1 //! Candidate selection. See the [rustc dev guide] for more information on how this works.
3 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
5 use self::EvaluationResult::*;
7 use super::{SelectionError, SelectionResult};
8 use rustc_errors::ErrorGuaranteed;
12 use rustc_hir::def_id::DefId;
13 use rustc_query_system::cache::Cache;
15 pub type SelectionCache<'tcx> = Cache<
16 // This cache does not use `ParamEnvAnd` in its keys because `ParamEnv::and` can replace
17 // caller bounds with an empty list if the `TraitPredicate` looks global, which may happen
18 // after erasing lifetimes from the predicate.
19 (ty::ParamEnv<'tcx>, ty::TraitPredicate<'tcx>),
20 SelectionResult<'tcx, SelectionCandidate<'tcx>>,
23 pub type EvaluationCache<'tcx> = Cache<
24 // See above: this cache does not use `ParamEnvAnd` in its keys due to sometimes incorrectly
25 // caching with the wrong `ParamEnv`.
26 (ty::ParamEnv<'tcx>, ty::PolyTraitPredicate<'tcx>),
30 /// The selection process begins by considering all impls, where
31 /// clauses, and so forth that might resolve an obligation. Sometimes
32 /// we'll be able to say definitively that (e.g.) an impl does not
33 /// apply to the obligation: perhaps it is defined for `usize` but the
34 /// obligation is for `i32`. In that case, we drop the impl out of the
35 /// list. But the other cases are considered *candidates*.
37 /// For selection to succeed, there must be exactly one matching
38 /// candidate. If the obligation is fully known, this is guaranteed
39 /// by coherence. However, if the obligation contains type parameters
40 /// or variables, there may be multiple such impls.
42 /// It is not a real problem if multiple matching impls exist because
43 /// of type variables - it just means the obligation isn't sufficiently
44 /// elaborated. In that case we report an ambiguity, and the caller can
45 /// try again after more type information has been gathered or report a
46 /// "type annotations needed" error.
48 /// However, with type parameters, this can be a real problem - type
49 /// parameters don't unify with regular types, but they *can* unify
50 /// with variables from blanket impls, and (unless we know its bounds
51 /// will always be satisfied) picking the blanket impl will be wrong
52 /// for at least *some* substitutions. To make this concrete, if we have
55 /// trait AsDebug { type Out: fmt::Debug; fn debug(self) -> Self::Out; }
56 /// impl<T: fmt::Debug> AsDebug for T {
58 /// fn debug(self) -> fmt::Debug { self }
60 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
63 /// we can't just use the impl to resolve the `<T as AsDebug>` obligation
64 /// -- a type from another crate (that doesn't implement `fmt::Debug`) could
65 /// implement `AsDebug`.
67 /// Because where-clauses match the type exactly, multiple clauses can
68 /// only match if there are unresolved variables, and we can mostly just
69 /// report this ambiguity in that case. This is still a problem - we can't
70 /// *do anything* with ambiguities that involve only regions. This is issue
73 /// If a single where-clause matches and there are no inference
74 /// variables left, then it definitely matches and we can just select
77 /// In fact, we even select the where-clause when the obligation contains
78 /// inference variables. The can lead to inference making "leaps of logic",
79 /// for example in this situation:
82 /// pub trait Foo<T> { fn foo(&self) -> T; }
83 /// impl<T> Foo<()> for T { fn foo(&self) { } }
84 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
86 /// pub fn foo<T>(t: T) where T: Foo<bool> {
87 /// println!("{:?}", <T as Foo<_>>::foo(&t));
89 /// fn main() { foo(false); }
92 /// Here the obligation `<T as Foo<$0>>` can be matched by both the blanket
93 /// impl and the where-clause. We select the where-clause and unify `$0=bool`,
94 /// so the program prints "false". However, if the where-clause is omitted,
95 /// the blanket impl is selected, we unify `$0=()`, and the program prints
98 /// Exactly the same issues apply to projection and object candidates, except
99 /// that we can have both a projection candidate and a where-clause candidate
100 /// for the same obligation. In that case either would do (except that
101 /// different "leaps of logic" would occur if inference variables are
102 /// present), and we just pick the where-clause. This is, for example,
103 /// required for associated types to work in default impls, as the bounds
104 /// are visible both as projection bounds and as where-clauses from the
105 /// parameter environment.
106 #[derive(PartialEq, Eq, Debug, Clone, TypeFoldable, TypeVisitable)]
107 pub enum SelectionCandidate<'tcx> {
109 /// `false` if there are no *further* obligations.
113 /// Implementation of transmutability trait.
114 TransmutabilityCandidate,
116 ParamCandidate(ty::PolyTraitPredicate<'tcx>),
117 ImplCandidate(DefId),
118 AutoImplCandidate(DefId),
120 /// This is a trait matching with a projected type as `Self`, and we found
121 /// an applicable bound in the trait definition. The `usize` is an index
122 /// into the list returned by `tcx.item_bounds`.
123 ProjectionCandidate(usize),
125 /// Implementation of a `Fn`-family trait by one of the anonymous types
126 /// generated for an `||` expression.
129 /// Implementation of a `Generator` trait by one of the anonymous types
130 /// generated for a generator.
133 /// Implementation of a `Fn`-family trait by one of the anonymous
134 /// types generated for a fn pointer type (e.g., `fn(int) -> int`)
139 /// Builtin implementation of `DiscriminantKind`.
140 DiscriminantKindCandidate,
142 /// Builtin implementation of `Pointee`.
145 TraitAliasCandidate(DefId),
147 /// Matching `dyn Trait` with a supertrait of `Trait`. The index is the
148 /// position in the iterator returned by
149 /// `rustc_infer::traits::util::supertraits`.
150 ObjectCandidate(usize),
152 /// Perform trait upcasting coercion of `dyn Trait` to a supertrait of `Trait`.
153 /// The index is the position in the iterator returned by
154 /// `rustc_infer::traits::util::supertraits`.
155 TraitUpcastingUnsizeCandidate(usize),
157 BuiltinObjectCandidate,
159 BuiltinUnsizeCandidate,
161 /// Implementation of `const Destruct`, optionally from a custom `impl const Drop`.
162 ConstDestructCandidate(Option<DefId>),
164 /// Witnesses the fact that a type is a tuple.
168 /// The result of trait evaluation. The order is important
169 /// here as the evaluation of a list is the maximum of the
172 /// The evaluation results are ordered:
173 /// - `EvaluatedToOk` implies `EvaluatedToOkModuloRegions`
174 /// implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
175 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
176 /// - the "union" of evaluation results is equal to their maximum -
177 /// all the "potential success" candidates can potentially succeed,
178 /// so they are noops when unioned with a definite error, and within
179 /// the categories it's easy to see that the unions are correct.
180 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq, HashStable)]
181 pub enum EvaluationResult {
182 /// Evaluation successful.
184 /// Evaluation successful, but there were unevaluated region obligations.
185 EvaluatedToOkModuloRegions,
186 /// Evaluation successful, but need to rerun because opaque types got
187 /// hidden types assigned without it being known whether the opaque types
188 /// are within their defining scope
189 EvaluatedToOkModuloOpaqueTypes,
190 /// Evaluation is known to be ambiguous -- it *might* hold for some
191 /// assignment of inference variables, but it might not.
193 /// While this has the same meaning as `EvaluatedToUnknown` -- we can't
194 /// know whether this obligation holds or not -- it is the result we
195 /// would get with an empty stack, and therefore is cacheable.
197 /// Evaluation failed because of recursion involving inference
198 /// variables. We are somewhat imprecise there, so we don't actually
199 /// know the real result.
201 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
203 /// Evaluation failed because we encountered an obligation we are already
204 /// trying to prove on this branch.
206 /// We know this branch can't be a part of a minimal proof-tree for
207 /// the "root" of our cycle, because then we could cut out the recursion
208 /// and maintain a valid proof tree. However, this does not mean
209 /// that all the obligations on this branch do not hold -- it's possible
210 /// that we entered this branch "speculatively", and that there
211 /// might be some other way to prove this obligation that does not
212 /// go through this cycle -- so we can't cache this as a failure.
214 /// For example, suppose we have this:
216 /// ```rust,ignore (pseudo-Rust)
217 /// pub trait Trait { fn xyz(); }
218 /// // This impl is "useless", but we can still have
219 /// // an `impl Trait for SomeUnsizedType` somewhere.
220 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
222 /// pub fn foo<T: Trait + ?Sized>() {
223 /// <T as Trait>::xyz();
227 /// When checking `foo`, we have to prove `T: Trait`. This basically
228 /// translates into this:
231 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
234 /// When we try to prove it, we first go the first option, which
235 /// recurses. This shows us that the impl is "useless" -- it won't
236 /// tell us that `T: Trait` unless it already implemented `Trait`
237 /// by some other means. However, that does not prevent `T: Trait`
238 /// does not hold, because of the bound (which can indeed be satisfied
239 /// by `SomeUnsizedType` from another crate).
241 // FIXME: when an `EvaluatedToRecur` goes past its parent root, we
242 // ought to convert it to an `EvaluatedToErr`, because we know
243 // there definitely isn't a proof tree for that obligation. Not
244 // doing so is still sound -- there isn't any proof tree, so the
245 // branch still can't be a part of a minimal one -- but does not re-enable caching.
247 /// Evaluation failed.
251 impl EvaluationResult {
252 /// Returns `true` if this evaluation result is known to apply, even
253 /// considering outlives constraints.
254 pub fn must_apply_considering_regions(self) -> bool {
255 self == EvaluatedToOk
258 /// Returns `true` if this evaluation result is known to apply, ignoring
259 /// outlives constraints.
260 pub fn must_apply_modulo_regions(self) -> bool {
261 self <= EvaluatedToOkModuloRegions
264 pub fn may_apply(self) -> bool {
266 EvaluatedToOkModuloOpaqueTypes
268 | EvaluatedToOkModuloRegions
270 | EvaluatedToUnknown => true,
272 EvaluatedToErr | EvaluatedToRecur => false,
276 pub fn is_stack_dependent(self) -> bool {
278 EvaluatedToUnknown | EvaluatedToRecur => true,
280 EvaluatedToOkModuloOpaqueTypes
282 | EvaluatedToOkModuloRegions
284 | EvaluatedToErr => false,
289 /// Indicates that trait evaluation caused overflow and in which pass.
290 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable)]
291 pub enum OverflowError {
292 Error(ErrorGuaranteed),
297 impl From<ErrorGuaranteed> for OverflowError {
298 fn from(e: ErrorGuaranteed) -> OverflowError {
299 OverflowError::Error(e)
303 TrivialTypeTraversalAndLiftImpls! {
307 impl<'tcx> From<OverflowError> for SelectionError<'tcx> {
308 fn from(overflow_error: OverflowError) -> SelectionError<'tcx> {
309 match overflow_error {
310 OverflowError::Error(e) => SelectionError::Overflow(OverflowError::Error(e)),
311 OverflowError::Canonical => SelectionError::Overflow(OverflowError::Canonical),
312 OverflowError::ErrorReporting => SelectionError::ErrorReporting,