Bad error message for missing DerefMut

[kmcallister]

use std::ops::{Deref, DerefMut};

struct Foo(u32);

impl Deref for Foo {
    type Target = u32;
    fn deref<'a>(&'a self) -> &'a u32 { &self.0 }
}

/*
impl DerefMut for Foo {
    fn deref_mut<'a>(&'a mut self) -> &'a mut u32 { &mut self.0 }
}
*/

fn main() {
    let mut foo = Foo(3);
    *foo = 4;
}

gives

foo.rs:18:5: 18:13 error: cannot assign to immutable borrowed content
foo.rs:18     *foo = 4;
              ^~~~~~~~

which is not too helpful. We shouldn't even try to borrow immutably in this case. Or at least tag the borrow with some metadata saying DerefMut wasn't available, so we can print a help: message here.

rustc 1.0.0-dev (ed530d7 2015-01-16 22:41:16 +0000)

Not sure this is actually easy to fix, but I think it'd be a good beginner project to spend a little time looking into it.

[ghost]

I'd like to try tackling this issue.

[ghost]

So after a lot of reading, I was wondering if it would just be better to present a warning in situations where the compiler prefers to use DerefMut but cannot. However, I am curious as to how one might add the metadata as @kmcallister suggested. I haven't yet understood a clear mechanism by which this could be done.

[steveklabnik]

Triage: no changes

[nikomatsakis]

Dup of #28419, closing in favor of that one as it has a bit more discussion.