Support platforms without 64-bit atomic types

[cjwatson]

As explained in https://doc.rust-lang.org/std/sync/atomic/#portability, some platforms don't have 64-bit atomic types. As a result of smallvec being used by pydantic-core, this crate is involved in some dependency chains in Debian where we currently support the armel and powerpc architectures, neither of which have these types.

Since the malloc_size_of_is_0! macro just implements a trait on the types it's called with, it should be fine to skip types that don't exist on the target platform.

Bumps MSRV to 1.60 for #[cfg(target_has_atomic = "...")].

[nicoburns]

Makes sense!

[cjwatson]

Are you OK with MSRV on this? I think #[cfg(target_has_atomic = "...")] was only stabilized in Rust 1.60.0, and your Cargo.toml declares rust-version = "1.56". But the check in .github/workflows/main.yml has toolchain: 1.68, so I'm not entirely sure what's going on there.

[cjwatson]

Looking through https://crates.io/crates/malloc-size-of/reverse_dependencies, it seems that thin-vec, smallbitvec, and app_units have MSRVs below 1.60. So does this need a major version bump?

(Except I just noticed that the Cargo Book recommends treating MSRV bumps as a minor change, i.e. just a patch version bump for a 0.x crate. Sorry, I'm a bit of a Rust novice.)

[nicoburns]

I just noticed that the Cargo Book recommends treating MSRV bumps as a minor change, i.e. just a patch version bump for a 0.x crate

That's still somewhat controversial in the Rust community. And some of our dependent crates do care about MSRV. However, I think we're ok because this MSRV should only kick in for crates enabling the malloc_size_of feature (which should only be Servo/Firefox, neither of which have strict MSRV requirements).

I think I'm going to merge/release this next week though. So I'm around just in case there are fires that need to be put out.

should only kick in for crates enabling the malloc_size_of feature

FYI, I'd expect you to not need to package this crate for the same reason.

[jdm]

FYI, I'd expect you to not need to package this crate for the same reason.

Yeah, I was struck by this as well. The dependency is disabled by default in smallvec; it would be worth your time to figure out why it's enabled in your case.