Difficult to use the presence of an optional dependency to use an optional dependency in a non-optional dependency

[Xaeroxe]

Phew! That issue title was a mouthful. So for context see this PR: rustgd/collision-rs#38

We want to export a feature called "serde" and only turn on the "serde" feature in cgmath if our "serde" feature is enabled. We were able to accomplish this with a few workarounds, but it was fairly non-obvious how to do what might become a common task.

In addition to this it was also discovered that proc-macro crate dependencies are silently ignored if placed under a dependency section using target.'cfg(...)'.dependencies I imagine this is due to needing the macro system to evaluate the cfg statement, but it's still annoying and behaves unexpectedly.

[Xaeroxe]

$ cargo --version
cargo 0.21.0 (5b4b8b2ae 2017-08-12)

[Xaeroxe]

Something that might help this situation would be having the ability to specify additional feature lists like ["serde_derive", "cgmath/serde"] with an optional dependency, much like you can with features that aren't dependencies.

Alternatively: Permit a feature named serde to exist alongside an optional dependency called serde if and only if that feature also has "serde" in its feature list so that the optional serde crate is used. (That's kind of a complicated ruleset so I'm not sure if I like this idea)

[Xaeroxe]

proc-macro crate dependencies are silently ignored

I wanted to provide a few more details on this. So with this project structure
collision-rs.zip. The following commands give the following output (rust stable as default with rust nighty installed as well)

$ cargo check
error: failed to learn about crate-type `proc-macro` early on

$ cargo +nightly check
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs

$ cargo +nightly check --features="serde"
   Compiling collision v0.11.0 (file:///C:/Users/JKiesel.CISDEV/collision-rs)
error[E0463]: can't find crate for `serde_derive`
  --> src\lib.rs:46:1
   |
46 | extern crate serde_derive;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to previous error

error: Could not compile `collision`.

To learn more, run the command again with --verbose.

[alexcrichton]

Thanks for the report! @Xaeroxe would this perhaps be solved by #1286?

[Xaeroxe]

@alexcrichton Yes, that RFC sounds great, but doesn't it break backwards compatibility?

[alexcrichton]

@Xaeroxe ok! In that case I'm going to close this in favor of that issue.

Strictly interpreted it does indeed break backwards compatibility, but there's a number of possible solutions we could employ to avoid the breaking change.