adding test

src/test/ui/parser/doc-inside-trait-item.rs

@@ -0,0 +1,6 @@
+trait User{
+    fn test();
+    /// empty doc
+    //~^ ERROR found a documentation comment that doesn't document anything
+}
+fn main() {}

src/test/ui/parser/doc-inside-trait-item.stderr

@@ -0,0 +1,11 @@
+error[E0584]: found a documentation comment that doesn't document anything
+  --> $DIR/doc-inside-trait-item.rs:3:5
+   |
+LL |     /// empty doc
+   |     ^^^^^^^^^^^^^
+   |
+   = help: doc comments must come before what they document, maybe a comment was intended with `//`?
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0584`.