Rollup merge of #84811 - scottmcm:rustdoc-trait-alias-fix, r=jyn514

RustDoc: Fix bounds linking trait.Foo instead of traitalias.Foo

Fixes #84782

The code was assuming `Trait` when adding bounds to the cache, so add a check on the DefId to see what its kind really is.

r? `@jyn514`

Before:
![image](https://user-images.githubusercontent.com/18526288/116775611-6a751e80-aa53-11eb-84d0-ed6b7782be3c.png)

After:
![image](https://user-images.githubusercontent.com/18526288/116802227-d19cdc80-ab00-11eb-8133-7b34dd750da2.png)

3 files changed, 15 insertions, 0 deletions

diff --git a/src/test/rustdoc/auxiliary/trait-alias-mention.rs b/src/test/rustdoc/auxiliary/trait-alias-mention.rs
new file mode 100644
index 00000000000..6df06c87a09
--- /dev/null
+++ b/src/test/rustdoc/auxiliary/trait-alias-mention.rs
@@ -0,0 +1,3 @@
+#![feature(trait_alias)]
+
+pub trait SomeAlias = std::fmt::Debug + std::marker::Copy;
diff --git a/src/test/rustdoc/trait-alias-mention.rs b/src/test/rustdoc/trait-alias-mention.rs
new file mode 100644
index 00000000000..6da0dc68785
--- /dev/null
+++ b/src/test/rustdoc/trait-alias-mention.rs
@@ -0,0 +1,10 @@
+// aux-build:trait-alias-mention.rs
+// build-aux-docs
+
+#![crate_name = "foo"]
+
+extern crate trait_alias_mention;
+
+// @has foo/fn.mention_alias_in_bounds.html '//a[@href="../trait_alias_mention/traitalias.SomeAlias.html"]' 'SomeAlias'
+pub fn mention_alias_in_bounds<T: trait_alias_mention::SomeAlias>() {
+}
diff --git a/src/test/rustdoc/trait_alias.rs b/src/test/rustdoc/trait_alias.rs
index 98b8d879ac0..6cd4a1a0afa 100644
--- a/src/test/rustdoc/trait_alias.rs
+++ b/src/test/rustdoc/trait_alias.rs
@@ -19,3 +19,5 @@ pub trait CopyAlias = Copy;
 pub trait Alias2 = Copy + Debug;
 // @has foo/traitalias.Foo.html '//section[@id="main"]/pre' 'trait Foo<T> = Into<T> + Debug;'
 pub trait Foo<T> = Into<T> + Debug;
+// @has foo/fn.bar.html '//a[@href="traitalias.Alias2.html"]' 'Alias2'
+pub fn bar<T>() where T: Alias2 {}