Don't unconditionally build alloc for no-std targets

[jyn514]

It's possible for targets to only support core and not alloc. Instead of building alloc unconditionally, pass a list of crates to build into std_cargo, and only pass -p alloc if the list of crates wasn't already filtered to a subset.

The original use case was to reuse std_cargo for a rustc_driver that doesn't emit metadata. But this seems like a reasonable change regardless.

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[Mark-Simulacrum]

Shouldn't this also pass dependencies, for the same reasons as the comment above?

[jyn514]

I think the comment is wrong, not the code. That comment used to be in check::Std and only really makes sense there. check::Std has the same behavior here before and after, and AFAICT the behavior is correct: see this comment in std_crates_for_run_make:

rust/src/bootstrap/src/core/build_steps/compile.rs

Lines 452 to 469 in 8338f9f

	/// Resolves standard library crates for `Std::run_make` for any build kind (like check, doc,
	/// build, clippy, etc.).
	pub fn std_crates_for_run_make(run: &RunConfig<'_>) -> Vec<String> {
	let mut crates = run.make_run_crates(builder::Alias::Library);
	// For no_std targets, we only want to check core and alloc
	// Regardless of core/alloc being selected explicitly or via the "library" default alias,
	// we only want to keep these two crates.
	// The set of no_std crates should be kept in sync with what `Builder::std_cargo` does.
	// Note: an alternative design would be to return an enum from this function (Default vs Subset)
	// of crates. However, several steps currently pass `-p <package>` even if all crates are
	// selected, because Cargo behaves differently in that case. To keep that behavior without
	// making further changes, we pre-filter the no-std crates here.
	let target_is_no_std = run.builder.no_std(run.target).unwrap_or(false);
	if target_is_no_std {
	crates.retain(|c| c == "core" || c == "alloc");
	}
	crates

I'm going to move this comment from std_cargo to check::Std::make_run.