diff --git a/src/librustc/traits/fulfill.rs b/src/librustc/traits/fulfill.rs index 78e47693caa..fbc393cbd96 100644 --- a/src/librustc/traits/fulfill.rs +++ b/src/librustc/traits/fulfill.rs @@ -251,6 +251,9 @@ impl<'a, 'gcx, 'tcx> FulfillmentContext<'tcx> { }); debug!("select: outcome={:?}", outcome); + // FIXME: if we kept the original cache key, we could mark projection + // obligations as complete for the projection cache here. + errors.extend( outcome.errors.into_iter() .map(|e| to_fulfillment_error(e))); diff --git a/src/librustc/traits/project.rs b/src/librustc/traits/project.rs index e70258007e4..dbf04d72439 100644 --- a/src/librustc/traits/project.rs +++ b/src/librustc/traits/project.rs @@ -121,11 +121,13 @@ struct ProjectionTyCandidateSet<'tcx> { /// /// for<...> ::U == V /// -/// If successful, this may result in additional obligations. +/// If successful, this may result in additional obligations. Also returns +/// the projection cache key used to track these additional obligations. pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>( selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>, obligation: &PolyProjectionObligation<'tcx>) - -> Result>>, MismatchedProjectionTypes<'tcx>> + -> Result>>, + MismatchedProjectionTypes<'tcx>> { debug!("poly_project_and_unify_type(obligation={:?})", obligation); @@ -161,7 +163,8 @@ pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>( fn project_and_unify_type<'cx, 'gcx, 'tcx>( selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>, obligation: &ProjectionObligation<'tcx>) - -> Result>>, MismatchedProjectionTypes<'tcx>> + -> Result>>, + MismatchedProjectionTypes<'tcx>> { debug!("project_and_unify_type(obligation={:?})", obligation); @@ -396,6 +399,7 @@ fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>( let infcx = selcx.infcx(); let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty); + let cache_key = ProjectionCacheKey { ty: projection_ty }; debug!("opt_normalize_projection_type(\ projection_ty={:?}, \ @@ -411,7 +415,7 @@ fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>( // bounds. It might be the case that we want two distinct caches, // or else another kind of cache entry. - match infcx.projection_cache.borrow_mut().try_start(projection_ty) { + match infcx.projection_cache.borrow_mut().try_start(cache_key) { Ok(()) => { } Err(ProjectionCacheEntry::Ambiguous) => { // If we found ambiguity the last time, that generally @@ -522,7 +526,7 @@ fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>( obligations, } }; - infcx.projection_cache.borrow_mut().complete(projection_ty, &result); + infcx.projection_cache.borrow_mut().insert_ty(cache_key, &result); Some(result) } Ok(ProjectedTy::NoProgress(projected_ty)) => { @@ -533,14 +537,14 @@ fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>( value: projected_ty, obligations: vec![] }; - infcx.projection_cache.borrow_mut().complete(projection_ty, &result); + infcx.projection_cache.borrow_mut().insert_ty(cache_key, &result); Some(result) } Err(ProjectionTyError::TooManyCandidates) => { debug!("opt_normalize_projection_type: \ too many candidates"); infcx.projection_cache.borrow_mut() - .ambiguous(projection_ty); + .ambiguous(cache_key); None } Err(ProjectionTyError::TraitSelectionError(_)) => { @@ -551,7 +555,7 @@ fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>( // reported later infcx.projection_cache.borrow_mut() - .error(projection_ty); + .error(cache_key); Some(normalize_to_error(selcx, param_env, projection_ty, cause, depth)) } } @@ -1323,8 +1327,62 @@ fn assoc_ty_def<'cx, 'gcx, 'tcx>( // # Cache +/// The projection cache. Unlike the standard caches, this can +/// include infcx-dependent type variables - therefore, we have to roll +/// the cache back each time we roll a snapshot back, to avoid assumptions +/// on yet-unresolved inference variables. Types with skolemized regions +/// also have to be removed when the respective snapshot ends. +/// +/// Because of that, projection cache entries can be "stranded" and left +/// inaccessible when type variables inside the key are resolved. We make no +/// attempt to recover or remove "stranded" entries, but rather let them be +/// (for the lifetime of the infcx). +/// +/// Entries in the projection cache might contain inference variables +/// that will be resolved by obligations on the projection cache entry - e.g. +/// when a type parameter in the associated type is constrained through +/// an "RFC 447" projection on the impl. +/// +/// When working with a fulfillment context, the derived obligations of each +/// projection cache entry will be registered on the fulfillcx, so any users +/// that can wait for a fulfillcx fixed point need not care about this. However, +/// users that don't wait for a fixed point (e.g. trait evaluation) have to +/// resolve the obligations themselves to make sure the projected result is +/// ok and avoid issues like #43132. +/// +/// If that is done, after evaluation the obligations, it is a good idea to +/// call `ProjectionCache::complete` to make sure the obligations won't be +/// re-evaluated and avoid an exponential worst-case. +/// +/// FIXME: we probably also want some sort of cross-infcx cache here to +/// reduce the amount of duplication. Let's see what we get with the Chalk +/// reforms. pub struct ProjectionCache<'tcx> { - map: SnapshotMap, ProjectionCacheEntry<'tcx>>, + map: SnapshotMap, ProjectionCacheEntry<'tcx>>, +} + +#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)] +pub struct ProjectionCacheKey<'tcx> { + ty: ty::ProjectionTy<'tcx> +} + +impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> { + pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>, + predicate: &ty::PolyProjectionPredicate<'tcx>) + -> Option + { + let infcx = selcx.infcx(); + // We don't do cross-snapshot caching of obligations with escaping regions, + // so there's no cache key to use + infcx.tcx.no_late_bound_regions(&predicate) + .map(|predicate| ProjectionCacheKey { + // We don't attempt to match up with a specific type-variable state + // from a specific call to `opt_normalize_projection_type` - if + // there's no precise match, the original cache entry is "stranded" + // anyway. + ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty) + }) + } } #[derive(Clone, Debug)] @@ -1337,7 +1395,7 @@ enum ProjectionCacheEntry<'tcx> { // NB: intentionally not Clone pub struct ProjectionCacheSnapshot { - snapshot: Snapshot + snapshot: Snapshot, } impl<'tcx> ProjectionCache<'tcx> { @@ -1356,7 +1414,7 @@ impl<'tcx> ProjectionCache<'tcx> { } pub fn rollback_skolemized(&mut self, snapshot: &ProjectionCacheSnapshot) { - self.map.partial_rollback(&snapshot.snapshot, &|k| k.has_re_skol()); + self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_skol()); } pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) { @@ -1366,7 +1424,7 @@ impl<'tcx> ProjectionCache<'tcx> { /// Try to start normalize `key`; returns an error if /// normalization already occurred (this error corresponds to a /// cache hit, so it's actually a good thing). - fn try_start(&mut self, key: ty::ProjectionTy<'tcx>) + fn try_start(&mut self, key: ProjectionCacheKey<'tcx>) -> Result<(), ProjectionCacheEntry<'tcx>> { if let Some(entry) = self.map.get(&key) { return Err(entry.clone()); @@ -1377,25 +1435,51 @@ impl<'tcx> ProjectionCache<'tcx> { } /// Indicates that `key` was normalized to `value`. - fn complete(&mut self, key: ty::ProjectionTy<'tcx>, value: &NormalizedTy<'tcx>) { - debug!("ProjectionCacheEntry::complete: adding cache entry: key={:?}, value={:?}", + fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: &NormalizedTy<'tcx>) { + debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}", key, value); let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value.clone())); assert!(!fresh_key, "never started projecting `{:?}`", key); } + /// Mark the relevant projection cache key as having its derived obligations + /// complete, so they won't have to be re-computed (this is OK to do in a + /// snapshot - if the snapshot is rolled back, the obligations will be + /// marked as incomplete again). + pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) { + let ty = match self.map.get(&key) { + Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => { + debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}", + key, ty); + ty.value + } + ref value => { + // Type inference could "strand behind" old cache entries. Leave + // them alone for now. + debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}", + key, value); + return + } + }; + + self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized { + value: ty, + obligations: vec![] + })); + } + /// Indicates that trying to normalize `key` resulted in /// ambiguity. No point in trying it again then until we gain more /// type information (in which case, the "fully resolved" key will /// be different). - fn ambiguous(&mut self, key: ty::ProjectionTy<'tcx>) { + fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) { let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous); assert!(!fresh, "never started projecting `{:?}`", key); } /// Indicates that trying to normalize `key` resulted in /// error. - fn error(&mut self, key: ty::ProjectionTy<'tcx>) { + fn error(&mut self, key: ProjectionCacheKey<'tcx>) { let fresh = self.map.insert(key, ProjectionCacheEntry::Error); assert!(!fresh, "never started projecting `{:?}`", key); } diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs index 46bdb1344b2..7d555f1d0be 100644 --- a/src/librustc/traits/select.rs +++ b/src/librustc/traits/select.rs @@ -16,7 +16,7 @@ use self::EvaluationResult::*; use super::coherence; use super::DerivedObligationCause; use super::project; -use super::project::{normalize_with_depth, Normalized}; +use super::project::{normalize_with_depth, Normalized, ProjectionCacheKey}; use super::{PredicateObligation, TraitObligation, ObligationCause}; use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation}; use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch}; @@ -655,8 +655,14 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> { let project_obligation = obligation.with(data.clone()); match project::poly_project_and_unify_type(self, &project_obligation) { Ok(Some(subobligations)) => { - self.evaluate_predicates_recursively(previous_stack, - subobligations.iter()) + let result = self.evaluate_predicates_recursively(previous_stack, + subobligations.iter()); + if let Some(key) = + ProjectionCacheKey::from_poly_projection_predicate(self, data) + { + self.infcx.projection_cache.borrow_mut().complete(key); + } + result } Ok(None) => { EvaluatedToAmbig