update comments

tests/ui/type-alias-impl-trait/wf-check-definition-site.rs

@@ -4,6 +4,11 @@
 //
 // `Opaque<'a> = Static<&'a str>`, vs
 // `Opaque<'a> = Static<&'static str>`.
+//
+// The hidden type of the opaque ends up as `Static<'?0 str>`. When
+// computing member constraints we end up choosing `'a` for `?0` unless
+// `?0` is already required to outlive `'a`. We achieve this by checking
+// that `Static<'?0 str>` is well-formed.
 #![feature(type_alias_impl_trait)]
 
 struct Static<T: 'static>(T);

tests/ui/type-alias-impl-trait/wf-nested.rs

@@ -1,12 +1,8 @@
 // Well-formedness of nested opaque types, i.e. `impl Sized` in
-// `type Outer = impl Trait<Assoc = impl Sized>`.
-// See the comments below.
-//
-//@ revisions: pass pass_sound fail
-//@ [pass] check-fail
-//@ [pass_sound] check-fail
-//@ [fail] check-fail
-
+// `type Outer = impl Trait<Assoc = impl Sized>`. We check that
+// the nested type is well-formed, even though this would also
+// be implied by the item bounds of the opaque being
+// well-formed. See the comments below.
 #![feature(type_alias_impl_trait)]
 
 struct IsStatic<T: 'static>(T);
@@ -23,46 +19,26 @@ impl<T> Trait<&'static T> for () {
     type Out = IsStatic<T>;
 }
 
-// The hidden type for `impl Sized` is `IsStatic<T>`, which requires `T: 'static`.
-// We know it is well-formed because it can *only* be referenced as a projection:
-// <OuterOpaque<T> as Trait<&'static T>>::Out`.
-// So any instantiation of the type already requires proving `T: 'static`.
-#[cfg(pass)]
-mod pass {
-    use super::*;
-    type OuterOpaque<T> = impl Trait<&'static T, Out = impl Sized>;
-    fn define<T>() -> OuterOpaque<T> {}
-    //[pass]~^ ERROR `T` may not live long enough
-
-    fn define_rpit<T>() -> impl Trait<&'static T, Out = impl Sized> {}
-    //[pass]~^ ERROR the parameter type `T` may not live long enough
-}
-
-// Test the soundness of `pass` - We should require `T: 'static` at the use site.
-#[cfg(pass_sound)]
-mod pass_sound {
-    use super::*;
-    type OuterOpaque<T> = impl Trait<&'static T, Out = impl Sized>;
-    fn define<T>() -> OuterOpaque<T> {}
-    //[pass_sound]~^ ERROR `T` may not live long enough
 
-    fn test<T>() {
-        let outer = define::<T>();
-        let _ = outer.get();
-        //[pass_sound]~^ ERROR `T` may not live long enough
-        //[pass_sound]~| ERROR `T` may not live long enough
-    }
-}
-
-// Similar to `pass` but here `impl Sized` can be referenced directly as
-// InnerOpaque<T>, so we require an explicit bound `T: 'static`.
-#[cfg(fail)]
-mod fail {
-    use super::*;
-    type InnerOpaque<T> = impl Sized;
-    type OuterOpaque<T> = impl Trait<&'static T, Out = InnerOpaque<T>>;
-    fn define<T>() -> OuterOpaque<T> {}
-    //[fail]~^ ERROR the parameter type `T` may not live long enough
-}
+// We could theoretically allow this (and previously did), as even
+// though the nested opaque is not well-formed, it can only be
+// used by normalizing the projection
+//    <OuterOpaque1<T> as Trait<&'static T>>::Out
+// Assuming that we check that this projection is well-formed, the wf
+// of the nested opaque is implied.
+type OuterOpaque1<T> = impl Trait<&'static T, Out = impl Sized>;
+fn define<T>() -> OuterOpaque1<T> {}
+//~^ ERROR `T` may not live long enough
+
+fn define_rpit<T>() -> impl Trait<&'static T, Out = impl Sized> {}
+//~^ ERROR the parameter type `T` may not live long enough
+
+// Similar to `define` but here `impl Sized` can be referenced directly as
+// InnerOpaque<T>, so the `'static` bound is definitely required for
+// soundness.
+type InnerOpaque<T> = impl Sized;
+type OuterOpaque2<T> = impl Trait<&'static T, Out = InnerOpaque<T>>;
+fn define_nested_rpit<T>() -> OuterOpaque2<T> {}
+//~^ ERROR the parameter type `T` may not live long enough
 
 fn main() {}

tests/ui/type-alias-impl-trait/wf-nested.stderr

@@ -0,0 +1,45 @@
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/wf-nested.rs:30:35
+   |
+LL | fn define<T>() -> OuterOpaque1<T> {}
+   |                                   ^^
+   |                                   |
+   |                                   the parameter type `T` must be valid for the static lifetime...
+   |                                   ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL | fn define<T: 'static>() -> OuterOpaque1<T> {}
+   |            +++++++++
+
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/wf-nested.rs:33:65
+   |
+LL | fn define_rpit<T>() -> impl Trait<&'static T, Out = impl Sized> {}
+   |                                                                 ^^
+   |                                                                 |
+   |                                                                 the parameter type `T` must be valid for the static lifetime...
+   |                                                                 ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL | fn define_rpit<T: 'static>() -> impl Trait<&'static T, Out = impl Sized> {}
+   |                 +++++++++
+
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/wf-nested.rs:41:47
+   |
+LL | fn define_nested_rpit<T>() -> OuterOpaque2<T> {}
+   |                                               ^^
+   |                                               |
+   |                                               the parameter type `T` must be valid for the static lifetime...
+   |                                               ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound
+   |
+LL | fn define_nested_rpit<T: 'static>() -> OuterOpaque2<T> {}
+   |                        +++++++++
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0310`.