Add mixin compositing

[AlexisTM]

Introduction

In our project, we would like to create mixins from other mixins.

What that means is: When using main-mixin, we want to use a set of mixins.

This allows for a single mixin to be defining, let's say: The output folders, the toolchain, specific build functions (prod build vs dev build), etc.

Example

Currently, I am making the main-mixin with all the default commands, output directory & co, but changing some arguments by using another mixin before it. For example:

{
    "build": {
        "legacy": {
            "cmake-args": [], // Many things here
            "install-base": "/opt/company/workspace/install/legacy",
            "build-base": "/opt/company/workspace/build/legacy",
            "log-base": "/opt/company/workspace/log/legacy",
        },
        "thirdparty": {
            "install-base": "/opt/company/thirdparty/install/legacy",
            "build-base": "/opt/company/thirdparty/build/legacy",
            "log-base": "/opt/company/thirdparty/log/legacy",
        },
    }
}

Would be called as: colcon build --mixin thirdparty legacy.

Prefered solution

{
    "build": {
        "main": {
            "cmake-args": [], // Many things here
            "mixin": ["legacy_folder"],
        },
        "debug": {
            "cmake-args": [], // Many things here
            "mixin": ["legacy_folder", "asan", "ubsan"],
        },
        "thirdparty": {
            "cmake-args": [], // Many things here
            "mixin": ["thirdparty_folder"],
        },
        "legacy_folder": {
            "install-base": "/opt/company/workspace/install/legacy",
            "build-base": "/opt/company/workspace/build/legacy",
            "log-base": "/opt/company/workspace/log/legacy",
        },
        "thirdparty_folder": {
            "install-base": "/opt/company/thirdparty/install/legacy",
            "build-base": "/opt/company/thirdparty/build/legacy",
            "log-base": "/opt/company/thirdparty/log/legacy",
        },
    }
}

[giuliano-97]

oh this would be a very very useful feature, is there any plan in the roadmap to implement this?

[Sommiey]

In colcon-mixin, when creating a composite mixin that includes multiple sub-mixins, how does the system handle conflicts between overlapping fields (e.g., install-base or cmake-args)?

Is there a recommended pattern for ordering mixins or overriding specific values to ensure predictable build behavior?

Additionally, are there best practices for designing reusable atomic mixins to maximize composability without introducing complex dependencies?