Add `String<A>` type with custom allocator parameter by shua · Pull Request #149328 · rust-lang/rust

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

rustbot has assigned @Mark-Simulacrum.
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

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.

shua · 2026-01-21T21:18:58Z

a note for my future self: these tests fail locally, but do not fail in CI

[debuginfo-lldb] tests/debuginfo/by-value-non-immediate-argument.rs
[debuginfo-lldb] tests/debuginfo/function-arg-initialization.rs
[debuginfo-lldb] tests/debuginfo/function-prologue-stepping-regular.rs
[debuginfo-lldb] tests/debuginfo/macro-stepping.rs#no-SingleUseConsts-mir-pass
[debuginfo-lldb] tests/debuginfo/method-on-enum.rs
[debuginfo-lldb] tests/debuginfo/pretty-std.rs
src/tools/clippy/tests/ui/cognitive_complexity.rs

shua · 2026-01-21T22:40:13Z

@rustbot ready

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.

rustbot · 2026-02-17T20:52:46Z

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.

rust-bors · 2026-02-18T17:27:16Z

☔ The latest upstream changes (presumably #152785) made this pull request unmergeable. Please resolve the merge conflicts.

Amanieu · 2026-02-25T15:01:00Z

Sorry for the late review! The code LGTM, but I want to run it through crater to be sure there is no additional unexpected breakage.

@bors try

rust-bors · 2026-02-25T15:01:04Z

🔒 Merge conflict

A merge attempt failed due to a merge conflict. Please rebase on top of the latest base
branch, and let the reviewer approve again.

How do I rebase?

Assuming self is your fork and upstream is this repository,
you can resolve the conflict following these steps:

git checkout stralloc (switch to your branch)
git fetch upstream HEAD (retrieve the latest base branch)
git rebase upstream/HEAD -p (rebase on top of it)
Follow the on-screen instruction to resolve conflicts (check git status if you got lost).
git push self stralloc --force-with-lease (update this PR)

You may also read
Git Rebasing to Resolve Conflicts by Drew Blessing
for a short tutorial.

Please avoid the "Resolve conflicts" button on GitHub.
It uses git merge instead of git rebase which makes the PR commit history more difficult to read.

Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Cargo.lock conflict is
handled during merge and rebase. This is normal, and you should still perform step 5 to update this PR.

Amanieu · 2026-02-25T15:02:20Z

Could you run @bors try once you have rebased?

@bors delegate+

rust-bors · 2026-02-25T15:02:23Z

Unknown argument "once". Did you mean to use @bors jobs=<jobs>|parent=<parent>? Run @bors help to see available commands.

rust-bors · 2026-02-25T15:02:23Z

✌️ @shua, you can now approve this pull request!

If @Amanieu told you to "r=me" after making some further change, then please make that change and post @bors r=Amanieu.

Amanieu · 2026-02-25T15:02:33Z

@bors delegate+

rust-bors · 2026-02-25T15:02:36Z

✌️ @shua, you can now approve this pull request!

If @Amanieu told you to "r=me" after making some further change, then please make that change and post @bors r=Amanieu.

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