Add DEPENDS_EXPLICIT_ONLY to remove implicit dependencies

[TonyWelte]

Description

This PR addresses one of the build speedup mentioned in #859.

When add_custom_command is used to generate files that are then used by a target, the add_custom_command becomes implicitly dependent on the target dependencies. In rosidl add_custom_command is used to generate files from the IDL files, the generator does not depend on anything else (except rosidl_generator_c[pp] whose generators depend on rosidl_generator_type_description).

This PR (and the equivalent for rosidl_typesupport, rosidl_typesupport_fastrtps and rosidl_python) adds the DEPENDS_EXPLICIT_ONLY option of add_custom_command to most generators (except for C/CPP).

Note

This option is only useful when using Ninja. When testing use --cmake-args -GNinja

Since the generators are no longer constrained by unnecessary dependencies, they can start immediately. This speeds up the build significantly (2x for px4_msgs on my machine).

Now	rolling

Warning

DEPENDS_EXPLICIT_ONLY requires cmake 3.27. The version that ships with Ubuntu 24.04 is 3.28, not sure about other supported distributions.
Right now I've bumped the required version to 3.27 but I could instead only add the option when the cmake version is sufficient (if that's preferable ?).

Note

I wanted to also add the option to rosidl_generator_c[pp] and explicitly specify the dependency to rosidl_generator_type_description. I couldn't figure out how to make it work.

Is this user-facing behavior change?

Did you use Generative AI?

no

Additional Information

I've already created PRs for the other packages. I've set then to Draft until this PR is reviewed.

• Add DEPENDS_EXPLICIT_ONLY to remove implicit dependencies rosidl_typesupport#168
• Add DEPENDS_EXPLICIT_ONLY to remove implicit dependencies rosidl_typesupport_fastrtps#136
• Add DEPENDS_EXPLICIT_ONLY to remove implicit dependencies rosidl_python#238

[InvincibleRMC]

Looking at rhel9 it only supports 3.26.5. The upcoming rhel10 is on 3.30.5. So you would probably have to make it conditional or wait for the switch to rhel10.

Refs:
rhel9
rhel10