add a note to the error message

src/libsyntax/parse/parser.rs

@@ -5388,7 +5388,10 @@ fn parse_fn_args(&mut self, named_args: bool, allow_variadic: bool)
         self.expect(&token::OpenDelim(token::Paren))?;
 
         if let Ok(Some(_)) = self.parse_self_arg() {
-            return Err(self.fatal("unexpected `self` argument in bare function"))
+            let mut err = self.struct_span_err(self.prev_span
+                , "unexpected `self` argument in bare function");
+            err.span_label(self.prev_span, "invalid argument in bare function");
+            return Err(err);
         }
 
         let sp = self.span;

src/test/ui/bare-function-self.rs

@@ -1,4 +1,5 @@
 fn a(&self) { }
 //~^ ERROR unexpected `self` argument in bare function
+//~| NOTE invalid argument in bare function
 
 fn main() { }

src/test/ui/bare-function-self.stderr

@@ -1,8 +1,8 @@
 error: unexpected `self` argument in bare function
-  --> $DIR/bare-function-self.rs:12:11
+  --> $DIR/bare-function-self.rs:1:7
    |
 LL | fn a(&self) { }
-   |           ^
+   |       ^^^^ invalid argument in bare function
 
 error: aborting due to previous error