Address review comments

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1280,14 +1280,14 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             concrete_is_opaque
                         );
 
-                        // concrete_is_opaque is 'true' when we're using an existential
-                        // type without 'revelaing' it. For example, code like this:
+                        // concrete_is_opaque is `true` when we're using an existential
+                        // type without 'revealing' it. For example, code like this:
                         //
                         // existential type Foo: Debug;
                         // fn foo1() -> Foo { ... }
                         // fn foo2() -> Foo { foo1() }
                         //
-                        // In 'foo2', we're not revealing the type of 'Foo' - we're
+                        // In `foo2`, we're not revealing the type of `Foo` - we're
                         // just treating it as the opaque type.
                         //
                         // When this occurs, we do *not* want to try to equate

src/librustc_typeck/check/writeback.rs

@@ -587,6 +587,11 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
             }
 
             if !opaque_defn.substs.has_local_value() {
+                // We only want to add an entry into `concrete_existential_types`
+                // if we actually found a defining usage of this existential type.
+                // Otherwise, we do nothing - we'll either find a defining usage
+                // in some other location, or we'll end up emitting an error due
+                // to the lack of defining usage
                 if !skip_add {
                     let new = ty::ResolvedOpaqueTy {
                         concrete_type: definition_ty,

src/test/ui/existential_types/existential_type_const.rs

@@ -1,3 +1,5 @@
+// check-pass
+
 #![feature(existential_type)]
 #![feature(impl_trait_in_bindings)]
 //~^ WARN the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash

src/test/ui/existential_types/existential_type_fns.rs

@@ -1,3 +1,5 @@
+// check-pass
+
 #![feature(existential_type)]
 
 // Regression test for issue #61863

src/test/ui/existential_types/existential_type_tuple.rs

@@ -1,5 +1,6 @@
-#![feature(existential_type)]
+// check-pass
 
+#![feature(existential_type)]
 #![allow(dead_code)]
 
 pub trait MyTrait {}