Implement `TryFrom<char>` for `usize`. #146792

bjoernager · 2025-09-19T19:08:37Z

Feature gate: usize_try_from_char.

This PR implements TryFrom<char> for usize.

usize is currently the only, unsigned, integral type to not implement TryFrom<char>. Technically, this conversion is trivial and can already be expressed with some indirection.

I think it useful to be able to describe this set of types with this interface.

rustbot · 2025-09-19T19:08:40Z

Failed to set assignee to ibraheemdev: cannot assign: error sending request

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rustbot · 2025-09-19T19:08:41Z

r? @ibraheemdev

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bjoernager · 2025-09-19T19:09:40Z

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

r? libs-api

dtolnay

Thanks!

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dtolnay · 2025-10-08T03:30:43Z

@rfcbot fcp merge

rust-rfcbot · 2025-10-08T03:30:45Z

Team member @dtolnay 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 · 2025-11-20T23:36:53Z

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

rust-rfcbot · 2025-11-30T23:37:15Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

teor2345 · 2025-12-01T02:17:35Z

library/core/src/char/convert.rs

+/// `usize`.
+///
+/// Generally speaking, this conversion can be seen as obtaining the character's corresponding
+/// UTF-32 code point to the extent representable by pointers addresses.


Grammar nitpick:

Suggested change

/// UTF-32 code point to the extent representable by pointers addresses.

/// UTF-32 code point to the extent representable by pointer addresses.

But it might be even better to use the same wording as the usize definition:

Suggested change

/// UTF-32 code point to the extent representable by pointers addresses.

/// UTF-32 code point to the extent representable by pointer-sized integers.

"Pointer-sized integers" is a bit ambiguous, e.g. any pointer type with non-ZST metadata will be larger than usize.

rustbot · 2025-12-29T09:46:26Z

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.

bjoernager · 2025-12-29T09:51:03Z

Sorry for the delay, this PR should be mergeable now. FCP has long passed.

@rustbot label -needs-fcp

bjoernager · 2025-12-29T17:23:03Z

Can this be backported to 1.93.0 (still in beta), given that the FCP passed before the branch?

dtolnay

Thank you.

Regarding a beta backport:

The FCP completion date is not a factor that I know of us having used to justify any previous library API backport, or if it has happened, it is not routine.
We are into the 3rd week of the 6 week release cycle so it is getting to be late to backport anything other than bugfixes.
For most purposes this feature is a minor usability improvement of writing usize::try_from(ch) instead of usize::try_from(u32::from(ch)). For a backport I would want to ascertain what makes this feature (relative to the typical other things landing in nightly this week) something that users would want to use as soon as possible.

View changes since this review

dtolnay · 2025-12-30T02:03:54Z

@bors r+

bors · 2025-12-30T02:03:57Z

📌 Commit cb5f95c has been approved by dtolnay

It is now in the queue for this repository.

Rollup of 3 pull requests Successful merges: - #146792 (Implement `TryFrom<char>` for `usize`.) - #150484 (Mark set_times as unavailable for RTEMS target) - #150498 (mir_build: Remove several remnants of `#![feature(inline_const_pat)]`) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #146792 - bjoernager:usize-try-from-char, r=dtolnay Implement `TryFrom<char>` for `usize`. Feature gate: `usize_try_from_char`. This PR implements `TryFrom<char>` for `usize`. `usize` is currently the only, unsigned, integral type to not implement `TryFrom<char>`. Technically, this conversion is trivial and can already be expressed with some indirection. I think it useful to be able to describe this set of types with this interface.

rustbot assigned ibraheemdev Sep 19, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 19, 2025

rustbot added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. and removed T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 19, 2025

rustbot assigned dtolnay and unassigned ibraheemdev Sep 19, 2025

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bjoernager force-pushed the usize-try-from-char branch from 226e29a to 23ae20b Compare September 19, 2025 19:17

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bjoernager force-pushed the usize-try-from-char branch from 23ae20b to cb66ec2 Compare September 19, 2025 19:59

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bjoernager force-pushed the usize-try-from-char branch from cb66ec2 to 306fad6 Compare September 19, 2025 20:49

rustbot added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Sep 21, 2025

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dtolnay approved these changes Oct 8, 2025

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rust-rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Oct 8, 2025

rust-rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Nov 20, 2025

rustbot mentioned this pull request Nov 30, 2025

Tracking issue for release notes of #146792: Implement TryFrom<char> for usize. #149491

Open

teor2345 reviewed Dec 1, 2025

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Implement 'TryFrom<char>' for 'usize';

cb5f95c

bjoernager force-pushed the usize-try-from-char branch from 306fad6 to cb5f95c Compare December 29, 2025 09:46

rustbot removed the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Dec 29, 2025

dtolnay approved these changes Dec 30, 2025

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 30, 2025

jhpratt mentioned this pull request Dec 30, 2025

Rollup of 3 pull requests #150500

Merged

bors merged commit 09baabc into rust-lang:main Dec 30, 2025
11 checks passed

rustbot added this to the 1.94.0 milestone Dec 30, 2025

	/// UTF-32 code point to the extent representable by pointers addresses.
	/// UTF-32 code point to the extent representable by pointer addresses.

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Implement `TryFrom<char>` for `usize`. #146792

Implement `TryFrom<char>` for `usize`. #146792

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