Example package.xml building with colcon-cargo?

[ruffsl]

I'm trying to write a pure Rust package and build it in a colcon workspace. However, I can't seem to add a basic package.xml without colcon thinking it somehow deals with Cmake:

colcon build --symlink-install --packages-select bbr_cli
Starting >>> bbr_cli 
--- stderr: bbr_cli
CMake Error: The source directory "/home/ruffsl/ws/bbr_ros2/src/bbr_ros2/bbr_cli" does not appear to contain CMakeLists.txt.
Specify --help for usage, or press the help button on the CMake GUI.
---
Failed   <<< bbr_cli	[ Exited with code 1 ]

Summary: 0 packages finished [0.22s]
  1 package failed: bbr_cli
  1 package had stderr output: bbr_cli

Having the colcon-cargo extension installed, the cargo project seems to build fine without a package.xml but if I add a skeleton package.xml like so, I get the error above:

<?xml version="1.0"?>
<package format="2">
  <name>bbr_cli</name>
  <version>0.0.1</version>
  <description>Command Line Interface for BBR</description>
  <maintainer email="roxfoxpox@gmail.com">Ruffin White</maintainer>
  <license>Apache License 2.0</license>

</package>

cc @lelongg

[lelongg]

colcon use different extensions to build different packages type. Extensions are tried according to their priority. If an extension identify a package, it will be used to build the package and extensions with less priority won't be triggered.

In this case, I believe that colcon-ros has higher priority than colcon-cargo and colcon-ros identify any package containing a package.xml file as its own, so colcon-cargo is ignored when such a file is present (https://github.com/colcon/colcon-ros/blob/7e51402a233e44350d0e957524bba7f8746d6b1d/colcon_ros/package_identification/ros.py#L28).

I think that the solution would be to add a new build_type, such as ament_cargo, to colcon-ros in order to correctly deal with ROS packages containing Rust code.

[ruffsl]

I think that the solution would be to add a new build_type, such as ament_cargo, to colcon-ros in order to correctly deal with ROS packages containing Rust code.

That's what I was thinking as well, but I wasn't sure where to start. Is there a document that details the requirements for adding a new ament build type? The ament_cmake repo is huge, and it looks like the ament_python repo is deprecated, so I could use some guidance on mimicking this for cargo.

cc @dirk-thomas

[dirk-thomas]

ament_cmake is a separate build type since CMake is being invoked with ament_cmake specific arguments, e.g. https://github.com/colcon/colcon-ros/blob/7e51402a233e44350d0e957524bba7f8746d6b1d/colcon_ros/task/ament_cmake/build.py#L58-L60

Same for ament_python: e.g. https://github.com/colcon/colcon-ros/blob/7e51402a233e44350d0e957524bba7f8746d6b1d/colcon_ros/task/ament_python/build.py#L38-L41

The question is if a ROS-specific cargo package needs any specific arguments / logic compared to a vanilla cargo package? If yes, this need to be implemented in a separate task (if that should go into colcon-ros or better be contributed through a separate extension is a different question).

[nnmm]

if that should go into colcon-ros or better be contributed through a separate extension is a different question

Could somebody elaborate? My reading of colcon/colcon-ros#24 was that separate extensions are preferred now. Context: I'm planning to rewrite a prototype for cargo support in colcon-ros into a separate extension.

[esteve]

@nnmm separate extensions are the recommended way. See for example https://github.com/colcon/colcon-ros-grade which leverages https://github.com/colcon/colcon-grade to support ament_gradle packages (Gradle packages that use the ROS and ament infrastructure for resolving dependencies and accesss the ament index)

[leocencetti]

Hello, I am also facing the same issue. package.xml files also allow declaring system dependencies that can then be easily managed with rosdep, making the build experience very streamline. Has there been any movement on this?