improve error when self is used as not the first argument

src/libsyntax/parse/parser.rs

@@ -1824,6 +1824,14 @@ impl<'a> Parser<'a> {
     fn parse_arg_general(&mut self, require_name: bool) -> PResult<'a, Arg> {
         maybe_whole!(self, NtArg, |x| x);
 
+        if let Ok(Some(_)) = self.parse_self_arg() {
+            let mut err = self.struct_span_err(self.prev_span,
+                "unexpected `self` argument in function");
+            err.span_label(self.prev_span,
+                "`self` is only valid as the first argument of a trait function");
+            return Err(err);
+        }
+
         let (pat, ty) = if require_name || self.is_named_argument() {
             debug!("parse_arg_general parse_pat (require_name:{})",
                    require_name);
@@ -5387,13 +5395,6 @@ impl<'a> Parser<'a> {
                      -> PResult<'a, (Vec<Arg> , bool)> {
         self.expect(&token::OpenDelim(token::Paren))?;
         
-        if let Ok(Some(_)) = self.parse_self_arg() {
-            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;
         let mut variadic = false;
         let args: Vec<Option<Arg>> =

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

@@ -1,5 +0,0 @@
-fn a(&self) { }
-//~^ ERROR unexpected `self` argument in bare function
-//~| NOTE invalid argument in bare function
-
-fn main() { }

src/test/ui/invalid-self-argument/bare-fn-start.rs

@@ -0,0 +1,5 @@
+fn a(&self) { }
+//~^ ERROR unexpected `self` argument in function
+//~| NOTE `self` is only valid as the first argument of a trait function
+
+fn main() { }

src/test/ui/invalid-self-argument/bare-fn.rs

@@ -0,0 +1,5 @@
+fn b(foo: u32, &mut self) { }
+//~^ ERROR unexpected `self` argument in function
+//~| NOTE `self` is only valid as the first argument of a trait function
+
+fn main() { }

src/test/ui/invalid-self-argument/trait-fn.rs

@@ -0,0 +1,11 @@
+struct Foo {}
+
+impl Foo {
+    fn c(foo: u32, self) {}
+    //~^ ERROR unexpected `self` argument in function
+    //~| NOTE `self` is only valid as the first argument of a trait function
+
+    fn good(&mut self, foo: u32) {}
+}
+
+fn main() { }