build-dependencies and dependencies should not have features unified

[whitequark]

Consider this real-world example from crc-rs:

[dependencies]
build_const = { version = "0.2", default-features = false }

[build-dependencies]
build_const = "0.2"

The build dependency, of course, needs (and has) the std feature. The runtime dependency does not. Moreover, on my platform, there is no std. However, cargo treats them as the same package, and as a result it is impossible to actually build crc-rs at all.

[vitiral]

wow, I certainly would not have anticipated this (and appear to have not tested it...). Yay me!

[gnzlbg]

Not only dev-dependencies, features of platform-specific dependencies are also unified:

With this Cargo.toml:

[package]
name = "cargo_fubar"
version = "0.1.0"
authors = ["fubar <fubar@fubar.com>"]

[target.'cfg(target_os = "macos")'.dependencies]
mach = "0.1.*"

[dependencies.libc]
version = "0.2"
default-features = false

cargo build --no-default-features --verbose --target x86_64-unknown-linux-gnu outputs:

 Compiling libc v0.2.36
     Running `rustc --crate-name libc /foo/.cargo/registry/src/github.com-1ecc6299db9ec823/libc-0.2.36/src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="default"' --cfg 'feature="use_std"' -C metadata=af3eb212fae2904c -C extra-filename=-af3eb212fae2904c --out-dir /foo/projects/sideprojects/cargo_fubar/target/x86_64-unknown-linux-gnu/debug/deps --target x86_64-unknown-linux-gnu -L dependency=/foo/cargo_fubar/target/x86_64-unknown-linux-gnu/debug/deps -L dependency=/foo/cargo_fubar/target/debug/deps --cap-lints allow`

Note how I have a platform-specific dependency for MacOSX which should absolutely have nothing to do with linux builds, yet --cfg 'feature="default"' --cfg 'feature="use_std"' is enabled for libc even though [dependencies.libc] and the cargo build --no-default-features explicitly disable all features.

Removing the platform specific dependency and recompiling for linux produces:

Compiling libc v0.2.36
     Running `rustc --crate-name libc /foo/.cargo/registry/src/github.com-1ecc6299db9ec823/libc-0.2.36/src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=ebd4a22423d44421 -C extra-filename=-ebd4a22423d44421 --out-dir /foo/cargo_fubar/target/x86_64-unknown-linux-gnu/debug/deps --target x86_64-unknown-linux-gnu -L dependency=/foo/cargo_fubar/target/x86_64-unknown-linux-gnu/debug/deps -L dependency=/foo/cargo_fubar/target/debug/deps --cap-lints allow`

Any workarounds?

[gnzlbg]

@alexcrichton I disagree about this being a feature request, I think this is a pretty severe bug in cargo and should be tagged as such.

[gnzlbg]

So I've added a test for this in PR #5007. If nobody beats me to it (and I hope somebody will) I'll either submit a new PR with a fix or add it to that PR (if it doesn't get merged).

[gnzlbg]

So fixing this is very hard, it looks like cargo was designed under the assumption that the features of all dependencies, dev-dependencies, target.dependencies,.... ought to be unified to be able to generate a single dependency graph for build, test, --target, etc.

What I've ended up doing as a workaround is having multiple Cargo.toml files for a single project... I have a Cargo_test.toml, Cargo_build_target_xxx.toml, etc. This sucks, but it work. However, I can't release to crates.io under this setup, because I don't have a single Cargo.toml for all my targets, and adding multiple targets messes the features...

[gnzlbg]

Someone should raise this with the tools team. I don't know how anybody manages to use cargo for targeting any #[no_std] environment.

If your crate happens to use std in any way for, for example, testing or benchmarking, chances are its always being built with std linked in, and forcing other crates to be linked with std as well :/

[vitiral]

@gnzlbg that's an interesting workaround, however it wouldn't fix the issue if your build.rs needed it.

For testing/benchmarking only could you have Cargo.toml be your "released application" and CargoTest.toml be your "test" one?

[SimonSapin]

I think this is related to #4463.