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

[upsuper]

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.