Remove align cargo feature

[gnzlbg]

I'd like to remove the align cargo feature (or make it a dummy), and automatically use repr(align) if the Rust version is new enough (newer than Rust 1.25.0).

In particular, I'd like for libc to not expose the C APIs that use repr(align) if repr(align) is not available anymore. This would be a breaking change, but using these APIs without the proper alignment is undefined behavior anyways (unless one is super super super careful).

Users that want to use these APIs and are currently on Rust versions older than Rust 1.25.0 can either stick to an older libc version or upgrade to at least Rust 1.25.0.

[gnzlbg]

Well, this is worse than I thought. Using repr(align(N)) anywhere (even behind a #[cfg_attr]) requires at least Rust 1.25.0 . That is, any target that wants to optionally use repr(align) needs to bump the minimum required Rust version to 1.25.0 (~1 year old).

cc @alexcrichton - a couple of targets already do this, maybe we should bump the minimum supported libc version to Rust 1.25.0 and call it a day. It might well be that libc works with older Rust versions in some targets, but I don't think we should guarantee that. In any case, gating align behind a cargo feature doesn't do anything, since Rust 1.25 is required for the cargo feature to actually work.

[gnzlbg]

Alternatively, we could move all code that uses repr(align(N)) into separate modules, and put the module declaration behind the feature gate. For example,

#[cfg(libc_align)]
mod align;

// align.rs
#[repr(align(N))] struct Foo;

[Mark-Simulacrum]

The modules would/could be completely transparent to downstream users, right? Via re-exports?

[gnzlbg]

Yes, I am already working on that, but it is a pain due to how many targets are already depending on it.

[josephlr]

@gnzlbg I think bumping libc's supported version to 1.25 is not unreasonable. Of the most popular crates, lazy_static has 1.24 as the min version, bitflags has 1.20, rand has 1.22. However, serde, syn, and log all have much lower required versions (~1.15)

[alexcrichton]

1.25.0 is a bit too recent I think for libc, but we could detect this in the build script and auto-enable the feature I believe?

[gnzlbg]

FWIW libc now auto-detect whether repr(align) is available. The align feature still works as it worked before.

I briefly thought about deprecating it, but I don't think we can ever remove it without breaking projects that want to be able to work properly on a very wide range of libc versions.