Avoid ICEing when associated type bound trait is missing

Author: spastorino

compiler/rustc_typeck/src/collect.rs

@@ -652,13 +652,21 @@ impl ItemCtxt<'tcx> {
         match b {
             hir::GenericBound::Trait(poly_trait_ref, _) => {
                 let trait_ref = &poly_trait_ref.trait_ref;
-                let trait_did = trait_ref.trait_def_id().unwrap();
-                super_traits_of(self.tcx, trait_did).any(|trait_did| {
-                    self.tcx
-                        .associated_items(trait_did)
-                        .find_by_name_and_kind(self.tcx, assoc_name, ty::AssocKind::Type, trait_did)
-                        .is_some()
-                })
+                if let Some(trait_did) = trait_ref.trait_def_id() {
+                    super_traits_of(self.tcx, trait_did).any(|trait_did| {
+                        self.tcx
+                            .associated_items(trait_did)
+                            .find_by_name_and_kind(
+                                self.tcx,
+                                assoc_name,
+                                ty::AssocKind::Type,
+                                trait_did,
+                            )
+                            .is_some()
+                    })
+                } else {
+                    false
+                }
             }
             _ => false,
         }

src/test/ui/associated-type-bounds/missing-trait-bound-for-assoc-fails.rs

@@ -0,0 +1,10 @@
+#[allow(dead_code)]
+fn foo<M>(_m: M)
+where
+    M::Item: Temp,
+    //~^ ERROR cannot find trait `Temp` in this scope [E0405]
+    //~| ERROR associated type `Item` not found for `M` [E0220]
+{
+}
+
+fn main() {}

src/test/ui/associated-type-bounds/missing-trait-bound-for-assoc-fails.stderr

@@ -0,0 +1,16 @@
+error[E0405]: cannot find trait `Temp` in this scope
+  --> $DIR/missing-trait-bound-for-assoc-fails.rs:4:14
+   |
+LL |     M::Item: Temp,
+   |              ^^^^ not found in this scope
+
+error[E0220]: associated type `Item` not found for `M`
+  --> $DIR/missing-trait-bound-for-assoc-fails.rs:4:8
+   |
+LL |     M::Item: Temp,
+   |        ^^^^ associated type `Item` not found
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0220, E0405.
+For more information about an error, try `rustc --explain E0220`.