Compiler panics on missing 'fn'.

[ghost]

Lets say you want to write this:

struct A {
    n: i32,
}

impl A {
    fn n(&self) -> i32 {
        self.n
    }
}

but you forget the fn

struct A {
    n: i32,
}

impl A {
    n(&self) -> i32 {
        self.n
    }
}

then the compiler responds with "note: the compiler unexpectedly panicked. this is a bug." I don't know if its possible but a "missing fn" message would be ideal or at least a "expected one of ! or ::, found ("

[petrochenkov]

This should be fixed in #41282 and the fix is going to appear in the next nightly.