Do not lock the artifact-dir for check builds

[ranger-ross]

What does this PR try to resolve?

This PR modifies the locking logic to avoid locking artifact-dir for check builds.
Part of #4282

Note: This change is not behind -Zbuild-dir-new-layout or -Zfine-grain-locking unstable flags.

How to test and review this PR?

See the tests included in the PR.
You can run cargo check to verify.

r? @epage

[weihanglo]

This should be in check command test file maybe?
https://github.com/rust-lang/cargo/blob/master/tests/testsuite/check.rs

[ranger-ross]

ah good idea. I was thinking about putting it in https://github.com/rust-lang/cargo/blob/master/tests/testsuite/artifact_dep.rs but noticed that file only has test for the unstable --artifact-dir flag.

master/tests/testsuite/check.rs seems like a good place for this

[weihanglo]

Should we also add one test asserting that no files are generated to the artifact directory from cargo check?

(Maybe we already have one though)

[ranger-ross]

Added a test in da0dfd3

[epage]

Instead of conditionally grabbing the lock, can we put the entire artifact layout in an Option? That has the potential to help us find bugs but unsure how messy that would be.

[ranger-ross]

I looked into this and it got pretty hairy. Switching from T to Option<T> is not too bad but there quiet a few places during compilation where the paths are used but not written to.

For example, in build_runner::prepare we build up the root_output

cargo/src/cargo/core/compiler/build_runner/mod.rs

Lines 395 to 397 in 8c4a25c

	self.compilation
	.root_output
	.insert(kind, layout.artifact_dir().dest().to_path_buf());

and we use this in fill_env()

cargo/src/cargo/core/compiler/compilation.rs

Lines 308 to 311 in 8c4a25c

	search_path.extend(super::filter_dynamic_search_path(
	self.native_dirs.iter(),
	&self.root_output[&kind],
	));

[epage]

Instead of intent, can we check if artifacts are produced? Do we know that?

[ranger-ross]

I checked and I think it may be possible to use some of the logic from CompilationFiles output but we would likely need to duplicate it as we need to construct Layout before CompilationFiles.

See https://github.com/rust-lang/cargo/blob/master/src/cargo/core/compiler/build_runner/compilation_files.rs#L405

Let me know if there is a better way to get this data.