Add test for doc alias attribute validation

Author: GuillaumeGomez

src/test/rustdoc-ui/check-doc-alias-attr.rs

@@ -0,0 +1,9 @@
+#![feature(doc_alias)]
+
+#[doc(alias = "foo")] // ok!
+pub struct Bar;
+
+#[doc(alias)] //~ ERROR
+#[doc(alias = 0)] //~ ERROR
+#[doc(alias("bar"))] //~ ERROR
+pub struct Foo;

src/test/rustdoc-ui/check-doc-alias-attr.stderr

@@ -0,0 +1,20 @@
+error: doc alias attribute expects a string: #[doc(alias = "0")]
+  --> $DIR/check-doc-alias-attr.rs:6:7
+   |
+LL | #[doc(alias)]
+   |       ^^^^^
+
+error: doc alias attribute expects a string: #[doc(alias = "0")]
+  --> $DIR/check-doc-alias-attr.rs:7:7
+   |
+LL | #[doc(alias = 0)]
+   |       ^^^^^^^^^
+
+error: doc alias attribute expects a string: #[doc(alias = "0")]
+  --> $DIR/check-doc-alias-attr.rs:8:7
+   |
+LL | #[doc(alias("bar"))]
+   |       ^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+