Add check for doc alias on assoc const in trait impl

compiler/rustc_passes/src/check_attr.rs

@@ -250,13 +250,23 @@ impl CheckAttrVisitor<'tcx> {
                                     None
                                 }
                             }
+                            Target::AssocConst => {
+                                let parent_hir_id = self.tcx.hir().get_parent_item(hir_id);
+                                let containing_item = self.tcx.hir().expect_item(parent_hir_id);
+                                // We can't link to trait impl's consts.
+                                let err = "associated constant in trait implementation block";
+                                match containing_item.kind {
+                                    ItemKind::Impl { of_trait: Some(_), .. } => Some(err),
+                                    _ => None,
+                                }
+                            }
                             _ => None,
                         } {
                             self.tcx
                                 .sess
                                 .struct_span_err(
                                     meta.span(),
-                                    &format!("`#[doc(alias = \"...\")]` isn't allowed on {}", err,),
+                                    &format!("`#[doc(alias = \"...\")]` isn't allowed on {}", err),
                                 )
                                 .emit();
                         }

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
+