Fix cargo rustc for test with implicit binary.

[ehuss]

Fixes #5502

[rust-highfive]

r? @matklad

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[matklad]

Fascinating! If I understand correctly, previously, integration tests and binaries could be built in parallel, because binaries were a prerequisite for running the test, now it is a per-requisite for building the test. I wonder if it is significant? One way to check this would be to do time cargo test --test testsuite --norun after modifying the lib.rs file. With this setup, we first should build cargo-the-library, and then the binary and the test either in parallel or sequentially. If the difference is significant, we might think about how to preserve the parallelism while fixing the bug. Or am I completely off base here about build parallelism changes?

That said, given that this code previously (before dependencies profiles) lived in compute_deps anyway, I am fine with merging it as is.

[ehuss]

I wonder if it is significant?

I thought the same thing, but fortunately JobQueue has a special case for this situation:

cargo/src/cargo/core/compiler/job_queue.rs

Lines 455 to 464 in caa7ac1

	Ok(targets
	.iter()
	.filter_map(|unit| {
	// Binaries aren't actually needed to *compile* tests, just to run
	// them, so we don't include this dependency edge in the job graph.
	if self.target.is_test() && unit.target.is_bin() {
	None
	} else {
	Some(Key::new(unit))
	}

There are other ways to fix this. We could add special-case code to detect this scenario in compile_ws where it does a simple length check, but I think that would be awkward. This solution will affect #5460 (will need to explicitly add bins to the list to uplift), so it's not without impact. I couldn't really think of a better way to do it, though.

[matklad]

Ok!

@bors r+

[bors]

📌 Commit a187ba8 has been approved by matklad

[bors]

⌛ Testing commit a187ba8 with merge 7067bc8...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: matklad
Pushing 7067bc8 to master...