plan should warn if target that has no default github runner is specified w/o a configured custom runner

[ashleygwilliams]

we currently support an infinite number of targets in the targets array, but github has a finite and specified number of default runners. if someone adds a target that does not have a default runner, AND does not specify a custom runner, we should have plan error. ideally, this means that cargo-dist will catch the error on the configuration update and not when someone is trying to release.

[Gankra]

So this is an interesting one in that there's a few different interacting checks.

Here we do a preliminary selection of what github runner to use for a given target triple, but in a very loose/permissive way:

cargo-dist/cargo-dist/src/backend/ci/github.rs

Lines 292 to 313 in e64776d

	fn github_runner_for_target(
	target: &TargetTriple,
	custom_runners: &HashMap<String, String>,
	) -> Option<GithubRunner> {
	if let Some(runner) = custom_runners.get(target) {
	return Some(runner.to_owned());
	}
	// We want to default to older runners to minimize the places
	// where random system dependencies can creep in and be very
	// recent. This helps with portability!
	if target.contains("linux") {
	Some(GITHUB_LINUX_RUNNER.to_owned())
	} else if target.contains("x86_64-apple") {
	Some(GITHUB_MACOS_INTEL_RUNNER.to_owned())
	} else if target.contains("aarch64-apple") {
	Some(GITHUB_MACOS_ARM64_RUNNER.to_owned())
	} else if target.contains("windows") {
	Some(GITHUB_WINDOWS_RUNNER.to_owned())
	} else {
	None
	}

And here we check that result and warn if it returned None:

cargo-dist/cargo-dist/src/backend/ci/github.rs

Lines 271 to 273 in e64776d

	let default = GITHUB_LINUX_RUNNER;
	warn!("not sure which github runner should be used for {target}, assuming {default}");
	default.to_owned()

So we do have a warning and it does fire after init/plan, but the "real" issue is that the preliminary selection is really generous, and arguably should be changed to be more strict, especially now that we have a proper config for the user to disambiguate (the looseness here was basically trying to let stuff work in a world where the user was doomed if we didn't pick something).

Also of note is this code that selects and expression for how to install cargo-dist for a given target triple, again very loose:

cargo-dist/cargo-dist/src/backend/ci/github.rs

Lines 316 to 331 in e64776d

	/// Select the cargo-dist installer approach for a given Github Runner
	fn install_dist_for_targets<'a>(
	targets: &'a [&'a TargetTriple],
	install_sh: &'a str,
	install_ps1: &'a str,
	) -> &'a str {
	for target in targets {
	if target.contains("linux") || target.contains("apple") {
	return install_sh;
	} else if target.contains("windows") {
	return install_ps1;
	}
	}
	unreachable!("internal error: unknown target triple!?")
	}

This code is ~fine for now since we need to have builds for the platform for it to work.

---

So the "fix" here is:

• change github_runner_for_target to an explicit allow-list of full target triples, using constants defined in axoproject https://docs.rs/axoproject/latest/axoproject/platforms/index.html
• improve the warning to mention the custom runner setting
• maybe make the warning a hard error?