Confusing error message on missing `self`, potentially indicating bigger issues with type parameters

[ironcev]

E.g., compiling this code:

library;

struct S { }

impl S {
    fn use_self() {
        let _ = self.x();
    }

    fn x(self) -> u64 {
        0
    }
}

emits:

5 | impl S {
6 |     fn use_self() {
7 |         let _ = self.x();
  |                 ^^^^ Identifier "self" was used as a variable, but it is actually a generic type parameter.

The error message is confusing, but aside from that, it looks like we might have a bigger issue here with injecting type parameters, because self is not a type parameter.