Consider this code:

fn main() { let a = (); let b = (); dbg!(std::ptr::eq(&a, &b)); }

It will output false in debug build, but true in release build.

I've tried other Zero-Sized Types (ZST), like an empty array and a struct with no field, and the result is still the same.

Interestingly, the code below will consistently output false in both debug build and release build:

fn main() { let a = (); let b = (); dbg!((&a as *const _) == (&b as *const _)); }

Playground

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rustc --version --verbose:

rustc 1.51.0 (2fd73fabe 2021-03-23) binary: rustc commit-hash: 2fd73fabe469357a12c2c974c140f67e7cdd76d0 commit-date: 2021-03-23 host: x86_64-apple-darwin release: 1.51.0 LLVM version: 11.0.1 

(Rust 1.51, 1.53, and nightly (2021-07-25) have this behavior.)