debug_assert_with_mut_call: false positive on immutable method call

Code:

pub fn f() {
    let v = &mut &mut Vec::<()>::new();
    debug_assert!(v.is_empty()); //~ ERROR do not call a function with mutable arguments inside of `debug_assert!`
}

Error:

error: do not call a function with mutable arguments inside of `debug_assert!`
  --> rust/src/lib.rs:17:19
   |
17 |     debug_assert!(v.is_empty());
   |                   ^^^^^^^^^^^^
   |
   = note: `#[deny(clippy::debug_assert_with_mut_call)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#debug_assert_with_mut_call

Reproduce the reported example with cargo clippy and search the debug_assert_with_mut_call lint implementation and its tests. Check how the lint treats the immutable is_empty call through nested mutable references; done means this example is no longer flagged while genuine mutable calls remain diagnosed.