Add no_std support

[CrazyboyQCD]

No description provided.

[MarijnS95]

Nice to see gpu-allocator usable on no_std with relatively minor changes! Thanks for doing the setup work here.

Just not sure if we're happy to commit to this rather tricky list of mutually exclusive features here. Consider someone uses gpu-allocator in their library crate and sets default-features = false to make it compile with a minimal set and no_std-compliance by default. They now have to enable the hashbrown feature to get it to compile.
When some other application crate uses that library crate, and also uses gpu-allocator with std feature to get backtrace support, this will now no longer compile because those are mutually exclusive.
I've suggested a change to make the hashbrown and std features coexist.

---

Can you add these lints to ensure this'll be easier to maintain in the future:

 #![warn(
    clippy::alloc_instead_of_core,
    clippy::std_instead_of_alloc,
    clippy::std_instead_of_core,
)]

And update the CI to ensure that it tests both no_std as well as std: we can add these to the matrix in the check_msrv, test and clippy jobs.

---

Finally, I ended up not spending any comments on it but instead recommend you to undo a few unnecessary import reorderings with:

cargo fmt -- --config imports_granularity=Crate --config group_imports=StdExternalCrate

The output isn't ideal (you have to group visualizer mod+use back together for example) but at least the ordering is back in check: log goes separate from the std/core/alloc "standard libraries" for example.

[MarijnS95]

Keep thiserror at 1 for now instead of pushing this drive-by change?

[CrazyboyQCD]

std can be disabled in thiserror since v2, but its msrv is 1.81+ dtolnay/thiserror#373.

But we can use derive_more instead, its msrv is 1.75+.

[CrazyboyQCD]

@MarijnS95
Should we use derive_more for bumping lower MSRV?

[MarijnS95]

Almost forgot to mention: can you please drop the chore: prefix from your commit messages? It's not used by this project and does not have any meaning in this context.

a routine task, especially a household one.

I don't think a one-off no_std conversion in a crate is considered a "routine task" 🙂

[CrazyboyQCD]

msrv is required to bump to either 1.75+(if we switch from thiserror to derive_more) or 1.81+ to get CI work.

[MarijnS95]

Nice update!

Only two things remaining: the formatting of the reports is broken, please test your changes to these before pushing them. I've suggested how to fix this in one file.

The previous review contained a request to update the CI to ensure that we test both with and without these new feature combinations, that would be very helpful to prevent breakage going forward. Right now we always build with specific backends and --no-default-features, meaning neither "hashbrown" nor "std" (nor a combination) ever gets exercised properly.