Improve diagnostic for missing `IndexMut: HashMap`

[estebank]

The following is invalid

let mut unique_things: HashMap<usize, bool>  = HashMap::new();
unique_things[&5] = false;

error[E0594]: cannot assign to data in an index of `HashMap<usize, bool>`
   --> src/main.rs:147:2
    |
147 |     unique_things[&5] = false;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ cannot assign
    |
    = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `HashMap<usize, bool>`

Because the entry &5 must exist before it can be assigned to. We should propose the use of the Entry APIs or .insert method.

https://users.rust-lang.org/t/problematic-errormessage-indexmut-is-required-to-modify-indexed-content/80158

[ChayimFriedman2]

@rustbot claim

[ChayimFriedman2]

The code responsible for generating this error is in compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs, but I don't feel like polluting it with detection of HashMap (and BTreeMap). Do you have a better idea?

[estebank]

It is not that uncommon for us to have weird one-off detection of types, although we do try to avoid it. Given that this is about HashMap and something a newcomer might reasonably attempt, I feel it is reasonable to do so. You should move the logic to its own method to avoid cluttering the happy path, though.

An option would be to check if the involved type has an .insert method that takes the relevant types, and try to rely on that convention so it will work with third party types as well, but even then I'd still want to tell people about Entry so you'd need to check for HashMap anyways.

[estebank]

CC #52788, #45491