Permit library-only and binary-only dependencies

[jhpratt]

Rust permits a package to contain both a library and a binary. However, any dependencies are necessarily shared, which is frequently undesirable. The only available alternative is to split the package in two, at which point the name (on crates.io) must be different.

My specific use case is avoiding deps like clap for anyone building a library, which adds needless overhead when building (even if only in non-incremental builds).

How feasible would it be to have [bin-dependencies] and [lib-dependencies] sections? These sections would extend the existing [dependencies] for backwards compatibility. Alternatively, a dependency could be specified with { bin = true, lib = false } or similar.

[lqd]

This issue should probably be moved to cargo's repository.

[Skgland]

As the binary implicitly depends on the lib I don't think [lib-dependencies] would be necessary, as they would implicitly be transitive dependencies of the bin.
One could make the binary only dependencies optional dependencies and require a feature for the binary that enables these dependencies.

Note that there is rust-lang/cargo#1982 that is basically a dupplicate, and there was rust-lang/cargo#10409 which was closed due to significant overlap with rust-lang/cargo#4663

[jhpratt]

I forgot cargo was its own repo. Closing in favor of that long-standing issue.