A dependency in both [dependencies] and [build-dependencies] has both feature sets combined

[chbaker0]

Problem

I have a no_std crate which depends on another crate, arrayvec. arrayvec only uses std if the corresponding crate feature is enabled (which is on by default). I disable its default features in my crate's Cargo.toml, which should make it no_std.

My crate additionally has a build-dependency on nasm-rs which itself depends on arrayvec. It uses the default features, meaning nasm-rs's arrayvec should use std.

I'm finding that this combination makes my build fail. Cargo attempts to use arrayvec with std in my normal build.

I expected Cargo to build arrayvec without default features in the main build, and with default features for build.rs. It seems to be combining the feature sets instead.

A crate that repros this error: https://github.com/chbaker0/build-dep-failure-repro

Steps

1. Attempt to build https://github.com/chbaker0/build-dep-failure-repro on latest nightly.
2. Observe the failure:

   Compiling arrayvec v0.5.1
error[E0463]: can't find crate for `std`
  |
  = note: the `i686-unknown-none-4984653564076209026` target may not be installed

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: could not compile `arrayvec`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed

3. Comment out dependency nasm-rs:

...
[build-dependencies]
# Commenting out this dependency makes the build succeed.
# nasm-rs = "0.2.0"

4. Observe successful build:

$ cargo build
   Compiling arrayvec v0.5.1
   Compiling build-dep-failure-repro v0.1.0 (/home/collin/development/build-dep-failure-repro)
    Finished dev [unoptimized + debuginfo] target(s) in 0.26s

Possible Solution(s)

Notes

Output of cargo version:
cargo 1.48.0-nightly (75615f8 2020-09-29)

I'm using a custom build target which might be part of the problem.

[ehuss]

Thanks for making the repro! There is an nightly-only feature (-Zfeatures=host_dep) which will decouple the features from build-dependencies. You can read about it in the docs and follow the tracking issue (#7915) for updates.

[chbaker0]

Ah my bad! I should've searched more

[ehuss]

Closing this in favor of the tracking issue #7915.