Use of environment variables can make use of .cargo/config impossible

[jonas-schievink]

The Docker containers used by cross set various environment variables that cannot be overridden in any way. This causes various issues, because this makes Cargo silently ignore the configuration from .cargo/config.

• On x86_64-unknown-linux-musl, cross sets CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL_LINKER. This makes it impossible to use a custom linker wrapper by setting the linker key. Instead, I had to use the rustflags key to pass -Clinker.

• On aarch64-unknown-linux-musl, cross sets CARGO_TARGET_AARCH64_UNKNOWN_LINUX_MUSL_LINKER as well as RUSTFLAGS (!), which makes it impossible to use a custom linker without replacing rustc with a custom wrapper (which can be made to work by setting the RUSTC env var, but is a lot of work and a pretty gross hack).

[jonas-schievink]

Ah I should note that most of this is only a problem due to rust-lang/cargo#5376

[Alexhuszagh]

This is fixable by using the environment variable passthrough, or by extending the Docker image (less than ideal). It might be a good idea to allow overrides of these variables in Cross.toml. For example, overriding CARGO_TARGET_AARCH64_UNKNOWN_LINUX_MUSL_LINKER should be possible by adding it to [target.(...).env.passthrough].

We currently allow overriding the runner (sort of), but it might be good to also add keywords to override the linker and rustflags.

cross/src/cross_toml.rs

Lines 29 to 35 in c8df353

	pub struct CrossTargetConfig {
	xargo: Option<bool>,
	image: Option<String>,
	runner: Option<String>,
	#[serde(default)]
	env: CrossEnvConfig,
	}

[Emilgardis]

#682 would help for this

[Alexhuszagh]

This has been fixed as of #869 and it is documented on the wiki now.