1 //! Candidate selection. See the [rustc dev guide] for more information on how this works.
3 //! [rustc dev guide]: https://rust-lang.github.io/rustc-dev-guide/traits/resolution.html#selection
5 use self::EvaluationResult::*;
7 use super::{SelectionError, SelectionResult};
9 use crate::dep_graph::DepNodeIndex;
10 use crate::ty::{self, TyCtxt};
12 use rustc_data_structures::fx::FxHashMap;
13 use rustc_data_structures::sync::Lock;
14 use rustc_hir::def_id::DefId;
16 #[derive(Clone, Default)]
17 pub struct SelectionCache<'tcx> {
20 ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>,
21 WithDepNode<SelectionResult<'tcx, SelectionCandidate<'tcx>>>,
26 impl<'tcx> SelectionCache<'tcx> {
27 /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
29 *self.hashmap.borrow_mut() = Default::default();
33 /// The selection process begins by considering all impls, where
34 /// clauses, and so forth that might resolve an obligation. Sometimes
35 /// we'll be able to say definitively that (e.g.) an impl does not
36 /// apply to the obligation: perhaps it is defined for `usize` but the
37 /// obligation is for `int`. In that case, we drop the impl out of the
38 /// list. But the other cases are considered *candidates*.
40 /// For selection to succeed, there must be exactly one matching
41 /// candidate. If the obligation is fully known, this is guaranteed
42 /// by coherence. However, if the obligation contains type parameters
43 /// or variables, there may be multiple such impls.
45 /// It is not a real problem if multiple matching impls exist because
46 /// of type variables - it just means the obligation isn't sufficiently
47 /// elaborated. In that case we report an ambiguity, and the caller can
48 /// try again after more type information has been gathered or report a
49 /// "type annotations needed" error.
51 /// However, with type parameters, this can be a real problem - type
52 /// parameters don't unify with regular types, but they *can* unify
53 /// with variables from blanket impls, and (unless we know its bounds
54 /// will always be satisfied) picking the blanket impl will be wrong
55 /// for at least *some* substitutions. To make this concrete, if we have
57 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
58 /// impl<T: fmt::Debug> AsDebug for T {
60 /// fn debug(self) -> fmt::Debug { self }
62 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
64 /// we can't just use the impl to resolve the `<T as AsDebug>` obligation
65 /// -- a type from another crate (that doesn't implement `fmt::Debug`) could
66 /// implement `AsDebug`.
68 /// Because where-clauses match the type exactly, multiple clauses can
69 /// only match if there are unresolved variables, and we can mostly just
70 /// report this ambiguity in that case. This is still a problem - we can't
71 /// *do anything* with ambiguities that involve only regions. This is issue
74 /// If a single where-clause matches and there are no inference
75 /// variables left, then it definitely matches and we can just select
78 /// In fact, we even select the where-clause when the obligation contains
79 /// inference variables. The can lead to inference making "leaps of logic",
80 /// 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); }
91 /// Here the obligation `<T as Foo<$0>>` can be matched by both the blanket
92 /// impl and the where-clause. We select the where-clause and unify `$0=bool`,
93 /// so the program prints "false". However, if the where-clause is omitted,
94 /// the blanket impl is selected, we unify `$0=()`, and the program prints
97 /// Exactly the same issues apply to projection and object candidates, except
98 /// that we can have both a projection candidate and a where-clause candidate
99 /// for the same obligation. In that case either would do (except that
100 /// different "leaps of logic" would occur if inference variables are
101 /// present), and we just pick the where-clause. This is, for example,
102 /// required for associated types to work in default impls, as the bounds
103 /// are visible both as projection bounds and as where-clauses from the
104 /// parameter environment.
105 #[derive(PartialEq, Eq, Debug, Clone, TypeFoldable)]
106 pub enum SelectionCandidate<'tcx> {
108 /// `false` if there are no *further* obligations.
111 ParamCandidate(ty::PolyTraitRef<'tcx>),
112 ImplCandidate(DefId),
113 AutoImplCandidate(DefId),
115 /// This is a trait matching with a projected type as `Self`, and
116 /// we found an applicable bound in the trait definition.
119 /// Implementation of a `Fn`-family trait by one of the anonymous types
120 /// generated for a `||` expression.
123 /// Implementation of a `Generator` trait by one of the anonymous types
124 /// generated for a generator.
127 /// Implementation of a `Fn`-family trait by one of the anonymous
128 /// types generated for a fn pointer type (e.g., `fn(int) -> int`)
131 TraitAliasCandidate(DefId),
135 BuiltinObjectCandidate,
137 BuiltinUnsizeCandidate,
140 /// The result of trait evaluation. The order is important
141 /// here as the evaluation of a list is the maximum of the
144 /// The evaluation results are ordered:
145 /// - `EvaluatedToOk` implies `EvaluatedToOkModuloRegions`
146 /// implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
147 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
148 /// - the "union" of evaluation results is equal to their maximum -
149 /// all the "potential success" candidates can potentially succeed,
150 /// so they are noops when unioned with a definite error, and within
151 /// the categories it's easy to see that the unions are correct.
152 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq, HashStable)]
153 pub enum EvaluationResult {
154 /// Evaluation successful.
156 /// Evaluation successful, but there were unevaluated region obligations.
157 EvaluatedToOkModuloRegions,
158 /// Evaluation is known to be ambiguous -- it *might* hold for some
159 /// assignment of inference variables, but it might not.
161 /// While this has the same meaning as `EvaluatedToUnknown` -- we can't
162 /// know whether this obligation holds or not -- it is the result we
163 /// would get with an empty stack, and therefore is cacheable.
165 /// Evaluation failed because of recursion involving inference
166 /// variables. We are somewhat imprecise there, so we don't actually
167 /// know the real result.
169 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
171 /// Evaluation failed because we encountered an obligation we are already
172 /// trying to prove on this branch.
174 /// We know this branch can't be a part of a minimal proof-tree for
175 /// the "root" of our cycle, because then we could cut out the recursion
176 /// and maintain a valid proof tree. However, this does not mean
177 /// that all the obligations on this branch do not hold -- it's possible
178 /// that we entered this branch "speculatively", and that there
179 /// might be some other way to prove this obligation that does not
180 /// go through this cycle -- so we can't cache this as a failure.
182 /// For example, suppose we have this:
184 /// ```rust,ignore (pseudo-Rust)
185 /// pub trait Trait { fn xyz(); }
186 /// // This impl is "useless", but we can still have
187 /// // an `impl Trait for SomeUnsizedType` somewhere.
188 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
190 /// pub fn foo<T: Trait + ?Sized>() {
191 /// <T as Trait>::xyz();
195 /// When checking `foo`, we have to prove `T: Trait`. This basically
196 /// translates into this:
199 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
202 /// When we try to prove it, we first go the first option, which
203 /// recurses. This shows us that the impl is "useless" -- it won't
204 /// tell us that `T: Trait` unless it already implemented `Trait`
205 /// by some other means. However, that does not prevent `T: Trait`
206 /// does not hold, because of the bound (which can indeed be satisfied
207 /// by `SomeUnsizedType` from another crate).
209 // FIXME: when an `EvaluatedToRecur` goes past its parent root, we
210 // ought to convert it to an `EvaluatedToErr`, because we know
211 // there definitely isn't a proof tree for that obligation. Not
212 // doing so is still sound -- there isn't any proof tree, so the
213 // branch still can't be a part of a minimal one -- but does not re-enable caching.
215 /// Evaluation failed.
219 impl EvaluationResult {
220 /// Returns `true` if this evaluation result is known to apply, even
221 /// considering outlives constraints.
222 pub fn must_apply_considering_regions(self) -> bool {
223 self == EvaluatedToOk
226 /// Returns `true` if this evaluation result is known to apply, ignoring
227 /// outlives constraints.
228 pub fn must_apply_modulo_regions(self) -> bool {
229 self <= EvaluatedToOkModuloRegions
232 pub fn may_apply(self) -> bool {
234 EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToUnknown => {
238 EvaluatedToErr | EvaluatedToRecur => false,
242 pub fn is_stack_dependent(self) -> bool {
244 EvaluatedToUnknown | EvaluatedToRecur => true,
246 EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToErr => false,
251 /// Indicates that trait evaluation caused overflow.
252 #[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable)]
253 pub struct OverflowError;
255 impl<'tcx> From<OverflowError> for SelectionError<'tcx> {
256 fn from(OverflowError: OverflowError) -> SelectionError<'tcx> {
257 SelectionError::Overflow
261 #[derive(Clone, Default)]
262 pub struct EvaluationCache<'tcx> {
264 FxHashMap<ty::ParamEnvAnd<'tcx, ty::PolyTraitRef<'tcx>>, WithDepNode<EvaluationResult>>,
268 impl<'tcx> EvaluationCache<'tcx> {
269 /// Actually frees the underlying memory in contrast to what stdlib containers do on `clear`
270 pub fn clear(&self) {
271 *self.hashmap.borrow_mut() = Default::default();
275 #[derive(Clone, Eq, PartialEq)]
276 pub struct WithDepNode<T> {
277 dep_node: DepNodeIndex,
281 impl<T: Clone> WithDepNode<T> {
282 pub fn new(dep_node: DepNodeIndex, cached_value: T) -> Self {
283 WithDepNode { dep_node, cached_value }
286 pub fn get(&self, tcx: TyCtxt<'_>) -> T {
287 tcx.dep_graph.read_index(self.dep_node);
288 self.cached_value.clone()