Supplying an &&str when an &str is expected missing a suggestion

[gilescope]

    let a = std::collections::HashMap::<String,String>::new();
    let s = "hello";
    let _b = a[&s];
               ^ & not needed, this is already a reference

playground

But the error we currently get is:

error[E0277]: the trait bound `std::string::String: std::borrow::Borrow<&str>` is not satisfied
 --> src/main.rs:4:14
  |
4 |     let _b = a[&s];
  |              ^^^^^ the trait `std::borrow::Borrow<&str>` is not implemented for `std::string::String`
  |
  = help: the following implementations were found:
            <std::string::String as std::borrow::Borrow<str>>
  = note: required because of the requirements on the impl of `std::ops::Index<&&str>` for `std::collections::HashMap<std::string::String, std::string::String>`

Adding one too many layers of indirection is fairly common in rust especially with String / &str so it would be excellent if Rust could hint to the user how to fix this.

[gilescope]

If the index was just a normal method, the below code compiles with no errors:

fn fn_with_ref_arg(_s: &str) {}

fn main() {
    let s = "hello";
    let _b = fn_with_ref_arg(&s);
}

I'm assuming that's because the compiler will dereference any number of times till it gets a type match.

Rather than improve the error message, should we be dereferencing indexes as many times as necessary for consistency?