src/librustc/middle/traits/mod.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Trait Resolution. See the Book for more.
  12
  13 pub use self::SelectionError::*;
  14 pub use self::FulfillmentErrorCode::*;
  15 pub use self::Vtable::*;
  16 pub use self::ObligationCauseCode::*;
  17
  18 use middle::free_region::FreeRegionMap;
  19 use middle::subst;
  20 use middle::ty::{self, HasProjectionTypes, Ty};
  21 use middle::ty_fold::TypeFoldable;
  22 use middle::infer::{self, fixup_err_to_string, InferCtxt};
  23 use std::rc::Rc;
  24 use syntax::ast;
  25 use syntax::codemap::{Span, DUMMY_SP};
  26
  27 pub use self::error_reporting::report_fulfillment_errors;
  28 pub use self::error_reporting::report_overflow_error;
  29 pub use self::error_reporting::report_selection_error;
  30 pub use self::error_reporting::suggest_new_overflow_limit;
  31 pub use self::coherence::orphan_check;
  32 pub use self::coherence::overlapping_impls;
  33 pub use self::coherence::OrphanCheckErr;
  34 pub use self::fulfill::{FulfillmentContext, FulfilledPredicates, RegionObligation};
  35 pub use self::project::MismatchedProjectionTypes;
  36 pub use self::project::normalize;
  37 pub use self::project::Normalized;
  38 pub use self::object_safety::is_object_safe;
  39 pub use self::object_safety::object_safety_violations;
  40 pub use self::object_safety::ObjectSafetyViolation;
  41 pub use self::object_safety::MethodViolationCode;
  42 pub use self::object_safety::is_vtable_safe_method;
  43 pub use self::select::SelectionContext;
  44 pub use self::select::SelectionCache;
  45 pub use self::select::{MethodMatchResult, MethodMatched, MethodAmbiguous, MethodDidNotMatch};
  46 pub use self::select::{MethodMatchedData}; // intentionally don't export variants
  47 pub use self::util::elaborate_predicates;
  48 pub use self::util::get_vtable_index_of_object_method;
  49 pub use self::util::trait_ref_for_builtin_bound;
  50 pub use self::util::predicate_for_trait_def;
  51 pub use self::util::supertraits;
  52 pub use self::util::Supertraits;
  53 pub use self::util::supertrait_def_ids;
  54 pub use self::util::SupertraitDefIds;
  55 pub use self::util::transitive_bounds;
  56 pub use self::util::upcast;
  57
  58 mod coherence;
  59 mod error_reporting;
  60 mod fulfill;
  61 mod project;
  62 mod object_safety;
  63 mod select;
  64 mod util;
  65
  66 /// An `Obligation` represents some trait reference (e.g. `int:Eq`) for
  67 /// which the vtable must be found.  The process of finding a vtable is
  68 /// called "resolving" the `Obligation`. This process consists of
  69 /// either identifying an `impl` (e.g., `impl Eq for int`) that
  70 /// provides the required vtable, or else finding a bound that is in
  71 /// scope. The eventual result is usually a `Selection` (defined below).
  72 #[derive(Clone, PartialEq, Eq)]
  73 pub struct Obligation<'tcx, T> {
  74     pub cause: ObligationCause<'tcx>,
  75     pub recursion_depth: usize,
  76     pub predicate: T,
  77 }
  78
  79 pub type PredicateObligation<'tcx> = Obligation<'tcx, ty::Predicate<'tcx>>;
  80 pub type TraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tcx>>;
  81
  82 /// Why did we incur this obligation? Used for error reporting.
  83 #[derive(Clone, PartialEq, Eq)]
  84 pub struct ObligationCause<'tcx> {
  85     pub span: Span,
  86
  87     // The id of the fn body that triggered this obligation. This is
  88     // used for region obligations to determine the precise
  89     // environment in which the region obligation should be evaluated
  90     // (in particular, closures can add new assumptions). See the
  91     // field `region_obligations` of the `FulfillmentContext` for more
  92     // information.
  93     pub body_id: ast::NodeId,
  94
  95     pub code: ObligationCauseCode<'tcx>
  96 }
  97
  98 #[derive(Clone, PartialEq, Eq)]
  99 pub enum ObligationCauseCode<'tcx> {
 100     /// Not well classified or should be obvious from span.
 101     MiscObligation,
 102
 103     /// In an impl of trait X for type Y, type Y must
 104     /// also implement all supertraits of X.
 105     ItemObligation(ast::DefId),
 106
 107     /// Obligation incurred due to an object cast.
 108     ObjectCastObligation(/* Object type */ Ty<'tcx>),
 109
 110     /// Various cases where expressions must be sized/copy/etc:
 111     AssignmentLhsSized,        // L = X implies that L is Sized
 112     StructInitializerSized,    // S { ... } must be Sized
 113     VariableType(ast::NodeId), // Type of each variable must be Sized
 114     ReturnType,                // Return type must be Sized
 115     RepeatVec,                 // [T,..n] --> T must be Copy
 116
 117     // Captures of variable the given id by a closure (span is the
 118     // span of the closure)
 119     ClosureCapture(ast::NodeId, Span, ty::BuiltinBound),
 120
 121     // Types of fields (other than the last) in a struct must be sized.
 122     FieldSized,
 123
 124     // static items must have `Sync` type
 125     SharedStatic,
 126
 127
 128     BuiltinDerivedObligation(DerivedObligationCause<'tcx>),
 129
 130     ImplDerivedObligation(DerivedObligationCause<'tcx>),
 131
 132     CompareImplMethodObligation,
 133 }
 134
 135 #[derive(Clone, PartialEq, Eq)]
 136 pub struct DerivedObligationCause<'tcx> {
 137     /// The trait reference of the parent obligation that led to the
 138     /// current obligation. Note that only trait obligations lead to
 139     /// derived obligations, so we just store the trait reference here
 140     /// directly.
 141     parent_trait_ref: ty::PolyTraitRef<'tcx>,
 142
 143     /// The parent trait had this cause
 144     parent_code: Rc<ObligationCauseCode<'tcx>>
 145 }
 146
 147 pub type Obligations<'tcx, O> = Vec<Obligation<'tcx, O>>;
 148 pub type PredicateObligations<'tcx> = Vec<PredicateObligation<'tcx>>;
 149 pub type TraitObligations<'tcx> = Vec<TraitObligation<'tcx>>;
 150
 151 pub type Selection<'tcx> = Vtable<'tcx, PredicateObligation<'tcx>>;
 152
 153 #[derive(Clone,Debug)]
 154 pub enum SelectionError<'tcx> {
 155     Unimplemented,
 156     OutputTypeParameterMismatch(ty::PolyTraitRef<'tcx>,
 157                                 ty::PolyTraitRef<'tcx>,
 158                                 ty::type_err<'tcx>),
 159     TraitNotObjectSafe(ast::DefId),
 160 }
 161
 162 pub struct FulfillmentError<'tcx> {
 163     pub obligation: PredicateObligation<'tcx>,
 164     pub code: FulfillmentErrorCode<'tcx>
 165 }
 166
 167 #[derive(Clone)]
 168 pub enum FulfillmentErrorCode<'tcx> {
 169     CodeSelectionError(SelectionError<'tcx>),
 170     CodeProjectionError(MismatchedProjectionTypes<'tcx>),
 171     CodeAmbiguity,
 172 }
 173
 174 /// When performing resolution, it is typically the case that there
 175 /// can be one of three outcomes:
 176 ///
 177 /// - `Ok(Some(r))`: success occurred with result `r`
 178 /// - `Ok(None)`: could not definitely determine anything, usually due
 179 ///   to inconclusive type inference.
 180 /// - `Err(e)`: error `e` occurred
 181 pub type SelectionResult<'tcx, T> = Result<Option<T>, SelectionError<'tcx>>;
 182
 183 /// Given the successful resolution of an obligation, the `Vtable`
 184 /// indicates where the vtable comes from. Note that while we call this
 185 /// a "vtable", it does not necessarily indicate dynamic dispatch at
 186 /// runtime. `Vtable` instances just tell the compiler where to find
 187 /// methods, but in generic code those methods are typically statically
 188 /// dispatched -- only when an object is constructed is a `Vtable`
 189 /// instance reified into an actual vtable.
 190 ///
 191 /// For example, the vtable may be tied to a specific impl (case A),
 192 /// or it may be relative to some bound that is in scope (case B).
 193 ///
 194 ///
 195 /// ```
 196 /// impl<T:Clone> Clone<T> for Option<T> { ... } // Impl_1
 197 /// impl<T:Clone> Clone<T> for Box<T> { ... }    // Impl_2
 198 /// impl Clone for int { ... }             // Impl_3
 199 ///
 200 /// fn foo<T:Clone>(concrete: Option<Box<int>>,
 201 ///                 param: T,
 202 ///                 mixed: Option<T>) {
 203 ///
 204 ///    // Case A: Vtable points at a specific impl. Only possible when
 205 ///    // type is concretely known. If the impl itself has bounded
 206 ///    // type parameters, Vtable will carry resolutions for those as well:
 207 ///    concrete.clone(); // Vtable(Impl_1, [Vtable(Impl_2, [Vtable(Impl_3)])])
 208 ///
 209 ///    // Case B: Vtable must be provided by caller. This applies when
 210 ///    // type is a type parameter.
 211 ///    param.clone();    // VtableParam
 212 ///
 213 ///    // Case C: A mix of cases A and B.
 214 ///    mixed.clone();    // Vtable(Impl_1, [VtableParam])
 215 /// }
 216 /// ```
 217 ///
 218 /// ### The type parameter `N`
 219 ///
 220 /// See explanation on `VtableImplData`.
 221 #[derive(Clone)]
 222 pub enum Vtable<'tcx, N> {
 223     /// Vtable identifying a particular impl.
 224     VtableImpl(VtableImplData<'tcx, N>),
 225
 226     /// Vtable for default trait implementations
 227     /// This carries the information and nested obligations with regards
 228     /// to a default implementation for a trait `Trait`. The nested obligations
 229     /// ensure the trait implementation holds for all the constituent types.
 230     VtableDefaultImpl(VtableDefaultImplData<N>),
 231
 232     /// Successful resolution to an obligation provided by the caller
 233     /// for some type parameter. The `Vec<N>` represents the
 234     /// obligations incurred from normalizing the where-clause (if
 235     /// any).
 236     VtableParam(Vec<N>),
 237
 238     /// Virtual calls through an object
 239     VtableObject(VtableObjectData<'tcx>),
 240
 241     /// Successful resolution for a builtin trait.
 242     VtableBuiltin(VtableBuiltinData<N>),
 243
 244     /// Vtable automatically generated for a closure. The def ID is the ID
 245     /// of the closure expression. This is a `VtableImpl` in spirit, but the
 246     /// impl is generated by the compiler and does not appear in the source.
 247     VtableClosure(VtableClosureData<'tcx, N>),
 248
 249     /// Same as above, but for a fn pointer type with the given signature.
 250     VtableFnPointer(ty::Ty<'tcx>),
 251 }
 252
 253 /// Identifies a particular impl in the source, along with a set of
 254 /// substitutions from the impl's type/lifetime parameters. The
 255 /// `nested` vector corresponds to the nested obligations attached to
 256 /// the impl's type parameters.
 257 ///
 258 /// The type parameter `N` indicates the type used for "nested
 259 /// obligations" that are required by the impl. During type check, this
 260 /// is `Obligation`, as one might expect. During trans, however, this
 261 /// is `()`, because trans only requires a shallow resolution of an
 262 /// impl, and nested obligations are satisfied later.
 263 #[derive(Clone, PartialEq, Eq)]
 264 pub struct VtableImplData<'tcx, N> {
 265     pub impl_def_id: ast::DefId,
 266     pub substs: subst::Substs<'tcx>,
 267     pub nested: Vec<N>
 268 }
 269
 270 #[derive(Clone, PartialEq, Eq)]
 271 pub struct VtableClosureData<'tcx, N> {
 272     pub closure_def_id: ast::DefId,
 273     pub substs: subst::Substs<'tcx>,
 274     /// Nested obligations. This can be non-empty if the closure
 275     /// signature contains associated types.
 276     pub nested: Vec<N>
 277 }
 278
 279 #[derive(Clone)]
 280 pub struct VtableDefaultImplData<N> {
 281     pub trait_def_id: ast::DefId,
 282     pub nested: Vec<N>
 283 }
 284
 285 #[derive(Clone)]
 286 pub struct VtableBuiltinData<N> {
 287     pub nested: Vec<N>
 288 }
 289
 290 /// A vtable for some object-safe trait `Foo` automatically derived
 291 /// for the object type `Foo`.
 292 #[derive(PartialEq,Eq,Clone)]
 293 pub struct VtableObjectData<'tcx> {
 294     /// the object type `Foo`.
 295     pub object_ty: Ty<'tcx>,
 296
 297     /// `Foo` upcast to the obligation trait. This will be some supertrait of `Foo`.
 298     pub upcast_trait_ref: ty::PolyTraitRef<'tcx>,
 299 }
 300
 301 /// Creates predicate obligations from the generic bounds.
 302 pub fn predicates_for_generics<'tcx>(cause: ObligationCause<'tcx>,
 303                                      generic_bounds: &ty::InstantiatedPredicates<'tcx>)
 304                                      -> PredicateObligations<'tcx>
 305 {
 306     util::predicates_for_generics(cause, 0, generic_bounds)
 307 }
 308
 309 /// Determines whether the type `ty` is known to meet `bound` and
 310 /// returns true if so. Returns false if `ty` either does not meet
 311 /// `bound` or is not known to meet bound (note that this is
 312 /// conservative towards *no impl*, which is the opposite of the
 313 /// `evaluate` methods).
 314 pub fn type_known_to_meet_builtin_bound<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
 315                                                  typer: &ty::ClosureTyper<'tcx>,
 316                                                  ty: Ty<'tcx>,
 317                                                  bound: ty::BuiltinBound,
 318                                                  span: Span)
 319                                                  -> bool
 320 {
 321     debug!("type_known_to_meet_builtin_bound(ty={:?}, bound={:?})",
 322            ty,
 323            bound);
 324
 325     let mut fulfill_cx = FulfillmentContext::new(false);
 326
 327     // We can use a dummy node-id here because we won't pay any mind
 328     // to region obligations that arise (there shouldn't really be any
 329     // anyhow).
 330     let cause = ObligationCause::misc(span, ast::DUMMY_NODE_ID);
 331
 332     fulfill_cx.register_builtin_bound(infcx, ty, bound, cause);
 333
 334     // Note: we only assume something is `Copy` if we can
 335     // *definitively* show that it implements `Copy`. Otherwise,
 336     // assume it is move; linear is always ok.
 337     match fulfill_cx.select_all_or_error(infcx, typer) {
 338         Ok(()) => {
 339             debug!("type_known_to_meet_builtin_bound: ty={:?} bound={:?} success",
 340                    ty,
 341                    bound);
 342             true
 343         }
 344         Err(e) => {
 345             debug!("type_known_to_meet_builtin_bound: ty={:?} bound={:?} errors={:?}",
 346                    ty,
 347                    bound,
 348                    e);
 349             false
 350         }
 351     }
 352 }
 353
 354 /// Normalizes the parameter environment, reporting errors if they occur.
 355 pub fn normalize_param_env_or_error<'a,'tcx>(unnormalized_env: ty::ParameterEnvironment<'a,'tcx>,
 356                                              cause: ObligationCause<'tcx>)
 357                                              -> ty::ParameterEnvironment<'a,'tcx>
 358 {
 359     // I'm not wild about reporting errors here; I'd prefer to
 360     // have the errors get reported at a defined place (e.g.,
 361     // during typeck). Instead I have all parameter
 362     // environments, in effect, going through this function
 363     // and hence potentially reporting errors. This ensurse of
 364     // course that we never forget to normalize (the
 365     // alternative seemed like it would involve a lot of
 366     // manual invocations of this fn -- and then we'd have to
 367     // deal with the errors at each of those sites).
 368     //
 369     // In any case, in practice, typeck constructs all the
 370     // parameter environments once for every fn as it goes,
 371     // and errors will get reported then; so after typeck we
 372     // can be sure that no errors should occur.
 373
 374     let tcx = unnormalized_env.tcx;
 375     let span = cause.span;
 376     let body_id = cause.body_id;
 377
 378     debug!("normalize_param_env_or_error(unnormalized_env={:?})",
 379            unnormalized_env);
 380
 381     let predicates: Vec<_> =
 382         util::elaborate_predicates(tcx, unnormalized_env.caller_bounds.clone())
 383         .filter(|p| !p.is_global()) // (*)
 384         .collect();
 385
 386     // (*) Any predicate like `i32: Trait<u32>` or whatever doesn't
 387     // need to be in the *environment* to be proven, so screen those
 388     // out. This is important for the soundness of inter-fn
 389     // caching. Note though that we should probably check that these
 390     // predicates hold at the point where the environment is
 391     // constructed, but I am not currently doing so out of laziness.
 392     // -nmatsakis
 393
 394     debug!("normalize_param_env_or_error: elaborated-predicates={:?}",
 395            predicates);
 396
 397     let elaborated_env = unnormalized_env.with_caller_bounds(predicates);
 398
 399     let infcx = infer::new_infer_ctxt(tcx);
 400     let predicates = match fully_normalize(&infcx, &elaborated_env, cause,
 401                                            &elaborated_env.caller_bounds) {
 402         Ok(predicates) => predicates,
 403         Err(errors) => {
 404             report_fulfillment_errors(&infcx, &errors);
 405             return unnormalized_env; // an unnormalized env is better than nothing
 406         }
 407     };
 408
 409     let free_regions = FreeRegionMap::new();
 410     infcx.resolve_regions_and_report_errors(&free_regions, body_id);
 411     let predicates = match infcx.fully_resolve(&predicates) {
 412         Ok(predicates) => predicates,
 413         Err(fixup_err) => {
 414             // If we encounter a fixup error, it means that some type
 415             // variable wound up unconstrained. I actually don't know
 416             // if this can happen, and I certainly don't expect it to
 417             // happen often, but if it did happen it probably
 418             // represents a legitimate failure due to some kind of
 419             // unconstrained variable, and it seems better not to ICE,
 420             // all things considered.
 421             let err_msg = fixup_err_to_string(fixup_err);
 422             tcx.sess.span_err(span, &err_msg);
 423             return elaborated_env; // an unnormalized env is better than nothing
 424         }
 425     };
 426
 427     elaborated_env.with_caller_bounds(predicates)
 428 }
 429
 430 pub fn fully_normalize<'a,'tcx,T>(infcx: &InferCtxt<'a,'tcx>,
 431                                   closure_typer: &ty::ClosureTyper<'tcx>,
 432                                   cause: ObligationCause<'tcx>,
 433                                   value: &T)
 434                                   -> Result<T, Vec<FulfillmentError<'tcx>>>
 435     where T : TypeFoldable<'tcx> + HasProjectionTypes
 436 {
 437     debug!("normalize_param_env(value={:?})", value);
 438
 439     let mut selcx = &mut SelectionContext::new(infcx, closure_typer);
 440     let mut fulfill_cx = FulfillmentContext::new(false);
 441     let Normalized { value: normalized_value, obligations } =
 442         project::normalize(selcx, cause, value);
 443     debug!("normalize_param_env: normalized_value={:?} obligations={:?}",
 444            normalized_value,
 445            obligations);
 446     for obligation in obligations {
 447         fulfill_cx.register_predicate_obligation(selcx.infcx(), obligation);
 448     }
 449     try!(fulfill_cx.select_all_or_error(infcx, closure_typer));
 450     let resolved_value = infcx.resolve_type_vars_if_possible(&normalized_value);
 451     debug!("normalize_param_env: resolved_value={:?}", resolved_value);
 452     Ok(resolved_value)
 453 }
 454
 455 impl<'tcx,O> Obligation<'tcx,O> {
 456     pub fn new(cause: ObligationCause<'tcx>,
 457                trait_ref: O)
 458                -> Obligation<'tcx, O>
 459     {
 460         Obligation { cause: cause,
 461                      recursion_depth: 0,
 462                      predicate: trait_ref }
 463     }
 464
 465     fn with_depth(cause: ObligationCause<'tcx>,
 466                   recursion_depth: usize,
 467                   trait_ref: O)
 468                   -> Obligation<'tcx, O>
 469     {
 470         Obligation { cause: cause,
 471                      recursion_depth: recursion_depth,
 472                      predicate: trait_ref }
 473     }
 474
 475     pub fn misc(span: Span, body_id: ast::NodeId, trait_ref: O) -> Obligation<'tcx, O> {
 476         Obligation::new(ObligationCause::misc(span, body_id), trait_ref)
 477     }
 478
 479     pub fn with<P>(&self, value: P) -> Obligation<'tcx,P> {
 480         Obligation { cause: self.cause.clone(),
 481                      recursion_depth: self.recursion_depth,
 482                      predicate: value }
 483     }
 484 }
 485
 486 impl<'tcx> ObligationCause<'tcx> {
 487     pub fn new(span: Span,
 488                body_id: ast::NodeId,
 489                code: ObligationCauseCode<'tcx>)
 490                -> ObligationCause<'tcx> {
 491         ObligationCause { span: span, body_id: body_id, code: code }
 492     }
 493
 494     pub fn misc(span: Span, body_id: ast::NodeId) -> ObligationCause<'tcx> {
 495         ObligationCause { span: span, body_id: body_id, code: MiscObligation }
 496     }
 497
 498     pub fn dummy() -> ObligationCause<'tcx> {
 499         ObligationCause { span: DUMMY_SP, body_id: 0, code: MiscObligation }
 500     }
 501 }
 502
 503 impl<'tcx, N> Vtable<'tcx, N> {
 504     pub fn nested_obligations(self) -> Vec<N> {
 505         match self {
 506             VtableImpl(i) => i.nested,
 507             VtableParam(n) => n,
 508             VtableBuiltin(i) => i.nested,
 509             VtableDefaultImpl(d) => d.nested,
 510             VtableClosure(c) => c.nested,
 511             VtableObject(_) | VtableFnPointer(..) => vec![]
 512         }
 513     }
 514
 515     pub fn map<M, F>(self, f: F) -> Vtable<'tcx, M> where F: FnMut(N) -> M {
 516         match self {
 517             VtableImpl(i) => VtableImpl(VtableImplData {
 518                 impl_def_id: i.impl_def_id,
 519                 substs: i.substs,
 520                 nested: i.nested.into_iter().map(f).collect()
 521             }),
 522             VtableParam(n) => VtableParam(n.into_iter().map(f).collect()),
 523             VtableBuiltin(i) => VtableBuiltin(VtableBuiltinData {
 524                 nested: i.nested.into_iter().map(f).collect()
 525             }),
 526             VtableObject(o) => VtableObject(o),
 527             VtableDefaultImpl(d) => VtableDefaultImpl(VtableDefaultImplData {
 528                 trait_def_id: d.trait_def_id,
 529                 nested: d.nested.into_iter().map(f).collect()
 530             }),
 531             VtableFnPointer(f) => VtableFnPointer(f),
 532             VtableClosure(c) => VtableClosure(VtableClosureData {
 533                 closure_def_id: c.closure_def_id,
 534                 substs: c.substs,
 535                 nested: c.nested.into_iter().map(f).collect()
 536             })
 537         }
 538     }
 539 }
 540
 541 impl<'tcx> FulfillmentError<'tcx> {
 542     fn new(obligation: PredicateObligation<'tcx>,
 543            code: FulfillmentErrorCode<'tcx>)
 544            -> FulfillmentError<'tcx>
 545     {
 546         FulfillmentError { obligation: obligation, code: code }
 547     }
 548 }
 549
 550 impl<'tcx> TraitObligation<'tcx> {
 551     fn self_ty(&self) -> ty::Binder<Ty<'tcx>> {
 552         ty::Binder(self.predicate.skip_binder().self_ty())
 553     }
 554 }