Add some conversion trait impls #145504

Jules-Bertholet · 2025-08-16T16:22:58Z

impl<T, const N: usize> From<[MaybeUninit<T>; N]> for MaybeUninit<[T; N]> (cc Tracking Issue for MaybeUninit methods for arrays #96097)
impl<T, const N: usize> AsRef<[MaybeUninit<T>; N]> for MaybeUninit<[T; N]> (cc Tracking Issue for MaybeUninit methods for arrays #96097)
impl<T, const N: usize> AsRef<[MaybeUninit<T>]> for MaybeUninit<[T; N]> (cc Tracking Issue for MaybeUninit methods for arrays #96097)
impl<T, const N: usize> AsMut<[MaybeUninit<T>; N]> for MaybeUninit<[T; N]> (cc Tracking Issue for MaybeUninit methods for arrays #96097)
impl<T, const N: usize> AsMut<[MaybeUninit<T>]> for MaybeUninit<[T; N]> (cc Tracking Issue for MaybeUninit methods for arrays #96097)
impl<T, const N: usize> From<MaybeUninit<[T; N]>> for [MaybeUninit<T>; N] (cc Tracking Issue for MaybeUninit methods for arrays #96097)
impl<T, const N: usize> AsRef<[Cell<T>; N]> for Cell<[T; N]> (equivalent of as_array_of_cells, cc Tracking Issue for Cell::as_array_of_cells #88248)
impl<T, const N: usize> AsRef<[Cell<T>]> for Cell<[T; N]>
impl<T> AsRef<[Cell<T>]> for Cell<[T]> (equivalent of as_slice_of_cells)

@rustbot label T-libs-api needs-fcp -T-libs

I’ve tried to only add impls that are unlikely to cause single-applicable-impl inference breakage.

rustbot · 2025-08-16T16:23:03Z

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

jieyouxu · 2025-08-17T01:49:17Z

r? libs-api

library/core/src/mem/maybe_uninit.rs

tgross35 · 2025-09-05T01:52:20Z

Preparing for crater

@bors try

@rustbot label +I-libs-api-nominated
r? libs

…r=<try> Add some conversion trait impls

tgross35 · 2025-09-05T02:47:42Z

library/core/src/ascii/ascii_char.rs

+impl AsMut<str> for AsciiChar {
+    #[inline(always)]
+    fn as_mut(&mut self) -> &mut str {
+        let ascii_ptr: *mut [Self] = crate::slice::from_mut(self);
+        let str_ptr = ascii_ptr as *mut str;
+        // SAFETY: Each ASCII codepoint in UTF-8 is encoded as one single-byte
+        // code unit having the same value as the ASCII byte.
+        unsafe { &mut *str_ptr }
+    }
+}


This could probably be implemented in a new as_mut_str method, similar to as_str

Also, could everything here use #[inline] without the always?

rust-bors · 2025-09-05T04:08:35Z

☀️ Try build successful (CI)
Build commit: 0498c15 (0498c157964e0946754e29e57ee9ef4527dc2a42, parent: af00ff2ce62b6617ed19305ae39e135ac71d0b22)

tgross35 · 2025-09-05T04:09:22Z

@craterbot check

Jules-Bertholet · 2025-09-10T12:58:12Z

That 1 regression seems spurious.

bors · 2025-09-13T08:31:15Z

☔ The latest upstream changes (presumably #146494) made this pull request unmergeable. Please resolve the merge conflicts.

- `impl AsRef<str> for AsciiChar` (unstable) - `impl AsMut<str> for AsciiChar` (unstable) - `impl<T, const N: usize> From<[MaybeUninit<T>; N]> for MaybeUninit<[T; N]>` - `impl<T, const N: usize> AsRef<[MaybeUninit<T>; N]> for MaybeUninit<[T; N]>` - `impl<T, const N: usize> AsRef<[MaybeUninit<T>]> for MaybeUninit<[T; N]>` - `impl<T, const N: usize> AsMut<[MaybeUninit<T>; N]> for MaybeUninit<[T; N]>` - `impl<T, const N: usize> AsMut<[MaybeUninit<T>]> for MaybeUninit<[T; N]>` - `impl<T, const N: usize> From<MaybeUninit<[T; N]>> for [MaybeUninit<T>; N]` - `impl<T, const N: usize> AsRef<[Cell<T>; N]> for Cell<[T; N]>` - `impl<T, const N: usize> AsRef<[Cell<T>]> for Cell<[T; N]>` - `impl<T> AsRef<[Cell<T>]> for Cell<[T]>`

rustbot · 2025-09-14T15:39:24Z

This PR was rebased onto a different master 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.

joshtriplett · 2025-09-23T15:49:45Z

We talked about these in today's @rust-lang/libs-api meeting.

The ones that stuck out to us were the impl AsRef<str> for AsciiChar and impl AsMut<str> for AsciiChar. Those seem correct, but we didn't see those specifically proposed anywhere, and it seemed odd to us to have impls for AsciiChar but not [AsciiChar] or [AsciiChar; N].

Could you elaborate on these, and point to where they might have been discussed? Was there a discussion of those specifically, or are you just proposing them because Char::as_str exists? If so, given that [Char]::as_str also exists, could you please provide the corresponding traits consistently for arrays/slices of Char?

joshtriplett · 2025-09-23T15:49:51Z

cc @the8472 for potential additional questions/comments.

Jules-Bertholet · 2025-09-24T01:40:31Z

Could you elaborate on these, and point to where they might have been discussed? Was there a discussion of those specifically, or are you just proposing them because Char::as_str exists?

Yes.

If so, given that [Char]::as_str also exists, could you please provide the corresponding traits consistently for arrays/slices of Char?

Unfortunately, alloc fails to build (“type annotations needed”) when I do that.

joshtriplett · 2025-09-30T15:18:44Z

Following up for clarity: other than AsRef<str> for AsciiChar and AsMut<str> for AsciiChar, we're fine approving all of these.

If we can't consistently add trait impls for [Char] as well due to inference failure, then please drop those two impls, and let's ship the rest.

Jules-Bertholet · 2025-10-01T15:13:35Z

please drop those two impls

Done.

the8472 · 2025-10-21T17:17:33Z

@rfcbot merge

rust-rfcbot · 2025-10-21T17:17:35Z

Team member @the8472 has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

tgross35 · 2025-10-21T17:34:10Z

@the8472 should fcp be restarted without the T-libs label?

the8472 · 2025-10-21T19:29:16Z

@rfcbot cancel

rust-rfcbot · 2025-10-21T19:29:18Z

@the8472 proposal cancelled.

the8472 · 2025-10-21T19:29:47Z

@rfcbot merge

rust-rfcbot · 2025-10-21T19:29:49Z

Team member @the8472 has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

rustbot assigned Mark-Simulacrum Aug 16, 2025

Jules-Bertholet mentioned this pull request Aug 16, 2025

Tracking Issue for MaybeUninit methods for arrays #96097

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Jules-Bertholet force-pushed the more_conversion_trait_impls branch from 7ef29d9 to 3a96604 Compare August 16, 2025 17:22

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Jules-Bertholet force-pushed the more_conversion_trait_impls branch 2 times, most recently from 676ff1f to bd960c9 Compare August 16, 2025 18:13

rustbot assigned m-ou-se and unassigned Mark-Simulacrum Aug 17, 2025

clarfonthey reviewed Aug 17, 2025

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rust-bors bot added a commit that referenced this pull request Sep 5, 2025

Auto merge of #145504 - Jules-Bertholet:more_conversion_trait_impls, …

0498c15

…r=<try> Add some conversion trait impls

rustbot unassigned m-ou-se Sep 5, 2025

rustbot added the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Sep 5, 2025

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Remove AsciiChar impls

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Jules-Bertholet commented Aug 16, 2025 •

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