create_rust_makefile not Cargo workspace aware, target dir is hardcoded

[wilsonzlin]

It appears that the create_rust_makefile is hardcoded to assume that the target dir is at target, instead of querying the cargo metadata. This makes it break when building a crate in a workspace (the cp command to copy the output library fails), which is reasonably common. I'm not familiar with Ruby, Rake, or how the Makefile is generated and executed so please correct me if I'm wrong.

Perhaps something like this could be useful? For example, cargo metadata | jq -r .target_directory.

So far, I have only been able to work around this by setting the RB_SYS_CARGO_TARGET_DIR environment variable to the absolute path of the workspace target dir. I tried setting do |r| r.target_dir = ... but this doesn't seem to have any effect.

[ianks]

Hey thanks for the report. We actually already query cargo metadata elsewhere in the crate, so perhaps we could leverage that in some way