const select_unpredictable

[clarfonthey]

Tracking issue: #145938

Since constants don't actually care about branch prediction, this just uses const_eval_select to use a normal if statement in const-time, instead of having to make the intrinsic work in const context.

[rustbot]

r? @tgross35

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[rust-log-analyzer]

The job aarch64-gnu-llvm-19-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    |
note: required by a bound in `select_predictable`
   --> library/core/src/hint.rs:807:12
    |
805 |     const fn select_predictable<T>(condition: bool, true_val: T, false_val: T) -> T
    |              ------------------ required by a bound in this function
806 |     where
807 |         T: [const] Destruct,
    |            ^^^^^^^^^^^^^^^^ required by this bound in `select_predictable`

error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_destruct)]`
   --> library/core/src/hint.rs:805:66
    |
805 |     const fn select_predictable<T>(condition: bool, true_val: T, false_val: T) -> T
    |                                                                  ^^^^^^^^^
    |
help: if the function is not (yet) meant to be exposed to stable const contexts, add `#[rustc_const_unstable]`
    |
805 +     #[rustc_const_unstable(feature = "...", issue = "...")]
806 |     const fn select_predictable<T>(condition: bool, true_val: T, false_val: T) -> T
    |

error: const function that might be (indirectly) exposed to stable cannot use `#[feature(const_destruct)]`
   --> library/core/src/hint.rs:805:53
    |
805 |     const fn select_predictable<T>(condition: bool, true_val: T, false_val: T) -> T
    |                                                     ^^^^^^^^
    |
help: if the function is not (yet) meant to be exposed to stable const contexts, add `#[rustc_const_unstable]`
    |
805 +     #[rustc_const_unstable(feature = "...", issue = "...")]
806 |     const fn select_predictable<T>(condition: bool, true_val: T, false_val: T) -> T
    |

For more information about this error, try `rustc --explain E0277`.
[RUSTC-TIMING] core test:false 23.229
error: could not compile `core` (lib) due to 3 previous errors

[bjorn3]

Wouldn't marking the select_unpredictable intrinsic itself as const work? It already has a fallback body equivalent to your select_predictable.

[clarfonthey]

Yeah, I probably should have thought of that first…

[clarfonthey]

I think that this would be blocked on #145725 then, if I did that. Which I'm fine doing, I think.

Since MaybeUninit::assume_init_drop requires ptr::drop_in_place.

[clarfonthey]

@rustbot blocked