Status / purpose of rosrust_codegen crate is unclear about rosrust

[adnanademovic]

rosrust_codegen is a direct dependency of rosrust. Its only responsibility is implementing the rosmsg_include! macro, since it's a procedural macro, and procedural macros need to be implemented in a crate of their own.

Though I will consider updating the README, the new README will probably just be "procedural macros for rosrust".

Thanks for the concern!

[tstellanova]

Since rosrust_codegen is a procedural macro crate, consider:

• Moving the source code for rosrust_codegen to its own repo instead of including within the rosrust repo
• Make it possible to use the rosrust_codegen without rosrust. Currently attempting to do this (by forcing a dependency on just the rosrust_codegen directory) results in:

error[E0433]: failed to resolve: use of undeclared type or module `rosrust`
 --> src/lib.rs:8:5
  |
8 |     rosmsg_include!(sensor_msgs/Image,sensor_msgs/Imu,geometry_msgs/TransformStamped);
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared type or module `rosrust`

The functionality provided by rosrust_codegen to generate rust types from ROS messages seems very useful on its own.

[adnanademovic]

Again, rosrust_codegen is a key part of rosrust and the only reason it's a separate crate is technical limitations of the crates system.
Depending on rosrust_codegen alone does not make sense since the generated messages implement traits from rosrust.
It's all similar to how serde and serde_derive interact.