src/librustc/traits/coherence.rs

   1 //! See Rustc Guide chapters on [trait-resolution] and [trait-specialization] for more info on how
   2 //! this works.
   3 //!
   4 //! [trait-resolution]: https://rust-lang.github.io/rustc-guide/traits/resolution.html
   5 //! [trait-specialization]: https://rust-lang.github.io/rustc-guide/traits/specialization.html
   6
   7 use crate::infer::{CombinedSnapshot, InferOk};
   8 use crate::hir::def_id::{DefId, LOCAL_CRATE};
   9 use crate::traits::{self, Normalized, SelectionContext, Obligation, ObligationCause};
  10 use crate::traits::IntercrateMode;
  11 use crate::traits::select::IntercrateAmbiguityCause;
  12 use crate::ty::{self, Ty, TyCtxt};
  13 use crate::ty::fold::TypeFoldable;
  14 use crate::ty::subst::Subst;
  15 use syntax::symbol::sym;
  16 use syntax_pos::DUMMY_SP;
  17
  18 /// Whether we do the orphan check relative to this crate or
  19 /// to some remote crate.
  20 #[derive(Copy, Clone, Debug)]
  21 enum InCrate {
  22     Local,
  23     Remote
  24 }
  25
  26 #[derive(Debug, Copy, Clone)]
  27 pub enum Conflict {
  28     Upstream,
  29     Downstream { used_to_be_broken: bool }
  30 }
  31
  32 pub struct OverlapResult<'tcx> {
  33     pub impl_header: ty::ImplHeader<'tcx>,
  34     pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
  35
  36     /// `true` if the overlap might've been permitted before the shift
  37     /// to universes.
  38     pub involves_placeholder: bool,
  39 }
  40
  41 pub fn add_placeholder_note(err: &mut errors::DiagnosticBuilder<'_>) {
  42     err.note(&format!(
  43         "this behavior recently changed as a result of a bug fix; \
  44          see rust-lang/rust#56105 for details"
  45     ));
  46 }
  47
  48 /// If there are types that satisfy both impls, invokes `on_overlap`
  49 /// with a suitably-freshened `ImplHeader` with those types
  50 /// substituted. Otherwise, invokes `no_overlap`.
  51 pub fn overlapping_impls<'tcx, F1, F2, R>(
  52     tcx: TyCtxt<'tcx>,
  53     impl1_def_id: DefId,
  54     impl2_def_id: DefId,
  55     intercrate_mode: IntercrateMode,
  56     on_overlap: F1,
  57     no_overlap: F2,
  58 ) -> R
  59 where
  60     F1: FnOnce(OverlapResult<'_>) -> R,
  61     F2: FnOnce() -> R,
  62 {
  63     debug!("overlapping_impls(\
  64            impl1_def_id={:?}, \
  65            impl2_def_id={:?},
  66            intercrate_mode={:?})",
  67            impl1_def_id,
  68            impl2_def_id,
  69            intercrate_mode);
  70
  71     let overlaps = tcx.infer_ctxt().enter(|infcx| {
  72         let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
  73         overlap(selcx, impl1_def_id, impl2_def_id).is_some()
  74     });
  75
  76     if !overlaps {
  77         return no_overlap();
  78     }
  79
  80     // In the case where we detect an error, run the check again, but
  81     // this time tracking intercrate ambuiguity causes for better
  82     // diagnostics. (These take time and can lead to false errors.)
  83     tcx.infer_ctxt().enter(|infcx| {
  84         let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
  85         selcx.enable_tracking_intercrate_ambiguity_causes();
  86         on_overlap(overlap(selcx, impl1_def_id, impl2_def_id).unwrap())
  87     })
  88 }
  89
  90 fn with_fresh_ty_vars<'cx, 'tcx>(
  91     selcx: &mut SelectionContext<'cx, 'tcx>,
  92     param_env: ty::ParamEnv<'tcx>,
  93     impl_def_id: DefId,
  94 ) -> ty::ImplHeader<'tcx> {
  95     let tcx = selcx.tcx();
  96     let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
  97
  98     let header = ty::ImplHeader {
  99         impl_def_id,
 100         self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
 101         trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
 102         predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
 103     };
 104
 105     let Normalized { value: mut header, obligations } =
 106         traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);
 107
 108     header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
 109     header
 110 }
 111
 112 /// Can both impl `a` and impl `b` be satisfied by a common type (including
 113 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
 114 fn overlap<'cx, 'tcx>(
 115     selcx: &mut SelectionContext<'cx, 'tcx>,
 116     a_def_id: DefId,
 117     b_def_id: DefId,
 118 ) -> Option<OverlapResult<'tcx>> {
 119     debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
 120
 121     selcx.infcx().probe(|snapshot| overlap_within_probe(selcx, a_def_id, b_def_id, snapshot))
 122 }
 123
 124 fn overlap_within_probe(
 125     selcx: &mut SelectionContext<'cx, 'tcx>,
 126     a_def_id: DefId,
 127     b_def_id: DefId,
 128     snapshot: &CombinedSnapshot<'_, 'tcx>,
 129 ) -> Option<OverlapResult<'tcx>> {
 130     // For the purposes of this check, we don't bring any placeholder
 131     // types into scope; instead, we replace the generic types with
 132     // fresh type variables, and hence we do our evaluations in an
 133     // empty environment.
 134     let param_env = ty::ParamEnv::empty();
 135
 136     let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
 137     let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
 138
 139     debug!("overlap: a_impl_header={:?}", a_impl_header);
 140     debug!("overlap: b_impl_header={:?}", b_impl_header);
 141
 142     // Do `a` and `b` unify? If not, no overlap.
 143     let obligations = match selcx.infcx().at(&ObligationCause::dummy(), param_env)
 144                                          .eq_impl_headers(&a_impl_header, &b_impl_header)
 145     {
 146         Ok(InferOk { obligations, value: () }) => obligations,
 147         Err(_) => return None
 148     };
 149
 150     debug!("overlap: unification check succeeded");
 151
 152     // Are any of the obligations unsatisfiable? If so, no overlap.
 153     let infcx = selcx.infcx();
 154     let opt_failing_obligation =
 155         a_impl_header.predicates
 156                      .iter()
 157                      .chain(&b_impl_header.predicates)
 158                      .map(|p| infcx.resolve_vars_if_possible(p))
 159                      .map(|p| Obligation { cause: ObligationCause::dummy(),
 160                                            param_env,
 161                                            recursion_depth: 0,
 162                                            predicate: p })
 163                      .chain(obligations)
 164                      .find(|o| !selcx.predicate_may_hold_fatal(o));
 165     // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
 166     // to the canonical trait query form, `infcx.predicate_may_hold`, once
 167     // the new system supports intercrate mode (which coherence needs).
 168
 169     if let Some(failing_obligation) = opt_failing_obligation {
 170         debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
 171         return None
 172     }
 173
 174     let impl_header = selcx.infcx().resolve_vars_if_possible(&a_impl_header);
 175     let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
 176     debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
 177
 178     let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
 179         Some(true) => true,
 180         _ => false,
 181     };
 182
 183     Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
 184 }
 185
 186 pub fn trait_ref_is_knowable<'tcx>(
 187     tcx: TyCtxt<'tcx>,
 188     trait_ref: ty::TraitRef<'tcx>,
 189 ) -> Option<Conflict> {
 190     debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
 191     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
 192         // A downstream or cousin crate is allowed to implement some
 193         // substitution of this trait-ref.
 194
 195         // A trait can be implementable for a trait ref by both the current
 196         // crate and crates downstream of it. Older versions of rustc
 197         // were not aware of this, causing incoherence (issue #43355).
 198         let used_to_be_broken =
 199             orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
 200         if used_to_be_broken {
 201             debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
 202         }
 203         return Some(Conflict::Downstream { used_to_be_broken });
 204     }
 205
 206     if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
 207         // This is a local or fundamental trait, so future-compatibility
 208         // is no concern. We know that downstream/cousin crates are not
 209         // allowed to implement a substitution of this trait ref, which
 210         // means impls could only come from dependencies of this crate,
 211         // which we already know about.
 212         return None;
 213     }
 214
 215     // This is a remote non-fundamental trait, so if another crate
 216     // can be the "final owner" of a substitution of this trait-ref,
 217     // they are allowed to implement it future-compatibly.
 218     //
 219     // However, if we are a final owner, then nobody else can be,
 220     // and if we are an intermediate owner, then we don't care
 221     // about future-compatibility, which means that we're OK if
 222     // we are an owner.
 223     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
 224         debug!("trait_ref_is_knowable: orphan check passed");
 225         return None;
 226     } else {
 227         debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
 228         return Some(Conflict::Upstream);
 229     }
 230 }
 231
 232 pub fn trait_ref_is_local_or_fundamental<'tcx>(
 233     tcx: TyCtxt<'tcx>,
 234     trait_ref: ty::TraitRef<'tcx>,
 235 ) -> bool {
 236     trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
 237 }
 238
 239 pub enum OrphanCheckErr<'tcx> {
 240     NoLocalInputType,
 241     UncoveredTy(Ty<'tcx>),
 242 }
 243
 244 /// Checks the coherence orphan rules. `impl_def_id` should be the
 245 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
 246 /// two conditions must be satisfied:
 247 ///
 248 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
 249 /// 2. Some local type must appear in `Self`.
 250 pub fn orphan_check<'tcx>(
 251     tcx: TyCtxt<'tcx>,
 252     impl_def_id: DefId,
 253 ) -> Result<(), OrphanCheckErr<'tcx>> {
 254     debug!("orphan_check({:?})", impl_def_id);
 255
 256     // We only except this routine to be invoked on implementations
 257     // of a trait, not inherent implementations.
 258     let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
 259     debug!("orphan_check: trait_ref={:?}", trait_ref);
 260
 261     // If the *trait* is local to the crate, ok.
 262     if trait_ref.def_id.is_local() {
 263         debug!("trait {:?} is local to current crate",
 264                trait_ref.def_id);
 265         return Ok(());
 266     }
 267
 268     orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
 269 }
 270
 271 /// Checks whether a trait-ref is potentially implementable by a crate.
 272 ///
 273 /// The current rule is that a trait-ref orphan checks in a crate C:
 274 ///
 275 /// 1. Order the parameters in the trait-ref in subst order - Self first,
 276 ///    others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
 277 /// 2. Of these type parameters, there is at least one type parameter
 278 ///    in which, walking the type as a tree, you can reach a type local
 279 ///    to C where all types in-between are fundamental types. Call the
 280 ///    first such parameter the "local key parameter".
 281 ///     - e.g., `Box<LocalType>` is OK, because you can visit LocalType
 282 ///       going through `Box`, which is fundamental.
 283 ///     - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
 284 ///       the same reason.
 285 ///     - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
 286 ///       not local), `Vec<LocalType>` is bad, because `Vec<->` is between
 287 ///       the local type and the type parameter.
 288 /// 3. Every type parameter before the local key parameter is fully known in C.
 289 ///     - e.g., `impl<T> T: Trait<LocalType>` is bad, because `T` might be
 290 ///       an unknown type.
 291 ///     - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
 292 ///       occurs before `T`.
 293 /// 4. Every type in the local key parameter not known in C, going
 294 ///    through the parameter's type tree, must appear only as a subtree of
 295 ///    a type local to C, with only fundamental types between the type
 296 ///    local to C and the local key parameter.
 297 ///     - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
 298 ///     is bad, because the only local type with `T` as a subtree is
 299 ///     `LocalType<T>`, and `Vec<->` is between it and the type parameter.
 300 ///     - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
 301 ///     the second occurrence of `T` is not a subtree of *any* local type.
 302 ///     - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
 303 ///     `LocalType<Vec<T>>`, which is local and has no types between it and
 304 ///     the type parameter.
 305 ///
 306 /// The orphan rules actually serve several different purposes:
 307 ///
 308 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
 309 ///    every type local to one crate is unknown in the other) can't implement
 310 ///    the same trait-ref. This follows because it can be seen that no such
 311 ///    type can orphan-check in 2 such crates.
 312 ///
 313 ///    To check that a local impl follows the orphan rules, we check it in
 314 ///    InCrate::Local mode, using type parameters for the "generic" types.
 315 ///
 316 /// 2. They ground negative reasoning for coherence. If a user wants to
 317 ///    write both a conditional blanket impl and a specific impl, we need to
 318 ///    make sure they do not overlap. For example, if we write
 319 ///    ```
 320 ///    impl<T> IntoIterator for Vec<T>
 321 ///    impl<T: Iterator> IntoIterator for T
 322 ///    ```
 323 ///    We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
 324 ///    We can observe that this holds in the current crate, but we need to make
 325 ///    sure this will also hold in all unknown crates (both "independent" crates,
 326 ///    which we need for link-safety, and also child crates, because we don't want
 327 ///    child crates to get error for impl conflicts in a *dependency*).
 328 ///
 329 ///    For that, we only allow negative reasoning if, for every assignment to the
 330 ///    inference variables, every unknown crate would get an orphan error if they
 331 ///    try to implement this trait-ref. To check for this, we use InCrate::Remote
 332 ///    mode. That is sound because we already know all the impls from known crates.
 333 ///
 334 /// 3. For non-#[fundamental] traits, they guarantee that parent crates can
 335 ///    add "non-blanket" impls without breaking negative reasoning in dependent
 336 ///    crates. This is the "rebalancing coherence" (RFC 1023) restriction.
 337 ///
 338 ///    For that, we only a allow crate to perform negative reasoning on
 339 ///    non-local-non-#[fundamental] only if there's a local key parameter as per (2).
 340 ///
 341 ///    Because we never perform negative reasoning generically (coherence does
 342 ///    not involve type parameters), this can be interpreted as doing the full
 343 ///    orphan check (using InCrate::Local mode), substituting non-local known
 344 ///    types for all inference variables.
 345 ///
 346 ///    This allows for crates to future-compatibly add impls as long as they
 347 ///    can't apply to types with a key parameter in a child crate - applying
 348 ///    the rules, this basically means that every type parameter in the impl
 349 ///    must appear behind a non-fundamental type (because this is not a
 350 ///    type-system requirement, crate owners might also go for "semantic
 351 ///    future-compatibility" involving things such as sealed traits, but
 352 ///    the above requirement is sufficient, and is necessary in "open world"
 353 ///    cases).
 354 ///
 355 /// Note that this function is never called for types that have both type
 356 /// parameters and inference variables.
 357 fn orphan_check_trait_ref<'tcx>(
 358     tcx: TyCtxt<'_>,
 359     trait_ref: ty::TraitRef<'tcx>,
 360     in_crate: InCrate,
 361 ) -> Result<(), OrphanCheckErr<'tcx>> {
 362     debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})",
 363            trait_ref, in_crate);
 364
 365     if trait_ref.needs_infer() && trait_ref.needs_subst() {
 366         bug!("can't orphan check a trait ref with both params and inference variables {:?}",
 367              trait_ref);
 368     }
 369
 370     if tcx.features().re_rebalance_coherence {
 371         // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
 372         // if at least one of the following is true:
 373         //
 374         // - Trait is a local trait
 375         // (already checked in orphan_check prior to calling this function)
 376         // - All of
 377         //     - At least one of the types T0..=Tn must be a local type.
 378         //      Let Ti be the first such type.
 379         //     - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
 380         //
 381         for input_ty in trait_ref.input_types() {
 382             debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
 383             if ty_is_local(tcx, input_ty, in_crate) {
 384                 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
 385                 return Ok(());
 386             } else if let ty::Param(_) = input_ty.sty {
 387                 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
 388                 return Err(OrphanCheckErr::UncoveredTy(input_ty))
 389             }
 390         }
 391         // If we exit above loop, never found a local type.
 392         debug!("orphan_check_trait_ref: no local type");
 393         Err(OrphanCheckErr::NoLocalInputType)
 394     } else {
 395         // First, create an ordered iterator over all the type
 396         // parameters to the trait, with the self type appearing
 397         // first.  Find the first input type that either references a
 398         // type parameter OR some local type.
 399         for input_ty in trait_ref.input_types() {
 400             if ty_is_local(tcx, input_ty, in_crate) {
 401                 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
 402
 403                 // First local input type. Check that there are no
 404                 // uncovered type parameters.
 405                 let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
 406                 for uncovered_ty in uncovered_tys {
 407                     if let Some(param) = uncovered_ty.walk()
 408                         .find(|t| is_possibly_remote_type(t, in_crate))
 409                     {
 410                         debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
 411                         return Err(OrphanCheckErr::UncoveredTy(param));
 412                     }
 413                 }
 414
 415                 // OK, found local type, all prior types upheld invariant.
 416                 return Ok(());
 417             }
 418
 419             // Otherwise, enforce invariant that there are no type
 420             // parameters reachable.
 421             if let Some(param) = input_ty.walk()
 422                 .find(|t| is_possibly_remote_type(t, in_crate))
 423             {
 424                 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
 425                 return Err(OrphanCheckErr::UncoveredTy(param));
 426             }
 427         }
 428         // If we exit above loop, never found a local type.
 429         debug!("orphan_check_trait_ref: no local type");
 430         Err(OrphanCheckErr::NoLocalInputType)
 431     }
 432 }
 433
 434 fn uncovered_tys<'tcx>(tcx: TyCtxt<'_>, ty: Ty<'tcx>, in_crate: InCrate) -> Vec<Ty<'tcx>> {
 435     if ty_is_local_constructor(ty, in_crate) {
 436         vec![]
 437     } else if fundamental_ty(ty) {
 438         ty.walk_shallow()
 439           .flat_map(|t| uncovered_tys(tcx, t, in_crate))
 440           .collect()
 441     } else {
 442         vec![ty]
 443     }
 444 }
 445
 446 fn is_possibly_remote_type(ty: Ty<'_>, _in_crate: InCrate) -> bool {
 447     match ty.sty {
 448         ty::Projection(..) | ty::Param(..) => true,
 449         _ => false,
 450     }
 451 }
 452
 453 fn ty_is_local(tcx: TyCtxt<'_>, ty: Ty<'_>, in_crate: InCrate) -> bool {
 454     ty_is_local_constructor(ty, in_crate) ||
 455         fundamental_ty(ty) && ty.walk_shallow().any(|t| ty_is_local(tcx, t, in_crate))
 456 }
 457
 458 fn fundamental_ty(ty: Ty<'_>) -> bool {
 459     match ty.sty {
 460         ty::Ref(..) => true,
 461         ty::Adt(def, _) => def.is_fundamental(),
 462         _ => false
 463     }
 464 }
 465
 466 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
 467     match in_crate {
 468         // The type is local to *this* crate - it will not be
 469         // local in any other crate.
 470         InCrate::Remote => false,
 471         InCrate::Local => def_id.is_local()
 472     }
 473 }
 474
 475 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
 476     debug!("ty_is_local_constructor({:?})", ty);
 477
 478     match ty.sty {
 479         ty::Bool |
 480         ty::Char |
 481         ty::Int(..) |
 482         ty::Uint(..) |
 483         ty::Float(..) |
 484         ty::Str |
 485         ty::FnDef(..) |
 486         ty::FnPtr(_) |
 487         ty::Array(..) |
 488         ty::Slice(..) |
 489         ty::RawPtr(..) |
 490         ty::Ref(..) |
 491         ty::Never |
 492         ty::Tuple(..) |
 493         ty::Param(..) |
 494         ty::Projection(..) => {
 495             false
 496         }
 497
 498         ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
 499             InCrate::Local => false,
 500             // The inference variable might be unified with a local
 501             // type in that remote crate.
 502             InCrate::Remote => true,
 503         },
 504
 505         ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
 506         ty::Foreign(did) => def_id_is_local(did, in_crate),
 507
 508         ty::Dynamic(ref tt, ..) => {
 509             if let Some(principal) = tt.principal() {
 510                 def_id_is_local(principal.def_id(), in_crate)
 511             } else {
 512                 false
 513             }
 514         }
 515
 516         ty::Error => true,
 517
 518         ty::UnnormalizedProjection(..) |
 519         ty::Closure(..) |
 520         ty::Generator(..) |
 521         ty::GeneratorWitness(..) |
 522         ty::Opaque(..) => {
 523             bug!("ty_is_local invoked on unexpected type: {:?}", ty)
 524         }
 525     }
 526 }