Features from build dependencies aren't propagated to normal dependencies

[alexkirsz]

After #2877 (350d249) was merged, we've started seeing an error in our compilation pipeline where a crate's build script would be built with a feature, but the crate itself wouldn't.

Here's a minimal repro case: https://github.com/alexkirsz/rules_rust_build_dep_repro

If you run cargo build --target=aarch64-apple-ios --verbose, you'll see that cargo builds mime_guess on both the host platform (aarch64-apple-darwin in my case) and the target platform (aarch64-apple-ios) with the "rev-mappings" feature enabled, even though it is theoretically only needed at compile-time on the host platform.

If you run bazel build -s --platforms=//:ios_arm64 //:mime_guess_test --keep_going, you'll see that rules_rust builds the build script of mime_guess with the "rev-mappings" feature enabled, but on the target platform, it builds mime_guess without the feature, resulting in a compilation error.

Instead, it should either:

1. Copy Cargo's behavior and always build with the complete set of features from normal and build deps; or
2. Build the build script twice for the host platform, once with the feature, and once without.

[sthornington]

I believe we are seeing this as well. I've been hacking away on a little fix for #2938 and noticed that, e.g., building something with a crate dependency on tokio was failing for lack of parking_lot, so it seems like the feature propagation might not be working not only for build scripts but also general dependencies?

[sthornington]

In my case I was able to fix the problem by changing a dependency on tokio to use the "full" feature set, but I am not sure why it was trying to build parts of it that needed parking_lot if that feature was not present ...