Dependencies that should not be built in the target arch are still built

[xxuejie]

Problem

https://github.com/xxuejie/cargo-target-dep-test This is a minimal repo reproducing the problem. As denoted here, the included dependency should not be built in riscv64 targets, yet it is still built.

Steps

1. git clone https://github.com/xxuejie/cargo-target-dep-test
2. cd cargo-target-dep-test
3. rustup target add riscv64imac-unknown-none-elf
4. cargo build --target riscv64imac-unknown-none-elf

This would result an error. Even though ckb-x64-simulator is marked as a dependency only in non-riscv64 envnvironment, it is still built here.

When those lines are commented out, the crate could be built just fine.

Possible Solution(s)

Target specific dependency should be handled propoerly.

Notes

Output of cargo version:
cargo 1.48.0 (65cbdd2 2020-10-14)

[ehuss]

Features for dependencies are unified, even if they are for targets that are not currently enabled. This means that the features of the molecule package that ckb-x64-simulator enables causes it to add optional dependencies that are not compatible with no_std.

You can work around this with -Zfeatures=itarget. The tracking issue for that is #7914.

[xxuejie]

That seems to be the reason for this problem, many thanks.

[ehuss]

Closing as a duplicate of #7914, and note it has now been stabilized.