improve err msg for "cannot coerce non-statically resolved bare fn"

[pnkfelix]

fn bar() {
    for i in range(0, 10u) {
        println!("{}", i)
    }
}

fn callit(f: ||) { f() }

fn main() {
    // callit(bar);
    let f = bar;
    callit(f)
}

play-pen

prints

<anon>:12:12: 12:13 error: cannot coerce non-statically resolved bare fn
<anon>:12     callit(f)
                     ^

We can do better here, e.g. by suggesting to the user that they make the closure construction explicit via eta-expansion, i.e. writing callit(||f()) (or callit(|x|g(x)), etc).

[huonw]

It would be nice to say "cannot coerce ... to closure" explicitly too.

[Kleptine]

I'm attempting this as an easy first fix. Are there guidelines for Rust's stance on error message length / style / complexity? I've got a simple fix that just makes the error message: "error: cannot coerce non-statically resolved bare fn to closure. Consider embedding the fn in a closure: callit(||f()) or callit(|x|f(g))".

Currently working to actually embed the names into the message.

[pnkfelix]

@Kleptine I think we often put the longer info into a separate Note, via sess.note or sess.span_note. Would that work out for your "Consider embedding ..." ?