rust-lang · Manishearth · May 23, 2020 · May 30, 2020 · Jun 6, 2020 · Jun 7, 2020
diff --git a/src/librustc_infer/infer/outlives/verify.rs b/src/librustc_infer/infer/outlives/verify.rs
@@ -1,9 +1,8 @@
 use crate::infer::outlives::env::RegionBoundPairs;
 use crate::infer::{GenericKind, VerifyBound};
-use crate::traits;
 use rustc_data_structures::captures::Captures;
 use rustc_hir::def_id::DefId;
-use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst};
+use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
 /// The `TypeOutlives` struct has the job of "lowering" a `T: 'a`
@@ -311,18 +310,14 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
     fn region_bounds_declared_on_associated_item(
         &self,
         assoc_item_def_id: DefId,
-    ) -> impl Iterator<Item = ty::Region<'tcx>> + 'cx + Captures<'tcx> {
+    ) -> impl Iterator<Item = ty::Region<'tcx>> {
         let tcx = self.tcx;
-        let assoc_item = tcx.associated_item(assoc_item_def_id);
-        let trait_def_id = assoc_item.container.assert_trait();
-        let trait_predicates = tcx.predicates_of(trait_def_id).predicates.iter().map(|(p, _)| *p);
-        let identity_substs = InternalSubsts::identity_for_item(tcx, assoc_item_def_id);
-        let identity_proj = tcx.mk_projection(assoc_item_def_id, identity_substs);
-        self.collect_outlives_from_predicate_list(
-            move |ty| ty == identity_proj,
-            traits::elaborate_predicates(tcx, trait_predicates).map(|o| o.predicate),
-        )
-        .map(|b| b.1)
+        let predicates = tcx.projection_predicates(assoc_item_def_id);
+        predicates
+            .into_iter()
+            .filter_map(|p| p.to_opt_type_outlives())
+            .filter_map(|p| p.no_bound_vars())
+            .map(|b| b.1)
     }
 
     /// Searches through a predicate list for a predicate `T: 'a`.

diff --git a/src/librustc_infer/traits/mod.rs b/src/librustc_infer/traits/mod.rs
@@ -25,8 +25,6 @@ pub use self::project::{
     Normalized, NormalizedTy, ProjectionCache, ProjectionCacheEntry, ProjectionCacheKey,
     ProjectionCacheStorage, Reveal,
 };
-crate use self::util::elaborate_predicates;
-
 pub use rustc_middle::traits::*;
 
 /// An `Obligation` represents some trait reference (e.g., `int: Eq`) for

diff --git a/src/librustc_middle/query/mod.rs b/src/librustc_middle/query/mod.rs
@@ -133,6 +133,23 @@ rustc_queries! {
             cache_on_disk_if { key.is_local() }
         }
 
+        /// Returns the list of predicates that can be used for
+        /// `SelectionCandidate::ProjectionCandidate` and
+        /// `ProjectionTyCandidate::TraitDef`.
+        /// Specifically this is the bounds (equivalent to) those
+        /// written on the trait's type definition, or those
+        /// after the `impl` keyword
+        ///
+        /// type X: Bound + 'lt
+        ///         ^^^^^^^^^^^
+        /// impl Debug + Display
+        ///      ^^^^^^^^^^^^^^^
+        ///
+        /// `key` is the `DefId` of the associated type or opaque type.
+        query projection_predicates(key: DefId) -> &'tcx ty::List<ty::Predicate<'tcx>> {
+            desc { |tcx| "finding projection predicates for `{}`", tcx.def_path_str(key) }
+        }
+
         query native_libraries(_: CrateNum) -> Lrc<Vec<NativeLib>> {
             desc { "looking up the native libraries of a linked crate" }
         }

diff --git a/src/librustc_middle/ty/flags.rs b/src/librustc_middle/ty/flags.rs
@@ -93,7 +93,6 @@ impl FlagComputation {
 
             &ty::Bound(debruijn, _) => {
                 self.add_binder(debruijn);
-                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
 
             &ty::Placeholder(..) => {
@@ -216,7 +215,6 @@ impl FlagComputation {
             }
             ty::ConstKind::Bound(debruijn, _) => {
                 self.add_binder(debruijn);
-                self.add_flags(TypeFlags::STILL_FURTHER_SPECIALIZABLE);
             }
             ty::ConstKind::Param(_) => {
                 self.add_flags(TypeFlags::HAS_CT_PARAM);

diff --git a/src/librustc_middle/ty/sty.rs b/src/librustc_middle/ty/sty.rs
@@ -1583,19 +1583,16 @@ impl RegionKind {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_INFER;
-                flags = flags | TypeFlags::STILL_FURTHER_SPECIALIZABLE;
             }
             ty::RePlaceholder(..) => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_PLACEHOLDER;
-                flags = flags | TypeFlags::STILL_FURTHER_SPECIALIZABLE;
             }
             ty::ReEarlyBound(..) => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;
                 flags = flags | TypeFlags::HAS_FREE_LOCAL_REGIONS;
                 flags = flags | TypeFlags::HAS_RE_PARAM;
-                flags = flags | TypeFlags::STILL_FURTHER_SPECIALIZABLE;
             }
             ty::ReFree { .. } => {
                 flags = flags | TypeFlags::HAS_FREE_REGIONS;

diff --git a/src/librustc_middle/ty/subst.rs b/src/librustc_middle/ty/subst.rs
@@ -333,6 +333,17 @@ impl<'a, 'tcx> InternalSubsts<'tcx> {
     /// in a different item, with `target_substs` as the base for
     /// the target impl/trait, with the source child-specific
     /// parameters (e.g., method parameters) on top of that base.
+    ///
+    /// For example given:
+    ///
+    /// trait X<S> { fn f<T>(); }
+    /// impl<U> X<U> for U { fn f<V>() {} }
+    ///
+    /// * If `self` is `[Self, S, T]`: the identity substs of `f` in the trait.
+    /// * If `source_ancestor` is the def_id of the trait.
+    /// * If `target_substs` is `[U]`, the substs for the impl.
+    /// * Then we will return `[U, T]`, the subst for `f` in the impl that
+    ///   are needed for it to match the trait.
     pub fn rebase_onto(
         &self,
         tcx: TyCtxt<'tcx>,

diff --git a/src/librustc_resolve/late/lifetimes.rs b/src/librustc_resolve/late/lifetimes.rs
@@ -883,7 +883,6 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                         })
                         .collect();
                     if !lifetimes.is_empty() {
-                        self.trait_ref_hack = true;
                         let next_early_index = self.next_early_index();
                         let scope = Scope::Binder {
                             lifetimes,
@@ -895,9 +894,10 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                         let result = self.with(scope, |old_scope, this| {
                             this.check_lifetime_params(old_scope, &bound_generic_params);
                             this.visit_ty(&bounded_ty);
+                            this.trait_ref_hack = true;
                             walk_list!(this, visit_param_bound, bounds);
+                            this.trait_ref_hack = false;
                         });
-                        self.trait_ref_hack = false;
                         result
                     } else {
                         self.visit_ty(&bounded_ty);
@@ -932,13 +932,15 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
         debug!("visit_poly_trait_ref(trait_ref={:?})", trait_ref);
 
         let should_pop_missing_lt = self.is_trait_ref_fn_scope(trait_ref);
-        if !self.trait_ref_hack
+
+        let trait_ref_hack = take(&mut self.trait_ref_hack);
+        if !trait_ref_hack
             || trait_ref.bound_generic_params.iter().any(|param| match param.kind {
                 GenericParamKind::Lifetime { .. } => true,
                 _ => false,
             })
         {
-            if self.trait_ref_hack {
+            if trait_ref_hack {
                 struct_span_err!(
                     self.tcx.sess,
                     trait_ref.span,
@@ -968,10 +970,11 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                 this.check_lifetime_params(old_scope, &trait_ref.bound_generic_params);
                 walk_list!(this, visit_generic_param, trait_ref.bound_generic_params);
                 this.visit_trait_ref(&trait_ref.trait_ref);
-            })
+            });
         } else {
             self.visit_trait_ref(&trait_ref.trait_ref);
         }
+        self.trait_ref_hack = trait_ref_hack;
         if should_pop_missing_lt {
             self.missing_named_lifetime_spots.pop();
         }

diff --git a/src/librustc_trait_selection/traits/project.rs b/src/librustc_trait_selection/traits/project.rs
@@ -895,9 +895,12 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
 
     let tcx = selcx.tcx();
     // Check whether the self-type is itself a projection.
-    let (def_id, substs) = match obligation_trait_ref.self_ty().kind {
-        ty::Projection(ref data) => (data.trait_ref(tcx).def_id, data.substs),
-        ty::Opaque(def_id, substs) => (def_id, substs),
+    // If so, extract what we know from the trait and try to come up with a good answer.
+    let bounds = match obligation_trait_ref.self_ty().kind {
+        ty::Projection(ref data) => {
+            tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs)
+        }
+        ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs),
         ty::Infer(ty::TyVar(_)) => {
             // If the self-type is an inference variable, then it MAY wind up
             // being a projected type, so induce an ambiguity.
@@ -907,17 +910,13 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
         _ => return,
     };
 
-    // If so, extract what we know from the trait and try to come up with a good answer.
-    let trait_predicates = tcx.predicates_of(def_id);
-    let bounds = trait_predicates.instantiate(tcx, substs);
-    let bounds = elaborate_predicates(tcx, bounds.predicates.into_iter()).map(|o| o.predicate);
     assemble_candidates_from_predicates(
         selcx,
         obligation,
         obligation_trait_ref,
         candidate_set,
         ProjectionTyCandidate::TraitDef,
-        bounds,
+        bounds.iter(),
     )
 }
 
@@ -1474,6 +1473,12 @@ fn confirm_impl_candidate<'cx, 'tcx>(
         );
         return Progress { ty: tcx.types.err, obligations: nested };
     }
+    // If we're trying to normalize `<Vec<u32> as X>::A<S>` using
+    //`impl<T> X for Vec<T> { type A<Y> = Box<Y>; }`, then:
+    //
+    // * `obligation.predicate.substs` is `[Vec<u32>, S]`
+    // * `substs` is `[u32]`
+    // * `substs` ends up as `[u32, S]`
     let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
     let substs =
         translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.defining_node);

diff --git a/src/librustc_trait_selection/traits/select/mod.rs b/src/librustc_trait_selection/traits/select/mod.rs
@@ -1273,9 +1273,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             placeholder_trait_predicate,
         );
 
-        let (def_id, substs) = match placeholder_trait_predicate.trait_ref.self_ty().kind {
-            ty::Projection(ref data) => (data.trait_ref(self.tcx()).def_id, data.substs),
-            ty::Opaque(def_id, substs) => (def_id, substs),
+        let tcx = self.infcx.tcx;
+        let predicates = match placeholder_trait_predicate.trait_ref.self_ty().kind {
+            ty::Projection(ref data) => {
+                tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs)
+            }
+            ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs),
             _ => {
                 span_bug!(
                     obligation.cause.span,
@@ -1285,32 +1288,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 );
             }
         };
-        debug!(
-            "match_projection_obligation_against_definition_bounds: \
-             def_id={:?}, substs={:?}",
-            def_id, substs
-        );
 
-        let predicates_of = self.tcx().predicates_of(def_id);
-        let bounds = predicates_of.instantiate(self.tcx(), substs);
-        debug!(
-            "match_projection_obligation_against_definition_bounds: \
-             bounds={:?}",
-            bounds
-        );
-
-        let elaborated_predicates =
-            util::elaborate_predicates(self.tcx(), bounds.predicates.into_iter());
-        let matching_bound = elaborated_predicates.filter_to_traits().find(|bound| {
-            self.infcx.probe(|_| {
-                self.match_projection(
-                    obligation,
-                    *bound,
-                    placeholder_trait_predicate.trait_ref,
-                    &placeholder_map,
-                    snapshot,
-                )
-            })
+        let matching_bound = predicates.iter().find_map(|bound| {
+            if let ty::PredicateKind::Trait(bound, _) = bound.kind() {
+                let bound = bound.to_poly_trait_ref();
+                if self.infcx.probe(|_| {
+                    self.match_projection(
+                        obligation,
+                        bound,
+                        placeholder_trait_predicate.trait_ref,
+                        &placeholder_map,
+                        snapshot,
+                    )
+                }) {
+                    return Some(bound);
+                }
+            }
+            None
         });
 
         debug!(