If I choose a supported target (let's say cross build --target=aarch64-unknown-linux-gnu), everything works like charm ‒ it automatically downloads the std for that target, downloads the docker image and builds it.

However, if I choose an unsupported target (aarch64-unknown-linux-musl), it just tries to call cargo without any complaint. Then of course cargo complains:

error[E0463]: can't find crate for `core`
  |
  = note: the `aarch64-unknown-linux-musl` target may not be installed

I'd expect to see one of:

• A message that such target is not supported
• A failed attempt at installing the target's std or the docker image