The rust plugin fails when using source_subdir

[Leo Arias]

When trying to make a rust snap that has uses source-subdir, it fails during fetch:

/home/ubuntu/workspace/canonical/snapcraft/integration_tests/snaps/rust-subdir/parts/rust-subdir/rust/bin/cargo fetch --manifest-path /home/ubuntu/workspace/canonical/snapcraft/integration_tests/snaps/rust-subdir/parts/rust-subdir/src/Cargo.toml
error: manifest path `/home/ubuntu/workspace/canonical/snapcraft/integration_tests/snaps/rust-subdir/parts/rust-subdir/src/Cargo.toml` does not exist

It's looking for the Cargo.toml file in source, not in source-subdir.

Steps to reproduce:

Go to integration_tests/snaps/rust-subdir
Edit the snapcraft.yaml to have:
    source: .
    source-subdir: subdir
Run snapcraft

[Sergio Schvezov (sergiusens)]

https://github.com/snapcore/snapcraft/pull/1296