Add test cases for unused math attrs

Author: notriddle

tests/rustdoc-ui/math/unused-doc-syntax.rs

@@ -0,0 +1,25 @@
+#![feature(rustdoc_texmath)]
+#![deny(unused_doc_comments)]
+
+#[doc(syntax="+tex_math_dollars")]
+pub fn foo() {}
+
+pub fn bar() {
+    #[doc(syntax="+tex_math_dollars")]
+    // there should be an unused_doc_comments line here
+    fn baz() {}
+    #[doc(syntax="+tex_math_dollars")]
+    // there should be an unused_doc_comments line here
+    struct Baz;
+}
+
+#[doc(syntax="+tex_math_dollars")]
+//~^ ERROR unused_doc_comments
+unsafe extern "C" {
+    pub unsafe fn quux();
+}
+
+unsafe extern "C" {
+    #[doc(syntax="+tex_math_dollars")]
+    pub unsafe fn zzyzx();
+}

tests/rustdoc-ui/math/unused-doc-syntax.stderr

@@ -0,0 +1,20 @@
+error: unused doc comment
+  --> $DIR/unused-doc-syntax.rs:16:1
+   |
+LL |   #[doc(syntax="+tex_math_dollars")]
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL | / unsafe extern "C" {
+LL | |     pub unsafe fn quux();
+LL | | }
+   | |_- rustdoc does not generate documentation for extern blocks
+   |
+   = help: use `//` for a plain comment
+note: the lint level is defined here
+  --> $DIR/unused-doc-syntax.rs:2:9
+   |
+LL | #![deny(unused_doc_comments)]
+   |         ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+