check/build on workspace forces build of package not required by current target

[temoto]

Problem

# project/Cargo.toml
[workspace]
members = ["cmd/iodin"]

# project/cmd/iodin/Cargo.toml
#...
[dependencies]
[target.'cfg(target_os = "macos")'.dependencies]
[target.'cfg(target_os = "linux")'.dependencies]
pigiron = { path = "../../hardware/pigiron" }  # impossible to build on macos

Steps

• cargo commands in binary directory work as expected

cd project/cmd/iodin
cargo check
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s

• but in workspace it tries to compile dependency that was not required by target

cd project
cargo check
   Compiling pigiron v0.1.0 (/Users/temoto/dev/github.com/temoto/vender/hardware/pigiron)
error: failed to run custom build command for `pigiron v0.1.0 (/Users/temoto/dev/github.com/temoto/vender/hardware/pigiron)`
...

Possible Solution(s)

For workaround I just use bash script instead of cargo at top level.

Notes

Output of cargo version:

• cargo 1.31.0-nightly (5dbac98 2018-10-08)
• toolchain: nightly-x86_64-apple-darwin (default)
• OS: OSX Darwin Kernel Version 16.7.0

[ehuss]

I think this is to be expected (albeit surprising), as all path dependencies are implicitly added as workspace members. One workaround should be to add it to the workspace exclude list. I don't think it would be feasible for target dependencies to be automatically excluded because we don't have the target configuration before the workspace is constructed.

[epage]

With #6179 and #9208, we'd have the tools to allow a package to be a no-op when building workspace members which I feel takes care of this problem. I'm closing in favor of those. If there is a reason we should keep this open, let us know!