Constified array::from_fn and ptr::drop_in_place

[fee1-dead]

• Add unit test
• Add tracking issue to feature

[rustbot]

r? @cuviper

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[rustbot]

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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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The Miri subtree was changed

cc @rust-lang/miri

[cuviper]

Isn't this a bad error? We're not actually calling the function that needs const drop, and this breaks stable code:

pub const fn get_drop<T>() -> unsafe fn(*mut T) {
    core::ptr::drop_in_place::<T>
}

[cuviper]

Similarly for array::from_fn:

pub const fn array_from_fn() -> fn(fn(usize) -> ()) -> [(); 42] {
    core::array::from_fn
}

... works on stable, but fails here for lack of ~const FnMut on the function pointer.

[cuviper]

Does this go back to the previous error after #109557?

[fee1-dead]

The const-eval working group has been discussing the status of const traits in the compiler as well as the standard library. We came to a consensus that all uses of ~const in the standard library should be temporary removed to allow for a clean impl of it to land in the compiler. See this zulip thread.

[cuviper]

If the current plan is to remove ~const, you may want to just close this PR.
@rustbot author