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

- `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 · 2025-10-21T19:29:49Z

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.

rust-rfcbot · 2025-10-27T16:15:33Z

🔔 This is now entering its final comment period, as per the review above. 🔔

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

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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

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Remove AsciiChar impls

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