`debug_assert_eq` still requires the type(s) implement `PartialEq` in release mode

[dcormier]

It can be surprising that debug_assert_eq still requires the type implement PartialEq in release mode.

I tried this code:

fn main() {
    let one = Thing;
    let two = Thing;

    debug_assert_eq!(one, two);
}

#[derive(Debug)]
#[cfg_attr(debug_assertions, derive(PartialEq))]
struct Thing;

I expected to see this happen: The debug_assert_eq macro would not require PartialEq when debug_assertions is not enabled (i.e. release mode).

Instead, this happened: In release mode, debug_assert_eq was expanded in such a way that PartialEq is still required to be implemented, even though it's never executed.

Meta

rustc --version --verbose:

rustc 1.89.0 (29483883e 2025-08-04)
binary: rustc
commit-hash: 29483883eed69d5fb4db01964cdf2af4d86e9cb2
commit-date: 2025-08-04
host: aarch64-apple-darwin
release: 1.89.0
LLVM version: 20.1.7

As well as in the playground on stable (1.90.0), 1.91.0-beta.4 (2025-09-27 aa7859c0dec2de6c9cab), and 1.92.0-nightly (2025-09-30 fa3155a644dd62e86582).

[tgross35]

This is expected: the docs for https://doc.rust-lang.org/std/macro.debug_assert.html state:

The result of expanding debug_assert! is always type checked.

Implementing PartialEq only in debug mode is quite an unusual pattern, why not just implement it all the time? If you really need this to work, you can gate a normal assert yourself rather than using debug_assert! (or define your own assert macro that acts this way):

#[cfg(debug_assertions)]
assert_eq!(...);

Cc @rust-lang/libs but I'm pretty sure we don't want to change the behavior here.