Warn if an "all" target is specified, but we don't match anything

[Bryysen]

If a combination of --bins, --benches, --examples, --tests flags have
been specified, but we didn't match on anything after resolving the unit-list,
we emit a warning to make it clear that cargo didn't do anything and that the
code is unchecked.

This is my first PR and there are a couple things that I'm unsure about

• The integration test covers only one case (ideally it should cover every combination of the above mentioned flags the user can pass). I figured since the warning function is so simple, it'd best not to clog the testsuite with unnecessary p.cargo().runs() and whatnot. If I should make the test more comprehensive I can do that, it's also very easy to write unit tests so i can do that as well if needed.
• I figure we don't actually have to check the --all-targets, but i'm doing so for consistency. I also didn't check for the --lib flag at all because (I'm assuming) if the user passes --lib and there are no libraries, we error.
  Edit: I notice (among other things) we sometimes silently skip certain units that have incompatible feature flags (see here) so maybe we should be checking the --lib flag after all, in the event that a library was silently skipped and we no-opped 🤔

And thanks to @ehuss for taking the time to answer my questions and helping me through the contribution process, much appreciated

Closes #9536

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Eh2406 (or someone else) soon.

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[Eh2406]

Looks like Eric was helping you design this, so assigning the revue to him.
@bors r? @ehuss

[ehuss]

Thanks, this looks great!

Don't worry too much about testing every situation. I think the test you have as-is looks good.

I don't think --lib needs to be checked, since it doesn't allow required-features, so it can't be silently skipped.

There is a situation where --all-targets can be hit. If you have a bin-only package, and the bin has required-features that are not met, then --all-targets will result in 0 units. If you want to add that check back in, I think that would be fine. Or if not, that's fine, too, since I think that should be extremely rare and I suspect the user can figure out what's going on from the warning.

[Bryysen]

I'll add it back in, it's a very cheap operation (another if branch). I'll also squash the commits into one.

[Bryysen]

Hopefully i'm doing this correctly, still new to the github workflow. Should be good to go.

[ehuss]

Looks good, thanks!

@bors r+

[bors]

📌 Commit da1c2f3 has been approved by ehuss

[bors]

⌛ Testing commit da1c2f3 with merge 46ba901...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing 46ba901 to master...