rustup and interfaction with Swatinem/rust-cache

[jennydaman]

Hi, I am wondering how to use this properly with Swatinem/rust-cache. The README says

actions-rust-cross/README.md

Lines 89 to 93 in ad283b2

	## Caching Rust Compilation Output
	You can use the [Swatinem/rust-cache](https://github.com/Swatinem/rust-cache) action with this one
	seamlessly, whether or not a specific build target needs `cross`. There is no special configuration
	that you need for this. It just works.

However, Swatinem/rust-cache README says

selecting a toolchain either by action or manual rustup calls should happen
before the plugin, as the cache uses the current rustc version as its cache key

https://github.com/Swatinem/rust-cache/blob/9bdad043e88c75890e36ad3bbc8d27f0090dd609/README.md?plain=1#L11-L12

I see that houseabsolute/actions-rust-cross calls dtolnay/rust-toolchain@master, which is a wrapper around rustup

actions-rust-cross/action.yml

Lines 43 to 46 in d788e00

	uses: dtolnay/rust-toolchain@master
	with:
	targets: ${{ inputs.target }}
	toolchain: ${{ inputs.toolchain }}

So if you have something like

- uses: Swatinem/rust-cache@v2
- uses: houseabsolute/actions-rust-cross@v0
  with:
    taget: x86_64-unknown-linux-musl

What ends up happening is rustup gets executed before Swatinem/rust-cache

[autarch]

Oh, that's a good question. I didn't actually notice this in the rust-cache docs! I've been using it the wrong way, I guess. See https://github.com/houseabsolute/ubi/blob/master/.github/workflows/ci.yml for an example.

It seems to be caching something. My guess is that it's using the rust version installed on the Actions image. I think the only way to fix this is to have actions-rust-cross integrate rust-cache. That's not very hard to do, so I'll look into that soon.

[jennydaman]

I think the only way to fix this is to have actions-rust-cross integrate rust-cache. That's not very hard to do, so I'll look into that soon.

That would be great! Supporting rust-cache would give houseabsolute/actions-rust-cross a feature over https://github.com/taiki-e/setup-cross-toolchain-action

[autarch]

So I looked into this a little more. I think that Swatinem/rust-cache is just not compatible with cross. The rust-cache action uses the output of rustc -vV as part of its cache key. But if the build is running via cross, then the output of rustc on the host isn't relevant to the compilation artifacts.

I'm not sure how to fix this. The "obvious" solution would be to tell rust-cache to run cross -vV instead. But that doesn't actually do anything useful, since it will just run cargo under the hood. What we really need is a way to ask cross to give us the same output for a given target, but cross only accepts a --target parameter when given a subcommand like build or test.

I tried capturing the output from the correct rustc with a simple crate using a build.rs script, but even then, it shows me info about an x86-64 host.

So I'm a bit stumped on this one. If there was a way to get the right rustc info from cross I think making a PR for rust-cache to use this info wouldn't be too hard. But I don't know how to get this info.