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constify from_fn, try_from_fn, try_map, map #147071

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constify from_fn, try_from_fn, try_map, map #147071

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@bend-n bend-n commented Sep 26, 2025 •
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adds the const_array feature
reimplements try_map in more or less the same way

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 26, 2025
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@bend-n bend-n marked this pull request as ready for review September 26, 2025 18:44
@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 26, 2025
@rustbot rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 26, 2025
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r? @ibraheemdev

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oli-obk
oli-obk requested changes Oct 11, 2025
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library/core/src/array/mod.rs Outdated
else {
// SAFETY: this slice will contain only initialized objects.
unsafe {
x.array_mut.get_unchecked_mut(..x.initialized).assume_init_drop();
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What part of this isn't const evaluable? The get_unchecked_mut?

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@bend-n bend-n Oct 11, 2025 •
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assume_init_drop, and changing it to be const had some rammifications for some reason, causing some tests to fail. #147071 (comment)

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Looks like just trivial stderr file changes. Feel free to just bless them.

But is that code path testable in const? Since it's only happening in a panic path, I think it's unreachable anyway in const contexts. They never unwind and never will unwind. So maybe just document that and move the entire drop method body into the non-const code path... Or can we keep it non-const and avoid any drop happening for it by "disarming it" in the success path?

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its used in try_from_fn, so it can get dropped in const context?

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Ah right. Hmm. Wondering how to write an observable test for this. I think if you use a type with a reference to a Cell and in its const drop impl increment that cell, you should see in a const context that the cell isn't incremented if try_map errors/returns None.

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Ah oops, wanted to write this test but forgot about it when I was on a computer.

Need to reinvestigate this to get it all back into cache.

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It oughtnt really be needed now, as the behaviour is identical to the non const version?

oli-obk
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@oli-obk oli-obk assigned oli-obk and unassigned ibraheemdev Oct 11, 2025
oli-obk
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rustbot commented Oct 11, 2025

The Miri subtree was changed

cc @rust-lang/miri

@bend-n bend-n force-pushed the const_array-ops branch 3 times, most recently from e95a529 to 0021344 Compare October 11, 2025 11:43
oli-obk
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@bend-n bend-n force-pushed the const_array-ops branch 2 times, most recently from 43edafa to 7bec7a4 Compare October 11, 2025 12:03
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bend-n
bend-n commented Oct 11, 2025
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library/core/src/array/drain.rs Outdated
where
F: [const] FnMut(T) -> U,
{
extern "rust-call" fn call_mut(&mut self, (i,): (usize,)) -> Self::Output {
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@bend-n bend-n Oct 11, 2025 •
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it would be possible to ignore this i and just use moved, but, uh, i think that would be somewhat confusing.

@bend-n bend-n mentioned this pull request Oct 12, 2025
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library/core/src/array/drain.rs Outdated
func(drain)
#[rustc_const_unstable(feature = "array_try_map", issue = "79711")]
#[unstable(feature = "array_try_map", issue = "79711")]
pub(super) struct Drain<'a, T, U, const N: usize, F: FnMut(T) -> U> {
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The previous version had lots of docs explaining the weirdness of the type. Please add docs explaining what this type does and that it's emulating iterators which don't exist in const yet. Also add const hack fixmes

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rustbot commented Nov 24, 2025

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

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@bend-n bend-n force-pushed the const_array-ops branch 2 times, most recently from f435a2d to 03360fe Compare November 24, 2025 15:49
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@bend-n bend-n force-pushed the const_array-ops branch 3 times, most recently from d2077d0 to 21593b2 Compare November 24, 2025 20:06
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@bend-n bend-n force-pushed the const_array-ops branch 6 times, most recently from 5abb182 to 598437f Compare November 26, 2025 06:29
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