std::String doesn't have an Allocator

geebee22 · March 26, 2026, 3:26am

I just noticed String ( unlike Vec, HashMap, BTreeMap etc ) doesn’t have an Allocator (yet).

Do you know if there any particular reason for this?

quinedot · March 26, 2026, 4:05am

Someone getting around to it plus whatever stabilization challenges need tackled.

github.com/rust-lang/rust

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

main ← shua:stralloc

opened 08:27PM - 25 Nov 25 UTC

shua

+1122 -690

*[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang…/rust/pull/149328)* This change is part of the `allocator_api` feature set [#32838](https://github.com/rust-lang/rust/issues/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](https://github.com/rust-lang/rust/pull/101551#issuecomment-2605604862) 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: 1. methods which don't allocate: here we just change `impl Foo for String` to `impl<A: Allocator> Foo for String<A>` 2. 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. 3. 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. [wg-allocators roadmap]: https://github.com/rust-lang/wg-allocators/issues/7 [libs-team ACP]: https://github.com/rust-lang/libs-team/issues/101 [workaround]: https://github.com/rust-lang/rust/issues/79499#issuecomment-1238578527 [rust-lang/rust#101551]: https://github.com/rust-lang/rust/pull/101551 [rust-lang/rust#79500]: https://github.com/rust-lang/rust/pull/79500 [rust-lang/rust#98931]: https://github.com/rust-lang/rust/issues/98931

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std::String doesn't have an Allocator

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