-Copt-level=2 may miscompile valid code

[shao-hua-li]

I tried this code:

pub fn a() -> i32 {
    return 0;
}
pub fn b() -> i32 {
    return 0;
}
pub fn main() {
    if Some(b as fn() -> i32) != Some(a as fn() -> i32) {
        println!("OK\n");
    }
}

I expected to see this happen: -Copt-level=0/1/2/3 should give the same result

Instead, this happened:

% rustc test.rs -Copt-level=1 &&./test
OK
% rustc test.rs -Copt-level=2 &&./test
%

Meta

rustc --version --verbose:

rustc 1.84.0-nightly (9322d183f 2024-10-14)
binary: rustc
commit-hash: 9322d183f45e0fd5a509820874cc5ff27744a479
commit-date: 2024-10-14
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1

[saethlin]

This is not a bug, you're tripping over rust-lang/unsafe-code-guidelines#522.

In short:

Functions, vtables, and consts do not have a unique stable address

The program above has different behavior because the functions are merged at some opt levels. Note that function addresses truly are unstable, not only can you get merges like this, a single function can have multiple addresses in a single program.