Add APIs for dealing with titlecase by Jules-Bertholet · Pull Request #122668 · rust-lang/rust

Jules-Bertholet · 2024-03-17T23:50:40Z

ACP: rust-lang/libs-team#354
Tracking issue: #153892

r? libs-api

@rustbot label T-libs -T-libs-api A-unicode

The last commit has some insta-stable PartialEq impls, therefore: @rustbot label -needs-fcp
Alternatively, I could split those out into a follow-up PR. (Edit: will do in follow-up)

bors · 2024-03-29T09:46:50Z

☔ The latest upstream changes (presumably #122616) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2024-04-20T23:11:30Z

☔ The latest upstream changes (presumably #122013) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-11-24T05:16:12Z

library/core/src/unicode/unicode_data.rs is generated by the src/tools/unicode-table-generator tool.

If you want to modify unicode_data.rs, please modify the tool then regenerate the library source file via ./x run src/tools/unicode-table-generator instead of editing unicode_data.rs manually.

Jules-Bertholet · 2025-11-24T14:19:55Z

The libs team gave a positive response to the ACP. @rustbot label -S-waiting-on-ACP S-waiting-on-review

rustbot · 2026-03-15T00:26:48Z

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.

Jules-Bertholet · 2026-03-15T20:41:24Z

@rustbot reroll

Mark-Simulacrum · 2026-03-16T21:15:34Z

library/core/src/char/methods.rs

-            c => c > '\x7f' && unicode::Alphabetic(c),
+            'A'..='Z' | 'a'..='z' => true,
+            '\0'..='\u{A9}' => false,
+            _ => unicode::Alphabetic(self),


I think this is from "Extend ASCII fast paths of char methods beyond ASCII"...

Can you confirm we continue to have exhaustive coverage in tests? I'm not sure how much the generated tests by the test generator call the public methods vs check that unicode::Alphabetic (for example) is accurate.

Also, I imagine that flipping the order of capital A-Z vs. lowercase a-z might influence codegen, and lowercase seems more likely to be common. Maybe worth doing something different there?

How did you decide on the particular threshold here (and in other modified functions)? Maybe we can split this out to a separate PR?

Also, I imagine that flipping the order of capital A-Z vs. lowercase a-z might influence codegen, and lowercase seems more likely to be common. Maybe worth doing something different there?

Good point, I fixed it.

How did you decide on the particular threshold here (and in other modified functions)?

I chose the highest value that would work, using https://util.unicode.org/UnicodeJsps/list-unicodeset.jsp to verify. E.g., for this function, https://util.unicode.org/UnicodeJsps/list-unicodeset.jsp?a=%5B%3AAlphabetic%3A%5D-%5B%3AASCII%3A%5D&abb=on says that the first non-ASCII alphabetic character is U+AA.

Can you confirm we continue to have exhaustive coverage in tests? I'm not sure how much the generated tests by the test generator call the public methods vs check that unicode::Alphabetic (for example) is accurate.

They didn't call the public methods, no. But should now

Mark-Simulacrum · 2026-03-16T21:18:58Z

library/core/src/char/mod.rs

+/// as returned by [`char::case`].
+#[unstable(feature = "titlecase", issue = "none")]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum CharCase {


nit: also #[must_use], probably.

Why? I don't think we usually label types like this #[must_use]?

Mark-Simulacrum · 2026-03-16T21:19:53Z

library/core/src/char/mod.rs

+    /// Titlecase. Corresponds to the `Titlecase_Letter` Unicode general category.
+    Title = 0b10,
+    /// Uppercase. Corresponds to the `Uppercase` Unicode property.
+    Upper = 0b11,


Can you add a comment about why these particular discriminants are chosen?

I added a doc comment explaining this.

rustbot · 2026-03-16T21:20:42Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

Jules-Bertholet · 2026-03-17T04:28:56Z

@rustbot ready

rustbot assigned m-ou-se Mar 17, 2024

Jules-Bertholet force-pushed the titlecase branch from 5f6fa45 to fe5d72b Compare March 29, 2024 12:48

Dylan-DPC removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 3, 2024

Jules-Bertholet force-pushed the titlecase branch from fe5d72b to b2fdfab Compare November 24, 2025 05:16

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Jules-Bertholet force-pushed the titlecase branch from b2fdfab to a2512b4 Compare November 24, 2025 05:30

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Jules-Bertholet force-pushed the titlecase branch from a2512b4 to 108a15c Compare February 15, 2026 19:33

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Jules-Bertholet added 4 commits March 14, 2026 19:20

Extend ASCII fast paths of char methods beyond ASCII

4eb9e66

Add char::is_cased

823b0f7

Add char::is_titlecase

2361f7a

Add char::case

af3c572

Jules-Bertholet force-pushed the titlecase branch from 108a15c to 15a00b3 Compare March 15, 2026 00:26

Add char::to_titlecase

ea8393a

Jules-Bertholet force-pushed the titlecase branch from 15a00b3 to ea8393a Compare March 15, 2026 00:31

rustbot removed the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Mar 15, 2026

Jules-Bertholet mentioned this pull request Mar 15, 2026

Tracking Issue for titlecase handling in core::char #153892

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4 tasks

rustbot removed the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Mar 15, 2026

rustbot assigned Mark-Simulacrum and unassigned m-ou-se Mar 15, 2026

Mark-Simulacrum requested changes Mar 16, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 16, 2026

Check lowercase first

c69ea1e

Jules-Bertholet force-pushed the titlecase branch from 514f3bb to c69ea1e Compare March 16, 2026 22:02

Jules-Bertholet added 2 commits March 16, 2026 22:19

Explain CharCase's discriminants

bb0ef2d

Extend case mapping fast path beyond ASCII as well

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Test stdlib functions with generated test data

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 17, 2026

Better document the practical use-cases for titlecase

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