• A workaround for cargo#5730

* Overview ** The problem Cargo can't deal with different kinds of dependencies having different features enabled. If you have both a regular dependency and a build dependency on a single library, and the build dependency has a different feature selection, that gets used for the regular one as well. This is a big problem for no_std projects, where you have the standard library available for the build script but not for the application itself.

The project's namesake is, of course, https://github.com/rust-lang/cargo/issues/5730.

* The solution A build script is just a program that looks at special environment variables and writes magical values to stdout. We can just write a regular program that does that. This library helps you manage such a program.

(If you're using .cargo/config to set compiler flags for embedded development and you try to do this by hand, you'll note Cargo's recursive config resolution scheme gives you the same flags for the build script, preventing it from working. This library also works around that problem by building out of /tmp.)

* Getting Started ** Dependencies =cargo-5730= intentionally has no dependencies, to avoid adding fuel to dependency management fire it's attempting to extinguish.

It currently runs only on Linux. * Building

+begin_src sh

cargo build

+end_src

** Usage 1. Make a new crate, inside your regular crate, that will hold your build script. #+beginsrc sh cargo new --bin build-script #+endsrc

1. Your old build.rs is now main.rs for this new crate

   +begin_src sh

   mv build.rs build-script/src/main.rs

   +end_src

2. Your old build-dependencies are now regular dependencies in this crate

   +begin_src sh

   emacs build-script/Cargo.toml

   +end_src

3. Add this library as the only build dependency in your main crate's =Cargo.toml=

   +begin_src toml

   [build-dependencies] cargo-5730 = { git = "https://github.com/auxoncorp/cargo-5730.git", branch = "master" }

   +end_src

4. Use this library to delegate to the build crate in main crate's build.rs

   +begin_src rust

   use cargo_5730;

   fn main() { cargo5730::runbuild_crate("build-script"); }

   +end_src

* Example See the =example= directory for a cargo project set up as described above. To see the library in action, compile it with =cargo build -vv=. Among the other output, you will see something like:

+begin_src

[bin-crate 0.1.0] cargo:rerun-if-changed=build-script [bin-crate 0.1.0] Copying build crate source from build-script to /tmp/build-script-c2717537984a6f51b94ee464d0cf90 [bin-crate 0.1.0] Compiling lib-crate v0.1.0 (/home/mullr/devel/cargo-5730/example/lib-crate) [bin-crate 0.1.0] Running CARGO_PKG_VERSION_MAJOR=0 CARGO=/home/mullr/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo CARGO_ [bin-crate 0.1.0] Compiling build-script v0.1.0 (/tmp/build-script-c2717537984a6f51b94ee464d0cf90) [bin-crate 0.1.0] Running CARGO_PKG_VERSION_MAJOR=0 CARGO=/home/mullr/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo CARGO_ [bin-crate 0.1.0] Finished dev [unoptimized + debuginfo] target(s) in 0.40s [bin-crate 0.1.0] Adding the numbers 1 and 2 [bin-crate 0.1.0] Build script says: the sum is 3 [bin-crate 0.1.0] Removing build crate staging dir: /tmp/build-script-c2717537984a6f51b94ee464d0cf90

+end_src

** License © 2019, Auxon Corporation Please see the LICENSE file for more details.