Add Deserialize impl for CStr

[jonhoo]

Fixes #810.

I've scoped the impl under feature="unstable" for now (don't know what the policy for that feature is), but this should possibly not be merged until rust-lang/rust#40380 lands and fully stabilizes CString::into_boxed_c_str. At that point, feature="unstable" should obviously be removed.

[dtolnay]

The method is in nightly but you are missing an underscore. 😉

[jonhoo]

Haha, I am indeed. Fix coming shortly.

[jonhoo]

I'm not entirely sure how to run the tests including the unstable ones though?

cargo test --features "serde/unstable

builds, but doesn't run them, and

cargo test --features "unstable"

errors out saying that there's no such feature?

[jonhoo]

And

rustup run nightly cargo t --features "unstable-testing"

comes back with a whole host of errors from compiletest, which I believe are unrelated to my changes.

[dtolnay]

Yep the unstable testing situation was pretty messed up. I think I fixed it in dd6989d.

[dtolnay]

The compiletest setup is unfortunately really fragile in how it detects and links dependencies. Try the following:

cd test_suite/deps
cargo clean
cargo +nightly build
cd ..
cargo +nightly test --features unstable

[jonhoo]

Oooh, I completely missed the +TOOLCHAIN feature release!

cargo clean didn't work inside deps/:

test_suite/deps $ cargo clean
error: package `/home/jon/dev/others/serde/serde_derive/Cargo.toml` is a member of the wrong workspace
expected: /home/jon/dev/others/serde/test_suite/deps/Cargo.toml
actual:   /home/jon/dev/others/serde/Cargo.toml

The other commands worked fine though, and all tests passed!

I guess technically we should now have a way of monitoring when into_boxed_c_str stabilizes, and then remove the unstable feature flag... Or maybe it's fine if I just monitor rust-lang/rust#40380 and submit another PR when that day comes.

[dtolnay]

Looks like the workspace issue is fixed in nightly: cargo +nightly clean should work.

I don't think we need an issue to track stabilizing this. Typically for every major release of Serde we decide what the minimum supported Rust version should be and then we remove as many of the #[cfg(feature = "unstable")] as possible.