lint internally mutable keys in maps

[llogiq]

I'm not sure if this is actually possible, but it is a logic error to change keys in hashmaps or btreemaps. So if we can identify common types with interior mutability, we could warn when finding those types as keys in a map.

[fhartwig]

It looks like the Cell types don't implement Ord or Hash, so it's probably quite difficult to accidentally use them in a key for any of the Map types.

[llogiq]

Yeah, I think #732 will suffice to ensure this, and is more to the point. Closing this.