Cannot build crates in workspace environment where sub-crates have different architectures.

[bjc]

Arguably this is a problem with cargo itself, but you can work around it easily in the shell.

Setup

1. create a cargo workspace
2. add a subworkspace
3. in the subworkspace, create a .cargo/config with, eg
   [build] target = "thumbv6-none-eabi"
4. try to build the subworkspace in emacs.

Results

You'll get a bunch of linker errors. I realize the setup is pretty vague, but you can find a sample project that has this problem here

Investigation

The reason this happens is because the various cargo commands, spawned from cargo-process--start specify default-directory as (or (cargo-process--workspace-root) default-directory), where the latter default-directory points to the sub-workspace root.

If the let binding on default-directory is removed from the subsequent compilation-start call (thus pointing default-directory at the result of cargo-process--project-root), the sub-workspaces will compile correctly.

I assume there's a reason for overriding default-directory before calling compilation-start, but I don't know what it is, as everything appears to work correctly without it, with the added bonus that the problem described above goes away.

[kwrooijen]

I honestly don't remember what the reason what to override default-directory. It could be that it had something to do with sub workspaces but I don't know.

I haven't used Rust in a long time and I myself am not using this Emacs package, so it's very hard for me to maintain it.