Add optional zerocopy trait derives

[joshlf]

No description provided.

[rustbot]

r? @JohnTitor

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[joshlf]

I'm uploading this as a draft early so I can run stuff through CI. Definitely not ready for review yet!

[joshlf]

TODO: Update tests to exercise this feature.

[joshlf]

@JohnTitor I do a have a question if you happen to have any insight: This PR adds a new Cargo feature which doesn't work on the MSRV of 1.13. I'm looking through the CI infrastructure but don't see an obvious place to put logic to test features only on more recent MSRVs. The README advertises various MSRVs for various features, so I assume such machinery must exist somewhere?

[JohnTitor]

You can tweak https://github.com/rust-lang/libc/blob/main/build.rs :)

[joshlf]

You can tweak https://github.com/rust-lang/libc/blob/main/build.rs :)

It looks like the current logic in that file automatically enables features unconditionally if the compiler version is high enough (ie, it doesn't require the user to enable the corresponding Cargo feature). Is that the behavior we want here? I assumed y'all would want this to remain optional, but if that's not the case, I'd be happy to make this unconditional following the logic used with the other features.

[JohnTitor]

Ah, hmm, fair enough.
So, let's just declare a normal Cargo feature then (with the MSRV note on README).

[joshlf]

Sounds good. Do you have a sense of how I could enable this feature in CI, but only when the toolchain version is 1.61 (zerocopy's MSRV) or greater?

[JohnTitor]

It'd be hacky but something like this should be possible:

libc/ci/build.sh

Lines 69 to 79 in 5e32df0

	# Test the 'const-extern-fn' feature on nightly
	if [ "${RUST}" = "nightly" ]; then
	if [ "${NO_STD}" != "1" ]; then
	cargo "+${RUST}" "${BUILD_CMD}" -vv --no-default-features --target "${TARGET}" \
	--features const-extern-fn
	else
	RUSTFLAGS="-A improper_ctypes_definitions" cargo "+${RUST}" "${BUILD_CMD}" \
	-Z build-std=core,alloc -vv --no-default-features \
	--target "${TARGET}" --features const-extern-fn
	fi
	fi

[joshlf]

Sounds good! Does this look good to you?

[joshlf]

Also, do these CI tests test on the MSRV (1.13)? I searched for code that enabled it, and found none:

$ rg '1\.13'
README.md
45:The minimum supported Rust toolchain version is currently **Rust 1.13.0**.

ci/build.sh
191:        if [ "${RUST}" != "1.13.0" ]; then

libc-test/build.rs
1198:            "MS_RMT_MASK" => true, // updated in glibc 2.22 and musl 1.1.13
1424:            "MS_RMT_MASK" => true, // updated in glibc 2.22 and musl 1.1.13

[joshlf]

Leaving a TODO for myself:

• Rename to zerocopy-0-7
• Add zerocopy-0-6 (to be removed before merge) to test that multiple version trains work

Since libc doesn't specify an edition, we have to use the legacy approach of importing macros via #[macro_use] extern crate zerocopy;, which may not play nicely with having macros from multiple versions of the same crate. Instead, we may need to update build.rs to detect when the Rust version is early enough that editions are supported, and instruct rustc to use a recent-enough version that we can explicitly import different versions under different names. This would be necessary in case a build requires multiple zerocopy versions simultaneously.

[joshlf]

See for more discussion: google/zerocopy#557

[JohnTitor]

Once rust-lang/libs-team#72 gets a conclusion, we can specify the edition.

[JohnTitor]

Also, do these CI tests test on the MSRV (1.13)? I searched for code that enabled it, and found none:

No, I remember that I removed it at some point as something on the test pipeline had been broken.

[bors]

☔ The latest upstream changes (presumably #3393) made this pull request unmergeable. Please resolve the merge conflicts.