CACHEDIR.TAG not created for some `*-apple-darwin` targets

[sandhose]

Cargo creates CACHEDIR.TAG files in some directories of the target/ folder, so that they are marked as excluded for backups. This is also used by the Swatinem/rust-cache action to know if the directory should be kept for cache or not

Turns out, for some reason with *-apple-darwin targets (at least), cargo-zigbuild doesn't create such a file.

Repo:

Create an empty bin crate:

> cargo new cachedir-repro
     Created binary (application) `cachedir-repro` package
> cd cachedir-repo/

It works as expected with cargo build on the aarch64-apple-darwin target:

> rm -rf target/
> cargo build --target aarch64-apple-darwin
...
> cat target/aarch64-apple-darwin/CACHEDIR.TAG
Signature: 8a477f597d28d172789f06886806bc55
# This file is a cache directory tag created by cargo.
# For information about cache directory tags see https://bford.info/cachedir/

It doesn't with cargo zigbuild on the aarch64-apple-darwin target:

> rm -rf target/
> cargo zigbuild --target aarch64-apple-darwin
...
> cat target/aarch64-apple-darwin/CACHEDIR.TAG
cat: target/aarch64-apple-darwin/CACHEDIR.TAG: No such file or directory

But it works with some targets:

> rm -rf target/
> cargo zigbuild --target aarch64-unknown-linux-gnu
...
> cat target/aarch64-unknown-linux-gnu/CACHEDIR.TAG
Signature: 8a477f597d28d172789f06886806bc55
# This file is a cache directory tag created by cargo.
# For information about cache directory tags see https://bford.info/cachedir/

This is affecting me because it means macOS builds are not properly cached in my CI, with a combination of cargo-zigbuild and Swatinem/rust-cache.

Note that the host target seems irrelevant, I tried both from a Linux host and a macOS host, and it affects AFAIK the aarch64-apple-darwin and the x86_64-apple-darwin targets

[messense]

I'm not sure why it's not created because eventually it's cargo driving the build, cargo-zigbuild only sets up some environment variables to use zig as c/c++ compiler and linker.

Nothing obvious I can do here.

[sandhose]

But the issue exists, can be reliably reproduced, has real consequences (in my case, macOS CI builds not being cached), and looks somehow specific to cargo-zigbuild, since regular equivalent cargo calls work as expected. I can try investigating a bit more on my free time, but I really think the issue should stay open in the meantime

[messense]

Sure, we can open it if you want to investigate.

Do note that I'll close it again if nothing comes up after a month or two.

[sandhose]

I found the culprit:

https://github.com/rust-cross/cargo-zigbuild/blob/0c4827bbf6bcacb0b5eb814be244b506a2961a98/src/zig.rs#L749-L777

Before invoking cargo build, the target/[profile]/deps/ directory is being created, so when cargo is invoked, it skips the target directory creation (done here)

I don't exactly know how one would fix this, but I can confirm commenting the last two lines is enough to fix the issue

[messense]

Thanks, see also https://github.com/rust-lang/cargo/issues/12441.