Using `&mut Trait` requires `mut` local variable

[alexcrichton]

This code:

use std::rt::io;

fn foo(a: &mut io::Writer) {
    a.write([])
}

fn main(){}

Yields this error:

foo.rs:4:4: 4:5 error: cannot borrow immutable argument as mutable
foo.rs:4     a.write([])
             ^
error: aborting due to previous error

Which doesn't make sense to me because a shouldn't have to be mutable because it's already a pointer to a mutable thing (which is what io::Writer wants)

cc @msullivan

[alexcrichton]

Yay, this works now!

Flagging as needstest