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

[Fabian Grünbichler]

> 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.

picking this up again while going through old bugs..

I think we could potentially even piggy-back on top of
Static-Built-Using - if we define some file/path where librust-*
packages can specify that packages actually linking them should have a
dependency/.. on a third package, then after S-B-U has been collected we
"just" need to check if any of the referenced librust packages have such
a file, and emit the corresponding substvars. technically, we could even
do it *in* dh-cargo-built-using, to avoid the round-trip..

maybe /usr/share/cargo/runtime-dependencies/$crate-$ver ? I think it
would be great to not have to patch build.rs for this use case, and
instead have a declarative file (usually generated by debcargo using
declarative input).