Allow customizing manifest path in Rake task

[cretz]

Cargo metadata at

rb-sys/gem/lib/rb_sys/cargo/metadata.rb

Line 141 in 1da94ec

def cargo_metadata

is run in the root, and with no way to configure --manifest-path (not env var configurable either that I can see), the Rake task basically requires that you have a Cargo.toml at the base dir.

Possible to provide an option to customize --manifest-path?

[ianks]

Currently it's not possible, but I would happily merge a PR with that functionality