Allow specifying target-dependent build.rs scripts

[nagisa]

Currently it is possible to specify something like following in Cargo.toml:

[target.'cfg(unix)'.build-dependencies.cc]
version = "1.0"

however it is not feasible to use this functionality, because you cannot conditionally extern use crates or specify per-target build.rs scripts (i.e. the following does not work):

[target.'cfg(unix)'.package]
build = "build/unix.rs"

[sfackler]

Why can't you conditionally compile the contents of the build script?

[nagisa]

The #[cfg(...)] attributes in build scripts only affects the build script in terms of where the build script will be executed (a.k.a. $HOST), whereas what I’m looking for is an ability to change the build script depending on the crate target (i.e. $BUILD).

The goal is that the build script example below would not depend on cc when cargo build --target=x86_64-pc-windows-* is invoked from a UNIX machine.

// #[$BUILD_cfg(unix)]
extern crate cc;

fn main() { ... }

...

A lot of the time if statements based on environment variables work, but you cannot remove items from a crate based on environment variables and so dependency on the cc crate must exist even if unused.

Does that explain the issue clearly enough?

NB: I’m using cc here as an example only, because that is a widespread and well known crate.

[sfackler]

Ah right, that makes sense. One option could be for cargo to set equivalent cfgs for the target - something like #[cfg(cargo_target(unix))], #[cfg(cargo_target(target_os = "ios"))], etc.

[nagisa]

Another good reason to have target-dependent build-scripts: currently use of build scripts will force compilation of a dependency that's used during cargo check, which costs a fair amount of extra time. This cost does not need to be paid for by targets that don't benefit from the build script.

[SamNormcoreWayne]

I met this issue when writing a build.rs and .cargo/config.toml for a project cross-compiling from x86_64 linux to aarch64 android.

The reason why I need target-dependable build.rs is:

// build.rs
println!("cargo:rustc-link-search=native=platform/specific/lib/path");
println!("cargo:rustc-link-lib=static=some_aarch64_lib");

which means I want to link different libs by platforms in build.rs in project level. However, .cargo/config.toml only supports sub-module level over-write according to my research.