[Pkg-rust-maintainers] Bug#924750: dh-cargo: Support transitive runtime-depends

[Ximin Luo]

Package: dh-cargo
Version: 17
Severity: important

Some crates (e.g. opener) cause a runtime-dependency *for any crate that uses
it*. For example cargo should really Depends: xdg-utils because it uses opener.

We currently have no way of expressing this. We could use a mechanism similar
to dh-cargo-built-using which embeds this information into the build.rs output.
We should support depends/recommends/suggests, arguably xdg-utils from the
above example should be a Recommends rather than a hard Depends.

-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (990, 'testing'), (500, 'unstable-debug'), (500, 'testing-debug'), (500, 'stable'), (300, 'unstable'), (100, 'experimental'), (1, 'experimental-debug')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.19.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8), LANGUAGE=en_GB:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages dh-cargo depends on:
ii  cargo      0.33.0-1
ii  debhelper  12.1.1
ii  perl       5.28.1-4
ii  python3    3.7.2-1

dh-cargo recommends no packages.

dh-cargo suggests no packages.

-- no debconf information