Lint against unnecessary borrow_mut when borrow is enough

What it does

Suggest replacing (try_)borrow_mut with (try_)borrow when the returned RefMut is not mutable and is not moved.

Advantage

Using borrow_mut but not using the mut ref is probably a bug

Drawbacks

No response

Example

use std::cell::RefCell;

fn f(cell: &RefCell<String>) {
    let Ok(s) = cell.try_borrow_mut() else {
        return;
    };
    println!("{}", s.len());
}

Could be written as:

use std::cell::RefCell;

fn f(cell: &RefCell<String>) {
    let Ok(s) = cell.try_borrow() else {
        return;
    };
    println!("{}", s.len());
}

Comparison with existing lints

No response

Additional Context

No response

Start with the issue's Rust example and compare the borrow_mut and borrow cases, then locate the existing Clippy lint entry point for borrow-related analysis. Done means the lint identifies unused mutability in the shown pattern without flagging cases where the mutable reference is needed or moved, with coverage for the demonstrated behavior.