Add tests for custom_code_classes_in_docs feature

tests/rustdoc-ui/custom_code_classes_in_docs-warning.rs

@@ -0,0 +1,19 @@
+// This test ensures that warnings are working as expected for "custom_code_classes_in_docs"
+// feature.
+
+#![feature(custom_code_classes_in_docs)]
+#![deny(warnings)]
+#![feature(no_core)]
+#![no_core]
+
+/// ```{. class= whatever=hehe #id} } {{
+/// main;
+/// ```
+//~^^^ ERROR missing class name after `.`
+//~| ERROR missing class name after `class=`
+//~| ERROR unsupported attribute `whatever=hehe`
+//~| ERROR unsupported attribute `#id`
+//~| ERROR unexpected `}` outside attribute block (`{}`)
+//~| ERROR unclosed attribute block (`{}`): missing `}` at the end
+//~| ERROR unexpected `{` inside attribute block (`{}`)
+pub fn foo() {}