Tracking Issue for const unreachable_unchecked

[gnzlbg]

Currently one cannot call unreachable_unchecked in constant functions, so one cannot add hints of the form:

const fn foo(x: i32) -> i32 {
    if x < 0 { unreachable_unchecked() } else { x * 2 } 
}

These types of hints can heavily influence code generation, which can substantially change the performance of const fn when they are invoked at run-time.

It would be a shame to have to write and call foo_compile_time and foo_run_time because the code generated for foo_compile_time is sub-par.

This issue has been assigned to @canova via this comment.

[oli-obk]

One cannot do any sort of conditional control flow, so unreachable_unchecked would currently be less than useless. Any test you can write would be undefined behaviour

[gnzlbg]

@oli-obk the RFC for if/match in const was merged and is pending an implementation right? When that's implemented, will unreachable_unchecked still be useless?

[oli-obk]

No it makes sense after we have conditional control flow.

[q2p]

As I understand if / match in const functions is now implemented.
Can unreachable_unchecked be made into a const fn now?

[oli-obk]

yes. Basically you just need to add the rustc_const_unstable attribute to the intrinsic and wrapper function. This is documented at https://rustc-dev-guide.rust-lang.org/stability.html?highlight=rustc_const_un#rustc_const_unstable Let me know if anything is unclear (in which case we should update the guide)

[canova]

Hi, It looks like previous PR is not being worked on currently. I'm working on this and will create a PR soon.
@rustbot claim

[josephlr]

As mentioned in #76209, this issue should be reopened (to track stablization)

[tesuji]

cc @RalfJung ^

[jhpratt]

After a short discussion on Zulip, it was decided that it's best to wait for const_panic to stabilize to avoid a potential footgun wherein users use this instead of panic!(), leading to UB at runtime.

I'm subscribed to the const_panic issue and intend to file a PR to stabilize this once the aforementioned happens.