improve error when self is used as not the first argument

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() { }