better error message for trailing doc comment in traits (issue #56766)

src/libsyntax/parse/parser.rs

@@ -5940,6 +5940,14 @@ impl<'a> Parser<'a> {
             self.expect(&token::OpenDelim(token::Brace))?;
             let mut trait_items = vec![];
             while !self.eat(&token::CloseDelim(token::Brace)) {
+                if let token::DocComment(_) = self.token {
+                    if self.look_ahead(1, |tok| tok == &token::Token::CloseDelim(token::Brace)) {
+                        self.span_fatal_err(self.span, Error::UselessDocComment).emit();
+                        self.bump();
+                        continue;
+                    }
+                }
+
                 let mut at_end = false;
                 match self.parse_trait_item(&mut at_end) {
                     Ok(item) => trait_items.push(item),
@@ -7676,7 +7684,7 @@ impl<'a> Parser<'a> {
             &mut self.token_cursor.stack[prev].last_token
         };
 
-        // Pull our the toekns that we've collected from the call to `f` above
+        // Pull our the tokens that we've collected from the call to `f` above
         let mut collected_tokens = match *last_token {
             LastToken::Collecting(ref mut v) => mem::replace(v, Vec::new()),
             LastToken::Was(_) => panic!("our vector went away?"),

src/test/ui/useless-doc-comment/trait-impl.rs

@@ -0,0 +1,14 @@
+//! issue #56766
+//!
+//! invalid doc comment at the end of a trait declaration
+
+trait Foo {
+    ///
+    fn foo(&self);
+    /// I am not documenting anything
+    //~^ ERROR: found a documentation comment that doesn't document anything [E0585]
+}
+
+fn main() {
+
+}

src/test/ui/useless-doc-comment/trait-impl.stderr

@@ -0,0 +1,11 @@
+error[E0585]: found a documentation comment that doesn't document anything
+  --> $DIR/trait-impl.rs:8:5
+   |
+LL |     /// I am not documenting anything
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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 E0585`.