Error message for mutating field of immutable struct could be clearer

struct Foo {
    pub v: Vec<String>
}

fn main() {
    let f = Foo { v: Vec::new() };
    f.v.push("cat".to_string());
}

The above code gives this compile error:

<anon>:8:5: 8:8 error: cannot borrow immutable field `f.v` as mutable
<anon>:8     f.v.push("cat".to_string());
             ^~~

From my perspective, this implies that the field somehow needs to be marked as mutable, when in fact the user error is that the struct is immutable.

I believe the error message could be improved by saying something like cannot borrow field vof immutable structf as mutable.
#18150 is "related".

[steveklabnik]

Triage: no change. I agree that "immutable field" is bad here.

[Mark-Simulacrum]

Probably needs an extension of #39139 to immutable bindings instead of just references. Unfamiliar with the code, but possibly easy?