cmp_null suggests incorrect code, and --fix applies it

[zkrising]

Summary

debug_assert!(ptr != std::ptr::null_mut()) will be suggested to be replaced with debug_assert!(ptr.is_null()), which has the opposite meaning

I found this by running clippy --fix on my codebase.

Reproducer

I tried this code:

let f: *mut i32 = std::ptr::null_mut();

debug_assert!(f != std::ptr::null_mut());

I expected to see this happen:

Use debug_assert!(!f.is_null())` instead

Instead, this happened:

warning: comparing with null is better expressed by the `.is_null()` method
   --> main.rs:165:17
    |
165 |         debug_assert!(f != std::ptr::null_mut());
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `f.is_null()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cmp_null
    = note: `#[warn(clippy::cmp_null)]` on by default

Version

rustc 1.89.0-nightly (4b27a04cc 2025-06-04)
binary: rustc
commit-hash: 4b27a04cc8ed4da10a546a871e23e665d03f7a79
commit-date: 2025-06-04
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

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