Add `String<A>` type with custom allocator parameter #149328

shua · 2025-11-25T20:27:30Z

This change is part of the allocator_api feature set #32838 (also see wg-allocators roadmap or libs-team ACP).
The previous attempts at adding an allocator parameter to string are at rust-lang/rust#101551, and rust-lang/rust#79500 (I think those authors should get much of the credit here, I am re-writing what they worked out in those threads).

workaround

There is a type inference ambiguity introduced when adding a generic parameter to a type which previously had none, even when that parameter has a default value (more details in rust-lang/rust#98931). I've done the same workaround as rust-lang/rust#101551, which is to make alloc::string::String a type alias to String<Global>, but I've arranged the modules a bit differently to make rebase/merges a bit easier.

This workaround unfortunately changes the type name of the String language item, and that would be user-facing in error or diagnostic output. I understand from this comment that this change is acceptable.

changes to existing API

Most of the methods on the original String have been implemented for the generic version instead. I don't foresee anything more moving from String<Global> to String<A>, as the remaining methods are all constructors which implicitly use the Global allocator.

There are three general types of changes:

methods which don't allocate: here we just change impl Foo for String to impl<A: Allocator> Foo for String<A>
converting to/from other allocator generic types like Vec<u8, A> and Box<str, A>: here we can use the existing allocator from those types.
methods which clone from some other type with an allocator: here it's ambiguous whether the end result should be String<A>, String<Global>, or String<impl Allocator + Default>, etc; in general I try to leave these out of this change, but where some analogous change was made to Vec<T, A> I follow that.

new methods

Some methods have been added to String<A> which are not strictly necessary to land this change, but are considered helpful enough to users, and close enough to existing precedent in Vec<T, A>. Specifically, 4 new constructors (new_in, with_capacity_in, try_with_capacity_in, from_raw_parts_in), 1 destructor (into_raw_parts_with_alloc), and 1 getter (allocator). Technically, the updated from_utf8_unchecked should be sufficient for constructing, but we can add some safe constructors so users don't have to sully themselves.

not implemented

Variants of from_utf{8,16}* which internally allocate or use Cow have been punted on this PR, maybe a followup would make sense to either rewrite them, or add some *_in variant.

rustbot · 2025-11-25T20:27:34Z

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

rustbot · 2025-11-25T20:27:37Z

r? @Mark-Simulacrum

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shua · 2025-11-25T20:35:05Z

maybe r? @Amanieu as the reviewer of the previous PR has more context

edit: whoops saw previous reviewer actually was Mark-Simulcrum, Amanieu was just last to leave a review. If it makes sense to swap it back, please do!

bors · 2025-11-26T11:30:07Z

☔ The latest upstream changes (presumably #147799) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-11-26T19:16:37Z

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.

This is a third attempt at implementing adding Allocator support to the std lib's `String`. Still stuck on the same issue with type inference failing on the newly generic `String<A>`, but I opted to do even less than the previous WIP work, and have added no new functions (`String<A>` can be constructed via `Vec<u8, A>` or `Box<str, A>`), and have moved only the struct definition to its own mod to make rebasing a bit easier if/when main changes underneath me.

This adds a diagnostic name `string_in_global` so that clippy can easily check for the alloc::string::String type alias.

added some code in linkchecker to check the generic::String docs when trying to resolve links to alloc::string::String type alias. There's some lazy-loading that the browser does, but linkchecker doesn't, so maybe some general-purpose solution would be better, but this seemed better than a big list of exceptions.

Yes, you could just use `unsafe { from_utf8_unchecked }``, but people get antsy about `unsafe`, so add some Vec ctor/dtor equivalents.

the links are changed in the original source, so not sure why the html being checked in CI still has them, maybe it checks docs from `main` as well, but if so, wouldn't `struct.String.html` still exist? Truly a pickle, but I'll add these exceptions and a note.

rust-log-analyzer · 2025-11-27T09:13:01Z

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)


...............................................F
======== tests/rustdoc-gui/item-decl-colors.goml ========

[ERROR] `tests/rustdoc-gui/item-decl-colors.goml` line 31
    from `tests/rustdoc-gui/item-decl-colors.goml` line 42: `.item-decl code .struct` not found: for command `assert-css: (".item-decl code .struct", {"color": |struct_color|}, ALL)`
[ERROR] `tests/rustdoc-gui/item-decl-colors.goml` line 31
    from `tests/rustdoc-gui/item-decl-colors.goml` line 56: `.item-decl code .struct` not found: for command `assert-css: (".item-decl code .struct", {"color": |struct_color|}, ALL)`
[ERROR] `tests/rustdoc-gui/item-decl-colors.goml` line 31
    from `tests/rustdoc-gui/item-decl-colors.goml` line 70: `.item-decl code .struct` not found: for command `assert-css: (".item-decl code .struct", {"color": |struct_color|}, ALL)`


<= doc-ui tests done: 47 succeeded, 97 failed, 0 filtered out

Error: ()

rustbot assigned Mark-Simulacrum Nov 25, 2025

rustbot added A-rustc-dev-guide Area: rustc-dev-guide A-rustdoc-search Area: Rustdoc's search feature labels Nov 25, 2025

rustbot assigned Amanieu and unassigned Mark-Simulacrum Nov 25, 2025

cyrgani added the A-allocators Area: Custom and system allocators label Nov 25, 2025