Add APIs for case folding to the standard library by Jules-Bertholet · Pull Request #154742 · rust-lang/rust

Jules-Bertholet · 2026-04-03T03:08:10Z

Libs-api requested these, so here they are.

New public API (gated behind #[feature(casefold)]):

impl char {
    pub fn to_casefold(self) -> ToCasefold;
}

impl str {
    pub fn to_casefold(&self) -> String;
    pub fn eq_ignore_case(&self) -> bool;
}

pub struct ToCasefold { ... }
impl Iterator for ToCasefold { type Item = char; ... }
impl DoubleEndedIterator for ToCasefold { ... }
impl FusedIterator for ToCasefold { }
impl ExactSizeIterator for ToCasefold { ... }
impl fmt::Display for ToCasefold { ... }

Notes

This only adds a negligible amount of static data to core::unicode. To accomplish that, we compute the case-folding for most characters as the lowercase of their uppercase; this double mapping adds some complexity to the implementation.
No normalization (e.g. NFC) is performed, so visually and semantically equivalent strings can compare unequal.
I have not put any effort into optimizing eq_ignore_case(); there may be a more performant implementation.
char::eq_ignore_case() is left to future work—it's a potential footgun, so we may want to think more deeply about how to expose and document that API.

@rustbot label T-libs-api A-unicode

rustbot · 2026-04-03T03:08:13Z

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

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.

rustbot · 2026-04-03T03:08:16Z

r? @scottmcm

rustbot has assigned @scottmcm.
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

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: @scottmcm, libs
@scottmcm, libs expanded to 8 candidates
Random selection from Mark-Simulacrum, jhpratt, scottmcm

scottmcm · 2026-04-16T00:34:19Z

@rustbot reroll

jhpratt · 2026-04-18T05:16:52Z

LGTM. r=me once CI passes.

Jules-Bertholet · 2026-04-18T15:45:05Z

r? libs-api

jhpratt · 2026-04-18T23:43:09Z

I don't mind, but any particular reason for the reassign? I thought it was good to go.

Jules-Bertholet · 2026-04-18T23:54:35Z

The API needs libs-API approval, I believe. They expressed interest in something like this, but there was never an ACP. (I also need to add a tracking issue after I get that)

Rollup of 8 pull requests Successful merges: - #157240 (Enable Enzyme for aarch64-apple-darwin) - #157276 (miri subtree update) - #154742 (Add APIs for case folding to the standard library) - #157130 (Use a `ArrayVec` in `CastTarget`) - #157195 (Move feature gating to the new attr parsing infrastructure) - #157256 (tests: adapt for LLVM codegen change) - #157265 (Update books) - #157277 (triagebot.toml: add LawnGnome to libs reviewers)

jhpratt · 2026-06-02T05:51:38Z

@bors r-

#157279 (comment)

rust-bors · 2026-06-02T05:51:44Z

This pull request was unapproved.

This PR was contained in a rollup (#157279), which was unapproved.

View changes since this unapproval

This reverts commit 1ec2ee9, which unfortunately prevented `convert_while_ascii` from vectorizing :( "LLVM gave, and LLVM hath taken away"

Jules-Bertholet · 2026-06-02T13:38:41Z

@rustbot ready

Mark-Simulacrum · 2026-06-06T14:35:06Z

@bors r+ rollup=iffy

rust-bors · 2026-06-06T14:35:09Z

📌 Commit e6d5e95 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

@rustbot

…imulacrum Add APIs for case folding to the standard library [Libs-api requested these](rust-lang#154287 (comment)), so here they are. New public API (gated behind `#[feature(casefold)]`): ```rust impl char { pub fn to_casefold(self) -> ToCasefold; } impl str { pub fn to_casefold(&self) -> String; pub fn eq_ignore_case(&self) -> bool; } pub struct ToCasefold { ... } impl Iterator for ToCasefold { type Item = char; ... } impl DoubleEndedIterator for ToCasefold { ... } impl FusedIterator for ToCasefold { } impl ExactSizeIterator for ToCasefold { ... } impl fmt::Display for ToCasefold { ... } ``` ## Notes - This only adds a negligible amount of static data to `core::unicode`. To accomplish that, we compute the case-folding for most characters as the lowercase of their uppercase; this double mapping adds some complexity to the implementation. - No normalization (e.g. NFC) is performed, so visually and semantically equivalent strings can compare unequal. - I have not put any effort into optimizing `eq_ignore_case()`; there may be a more performant implementation. - `char::eq_ignore_case()` is left to future work—it's a potential footgun, so we may want to think more deeply about how to expose and document that API. @rustbot label T-libs-api A-unicode

@rustbot

…imulacrum Add APIs for case folding to the standard library [Libs-api requested these](rust-lang#154287 (comment)), so here they are. New public API (gated behind `#[feature(casefold)]`): ```rust impl char { pub fn to_casefold(self) -> ToCasefold; } impl str { pub fn to_casefold(&self) -> String; pub fn eq_ignore_case(&self) -> bool; } pub struct ToCasefold { ... } impl Iterator for ToCasefold { type Item = char; ... } impl DoubleEndedIterator for ToCasefold { ... } impl FusedIterator for ToCasefold { } impl ExactSizeIterator for ToCasefold { ... } impl fmt::Display for ToCasefold { ... } ``` ## Notes - This only adds a negligible amount of static data to `core::unicode`. To accomplish that, we compute the case-folding for most characters as the lowercase of their uppercase; this double mapping adds some complexity to the implementation. - No normalization (e.g. NFC) is performed, so visually and semantically equivalent strings can compare unequal. - I have not put any effort into optimizing `eq_ignore_case()`; there may be a more performant implementation. - `char::eq_ignore_case()` is left to future work—it's a potential footgun, so we may want to think more deeply about how to expose and document that API. @rustbot label T-libs-api A-unicode

…uwer Rollup of 17 pull requests Successful merges: - #157251 (`rust-analyzer` subtree update) - #157533 (Subtree sync for rustc_codegen_cranelift) - #154742 (Add APIs for case folding to the standard library) - #155144 (mir_build: Add an extra intermediate step in MIR building for patterns ) - #157016 (add `extern "tail"` calling convention) - #157264 (diagnostics: Fix ICE building a trait ref in method suggestions) - #157386 (Parse deprecated note links separately in rustc_resolve) - #157483 (fix windows-gnu TLS leak) - #157488 (compiletest: inject `#![windows_subsystem = "windows"]` to debuginfo tests on Windows) - #157509 (remove solaris implementation for File::lock, it has the wrong semantics) - #157521 (Rename `SyncView::{as_pin => as_pin_ref}`) - #156136 (Move tests box) - #157365 (Revert "LLVM 23: Run AssignGUIDPass in some places") - #157471 (Debug assert that parsed attributes are in the `BUILTIN_ATTRIBUTE_MAP`) - #157485 (Rename `errors.rs` file to `diagnostics.rs` (1/N)) - #157494 (Convert `QueryRegionConstraint` into a struct) - #157526 (std tests: skip a slow test on Miri) Failed merges: - #155527 (Replace printables table with `unicode_data.rs` tables)

@rustbot

…imulacrum Add APIs for case folding to the standard library [Libs-api requested these](rust-lang#154287 (comment)), so here they are. New public API (gated behind `#[feature(casefold)]`): ```rust impl char { pub fn to_casefold(self) -> ToCasefold; } impl str { pub fn to_casefold(&self) -> String; pub fn eq_ignore_case(&self) -> bool; } pub struct ToCasefold { ... } impl Iterator for ToCasefold { type Item = char; ... } impl DoubleEndedIterator for ToCasefold { ... } impl FusedIterator for ToCasefold { } impl ExactSizeIterator for ToCasefold { ... } impl fmt::Display for ToCasefold { ... } ``` ## Notes - This only adds a negligible amount of static data to `core::unicode`. To accomplish that, we compute the case-folding for most characters as the lowercase of their uppercase; this double mapping adds some complexity to the implementation. - No normalization (e.g. NFC) is performed, so visually and semantically equivalent strings can compare unequal. - I have not put any effort into optimizing `eq_ignore_case()`; there may be a more performant implementation. - `char::eq_ignore_case()` is left to future work—it's a potential footgun, so we may want to think more deeply about how to expose and document that API. @rustbot label T-libs-api A-unicode

@rustbot

…imulacrum Add APIs for case folding to the standard library [Libs-api requested these](rust-lang#154287 (comment)), so here they are. New public API (gated behind `#[feature(casefold)]`): ```rust impl char { pub fn to_casefold(self) -> ToCasefold; } impl str { pub fn to_casefold(&self) -> String; pub fn eq_ignore_case(&self) -> bool; } pub struct ToCasefold { ... } impl Iterator for ToCasefold { type Item = char; ... } impl DoubleEndedIterator for ToCasefold { ... } impl FusedIterator for ToCasefold { } impl ExactSizeIterator for ToCasefold { ... } impl fmt::Display for ToCasefold { ... } ``` ## Notes - This only adds a negligible amount of static data to `core::unicode`. To accomplish that, we compute the case-folding for most characters as the lowercase of their uppercase; this double mapping adds some complexity to the implementation. - No normalization (e.g. NFC) is performed, so visually and semantically equivalent strings can compare unequal. - I have not put any effort into optimizing `eq_ignore_case()`; there may be a more performant implementation. - `char::eq_ignore_case()` is left to future work—it's a potential footgun, so we may want to think more deeply about how to expose and document that API. @rustbot label T-libs-api A-unicode

Rollup of 25 pull requests Successful merges: - #157251 (`rust-analyzer` subtree update) - #157533 (Subtree sync for rustc_codegen_cranelift) - #154742 (Add APIs for case folding to the standard library) - #155144 (mir_build: Add an extra intermediate step in MIR building for patterns ) - #156222 (Stabilize `Result::map_or_default` and `Option::map_or_default`) - #157016 (add `extern "tail"` calling convention) - #157264 (diagnostics: Fix ICE building a trait ref in method suggestions) - #157386 (Parse deprecated note links separately in rustc_resolve) - #157483 (fix windows-gnu TLS leak) - #157488 (compiletest: inject `#![windows_subsystem = "windows"]` to debuginfo tests on Windows) - #157509 (remove solaris implementation for File::lock, it has the wrong semantics) - #157521 (Rename `SyncView::{as_pin => as_pin_ref}`) - #156136 (Move tests box) - #156573 (Add unwinder_private_data_size for wasm64 target) - #156783 (docs: make `Rc::into_raw` clickable in `Rc::increment_strong_count` doc) - #156840 (Stabilize `PathBuf::into_string`) - #156936 (Remove FIXME about impl PinCoerceUnsized for UnsafePinned<T>) - #157365 (Revert "LLVM 23: Run AssignGUIDPass in some places") - #157380 (clarify compiler_fence (and fence) docs) - #157471 (Debug assert that parsed attributes are in the `BUILTIN_ATTRIBUTE_MAP`) - #157485 (Rename `errors.rs` file to `diagnostics.rs` (1/N)) - #157494 (Convert `QueryRegionConstraint` into a struct) - #157526 (std tests: skip a slow test on Miri) - #157531 (ci: bump x86_64-gnu base image to 26.04) - #157556 (Add `BTree::append()` change to 1.96.0 relnotes) Failed merges: - #155527 (Replace printables table with `unicode_data.rs` tables)

Rollup of 25 pull requests Successful merges: - rust-lang/rust#157251 (`rust-analyzer` subtree update) - rust-lang/rust#157533 (Subtree sync for rustc_codegen_cranelift) - rust-lang/rust#154742 (Add APIs for case folding to the standard library) - rust-lang/rust#155144 (mir_build: Add an extra intermediate step in MIR building for patterns ) - rust-lang/rust#156222 (Stabilize `Result::map_or_default` and `Option::map_or_default`) - rust-lang/rust#157016 (add `extern "tail"` calling convention) - rust-lang/rust#157264 (diagnostics: Fix ICE building a trait ref in method suggestions) - rust-lang/rust#157386 (Parse deprecated note links separately in rustc_resolve) - rust-lang/rust#157483 (fix windows-gnu TLS leak) - rust-lang/rust#157488 (compiletest: inject `#![windows_subsystem = "windows"]` to debuginfo tests on Windows) - rust-lang/rust#157509 (remove solaris implementation for File::lock, it has the wrong semantics) - rust-lang/rust#157521 (Rename `SyncView::{as_pin => as_pin_ref}`) - rust-lang/rust#156136 (Move tests box) - rust-lang/rust#156573 (Add unwinder_private_data_size for wasm64 target) - rust-lang/rust#156783 (docs: make `Rc::into_raw` clickable in `Rc::increment_strong_count` doc) - rust-lang/rust#156840 (Stabilize `PathBuf::into_string`) - rust-lang/rust#156936 (Remove FIXME about impl PinCoerceUnsized for UnsafePinned<T>) - rust-lang/rust#157365 (Revert "LLVM 23: Run AssignGUIDPass in some places") - rust-lang/rust#157380 (clarify compiler_fence (and fence) docs) - rust-lang/rust#157471 (Debug assert that parsed attributes are in the `BUILTIN_ATTRIBUTE_MAP`) - rust-lang/rust#157485 (Rename `errors.rs` file to `diagnostics.rs` (1/N)) - rust-lang/rust#157494 (Convert `QueryRegionConstraint` into a struct) - rust-lang/rust#157526 (std tests: skip a slow test on Miri) - rust-lang/rust#157531 (ci: bump x86_64-gnu base image to 26.04) - rust-lang/rust#157556 (Add `BTree::append()` change to 1.96.0 relnotes) Failed merges: - rust-lang/rust#155527 (Replace printables table with `unicode_data.rs` tables)

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 Apr 3, 2026

rustbot assigned scottmcm Apr 3, 2026

rustbot added A-Unicode Area: Unicode T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Apr 3, 2026