Add back missing trait test

Rustdoc has been able to deal with `impl trait` for almost a year now.

src/test/rustdoc-ui/coverage/traits.rs

@@ -2,6 +2,7 @@
 // check-pass
 
 #![feature(trait_alias)]
+#![feature(min_type_alias_impl_trait)]
 
 /// look at this trait right here
 pub trait ThisTrait {
@@ -30,10 +31,8 @@ impl ThisTrait for SomeStruct {
 /// but what about those aliases? i hear they're pretty exotic
 pub trait MyAlias = ThisTrait + Send + Sync;
 
-// FIXME(58624): once rustdoc can process opaque `impl Trait` types,
-// we need to make sure they're counted
-// /// woah, getting all opaque in here
-// pub type ThisExists = impl ThisTrait;
-//
-// /// why don't we get a little more concrete
-// pub fn defines() -> ThisExists { SomeStruct {} }
+/// woah, getting all opaque in here
+pub type ThisExists = impl ThisTrait;
+
+/// why don't we get a little more concrete
+pub fn defines() -> ThisExists { SomeStruct {} }

src/test/rustdoc-ui/coverage/traits.stdout

@@ -1,7 +1,7 @@
 +-------------------------------------+------------+------------+------------+------------+
 | File                                | Documented | Percentage |   Examples | Percentage |
 +-------------------------------------+------------+------------+------------+------------+
-| ...st/rustdoc-ui/coverage/traits.rs |          6 |      85.7% |          0 |       0.0% |
+| ...st/rustdoc-ui/coverage/traits.rs |          8 |      88.9% |          0 |       0.0% |
 +-------------------------------------+------------+------------+------------+------------+
-| Total                               |          6 |      85.7% |          0 |       0.0% |
+| Total                               |          8 |      88.9% |          0 |       0.0% |
 +-------------------------------------+------------+------------+------------+------------+