rustc: avoid checking Trait's predicates for WF(<T as Trait>::X).

Author: eddyb

src/librustc/ty/wf.rs

@@ -53,7 +53,7 @@ pub fn trait_obligations<'a, 'tcx>(
     item: Option<&'tcx hir::Item>,
 ) -> Vec<traits::PredicateObligation<'tcx>> {
     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item };
-    wf.compute_trait_ref(trait_ref, Elaborate::All);
+    wf.compute_trait_ref(trait_ref, Elaborate::All, false);
     wf.normalize()
 }
 
@@ -69,7 +69,7 @@ pub fn predicate_obligations<'a, 'tcx>(
     // (*) ok to skip binders, because wf code is prepared for it
     match *predicate {
         ty::Predicate::Trait(ref t) => {
-            wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
+            wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None, false); // (*)
         }
         ty::Predicate::RegionOutlives(..) => {
         }
@@ -163,14 +163,18 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
     }
 
     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
-    fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
+    fn compute_trait_ref(
+        &mut self,
+        trait_ref: &ty::TraitRef<'tcx>,
+        elaborate: Elaborate,
+        is_proj: bool,
+    ) {
         let tcx = self.infcx.tcx;
-        let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 
         let cause = self.cause(traits::MiscObligation);
         let param_env = self.param_env;
 
-        let item = &self.item;
+        let item = self.item;
         let extend_cause_with_original_assoc_item_obligation = |
             cause: &mut traits::ObligationCause<'_>,
             pred: &ty::Predicate<'_>,
@@ -313,26 +317,30 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
             }
         };
 
-        if let Elaborate::All = elaborate {
-            let trait_assoc_items = tcx.associated_items(trait_ref.def_id);
-
-            let predicates = obligations.iter()
-                .map(|obligation| obligation.predicate.clone())
-                .collect();
-            let implied_obligations = traits::elaborate_predicates(tcx, predicates);
-            let implied_obligations = implied_obligations.map(|pred| {
-                let mut cause = cause.clone();
-                extend_cause_with_original_assoc_item_obligation(
-                    &mut cause,
-                    &pred,
-                    trait_assoc_items.clone(),
-                );
-                traits::Obligation::new(cause, param_env, pred)
-            });
-            self.out.extend(implied_obligations);
-        }
+        if !is_proj {
+            let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
+
+            if let Elaborate::All = elaborate {
+                let trait_assoc_items = tcx.associated_items(trait_ref.def_id);
+
+                let predicates = obligations.iter()
+                    .map(|obligation| obligation.predicate.clone())
+                    .collect();
+                let implied_obligations = traits::elaborate_predicates(tcx, predicates);
+                let implied_obligations = implied_obligations.map(|pred| {
+                    let mut cause = cause.clone();
+                    extend_cause_with_original_assoc_item_obligation(
+                        &mut cause,
+                        &pred,
+                        trait_assoc_items.clone(),
+                    );
+                    traits::Obligation::new(cause, param_env, pred)
+                });
+                self.out.extend(implied_obligations);
+            }
 
-        self.out.extend(obligations);
+            self.out.extend(obligations);
+        }
 
         self.out.extend(trait_ref.substs.types()
             .filter(|ty| !ty.has_escaping_bound_vars())
@@ -350,7 +358,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         // WF and (b) the trait-ref holds.  (It may also be
         // normalizable and be WF that way.)
         let trait_ref = data.trait_ref(self.infcx.tcx);
-        self.compute_trait_ref(&trait_ref, Elaborate::None);
+        self.compute_trait_ref(&trait_ref, Elaborate::None, true);
 
         if !data.has_escaping_bound_vars() {
             let predicate = trait_ref.to_predicate();

src/test/ui/associated-types/associated-types-overridden-binding.stderr

@@ -6,13 +6,6 @@ LL | trait Foo: Iterator<Item = i32> {}
 LL | trait Bar: Foo<Item = u32> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0282]: type annotations needed
-  --> $DIR/associated-types-overridden-binding.rs:7:1
-   |
-LL | trait U32Iterator = I32Iterator<Item = u32>;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0282, E0284.
-For more information about an error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0284`.

src/test/ui/issues/issue-20831-debruijn.stderr

@@ -1,65 +1,3 @@
-error[E0308]: mismatched types
-  --> $DIR/issue-20831-debruijn.rs:28:5
-   |
-LL | /     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
-LL | |         // Not obvious, but there is an implicit lifetime here -------^
-LL | |
-LL | |
-...  |
-LL | |         self.sub = t;
-LL | |     }
-   | |_____^ lifetime mismatch
-   |
-   = note: expected type `'a`
-              found type `'_`
-note: the anonymous lifetime #2 defined on the method body at 28:5...
-  --> $DIR/issue-20831-debruijn.rs:28:5
-   |
-LL | /     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
-LL | |         // Not obvious, but there is an implicit lifetime here -------^
-LL | |
-LL | |
-...  |
-LL | |         self.sub = t;
-LL | |     }
-   | |_____^
-note: ...does not necessarily outlive the lifetime `'a` as defined on the impl at 26:6
-  --> $DIR/issue-20831-debruijn.rs:26:6
-   |
-LL | impl<'a> Publisher<'a> for MyStruct<'a> {
-   |      ^^
-
-error[E0308]: mismatched types
-  --> $DIR/issue-20831-debruijn.rs:28:5
-   |
-LL | /     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
-LL | |         // Not obvious, but there is an implicit lifetime here -------^
-LL | |
-LL | |
-...  |
-LL | |         self.sub = t;
-LL | |     }
-   | |_____^ lifetime mismatch
-   |
-   = note: expected type `'a`
-              found type `'_`
-note: the lifetime `'a` as defined on the impl at 26:6...
-  --> $DIR/issue-20831-debruijn.rs:26:6
-   |
-LL | impl<'a> Publisher<'a> for MyStruct<'a> {
-   |      ^^
-note: ...does not necessarily outlive the anonymous lifetime #2 defined on the method body at 28:5
-  --> $DIR/issue-20831-debruijn.rs:28:5
-   |
-LL | /     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
-LL | |         // Not obvious, but there is an implicit lifetime here -------^
-LL | |
-LL | |
-...  |
-LL | |         self.sub = t;
-LL | |     }
-   | |_____^
-
 error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
   --> $DIR/issue-20831-debruijn.rs:28:5
    |
@@ -92,7 +30,6 @@ LL | impl<'a> Publisher<'a> for MyStruct<'a> {
            expected Publisher<'_>
               found Publisher<'_>
 
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0308, E0495.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0495`.