Reviving conversation around trait fields (RFC#1546)

language design
3

Thanks for the follow-up! Still thinking about the privacy case.

How about if the types don't line up?

Since the compiler is evaluating the trait when it encounters the impl, it could be able to validate the types (and ops involved). Taking a leaf out of your example:

trait Foo {
    fn call(&self) -> &str { &self.x }
}

struct Bar {
    x: i32
}

// The compiler can emit type error for the method call()
// expected &str; found i32 for this impl
impl Foo for Bar {}

Similarly, the one below may also fail at the point of impl

trait Foo {
    fn by_hundred(&self) -> i32 { self.x / 100 }
}

struct Bar {
    x: String
}

// This too will fail with error message about std::ops::Div
// not present for String 
impl Foo for Bar {}