Need ability to add dependencies based on `#[cfg()]`

[codyps]

For example, in curl-rust:

Usage: https://github.com/carllerche/curl-rust/blob/5d0f5c8848e3cf1e12480a1923ae888e24d58f63/src/lib.rs#L10
Dependence: https://github.com/carllerche/curl-rust/blob/5d0f5c8848e3cf1e12480a1923ae888e24d58f63/curl-sys/Cargo.toml#L18-L30

Also see alexcrichton/curl-rust#30

In summary: without this, we end up with ad-hoc lists of triples in various Cargo.toml files, which breaks the build for targets with a non-matching triple that do match #[cfg(...)].

[alexcrichton]

To clarify, one may wish to say [target.unix.dependencies] and [target.windows.dependencies] instead of listing out triples. This will help correspond to cfg(unix) and cfg(windows) in the compiler but requires duplication unfortunately.

[carllerche]

👍 (sorry for the useless comment, but its the only way to search for issues!)

[alexcrichton]

Another alternative mentioned by @larsbergstrom would be:

[target."x86_64-*".dependencies]
# ...

[target."*-linux-*".dependencies]
# ...

[codyps]

I'd like to caution against trying to assume anything about targets from their triples.
Rustc currently just treats targets as arbitrary strings: their contents is entirely up to whomever is adding the target.

Trying to extract meaning about targets out of the triple will be fairly error prone & in some cases will completely fail (or be impossible).

Possibly: rustc would expose a way of determining the attributes of a given target (looking at it's builtin targets & the requested json target) and cargo would query that.

Alternately: we could remove all the builtin target knowledge and push it into json files installed in rustlib, and cargo could read them directly.