Mutable borrow of unit-like struct is a weird edge case

[comex]

// We need a mutable borrow to an empty struct because it implements a trait
// that takes &mut self.
struct Empty;
impl Iterator<int> for Empty {
    fn next(&mut self) -> Option<int> { None }
}

fn do_something_with(a : &mut Iterator<int>) {
    println!("{}", a.next())
}

fn main() {
    // This is OK...
    let mut a = Empty;
    do_something_with(&mut a);
    // But if I want to use an immediate expression, as is possible with
    // non-empty structs...
    // "error: cannot borrow immutable static item as mutable"
    do_something_with(&mut Empty);
}

It seems like it would make sense to allow mutably borrowing the Empty singleton.

[glaebhoerl]

Seems related to #10800, possibly a dupe.

[dmski]

No longer reproduces with master for me.
cc #11681

[alexcrichton]

Flagging as needstest