Cache toolchain, dependencies in PR/CI

[heaths]

Rust builds seem to be taking longer lately, presumably to set up older toolchains the agents may not have since we target older versions by design. But caching certain directories can help a lot. I use the following in nearly all my Rust repos and it makes a significant improvement in build time.

https://github.com/heaths/akv-cli-rs/blob/241c9ccc84be7ba277c5483192ba3f0714215c64/.github/workflows/ci.yml#L36-L45

    - name: Cache
      uses: actions/cache@v4
      with:
        path: |
          ~/.cargo/bin/
          ~/.cargo/registry/index/
          ~/.cargo/registry/cache/
          ~/.cargo/git/db/
          target/
        key: ${{ runner.os }}-${{ runner.arch }}-cargo-${{ hashFiles('**/Cargo.lock') }}

Using the right cache key - including the toolchain (version), platform, and a hash of Cargo.lock - we can handle all the variations we have.

[hallipr]

We have a similar task we can use in ADO:
https://learn.microsoft.com/en-us/azure/devops/pipelines/tasks/reference/cache-v2?view=azure-pipelines

Caches will be isolated per definition, per cache key. Cache keys can contain multiple segments, pipe delimited. If a segment contains a file path, the hash of the contents of the path are used in the cache key.

- task: Cache@2
  inputs:
    path: # string. Required. Path. 
    key: $(ToolChain)|Cargo.lock

[hallipr]

We'd want to run rustup first to resolve our toolchain name to a real version, then use that in the cache key to let toolchain revisions cache bust

[heaths]

We also need to include the platform/host. A cache for Windows won't work for linux, macOS, and vice versa.

[danieljurek]

This is worth a try. Caching too much can lead to problems. For example, we should probably expand $(ToolChain) from a string like stable into the specific version of the toolchain we want to use or you may end up with a toolchain version that used to be stable but is now old.

[heaths]

"stable" won't be good enough. That's why @hallipr mentioned above that we should parse out and expand that to the real version. Latest "stable" will change about every month.