I tried this code:

#[unsafe(no_mangle)] pub fn foo() -> u64 { let mut a = 0u64; for _ in 0..1024 { a ^= 1; } a }

I compiled it with -Copt-level=3 on godbolt, and got this assembly:

foo: mov eax, 1024 .LBB0_1: add eax, -32 jne .LBB0_1 xor eax, eax ret 

For some reason, the generated assembly counts from 1024 to 0 in a loop that does nothing. This seems wasteful.

Other operations that flip-flop between two states also have a similar effect. For example, a = -a;, or a = !a;.

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Compiler version on godbolt:

rustc 1.90.0 (1159e78c4 2025-09-14) binary: rustc commit-hash: 1159e78c4747b02ef996e55082b704c09b970588 commit-date: 2025-09-14 host: x86_64-unknown-linux-gnu release: 1.90.0 LLVM version: 20.1.8 Internal compiler ID: r1900