Improve error message for opaque captures

compiler/rustc_error_codes/src/error_codes/E0657.md

@@ -1,57 +1,26 @@
-A lifetime bound on a trait implementation was captured at an incorrect place.
+An `impl Trait` captured a higher-ranked lifetime, which is not supported.
+
+Currently, `impl Trait` types are only allowed to capture lifetimes from
+their parent items, and not from any `for<'a>` binders in scope.
 
 Erroneous code example:
 
 ```compile_fail,E0657
-trait Id<T> {}
-trait Lt<'a> {}
+trait BorrowInto<'a> {
+    type Target;
 
-impl<'a> Lt<'a> for () {}
-impl<T> Id<T> for T {}
-
-fn free_fn_capture_hrtb_in_impl_trait()
-    -> Box<for<'a> Id<impl Lt<'a>>> // error!
-{
-    Box::new(())
+    fn borrow_into(&'a self) -> Self::Target;
 }
 
-struct Foo;
-impl Foo {
-    fn impl_fn_capture_hrtb_in_impl_trait()
-        -> Box<for<'a> Id<impl Lt<'a>>> // error!
-    {
-        Box::new(())
+impl<'a> BorrowInto<'a> for () {
+    type Target = &'a ();
+
+    fn borrow_into(&'a self) -> Self::Target {
+        self
     }
 }
-```
 
-Here, you have used the inappropriate lifetime in the `impl Trait`,
-The `impl Trait` can only capture lifetimes bound at the fn or impl
-level.
-
-To fix this we have to define the lifetime at the function or impl
-level and use that lifetime in the `impl Trait`. For example you can
-define the lifetime at the function:
-
-```
-trait Id<T> {}
-trait Lt<'a> {}
-
-impl<'a> Lt<'a> for () {}
-impl<T> Id<T> for T {}
-
-fn free_fn_capture_hrtb_in_impl_trait<'b>()
-    -> Box<for<'a> Id<impl Lt<'b>>> // ok!
-{
-    Box::new(())
-}
-
-struct Foo;
-impl Foo {
-    fn impl_fn_capture_hrtb_in_impl_trait<'b>()
-        -> Box<for<'a> Id<impl Lt<'b>>> // ok!
-    {
-        Box::new(())
-    }
+fn opaque() -> impl for<'a> BorrowInto<'a, Target = impl Sized + 'a> {
+    ()
 }
 ```