Wrong post-build if .cargo/config contains build target

[MathiasKoch]

If .config/cargo contains a build target where std is not available, post-build will fail, as it tries to compile post-build for said target.

.cargo/config:

[build]
target = "thumbv7em-none-eabihf" # Cortex-M4F and Cortex-M7F (with FPU)

post_build.rs:

use std::{path::PathBuf, env};

fn main() {
    let current_dir = env::current_dir().unwrap();
    let current_parent = current_dir.parent().unwrap();
    println!("current_dir: {:?}", current_dir);
    println!("current_parent: {:?}", current_parent);
    println!("ok");
}

cargo post build -p factbird-rs:

Finished dev [optimized + debuginfo] target(s) in 0.03s
Running Post Build Script at /home/mathias/git/blackbird/factbird-mini/factbird-rs/post_build.rs
   Compiling post-build-script v0.1.0 (/home/mathias/git/blackbird/factbird-mini/target/post_build_script_manifest)
error[E0463]: can't find crate for `std`
  |
  = note: the `thumbv7em-none-eabihf` 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 `post-build-script`

To learn more, run the command again with --verbose.

I can't quite figure out if this is actually the same as #2 ?

As i see it, the post_build script should always be run using the host target, and never be influenced by .config/cargo target?

[phil-opp]

Thanks for reporting! The issue is that .cargo/config files apply to all cargo invocations in that directory. Unfortunately, there is currently no way (e.g. a command line flag) to unset a default target for a single invocation.

I also encountered this underlying problem many times, e.g. as described here: rust-lang/cargo#8757 (comment). I now think the best solution for this is https://internals.rust-lang.org/t/proposal-move-some-cargo-config-settings-to-cargo-toml/13336, which would allow to specify the default target as a package-specific key in the Cargo.toml. There is already an implementation for this in rust-lang/cargo#9030, let's hope it gets merged soon.

[MathiasKoch]

Hmm, okay.
So what i hear you say, is that there is not even a work-around currently?

Lets hope for rust-lang/cargo#9030 soon :)

[phil-opp]

The only workaround that I'm aware of is to not use a default target and instead pass it as --target argument on the command line. Alternatively, you can create a custom alias for this for easier typing (e.g. cargo arm-build or cargo xbuild).

[MathiasKoch]

It seems to work if i remove the default target, and builds with cargo post build --target ... --release, but the custom alias to make this easier, does not seem to work as expected, due to

cargo-post/src/main.rs

Lines 42 to 53 in ef1b4d0

	let build_script_call = match args.peek().map(Deref::deref) {
	Some(cmd) => match cmd {
	"b" | "build" | "xbuild" => BuildScriptCall::AfterCommand,
	"c" | "check" | "clean" | "doc" | "new" | "init" | "update" | "search"
	| "uninstall" => BuildScriptCall::NoCall,
	cmd if ["run", "test", "bench", "publish", "install"].contains(&cmd) => {
	panic!("`cargo post {}` is not supported yet", cmd)
	}
	cmd => panic!("unknown cargo command `cargo {}`", cmd),
	},
	None => BuildScriptCall::NoCall,
	};

An alias like

[alias]
br = "build --release --target thumbv7em-none-eabihf"

Will always result in unknown cargo command 'cargo br'

[phil-opp]

Have you tried including the post in the alias, i.e. br = "post build --release --target thumbv7em-none-eabihf"?

[MathiasKoch]

Ahh, no haven't tried that. Will give it a shot

EDIT
Confirmed, it works to include post