deprecate `std::char` constants and functions by folkertdev · Pull Request #153873 · rust-lang/rust

folkertdev · 2026-03-14T15:04:51Z

similar to how constants in those modules for numeric types have been deprecated. The std::char module contains:

Three stable constants that this PR deprecates. These already link to their associated constant equivalents.

MAX
REPLACEMENT_CHARACTER
UNICODE_VERSION

two unstable constants that this PR removes. The constants are already stablized as associated constants on char.

MAX_LEN_UTF8
MAX_LEN_UTF16

Four stable functions that this PR deprecates. These already link to their method equivalents.

fn decode_utf16
fn from_digit
fn from_u32
fn from_u32_unchecked⚠

discussion at #t-libs > should `std::char::{MIN, MAX}` be deprecated?.

r? libs-api

folkertdev · 2026-03-14T18:45:45Z

-/// The maximum number of bytes required to [encode](char::encode_utf8) a `char` to
-/// UTF-8 encoding.
-#[unstable(feature = "char_max_len", issue = "121714")]
-pub const MAX_LEN_UTF8: usize = char::MAX_LEN_UTF8;
-
-/// The maximum number of two-byte units required to [encode](char::encode_utf16) a `char`
-/// to UTF-16 encoding.
-#[unstable(feature = "char_max_len", issue = "121714")]
-pub const MAX_LEN_UTF16: usize = char::MAX_LEN_UTF16;


There is currently no core::char::consts module, so currently these constants are just gone. Should they be added as associated consts, or should that core::char::consts module be added?

note to self: the consts being gone completely is the cause of the char_max_len CI failure.

They already are associated consts: https://doc.rust-lang.org/stable/std/primitive.char.html#associatedconstant.MAX_LEN_UTF8

And were stabilized under the feature char_max_len_assoc

Nice, these can probably just be removed then.

similar to how constants in those modules for numeric types have been deprecated

rustbot · 2026-03-15T10:18:42Z

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.

Amanieu · 2026-03-17T15:55:18Z

@rfcbot merge libs-api

rust-rfcbot · 2026-03-17T15:55:22Z

Team member @Amanieu 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.

joshtriplett · 2026-04-07T20:05:06Z

The only downside of a deprecation is that we still don't support use char::REPLACEMENT_CHARACTER; for associated constants. But still, 👍.

joshtriplett · 2026-04-07T22:52:49Z

@rfcbot reviewed

rust-rfcbot · 2026-04-07T22:53:02Z

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

RalfJung · 2026-04-17T14:23:35Z

Three stable constants that this PR deprecates. These already link to their method equivalents.

What do you mean by "method equivalents"? They link to associated const alternatives, not methods.

folkertdev · 2026-04-17T14:28:43Z

Must have been a copy-paste error. fixed.

rust-rfcbot · 2026-04-17T22:54:39Z

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.

folkertdev · 2026-04-18T10:42:50Z

r? libs

Mark-Simulacrum · 2026-04-18T18:15:33Z

@bors r+ rollup

rust-bors · 2026-04-18T18:15:39Z

📌 Commit ee9b61c has been approved by Mark-Simulacrum

It is now in the queue for this repository.

…ark-Simulacrum deprecate `std::char` constants and functions similar to how constants in those modules for numeric types have been deprecated. The `std::char` module contains: Three stable constants that this PR deprecates. These already link to their associated constant equivalents. - `MAX` - `REPLACEMENT_CHARACTER` - `UNICODE_VERSION` two unstable constants that this PR removes. The constants are already stablized as associated constants on `char`. - `MAX_LEN_UTF8` - `MAX_LEN_UTF16` Four stable functions that this PR deprecates. These already link to their method equivalents. - `fn decode_utf16` - `fn from_digit` - `fn from_u32` - `fn from_u32_unchecked⚠` discussion at [#t-libs > should &rust-lang#96;std::char::{MIN, MAX}&rust-lang#96; be deprecated?](https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/should.20.60std.3A.3Achar.3A.3A.7BMIN.2C.20MAX.7D.60.20be.20deprecated.3F/with/579444750). r? libs-api

Rollup of 8 pull requests Successful merges: - #155370 (Add regression test for dead code elimination with drop + panic) - #154823 (Replace the spdx-rs dependency with a minimal in-tree SPDX tag-value parser) - #155352 (triagebot.toml: Sync `assign.owners` with `autolabel."T-compiler"`) - #155431 (Add temporary scope to assert_matches) - #152995 (ACP Implementation of PermissionsExt for Windows ) - #153873 (deprecate `std::char` constants and functions) - #154654 (Move `std::io::ErrorKind` to `core::io`) - #154865 (libtest: use binary search for --exact test filtering)

…ark-Simulacrum deprecate `std::char` constants and functions similar to how constants in those modules for numeric types have been deprecated. The `std::char` module contains: Three stable constants that this PR deprecates. These already link to their associated constant equivalents. - `MAX` - `REPLACEMENT_CHARACTER` - `UNICODE_VERSION` two unstable constants that this PR removes. The constants are already stablized as associated constants on `char`. - `MAX_LEN_UTF8` - `MAX_LEN_UTF16` Four stable functions that this PR deprecates. These already link to their method equivalents. - `fn decode_utf16` - `fn from_digit` - `fn from_u32` - `fn from_u32_unchecked⚠` discussion at [#t-libs > should &rust-lang#96;std::char::{MIN, MAX}&rust-lang#96; be deprecated?](https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/should.20.60std.3A.3Achar.3A.3A.7BMIN.2C.20MAX.7D.60.20be.20deprecated.3F/with/579444750). r? libs-api

Rollup of 9 pull requests Successful merges: - #155370 (Add regression test for dead code elimination with drop + panic) - #154823 (Replace the spdx-rs dependency with a minimal in-tree SPDX tag-value parser) - #154972 (Implement `core::arch::return_address` and tests) - #155294 (Add test for coalescing of diagnostic attribute duplicates) - #155352 (triagebot.toml: Sync `assign.owners` with `autolabel."T-compiler"`) - #155431 (Add temporary scope to assert_matches) - #152995 (ACP Implementation of PermissionsExt for Windows ) - #153873 (deprecate `std::char` constants and functions) - #154865 (libtest: use binary search for --exact test filtering)

…ark-Simulacrum deprecate `std::char` constants and functions similar to how constants in those modules for numeric types have been deprecated. The `std::char` module contains: Three stable constants that this PR deprecates. These already link to their associated constant equivalents. - `MAX` - `REPLACEMENT_CHARACTER` - `UNICODE_VERSION` two unstable constants that this PR removes. The constants are already stablized as associated constants on `char`. - `MAX_LEN_UTF8` - `MAX_LEN_UTF16` Four stable functions that this PR deprecates. These already link to their method equivalents. - `fn decode_utf16` - `fn from_digit` - `fn from_u32` - `fn from_u32_unchecked⚠` discussion at [#t-libs > should &rust-lang#96;std::char::{MIN, MAX}&rust-lang#96; be deprecated?](https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/should.20.60std.3A.3Achar.3A.3A.7BMIN.2C.20MAX.7D.60.20be.20deprecated.3F/with/579444750). r? libs-api

Rollup of 10 pull requests Successful merges: - #155370 (Add regression test for dead code elimination with drop + panic) - #154823 (Replace the spdx-rs dependency with a minimal in-tree SPDX tag-value parser) - #155294 (Add test for coalescing of diagnostic attribute duplicates) - #155352 (triagebot.toml: Sync `assign.owners` with `autolabel."T-compiler"`) - #155431 (Add temporary scope to assert_matches) - #152995 (ACP Implementation of PermissionsExt for Windows ) - #153873 (deprecate `std::char` constants and functions) - #154865 (libtest: use binary search for --exact test filtering) - #154979 (add #[must_use] macros for floats) - #155504 (Remove `AttributeLintKind` variants - part 2)

…uwer Rollup of 12 pull requests Successful merges: - #155370 (Add regression test for dead code elimination with drop + panic) - #154823 (Replace the spdx-rs dependency with a minimal in-tree SPDX tag-value parser) - #155294 (Add test for coalescing of diagnostic attribute duplicates) - #155352 (triagebot.toml: Sync `assign.owners` with `autolabel."T-compiler"`) - #155431 (Add temporary scope to assert_matches) - #153873 (deprecate `std::char` constants and functions) - #154865 (libtest: use binary search for --exact test filtering) - #154979 (add #[must_use] macros for floats) - #155486 (c-variadic: add roundtrip test) - #155504 (Remove `AttributeLintKind` variants - part 2) - #155510 (Update Tidy python executable path) - #155514 (codegen-options docs: remove -Csoft-float)

Rollup merge of #153873 - folkertdev:deprecate-char-max, r=Mark-Simulacrum deprecate `std::char` constants and functions similar to how constants in those modules for numeric types have been deprecated. The `std::char` module contains: Three stable constants that this PR deprecates. These already link to their associated constant equivalents. - `MAX` - `REPLACEMENT_CHARACTER` - `UNICODE_VERSION` two unstable constants that this PR removes. The constants are already stablized as associated constants on `char`. - `MAX_LEN_UTF8` - `MAX_LEN_UTF16` Four stable functions that this PR deprecates. These already link to their method equivalents. - `fn decode_utf16` - `fn from_digit` - `fn from_u32` - `fn from_u32_unchecked⚠` discussion at [#t-libs > should `std::char::{MIN, MAX}` be deprecated?](https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/should.20.60std.3A.3Achar.3A.3A.7BMIN.2C.20MAX.7D.60.20be.20deprecated.3F/with/579444750). r? libs-api

folkertdev added the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Mar 14, 2026

rustbot assigned BurntSushi Mar 14, 2026

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 Mar 14, 2026

folkertdev force-pushed the deprecate-char-max branch from 3d35ff3 to 00b5d46 Compare March 14, 2026 16:33

folkertdev changed the title ~~deprecate std::char::MAX~~ deprecate std::char constants and functions Mar 14, 2026

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folkertdev commented Mar 14, 2026

View reviewed changes

deprecate std::char constants and functions

ee9b61c

similar to how constants in those modules for numeric types have been deprecated

folkertdev force-pushed the deprecate-char-max branch from 00b5d46 to ee9b61c Compare March 15, 2026 10:18

Amanieu removed the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Mar 17, 2026

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 Mar 17, 2026

folkertdev added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Mar 24, 2026

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 Apr 7, 2026

rustbot mentioned this pull request Apr 17, 2026

Tracking issue for release notes of #153873: deprecate std::char constants and functions #155468

Open

rustbot assigned Mark-Simulacrum and unassigned BurntSushi Apr 18, 2026

rust-bors Bot 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 Apr 18, 2026

jhpratt mentioned this pull request Apr 19, 2026

Rollup of 8 pull requests #155503

Closed

jhpratt mentioned this pull request Apr 19, 2026

Rollup of 9 pull requests #155506

Closed

jhpratt mentioned this pull request Apr 19, 2026

Rollup of 10 pull requests #155511

Closed

JonathanBrouwer mentioned this pull request Apr 19, 2026

Rollup of 12 pull requests #155519

Merged

rust-bors Bot merged commit b07688d into rust-lang:main Apr 19, 2026
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rustbot added this to the 1.97.0 milestone Apr 19, 2026

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