Index chosen over IndexMut when coercing index type

[ramosbugs]

We've run into an issue where we have a type that implements both Index<&str> and IndexMut<&str>. Passing an index value of &String causes rustc to choose Index instead of IndexMut, even if a mutable reference is required. If we first call String::as_str and use that as the index, IndexMut is chosen, as expected.

Example repro by @idubrov (see https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9f0e5d703465d4b87218cac8b1eea608):

struct Indexable;

impl Indexable {
    fn boo(&mut self) {
    }
}

impl std::ops::Index<&str> for Indexable {
    type Output = Indexable;
    
    fn index(&self, field: &str) -> &Indexable {
        self
    }
}

impl std::ops::IndexMut<&str> for Indexable {
    fn index_mut(&mut self, field: &str) -> &mut Indexable {
        self
    }
}

fn main() {
    let mut v = Indexable;
    let field = "hello".to_string();

    // Works    
    v[field.as_str()].boo();
    
    // Doesn't work    
    v[&field].boo();
}

I expected to see this happen: v[&field] should invoke IndexMut on v since boo() expects a mutable reference.

Instead, this happened: v[&field] invoked Index and complained that IndexMut isn't implemented even though it is:

error[E0596]: cannot borrow data in an index of `Indexable` as mutable
  --> src/main.rs:30:5
   |
30 |     v[&field].boo();
   |     ^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `Indexable`

Meta

This is reproducible on stable Rust 1.41.0 and 1.43.0 and 1.45.0-nightly (2020-05-06 1836e3b42a5b2f37fd79).

cc: @estebank

[estebank]

I have a hacky solution to this in #72068. Looking this code up in godbolt I can reproduce it all the way to 1.0, earlier versions giving a type error (&str vs &String), then an ICE in the 1.2x (that I was causing during my experiments to fix this: error: internal compiler error: re-trying op failed) and finally the current error. The problem is caused (somehow) in convert_place_op_to_mutable slightly differing from try_index_step, but haven't figured how to fix that underlying difference.

[timotree3]

In case this helps to find the root problem, #65682 gives a similar error: "IndexMut is not implemented" even though it really is.

[estebank]

It seems closely related but #72068 sadly doesn't change the behavior on that ticket. I'll take a look at it though.