Add test for doc alias on associated const in trait impls

src/test/rustdoc-ui/doc-alias-assoc-const.rs

@@ -0,0 +1,22 @@
+#![feature(doc_alias)]
+#![feature(trait_alias)]
+
+pub struct Foo;
+
+pub trait Bar {
+    const BAZ: u8;
+}
+
+impl Bar for Foo {
+    #[doc(alias = "CONST_BAZ")] //~ ERROR
+    const BAZ: u8 = 0;
+}
+
+impl Foo {
+    #[doc(alias = "CONST_FOO")] // ok!
+    pub const FOO: u8 = 0;
+
+    pub fn bar() -> u8 {
+        Self::FOO
+    }
+}

src/test/rustdoc-ui/doc-alias-assoc-const.stderr

@@ -0,0 +1,8 @@
+error: `#[doc(alias = "...")]` isn't allowed on associated constant in trait implementation block
+  --> $DIR/doc-alias-assoc-const.rs:11:11
+   |
+LL |     #[doc(alias = "CONST_BAZ")]
+   |           ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+