Mention that `self` is only valid on "associated functions"
```
error: unexpected `self` argument in function
--> $DIR/self-in-function-arg.rs:1:15
|
LL | fn foo(x:i32, self: i32) -> i32 { self }
| ^^^^ not valid as function argument
|
= note: `self` is only valid as the first argument of an associated function
```
When it is a method, mention it must be first
```
error: unexpected `self` argument in function
--> $DIR/trait-fn.rs:4:20
|
LL | fn c(foo: u32, self) {}
| ^^^^ must be the first associated function argument
```
