Clean up special function const checks

[nbdd0121]

Mark them as const and #[rustc_do_not_const_check] instead of hard-coding them in const-eval checks.

r? @oli-obk
@rustbot label A-const-eval T-compiler

[Mark-Simulacrum]

I'm not very familiar with this code, but it looks like rustc_do_not_const_check also disables post-drop elaboration checking, but this doesn't add extra checks to that code in an obvious way, so maybe this has an unintended side effect of disabling that checking?

[nbdd0121]

I'm not very familiar with this code, but it looks like rustc_do_not_const_check also disables post-drop elaboration checking, but this doesn't add extra checks to that code in an obvious way, so maybe this has an unintended side effect of disabling that checking?

#[rustc_do_not_const_check] functions shouldn't be const checked at all regardless if const_precise_live_drops is enabled.

[fee1-dead]

Please remove the is_const_fn local and put this inside the block that sets it to true, since all the hooked fns are now const, we don't need to pass this as a parameter.

[fee1-dead]

I'm not very familiar with this code, but it looks like rustc_do_not_const_check also disables post-drop elaboration checking, but this doesn't add extra checks to that code in an obvious way, so maybe this has an unintended side effect of disabling that checking?

#[rustc_do_not_const_check] functions have bodies that are not run at compile time (they are "fake" const fns), since CTFE always intercepts those functions to call another actual const fn that can be run at compile time.

I added this because I needed to use ~const bounds in the const_eval_select intrinsic, which means it must be a const fn without a const body.

[fee1-dead]

r? @fee1-dead

[fee1-dead]

LGTM! Thanks!

[fee1-dead]

@bors r+

[bors]

📌 Commit 223f580 has been approved by fee1-dead

[bors]

⌛ Testing commit 223f580 with merge dd757b9...

[bors]

☀️ Test successful - checks-actions
Approved by: fee1-dead
Pushing dd757b9 to master...

[rust-timer]

Finished benchmarking commit (dd757b9): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression