cargo test --lib should not error when no lib is present

[iamroot99]

Problem

This is somewhat related to what #10958 requests, but the target and use-case are different.

I need to run the unit tests in several workspaces. The only way I am aware of that can achieve this is via

cargo test --bins --lib

The problem is that I am running this command over arbitrary-structured workspaces that may have 0 or more binaries and 0 or more libraries within them. The scenarios boil down to the 3 cases below:

• 0 bins, 1+ libs: cargo test completes with a warning
• 1+bins, 1+libs: cargo test completes
• 1+bins, 0 libs: cargo test fails

If there are 0 binaries, then the command completes with a warning, but if there are no libs in the workspace it will fail with the following error:

error: no library targets found in packages: bin1, bin2

Proposed Solution

I think there are two options here:

1. Modify --lib to warn instead of error when no libraries are present.
2. Introduce a --libs argument that warns when no libraries are present. This would keep the behaviour of --lib intact.

I would be happy to implement the change myself.

Notes

No response

[weihanglo]

There is already an open PR #14163, though the target selection discussion is still in discussion in #10958.