Add warning message when no tests are run.

[Kuly14]

Problem

When trying to run a specific test with a command like:

cargo test tests::test_lib

If you make a mistake in the path to the test it won't display any warning, it will just not run any tests:

As you can see in the screenshot if you make a typo in the path it will still look like it passed even though the test doesn't exist:

Proposed Solution

In my opinion, we should add a warning when the path leads to a nonexistent test e.g.:

WARNING: The test doesn't exist

Notes

No response

[ehuss]

This may be a difficult issue to address due to the way tests are coordinated. When running cargo test, there are multiple potential binaries that get executed (lib, bin, integration tests, etc.). Passing a test name filter shouldn't also require passing the cargo target to test. That means a test could run in one target, but all the others report 0 tests. I wouldn't want those emitting warnings that were too visible (as I would be concerned about that being confusing or too noisy).

This may be more feasible to address using the JSON output of the test harness, but that hasn't been stabilized yet or had much momentum in a while. And rust-lang/rust#108659 illustrates some of the difficulties of using the JSON interface, among other potential hazards.

[edmorley]

I'd love it if the warning could be optionally made fatal (eg via a cargo test CLI arg) - which would help prevent the case where tests are silently not being run in CI (in our case a module name changed, wasn't updated in the CI workflow, and wasn't spotted for some time since Cargo happily exits zero even though nothing was tested).