectionPredicate<'tcx>),
 326     RegionOutlives(ty::RegionOutlivesPredicate<'tcx>),
 327     TypeOutlives(ty::TypeOutlivesPredicate<'tcx>),
 328 }
 329
 330 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable, Lift)]
 331 pub enum WellFormed<'tcx> {
 332     Trait(ty::TraitPredicate<'tcx>),
 333     Ty(Ty<'tcx>),
 334 }
 335
 336 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable, Lift)]
 337 pub enum FromEnv<'tcx> {
 338     Trait(ty::TraitPredicate<'tcx>),
 339     Ty(Ty<'tcx>),
 340 }
 341
 342 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable, Lift)]
 343 pub enum DomainGoal<'tcx> {
 344     Holds(WhereClause<'tcx>),
 345     WellFormed(WellFormed<'tcx>),
 346     FromEnv(FromEnv<'tcx>),
 347     Normalize(ty::ProjectionPredicate<'tcx>),
 348 }
 349
 350 pub type PolyDomainGoal<'tcx> = ty::Binder<DomainGoal<'tcx>>;
 351
 352 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable)]
 353 pub enum QuantifierKind {
 354     Universal,
 355     Existential,
 356 }
 357
 358 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable, Lift)]
 359 pub enum GoalKind<'tcx> {
 360     Implies(Clauses<'tcx>, Goal<'tcx>),
 361     And(Goal<'tcx>, Goal<'tcx>),
 362     Not(Goal<'tcx>),
 363     DomainGoal(DomainGoal<'tcx>),
 364     Quantified(QuantifierKind, ty::Binder<Goal<'tcx>>),
 365     Subtype(Ty<'tcx>, Ty<'tcx>),
 366     CannotProve,
 367 }
 368
 369 pub type Goal<'tcx> = &'tcx GoalKind<'tcx>;
 370
 371 pub type Goals<'tcx> = &'tcx List<Goal<'tcx>>;
 372
 373 impl<'tcx> DomainGoal<'tcx> {
 374     pub fn into_goal(self) -> GoalKind<'tcx> {
 375         GoalKind::DomainGoal(self)
 376     }
 377
 378     pub fn into_program_clause(self) -> ProgramClause<'tcx> {
 379         ProgramClause {
 380             goal: self,
 381             hypotheses: ty::List::empty(),
 382             category: ProgramClauseCategory::Other,
 383         }
 384     }
 385 }
 386
 387 impl<'tcx> GoalKind<'tcx> {
 388     pub fn from_poly_domain_goal(
 389         domain_goal: PolyDomainGoal<'tcx>,
 390         tcx: TyCtxt<'tcx>,
 391     ) -> GoalKind<'tcx> {
 392         match domain_goal.no_bound_vars() {
 393             Some(p) => p.into_goal(),
 394             None => GoalKind::Quantified(
 395                 QuantifierKind::Universal,
 396                 domain_goal.map_bound(|p| tcx.mk_goal(p.into_goal())),
 397             ),
 398         }
 399     }
 400 }
 401
 402 /// This matches the definition from Page 7 of "A Proof Procedure for the Logic of Hereditary
 403 /// Harrop Formulas".
 404 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable)]
 405 pub enum Clause<'tcx> {
 406     Implies(ProgramClause<'tcx>),
 407     ForAll(ty::Binder<ProgramClause<'tcx>>),
 408 }
 409
 410 impl Clause<'tcx> {
 411     pub fn category(self) -> ProgramClauseCategory {
 412         match self {
 413             Clause::Implies(clause) => clause.category,
 414             Clause::ForAll(clause) => clause.skip_binder().category,
 415         }
 416     }
 417 }
 418
 419 /// Multiple clauses.
 420 pub type Clauses<'tcx> = &'tcx List<Clause<'tcx>>;
 421
 422 /// A "program clause" has the form `D :- G1, ..., Gn`. It is saying
 423 /// that the domain goal `D` is true if `G1...Gn` are provable. This
 424 /// is equivalent to the implication `G1..Gn => D`; we usually write
 425 /// it with the reverse implication operator `:-` to emphasize the way
 426 /// that programs are actually solved (via backchaining, which starts
 427 /// with the goal to solve and proceeds from there).
 428 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable)]
 429 pub struct ProgramClause<'tcx> {
 430     /// This goal will be considered true ...
 431     pub goal: DomainGoal<'tcx>,
 432
 433     /// ... if we can prove these hypotheses (there may be no hypotheses at all):
 434     pub hypotheses: Goals<'tcx>,
 435
 436     /// Useful for filtering clauses.
 437     pub category: ProgramClauseCategory,
 438 }
 439
 440 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable)]
 441 pub enum ProgramClauseCategory {
 442     ImpliedBound,
 443     WellFormed,
 444     Other,
 445 }
 446
 447 /// A set of clauses that we assume to be true.
 448 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable)]
 449 pub struct Environment<'tcx> {
 450     pub clauses: Clauses<'tcx>,
 451 }
 452
 453 impl Environment<'tcx> {
 454     pub fn with<G>(self, goal: G) -> InEnvironment<'tcx, G> {
 455         InEnvironment { environment: self, goal }
 456     }
 457 }
 458
 459 /// Something (usually a goal), along with an environment.
 460 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, HashStable, TypeFoldable)]
 461 pub struct InEnvironment<'tcx, G> {
 462     pub environment: Environment<'tcx>,
 463     pub goal: G,
 464 }
 465
 466 #[derive(Clone, Debug, TypeFoldable)]
 467 pub enum SelectionError<'tcx> {
 468     Unimplemented,
 469     OutputTypeParameterMismatch(
 470         ty::PolyTraitRef<'tcx>,
 471         ty::PolyTraitRef<'tcx>,
 472         ty::error::TypeError<'tcx>,
 473     ),
 474     TraitNotObjectSafe(DefId),
 475     ConstEvalFailure(ErrorHandled),
 476     Overflow,
 477 }
 478
 479 /// When performing resolution, it is typically the case that there
 480 /// can be one of three outcomes:
 481 ///
 482 /// - `Ok(Some(r))`: success occurred with result `r`
 483 /// - `Ok(None)`: could not definitely determine anything, usually due
 484 ///   to inconclusive type inference.
 485 /// - `Err(e)`: error `e` occurred
 486 pub type SelectionResult<'tcx, T> = Result<Option<T>, SelectionError<'tcx>>;
 487
 488 /// Given the successful resolution of an obligation, the `Vtable`
 489 /// indicates where the vtable comes from. Note that while we call this
 490 /// a "vtable", it does not necessarily indicate dynamic dispatch at
 491 /// runtime. `Vtable` instances just tell the compiler where to find
 492 /// methods, but in generic code those methods are typically statically
 493 /// dispatched -- only when an object is constructed is a `Vtable`
 494 /// instance reified into an actual vtable.
 495 ///
 496 /// For example, the vtable may be tied to a specific impl (case A),
 497 /// or it may be relative to some bound that is in scope (case B).
 498 ///
 499 /// ```
 500 /// impl<T:Clone> Clone<T> for Option<T> { ... } // Impl_1
 501 /// impl<T:Clone> Clone<T> for Box<T> { ... }    // Impl_2
 502 /// impl Clone for int { ... }             // Impl_3
 503 ///
 504 /// fn foo<T:Clone>(concrete: Option<Box<int>>,
 505 ///                 param: T,
 506 ///                 mixed: Option<T>) {
 507 ///
 508 ///    // Case A: Vtable points at a specific impl. Only possible when
 509 ///    // type is concretely known. If the impl itself has bounded
 510 ///    // type parameters, Vtable will carry resolutions for those as well:
 511 ///    concrete.clone(); // Vtable(Impl_1, [Vtable(Impl_2, [Vtable(Impl_3)])])
 512 ///
 513 ///    // Case B: Vtable must be provided by caller. This applies when
 514 ///    // type is a type parameter.
 515 ///    param.clone();    // VtableParam
 516 ///
 517 ///    // Case C: A mix of cases A and B.
 518 ///    mixed.clone();    // Vtable(Impl_1, [VtableParam])
 519 /// }
 520 /// ```
 521 ///
 522 /// ### The type parameter `N`
 523 ///
 524 /// See explanation on `VtableImplData`.
 525 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 526 pub enum Vtable<'tcx, N> {
 527     /// Vtable identifying a particular impl.
 528     VtableImpl(VtableImplData<'tcx, N>),
 529
 530     /// Vtable for auto trait implementations.
 531     /// This carries the information and nested obligations with regards
 532     /// to an auto implementation for a trait `Trait`. The nested obligations
 533     /// ensure the trait implementation holds for all the constituent types.
 534     VtableAutoImpl(VtableAutoImplData<N>),
 535
 536     /// Successful resolution to an obligation provided by the caller
 537     /// for some type parameter. The `Vec<N>` represents the
 538     /// obligations incurred from normalizing the where-clause (if
 539     /// any).
 540     VtableParam(Vec<N>),
 541
 542     /// Virtual calls through an object.
 543     VtableObject(VtableObjectData<'tcx, N>),
 544
 545     /// Successful resolution for a builtin trait.
 546     VtableBuiltin(VtableBuiltinData<N>),
 547
 548     /// Vtable automatically generated for a closure. The `DefId` is the ID
 549     /// of the closure expression. This is a `VtableImpl` in spirit, but the
 550     /// impl is generated by the compiler and does not appear in the source.
 551     VtableClosure(VtableClosureData<'tcx, N>),
 552
 553     /// Same as above, but for a function pointer type with the given signature.
 554     VtableFnPointer(VtableFnPointerData<'tcx, N>),
 555
 556     /// Vtable automatically generated for a generator.
 557     VtableGenerator(VtableGeneratorData<'tcx, N>),
 558
 559     /// Vtable for a trait alias.
 560     VtableTraitAlias(VtableTraitAliasData<'tcx, N>),
 561 }
 562
 563 impl<'tcx, N> Vtable<'tcx, N> {
 564     pub fn nested_obligations(self) -> Vec<N> {
 565         match self {
 566             VtableImpl(i) => i.nested,
 567             VtableParam(n) => n,
 568             VtableBuiltin(i) => i.nested,
 569             VtableAutoImpl(d) => d.nested,
 570             VtableClosure(c) => c.nested,
 571             VtableGenerator(c) => c.nested,
 572             VtableObject(d) => d.nested,
 573             VtableFnPointer(d) => d.nested,
 574             VtableTraitAlias(d) => d.nested,
 575         }
 576     }
 577
 578     pub fn borrow_nested_obligations(&self) -> &[N] {
 579         match &self {
 580             VtableImpl(i) => &i.nested[..],
 581             VtableParam(n) => &n[..],
 582             VtableBuiltin(i) => &i.nested[..],
 583             VtableAutoImpl(d) => &d.nested[..],
 584             VtableClosure(c) => &c.nested[..],
 585             VtableGenerator(c) => &c.nested[..],
 586             VtableObject(d) => &d.nested[..],
 587             VtableFnPointer(d) => &d.nested[..],
 588             VtableTraitAlias(d) => &d.nested[..],
 589         }
 590     }
 591
 592     pub fn map<M, F>(self, f: F) -> Vtable<'tcx, M>
 593     where
 594         F: FnMut(N) -> M,
 595     {
 596         match self {
 597             VtableImpl(i) => VtableImpl(VtableImplData {
 598                 impl_def_id: i.impl_def_id,
 599                 substs: i.substs,
 600                 nested: i.nested.into_iter().map(f).collect(),
 601             }),
 602             VtableParam(n) => VtableParam(n.into_iter().map(f).collect()),
 603             VtableBuiltin(i) => {
 604                 VtableBuiltin(VtableBuiltinData { nested: i.nested.into_iter().map(f).collect() })
 605             }
 606             VtableObject(o) => VtableObject(VtableObjectData {
 607                 upcast_trait_ref: o.upcast_trait_ref,
 608                 vtable_base: o.vtable_base,
 609                 nested: o.nested.into_iter().map(f).collect(),
 610             }),
 611             VtableAutoImpl(d) => VtableAutoImpl(VtableAutoImplData {
 612                 trait_def_id: d.trait_def_id,
 613                 nested: d.nested.into_iter().map(f).collect(),
 614             }),
 615             VtableClosure(c) => VtableClosure(VtableClosureData {
 616                 closure_def_id: c.closure_def_id,
 617                 substs: c.substs,
 618                 nested: c.nested.into_iter().map(f).collect(),
 619             }),
 620             VtableGenerator(c) => VtableGenerator(VtableGeneratorData {
 621                 generator_def_id: c.generator_def_id,
 622                 substs: c.substs,
 623                 nested: c.nested.into_iter().map(f).collect(),
 624             }),
 625             VtableFnPointer(p) => VtableFnPointer(VtableFnPointerData {
 626                 fn_ty: p.fn_ty,
 627                 nested: p.nested.into_iter().map(f).collect(),
 628             }),
 629             VtableTraitAlias(d) => VtableTraitAlias(VtableTraitAliasData {
 630                 alias_def_id: d.alias_def_id,
 631                 substs: d.substs,
 632                 nested: d.nested.into_iter().map(f).collect(),
 633             }),
 634         }
 635     }
 636 }
 637
 638 /// Identifies a particular impl in the source, along with a set of
 639 /// substitutions from the impl's type/lifetime parameters. The
 640 /// `nested` vector corresponds to the nested obligations attached to
 641 /// the impl's type parameters.
 642 ///
 643 /// The type parameter `N` indicates the type used for "nested
 644 /// obligations" that are required by the impl. During type-check, this
 645 /// is `Obligation`, as one might expect. During codegen, however, this
 646 /// is `()`, because codegen only requires a shallow resolution of an
 647 /// impl, and nested obligations are satisfied later.
 648 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 649 pub struct VtableImplData<'tcx, N> {
 650     pub impl_def_id: DefId,
 651     pub substs: SubstsRef<'tcx>,
 652     pub nested: Vec<N>,
 653 }
 654
 655 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 656 pub struct VtableGeneratorData<'tcx, N> {
 657     pub generator_def_id: DefId,
 658     pub substs: SubstsRef<'tcx>,
 659     /// Nested obligations. This can be non-empty if the generator
 660     /// signature contains associated types.
 661     pub nested: Vec<N>,
 662 }
 663
 664 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 665 pub struct VtableClosureData<'tcx, N> {
 666     pub closure_def_id: DefId,
 667     pub substs: SubstsRef<'tcx>,
 668     /// Nested obligations. This can be non-empty if the closure
 669     /// signature contains associated types.
 670     pub nested: Vec<N>,
 671 }
 672
 673 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 674 pub struct VtableAutoImplData<N> {
 675     pub trait_def_id: DefId,
 676     pub nested: Vec<N>,
 677 }
 678
 679 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 680 pub struct VtableBuiltinData<N> {
 681     pub nested: Vec<N>,
 682 }
 683
 684 /// A vtable for some object-safe trait `Foo` automatically derived
 685 /// for the object type `Foo`.
 686 #[derive(PartialEq, Eq, Clone, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 687 pub struct VtableObjectData<'tcx, N> {
 688     /// `Foo` upcast to the obligation trait. This will be some supertrait of `Foo`.
 689     pub upcast_trait_ref: ty::PolyTraitRef<'tcx>,
 690
 691     /// The vtable is formed by concatenating together the method lists of
 692     /// the base object trait and all supertraits; this is the start of
 693     /// `upcast_trait_ref`'s methods in that vtable.
 694     pub vtable_base: usize,
 695
 696     pub nested: Vec<N>,
 697 }
 698
 699 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 700 pub struct VtableFnPointerData<'tcx, N> {
 701     pub fn_ty: Ty<'tcx>,
 702     pub nested: Vec<N>,
 703 }
 704
 705 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 706 pub struct VtableTraitAliasData<'tcx, N> {
 707     pub alias_def_id: DefId,
 708     pub substs: SubstsRef<'tcx>,
 709     pub nested: Vec<N>,
 710 }
 711
 712 #[derive(Clone, Debug, PartialEq, Eq, Hash, HashStable)]
 713 pub enum ObjectSafetyViolation {
 714     /// `Self: Sized` declared on the trait.
 715     SizedSelf(SmallVec<[Span; 1]>),
 716
 717     /// Supertrait reference references `Self` an in illegal location
 718     /// (e.g., `trait Foo : Bar<Self>`).
 719     SupertraitSelf(SmallVec<[Span; 1]>),
 720
 721     /// Method has something illegal.
 722     Method(ast::Name, MethodViolationCode, Span),
 723
 724     /// Associated const.
 725     AssocConst(ast::Name, Span),
 726 }
 727
 728 impl ObjectSafetyViolation {
 729     pub fn error_msg(&self) -> Cow<'static, str> {
 730         match *self {
 731             ObjectSafetyViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
 732             ObjectSafetyViolation::SupertraitSelf(ref spans) => {
 733                 if spans.iter().any(|sp| *sp != DUMMY_SP) {
 734                     "it uses `Self` as a type parameter in this".into()
 735                 } else {
 736                     "it cannot use `Self` as a type parameter in a supertrait or `where`-clause"
 737                         .into()
 738                 }
 739             }
 740             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
 741                 format!("associated function `{}` has no `self` parameter", name).into()
 742             }
 743             ObjectSafetyViolation::Method(
 744                 name,
 745                 MethodViolationCode::ReferencesSelfInput(_),
 746                 DUMMY_SP,
 747             ) => format!("method `{}` references the `Self` type in its parameters", name).into(),
 748             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfInput(_), _) => {
 749                 format!("method `{}` references the `Self` type in this parameter", name).into()
 750             }
 751             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
 752                 format!("method `{}` references the `Self` type in its return type", name).into()
 753             }
 754             ObjectSafetyViolation::Method(
 755                 name,
 756                 MethodViolationCode::WhereClauseReferencesSelf,
 757                 _,
 758             ) => {
 759                 format!("method `{}` references the `Self` type in its `where` clause", name).into()
 760             }
 761             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) => {
 762                 format!("method `{}` has generic type parameters", name).into()
 763             }
 764             ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) => {
 765                 format!("method `{}`'s `self` parameter cannot be dispatched on", name).into()
 766             }
 767             ObjectSafetyViolation::AssocConst(name, DUMMY_SP) => {
 768                 format!("it contains associated `const` `{}`", name).into()
 769             }
 770             ObjectSafetyViolation::AssocConst(..) => "it contains this associated `const`".into(),
 771         }
 772     }
 773
 774     pub fn solution(&self) -> Option<(String, Option<(String, Span)>)> {
 775         Some(match *self {
 776             ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf(_) => {
 777                 return None;
 778             }
 779             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(sugg), _) => (
 780                 format!(
 781                     "consider turning `{}` into a method by giving it a `&self` argument or \
 782                      constraining it so it does not apply to trait objects",
 783                     name
 784                 ),
 785                 sugg.map(|(sugg, sp)| (sugg.to_string(), sp)),
 786             ),
 787             ObjectSafetyViolation::Method(
 788                 name,
 789                 MethodViolationCode::UndispatchableReceiver,
 790                 span,
 791             ) => (
 792                 format!("consider changing method `{}`'s `self` parameter to be `&self`", name),
 793                 Some(("&Self".to_string(), span)),
 794             ),
 795             ObjectSafetyViolation::AssocConst(name, _)
 796             | ObjectSafetyViolation::Method(name, ..) => {
 797                 (format!("consider moving `{}` to another trait", name), None)
 798             }
 799         })
 800     }
 801
 802     pub fn spans(&self) -> SmallVec<[Span; 1]> {
 803         // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
 804         // diagnostics use a `note` instead of a `span_label`.
 805         match self {
 806             ObjectSafetyViolation::SupertraitSelf(spans)
 807             | ObjectSafetyViolation::SizedSelf(spans) => spans.clone(),
 808             ObjectSafetyViolation::AssocConst(_, span)
 809             | ObjectSafetyViolation::Method(_, _, span)
 810                 if *span != DUMMY_SP =>
 811             {
 812                 smallvec![*span]
 813             }
 814             _ => smallvec![],
 815         }
 816     }
 817 }
 818
 819 /// Reasons a method might not be object-safe.
 820 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
 821 pub enum MethodViolationCode {
 822     /// e.g., `fn foo()`
 823     StaticMethod(Option<(&'static str, Span)>),
 824
 825     /// e.g., `fn foo(&self, x: Self)`
 826     ReferencesSelfInput(usize),
 827
 828     /// e.g., `fn foo(&self) -> Self`
 829     ReferencesSelfOutput,
 830
 831     /// e.g., `fn foo(&self) where Self: Clone`
 832     WhereClauseReferencesSelf,
 833
 834     /// e.g., `fn foo<A>()`
 835     Generic,
 836
 837     /// the method's receiver (`self` argument) can't be dispatched on
 838     UndispatchableReceiver,
 839 }