Add partial support for cross-compiling on Linux ARM runners

Until https://github.com/cross-rs/cross/issues/1628 is resolved, running cross on Linux ARM also
requires the user to provide a custom Docker image, so this is only sort of usable.

.github/workflows/test.yml

@@ -103,6 +103,32 @@ jobs:
             expect-stripped: ""
             can-execute: true
 
+          - platform_name: Linux-aarch64 with arm64 host
+            runs-on: ubuntu-24.04-arm
+            target: aarch64-unknown-linux-gnu
+            cache-cross-binary: true
+            expect-file-re: "aarch64"
+            expect-cross: ""
+            expect-stripped: "--expect-stripped"
+            can-execute: true
+
+          # This fails because of some sort of weird bug in cross. See
+          # https://github.com/cross-rs/cross/issues/1628 for more details. Until that's fixed
+          # cross-compiling on Linux ARM won't work without a custom Docker image, which I don't
+          # want to do just for the sake of testing this.
+          #
+          # - platform_name: Linux-aarch64 with arm64 host compile to x86-64
+          #   runs-on: ubuntu-24.04-arm
+          #   target: x86_64-unknown-linux-gnu
+          #   # Until cross produces arm64 binary releases we _have_ to set this when running on
+          #   # arm64.
+          #   cross-version: "c7dee4d"
+          #   cache-cross-binary: true
+          #   expect-file-re: "ELF.+x86-64"
+          #   expect-cross: "--expect-cross"
+          #   expect-stripped: ""
+          #   can-execute: true
+
           - platform_name: Linux-arm
             runs-on: ubuntu-24.04
             target: arm-unknown-linux-gnueabi

Changes.md

@@ -1,3 +1,9 @@
+## 1.0.3
+
+- Fixed a bug when running with an ARM Linux host where the action would use a cached `cross`
+  download for x86-64 Linux (or vice versa). Now the cache key for the `cross` binary includes both
+  the runner's architecture in addition to its OS.
+
 ## 1.0.2 - 2025-02-16
 
 - Added a new `force-use-cross` input, which does what it says. It will force the use of `cross`

README.md

@@ -163,6 +163,19 @@ you can avoid this issue by structuring your workflow as follows:
 
 When structured this way, it does not matter if the output of crate A is deleted in step 3.
 
+## Cross-Compiling from Linux ARM Runners
+
+In theory, this should work, and this action does implement some of the necessary work for this.
+However, there are a couple issues with this:
+
+1. As of 2025-02-17, the `cross` project does not publish Linux ARM binary releases. That means that
+   in order to use `cross` on a Linux ARM runner as part of this action, you must set
+   `cross-version` to a more recent commit from the `cross` repo.
+2. There is
+   [a bug in `cross` that means you must use a custom Docker image](https://github.com/cross-rs/cross/issues/1628)
+   when cross-compiling from a Linux ARM runner. See
+   [this other `cross` issue](https://github.com/cross-rs/cross/issues/751) for more details.
+
 ## Linting and Tidying this Code
 
 The code in this repo is linted and tidied with

action.yml

@@ -107,6 +107,15 @@ runs:
         targets: ${{ inputs.target }}
         toolchain: ${{ inputs.toolchain }}
 
+    - name: Install qemu-user emulator binaries if cross-compiling on arm64 host
+      shell: bash
+      run: |
+        set -e
+        set -x
+
+        docker run --privileged --rm tonistiigi/binfmt --install all
+      if: steps.determine-cross-compile.outputs.needs-cross == 'true' && runner.os == 'Linux' && contains(runner.arch, 'ARM')
+
     - name: Determine cross version
       id: determine-cross-version
       shell: bash
@@ -128,7 +137,7 @@ runs:
       uses: actions/cache@v4
       with:
         path: ${{ steps.set-cross-dir.outputs.cross-dir }}/cross
-        key: ${{ runner.os }}-${{ steps.determine-cross-version.outputs.cross-version }}
+        key: ${{ runner.os }}-${{ runner.arch }}-${{ steps.determine-cross-version.outputs.cross-version }}
       if: steps.determine-cross-compile.outputs.needs-cross == 'true' && inputs.cache-cross-binary == 'true'
 
     - name: Install cross if cross-compiling (*nix)