Cargo improvements and bugs

[jethrogb]

This is a tracking issue for proposed changes to cargo that could aid portability.

• Conditional compilation based on a dependent's choices.
• Target-specific cargo features
• #4

• Post https://github.com/rust-lang/rfcs/pull/1133 concerns
  • [ ] https://github.com/rust-lang/cargo/issues/5002: Explicit standard library dependencies
  • [ ] https://github.com/rust-lang/cargo/issues/5003: Implicit standard library dependencies
  • [x] https://github.com/rust-lang/cargo/issues/4499: @Ericson2314 left comment about [patch] for std-lib libraries, thinks issue might by closed in mistake cause cannot find docs
• [ ] https://github.com/rust-lang/cargo/issues/4866: cross compilation totally breaks build.rs
  • https://github.com/rust-lang/cargo/issues/4866#issuecomment-362930403 shows that Cargo.lock doesn't handle platform-specific dependencies properly either

[Ericson2314]

At the end of https://github.com/rust-lang/rfcs/pull/1133 I carved off some stand-alone issues that can be addressed relatively independently. (edit added above).

---

@jethrogb seeing you copied my issues in to the OP, I embellished that and removed from here. In general, I was wondering what you think of non-authors editing the OP. It's convenient to put things there, also but don't want to put words into other people's mouths, so no obvious solution IMO.

[jethrogb]

https://github.com/jamesmunns/rfcs/pull/1

https://internals.rust-lang.org/t/pre-rfc-std-aware-cargo/9450