Add support for custom allocator for (C)String

[TimDiekmann]

This follows the roadmap of the allocator WG to add custom allocators to collections.

currently blocked on:

• Stability annotation on defaulted generics fails #79499

r? @Amanieu

cc @rust-lang/wg-allocators
@rustbot modify labels: +A-allocators +T-libs

[Amanieu]

CI seems to be failing.

[TimDiekmann]

@rustbot modify labels: +S-blocked

[bors]

☔ The latest upstream changes (presumably #80510) made this pull request unmergeable. Please resolve the merge conflicts.

[shlevy]

@TimDiekmann Is there a workaround until this is merged to have owned str with a custom allocator?

[TimDiekmann]

@TimDiekmann Is there a workaround until this is merged to have owned str with a custom allocator?

No, sorry. You can implement/use a custom String class with the same API as a drop-in replacement and provide a From implementation for std::String, but this is rather an unsatisfactory solution.

[lachlansneff]

Any chance this could happen soonish?

[zachs18]

@TimDiekmann I made an initial partial re-implementation of this PR (#101551) on the current master branch using the workaround(?) I mentioned in the blocking issue.
I can close that PR if you'd rather rebase this PR onto current master or something else.

[TimDiekmann]

I currently don't have the time to rebase this branch, I'm sorry 🙁
I'll close this PR in favor of #101551

Feel free to base on this branch or whatever is the easiest for you 🙂