Unsatisfiable build-dependency on librust-heck-0.5+default-dev
There seems to be an error here. the version of librust-prost-dev in sid (build-)depends on librust-heck-0.4+default-dev. The version in experimental does depend on librust-heck-0.5+default-dev as it's first choice, but that's fine because version 0.5 of rust-heck is available in experimental.
