diff --git a/compiler/rustc_trait_selection/src/lib.rs b/compiler/rustc_trait_selection/src/lib.rs index 11d72106b221f..1af383e9200d0 100644 --- a/compiler/rustc_trait_selection/src/lib.rs +++ b/compiler/rustc_trait_selection/src/lib.rs @@ -23,6 +23,7 @@ #![feature(extract_if)] #![feature(if_let_guard)] #![feature(iter_intersperse)] +#![feature(iterator_try_reduce)] #![feature(let_chains)] #![feature(never_type)] #![feature(rustdoc_internals)] diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs index 635d3bc99b1ad..4ae635d522666 100644 --- a/compiler/rustc_trait_selection/src/traits/select/mod.rs +++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs @@ -448,7 +448,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { // Winnow, but record the exact outcome of evaluation, which // is needed for specialization. Propagate overflow if it occurs. - let mut candidates = candidates + let candidates = candidates .into_iter() .map(|c| match self.evaluate_candidate(stack, &c) { Ok(eval) if eval.may_apply() => { @@ -461,40 +461,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { .flat_map(Result::transpose) .collect::, _>>()?; - debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len()); - - let has_non_region_infer = stack.obligation.predicate.has_non_region_infer(); - - // If there are STILL multiple candidates, we can further - // reduce the list by dropping duplicates -- including - // resolving specializations. - if candidates.len() > 1 { - let mut i = 0; - while i < candidates.len() { - let should_drop_i = (0..candidates.len()).filter(|&j| i != j).any(|j| { - self.candidate_should_be_dropped_in_favor_of( - &candidates[i], - &candidates[j], - has_non_region_infer, - ) == DropVictim::Yes - }); - if should_drop_i { - debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len()); - candidates.swap_remove(i); - } else { - debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len()); - i += 1; - - // If there are *STILL* multiple candidates, give up - // and report ambiguity. - if i > 1 { - debug!("multiple matches, ambig"); - return Ok(None); - } - } - } - } - + debug!(?stack, ?candidates, "{} potentially applicable candidates", candidates.len()); // If there are *NO* candidates, then there are no impls -- // that we know of, anyway. Note that in the case where there // are unbound type variables within the obligation, it might @@ -511,13 +478,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { // to have emitted at least one. if stack.obligation.predicate.references_error() { debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous"); - return Ok(None); + Ok(None) + } else { + Err(Unimplemented) + } + } else { + let has_non_region_infer = stack.obligation.predicate.has_non_region_infer(); + if let Some(candidate) = self.winnow_candidates(has_non_region_infer, candidates) { + self.filter_reservation_impls(candidate) + } else { + Ok(None) } - return Err(Unimplemented); } - - // Just one candidate left. - self.filter_reservation_impls(candidates.pop().unwrap().candidate) } /////////////////////////////////////////////////////////////////////////// @@ -1794,18 +1766,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { } } -#[derive(Debug, Copy, Clone, PartialEq, Eq)] -enum DropVictim { - Yes, - No, -} - -impl DropVictim { - fn drop_if(should_drop: bool) -> DropVictim { - if should_drop { DropVictim::Yes } else { DropVictim::No } - } -} - /// ## Winnowing /// /// Winnowing is the process of attempting to resolve ambiguity by @@ -1813,131 +1773,142 @@ impl DropVictim { /// type variables and then we also attempt to evaluate recursive /// bounds to see if they are satisfied. impl<'tcx> SelectionContext<'_, 'tcx> { - /// Returns `DropVictim::Yes` if `victim` should be dropped in favor of - /// `other`. Generally speaking we will drop duplicate - /// candidates and prefer where-clause candidates. + /// If there are multiple ways to prove a trait goal, we make some + /// *fairly arbitrary* choices about which candidate is actually used. /// - /// See the comment for "SelectionCandidate" for more details. - #[instrument(level = "debug", skip(self))] - fn candidate_should_be_dropped_in_favor_of( + /// For more details, look at the implementation of this method :) + #[instrument(level = "debug", skip(self), ret)] + fn winnow_candidates( &mut self, - victim: &EvaluatedCandidate<'tcx>, - other: &EvaluatedCandidate<'tcx>, has_non_region_infer: bool, - ) -> DropVictim { - if victim.candidate == other.candidate { - return DropVictim::Yes; + mut candidates: Vec>, + ) -> Option> { + if candidates.len() == 1 { + return Some(candidates.pop().unwrap().candidate); } - // Check if a bound would previously have been removed when normalizing - // the param_env so that it can be given the lowest priority. See - // #50825 for the motivation for this. - let is_global = - |cand: ty::PolyTraitPredicate<'tcx>| cand.is_global() && !cand.has_bound_vars(); + // We prefer trivial builtin candidates, i.e. builtin impls without any nested + // requirements, over all others. This is a fix for #53123 and prevents winnowing + // from accidentally extending the lifetime of a variable. + let mut trivial_builtin = candidates + .iter() + .filter(|c| matches!(c.candidate, BuiltinCandidate { has_nested: false })); + if let Some(_trivial) = trivial_builtin.next() { + // There should only ever be a single trivial builtin candidate + // as they would otherwise overlap. + debug_assert_eq!(trivial_builtin.next(), None); + return Some(BuiltinCandidate { has_nested: false }); + } - // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`, - // or `DiscriminantKindCandidate` to anything else. + // The next highest priority is for non-global where-bounds. However, there are + // two additional rules here: + // - if there are also global where-bound which may apply, we bail with ambiguity + // instead of prefering the non-global where-bound. Note that if there are + // only global where-bounds, they get ignored + // - if there are two where-bounds which only differ in their bound vars, but are + // otherwise equal, we arbitrarily prefer one of them // - // This is a fix for #53123 and prevents winnowing from accidentally extending the - // lifetime of a variable. - match (&other.candidate, &victim.candidate) { - // FIXME(@jswrenn): this should probably be more sophisticated - (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => DropVictim::No, - - // (*) - (BuiltinCandidate { has_nested: false }, _) => DropVictim::Yes, - (_, BuiltinCandidate { has_nested: false }) => DropVictim::No, - - (ParamCandidate(other), ParamCandidate(victim)) => { - let same_except_bound_vars = other.skip_binder().trait_ref - == victim.skip_binder().trait_ref - && other.skip_binder().polarity == victim.skip_binder().polarity - && !other.skip_binder().trait_ref.has_escaping_bound_vars(); - if same_except_bound_vars { - // See issue #84398. In short, we can generate multiple ParamCandidates which are - // the same except for unused bound vars. Just pick the one with the fewest bound vars - // or the current one if tied (they should both evaluate to the same answer). This is - // probably best characterized as a "hack", since we might prefer to just do our - // best to *not* create essentially duplicate candidates in the first place. - DropVictim::drop_if(other.bound_vars().len() <= victim.bound_vars().len()) + // This is fairly messy but necessary for backwards compatability, see #50825 for why + // we need to handle global where-bounds like this. + let param_candidates = candidates + .iter() + .filter_map(|c| if let ParamCandidate(p) = c.candidate { Some(p) } else { None }); + let is_global = |c: ty::PolyTraitPredicate<'tcx>| c.is_global() && !c.has_bound_vars(); + let mut has_non_global = false; + let mut param_candidate = None; + for c in param_candidates { + has_non_global |= !is_global(c); + let has_fewer_bound_vars = + |this: ty::PolyTraitPredicate<'tcx>, other: ty::PolyTraitPredicate<'tcx>| { + this.skip_binder().trait_ref == other.skip_binder().trait_ref + && this.skip_binder().polarity == other.skip_binder().polarity + && !this.skip_binder().trait_ref.has_escaping_bound_vars() + }; + if let Some(prev) = param_candidate.replace(c) { + if has_fewer_bound_vars(c, prev) { + // Ok, prefer `c` over the previous entry + } else if has_fewer_bound_vars(prev, c) { + // Ok, keep `prev` instead of the new entry + param_candidate = Some(prev); } else { - DropVictim::No + // Ambiguity, two potentially different where-clauses + return None; } } - - ( - ParamCandidate(other_cand), - ImplCandidate(..) - | AutoImplCandidate - | ClosureCandidate { .. } - | AsyncClosureCandidate - | AsyncFnKindHelperCandidate - | CoroutineCandidate - | FutureCandidate - | IteratorCandidate - | AsyncIteratorCandidate - | FnPointerCandidate { .. } - | BuiltinObjectCandidate - | BuiltinUnsizeCandidate - | TraitUpcastingUnsizeCandidate(_) - | BuiltinCandidate { .. } - | TraitAliasCandidate - | ObjectCandidate(_) - | ProjectionCandidate(_), - ) => { - // We have a where clause so don't go around looking - // for impls. Arbitrarily give param candidates priority - // over projection and object candidates. + } + if let Some(predicate) = param_candidate { + if is_global(predicate) { + // If we only have global where-bounds, we prefer + // impls over them, otherwise we bail with ambiguity. // - // Global bounds from the where clause should be ignored - // here (see issue #50825). - DropVictim::drop_if(!is_global(*other_cand)) - } - (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(victim_cand)) => { - // Prefer these to a global where-clause bound - // (see issue #50825). - if is_global(*victim_cand) { DropVictim::Yes } else { DropVictim::No } - } - ( - ImplCandidate(_) - | AutoImplCandidate - | ClosureCandidate { .. } - | AsyncClosureCandidate - | AsyncFnKindHelperCandidate - | CoroutineCandidate - | FutureCandidate - | IteratorCandidate - | AsyncIteratorCandidate - | FnPointerCandidate { .. } - | BuiltinObjectCandidate - | BuiltinUnsizeCandidate - | TraitUpcastingUnsizeCandidate(_) - | BuiltinCandidate { has_nested: true } - | TraitAliasCandidate, - ParamCandidate(victim_cand), - ) => { - // Prefer these to a global where-clause bound - // (see issue #50825). - DropVictim::drop_if( - is_global(*victim_cand) && other.evaluation.must_apply_modulo_regions(), - ) + // This is only reachable if there's one higher-ranked + // and one non-higher ranked version of a global + // where-clause. + if has_non_global { + return None; + } + } else { + return Some(ParamCandidate(predicate)); } + } + + // Prefer alias-bounds over blanket impls for rigid associated types. This is + // fairly arbitrary but once again necessary for backwards compatibility. + // If there are multiple applicable candidates which don't affect type inference, + // choose the one with the lowest index. + let alias_bound = candidates + .iter() + .filter_map(|c| if let ProjectionCandidate(i) = c.candidate { Some(i) } else { None }) + .try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) }); + match alias_bound { + Some(Some(index)) => return Some(ProjectionCandidate(index)), + Some(None) => {} + None => return None, + } + + // Need to prioritize builtin trait object impls as `::type_id` + // should use the vtable method and not the method provided by the user-defined + // impl `impl Any for T { .. }`. This really shouldn't exist but is + // necessary due to #57893. We again arbitrarily prefer the applicable candidate + // with the lowest index. + let object_bound = candidates + .iter() + .filter_map(|c| if let ObjectCandidate(i) = c.candidate { Some(i) } else { None }) + .try_reduce(|c1, c2| if has_non_region_infer { None } else { Some(c1.min(c2)) }); + match object_bound { + Some(Some(index)) => return Some(ObjectCandidate(index)), + Some(None) => {} + None => return None, + } - (ProjectionCandidate(i), ProjectionCandidate(j)) - | (ObjectCandidate(i), ObjectCandidate(j)) => { - // Arbitrarily pick the lower numbered candidate for backwards - // compatibility reasons. Don't let this affect inference. - DropVictim::drop_if(i < j && !has_non_region_infer) + // Finally, handle overlapping user-written impls. + let impls = candidates.iter().filter_map(|c| { + if let ImplCandidate(def_id) = c.candidate { + Some((def_id, c.evaluation)) + } else { + None } - (ObjectCandidate(_), ProjectionCandidate(_)) - | (ProjectionCandidate(_), ObjectCandidate(_)) => { - bug!("Have both object and projection candidate") + }); + let mut impl_candidate = None; + for c in impls { + if let Some(prev) = impl_candidate.replace(c) { + if self.prefer_lhs_over_victim(has_non_region_infer, c, prev) { + // Ok, prefer `c` over the previous entry + } else if self.prefer_lhs_over_victim(has_non_region_infer, prev, c) { + // Ok, keep `prev` instead of the new entry + impl_candidate = Some(prev); + } else { + // Ambiguity, two potentially different where-clauses + return None; + } } - - // Arbitrarily give projection and object candidates priority. - ( - ObjectCandidate(_) | ProjectionCandidate(_), - ImplCandidate(..) + } + if let Some((def_id, _evaluation)) = impl_candidate { + // Don't use impl candidates which overlap with other candidates. + // This should pretty much only ever happen with malformed impls. + if candidates.iter().all(|c| match c.candidate { + BuiltinCandidate { has_nested: _ } + | TransmutabilityCandidate | AutoImplCandidate | ClosureCandidate { .. } | AsyncClosureCandidate @@ -1946,155 +1917,109 @@ impl<'tcx> SelectionContext<'_, 'tcx> { | FutureCandidate | IteratorCandidate | AsyncIteratorCandidate - | FnPointerCandidate { .. } - | BuiltinObjectCandidate - | BuiltinUnsizeCandidate + | FnPointerCandidate + | TraitAliasCandidate | TraitUpcastingUnsizeCandidate(_) - | BuiltinCandidate { .. } - | TraitAliasCandidate, - ) => DropVictim::Yes, - - ( - ImplCandidate(..) - | AutoImplCandidate - | ClosureCandidate { .. } - | AsyncClosureCandidate - | AsyncFnKindHelperCandidate - | CoroutineCandidate - | FutureCandidate - | IteratorCandidate - | AsyncIteratorCandidate - | FnPointerCandidate { .. } | BuiltinObjectCandidate - | BuiltinUnsizeCandidate - | TraitUpcastingUnsizeCandidate(_) - | BuiltinCandidate { .. } - | TraitAliasCandidate, - ObjectCandidate(_) | ProjectionCandidate(_), - ) => DropVictim::No, - - (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => { - // See if we can toss out `victim` based on specialization. - // While this requires us to know *for sure* that the `other` impl applies - // we still use modulo regions here. - // - // This is fine as specialization currently assumes that specializing - // impls have to be always applicable, meaning that the only allowed - // region constraints may be constraints also present on the default impl. - let tcx = self.tcx(); - if other.evaluation.must_apply_modulo_regions() - && tcx.specializes((other_def, victim_def)) - { - return DropVictim::Yes; - } - - match tcx.impls_are_allowed_to_overlap(other_def, victim_def) { - // For #33140 the impl headers must be exactly equal, the trait must not have - // any associated items and there are no where-clauses. - // - // We can just arbitrarily drop one of the impls. - Some(ty::ImplOverlapKind::FutureCompatOrderDepTraitObjects) => { - assert_eq!(other.evaluation, victim.evaluation); - DropVictim::Yes - } - // For candidates which already reference errors it doesn't really - // matter what we do 🤷 - Some(ty::ImplOverlapKind::Permitted { marker: false }) => { - DropVictim::drop_if(other.evaluation.must_apply_considering_regions()) - } - Some(ty::ImplOverlapKind::Permitted { marker: true }) => { - // Subtle: If the predicate we are evaluating has inference - // variables, do *not* allow discarding candidates due to - // marker trait impls. - // - // Without this restriction, we could end up accidentally - // constraining inference variables based on an arbitrarily - // chosen trait impl. - // - // Imagine we have the following code: - // - // ```rust - // #[marker] trait MyTrait {} - // impl MyTrait for u8 {} - // impl MyTrait for bool {} - // ``` - // - // And we are evaluating the predicate `<_#0t as MyTrait>`. - // - // During selection, we will end up with one candidate for each - // impl of `MyTrait`. If we were to discard one impl in favor - // of the other, we would be left with one candidate, causing - // us to "successfully" select the predicate, unifying - // _#0t with (for example) `u8`. - // - // However, we have no reason to believe that this unification - // is correct - we've essentially just picked an arbitrary - // *possibility* for _#0t, and required that this be the *only* - // possibility. - // - // Eventually, we will either: - // 1) Unify all inference variables in the predicate through - // some other means (e.g. type-checking of a function). We will - // then be in a position to drop marker trait candidates - // without constraining inference variables (since there are - // none left to constrain) - // 2) Be left with some unconstrained inference variables. We - // will then correctly report an inference error, since the - // existence of multiple marker trait impls tells us nothing - // about which one should actually apply. - DropVictim::drop_if( - !has_non_region_infer - && other.evaluation.must_apply_considering_regions(), - ) - } - None => DropVictim::No, - } + | BuiltinUnsizeCandidate => false, + // Non-global param candidates have already been handled, global + // where-bounds get ignored. + ParamCandidate(_) | ImplCandidate(_) => true, + ProjectionCandidate(_) | ObjectCandidate(_) => unreachable!(), + }) { + return Some(ImplCandidate(def_id)); + } else { + return None; } + } - (AutoImplCandidate, ImplCandidate(_)) | (ImplCandidate(_), AutoImplCandidate) => { - DropVictim::No - } + // Also try ignoring all global where-bounds and check whether we end + // with a unique candidate in this case. + let mut not_a_global_where_bound = candidates + .into_iter() + .filter(|c| !matches!(c.candidate, ParamCandidate(p) if is_global(p))); + not_a_global_where_bound + .next() + .map(|c| c.candidate) + .filter(|_| not_a_global_where_bound.next().is_none()) + } - (AutoImplCandidate, _) | (_, AutoImplCandidate) => { - bug!( - "default implementations shouldn't be recorded \ - when there are other global candidates: {:?} {:?}", - other, - victim - ); + fn prefer_lhs_over_victim( + &self, + has_non_region_infer: bool, + (lhs, lhs_evaluation): (DefId, EvaluationResult), + (victim, victim_evaluation): (DefId, EvaluationResult), + ) -> bool { + let tcx = self.tcx(); + // See if we can toss out `victim` based on specialization. + // + // While this requires us to know *for sure* that the `lhs` impl applies + // we still use modulo regions here. This is fine as specialization currently + // assumes that specializing impls have to be always applicable, meaning that + // the only allowed region constraints may be constraints also present on the default impl. + if lhs_evaluation.must_apply_modulo_regions() { + if tcx.specializes((lhs, victim)) { + return true; } + } - // Everything else is ambiguous - ( - ImplCandidate(_) - | ClosureCandidate { .. } - | AsyncClosureCandidate - | AsyncFnKindHelperCandidate - | CoroutineCandidate - | FutureCandidate - | IteratorCandidate - | AsyncIteratorCandidate - | FnPointerCandidate { .. } - | BuiltinObjectCandidate - | BuiltinUnsizeCandidate - | TraitUpcastingUnsizeCandidate(_) - | BuiltinCandidate { has_nested: true } - | TraitAliasCandidate, - ImplCandidate(_) - | ClosureCandidate { .. } - | AsyncClosureCandidate - | AsyncFnKindHelperCandidate - | CoroutineCandidate - | FutureCandidate - | IteratorCandidate - | AsyncIteratorCandidate - | FnPointerCandidate { .. } - | BuiltinObjectCandidate - | BuiltinUnsizeCandidate - | TraitUpcastingUnsizeCandidate(_) - | BuiltinCandidate { has_nested: true } - | TraitAliasCandidate, - ) => DropVictim::No, + match tcx.impls_are_allowed_to_overlap(lhs, victim) { + // For #33140 the impl headers must be exactly equal, the trait must not have + // any associated items and there are no where-clauses. + // + // We can just arbitrarily drop one of the impls. + Some(ty::ImplOverlapKind::FutureCompatOrderDepTraitObjects) => { + assert_eq!(lhs_evaluation, victim_evaluation); + true + } + // For candidates which already reference errors it doesn't really + // matter what we do 🤷 + Some(ty::ImplOverlapKind::Permitted { marker: false }) => { + lhs_evaluation.must_apply_considering_regions() + } + Some(ty::ImplOverlapKind::Permitted { marker: true }) => { + // Subtle: If the predicate we are evaluating has inference + // variables, do *not* allow discarding candidates due to + // marker trait impls. + // + // Without this restriction, we could end up accidentally + // constraining inference variables based on an arbitrarily + // chosen trait impl. + // + // Imagine we have the following code: + // + // ```rust + // #[marker] trait MyTrait {} + // impl MyTrait for u8 {} + // impl MyTrait for bool {} + // ``` + // + // And we are evaluating the predicate `<_#0t as MyTrait>`. + // + // During selection, we will end up with one candidate for each + // impl of `MyTrait`. If we were to discard one impl in favor + // of the other, we would be left with one candidate, causing + // us to "successfully" select the predicate, unifying + // _#0t with (for example) `u8`. + // + // However, we have no reason to believe that this unification + // is correct - we've essentially just picked an arbitrary + // *possibility* for _#0t, and required that this be the *only* + // possibility. + // + // Eventually, we will either: + // 1) Unify all inference variables in the predicate through + // some other means (e.g. type-checking of a function). We will + // then be in a position to drop marker trait candidates + // without constraining inference variables (since there are + // none left to constrain) + // 2) Be left with some unconstrained inference variables. We + // will then correctly report an inference error, since the + // existence of multiple marker trait impls tells us nothing + // about which one should actually apply. + !has_non_region_infer && lhs_evaluation.must_apply_considering_regions() + } + None => false, } } }