Choice of ABI for host code on Windows

[jschwe]

Assuming the following input:

      name: Install Rust
      uses: dtolnay/rust-toolchain@master
      with:
        toolchain: stable
        targets: x86_64-pc-windows-gnu

The following line

rust-toolchain/action.yml

Line 72 in e645b0c

- run: rustup default ${{steps.parse.outputs.toolchain}}

runs e.g. rustup default stable, but this is incorrect on windows if previously there was already a different stable toolchain (stable-x86_64-pc-windows-msvc) installed (as is the case in github actions). The default toolchain after the action has run will still be the one with msvc abi, which is likely not what most github actions users would expect.

Since the action does allow multiple targets to be installed, I'm not sure how best to fix this.

[dtolnay]

I think this is working correctly. Rustup's host toolchain on Windows is always x86_64-pc-windows-msvc by default. When you install a toolchain with support for the x86_64-pc-windows-gnu target, that is a cross-compilation toolchain. Code that is built for the host, such as build scripts and proc macros, are going to get built with MSVC ABI; code that is built for the target would get GNU ABI. Just like if you were targeting any other arbitrary non-Windows target with a cross-compiler.

If you want GNU ABI for host code, you would need to install toolchain: stable-x86_64-pc-windows-gnu.