clippy::replace_box suggests a MachineApplicable fix that is semantically questionable for Box<&mut T> #17549

Summary

Hi, thanks for the great tool!

Clippy’s replace_box lint flags Box::new(...) assignments to an existing Box and suggests writing to the inner value instead. The suggestion is marked MachineApplicable, but in the case where the boxed type is itself a mutable reference (Box<&mut T>), the suggested fix is semantically different from the original code and can be misleading regarding borrow/lifetime behavior.

Reproducer

fn main() {
    let mut raphael = 300;
    let mut samuel = Box::new(&mut raphael);

    **samuel += 500;

    let mut albert = 900;
    samuel = Box::new(&mut albert);
}

Clippy output

warning: creating a new box
  --> src/main.rs:9:5
   |
9  |     samuel = Box::new(&mut albert);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace existing content with inner value instead: `*samuel = &mut albert`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#replace_box

Issue
The two forms are not equivalent here:

// original (linted)
samuel = Box::new(&mut albert);

// suggested fix
*samuel = &mut albert;

Both assign a mutable reference into the Box, but the lint fires only on the Box::new(...) spelling and suggests rewriting it to the deref-assignment form. For Box<&mut T> this ignores the reference’s lifetime implications; the deref-assign form can behave differently (or even be rejected by the borrow checker) depending on the lifetimes involved. Since the suggestion is marked MachineApplicable, a user applying it may silently change semantics or end up with a borrow-check error for code that previously compiled.

Expected behavior
The suggestion should either:

be restricted to cases where the inner type is not itself a reference (or is otherwise lifetime-free), or
not be emitted as MachineApplicable for Box<&mut T>.

Version

Additional Labels

No response