Lint against `x.borrow()` / `x.borrow_mut()` in favor of `&x` and `&mut x` when fit

In a discussion in our Rust group today, we saw one wrote something like

// foo is owned, and foo.x has type HashMap
let x = foo.x.borrow_mut();
// something mutating through x

and then got trouble moving ownership of items from x (which is unrelated).

This code really confused me because I don't recall that HashMap has borrow_mut, until someone brought up that there is a blanket impl of BorrowMut for all type to their mutable reference.

It doesn't feel idiomatic to use this impl in normal code. If one wants to get a mutable reference, they should just use &mut. .borrow_mut() is more lengthy and confusing. Similar for .borrow().

Thus I suggest that Clippy should lint against use of .borrow() and .borrow_mut() via that blanket impl, and suggest using & and &mut instead correspondingly.

The issue names no source file, test, or entry point. Start by locating Clippy’s lint area and related borrow lints, then add coverage showing when .borrow() or .borrow_mut() should be replaced by references and verify the suggested replacements are accepted.