[RFC] Build/Host/Target dependencies

[mosteo]

Having separate sets of dependencies for the build toolset and the built code is popping up in several places recently. I'm opening this issue to centralize this discussion here.

This comment is the latest example:

Regarding cross compilation, the compiler itself (same is true for gnatformat, gnatprove, ...) should not be a "normal" dependency specified by depends-on, but instead a dependency for the build platform.

For cross compilation we typically distinguish three platforms:

• Build Platform: This is the platform on which the compilation tools are executed.
• Host Platform: This is the platform on which the code will eventually run.
• Target Platform: If this is a compiler, this is the platform that the compiler will generate code for.

So the regular depends-on would only specify host platform dependencies, while something like build-depends-on might specify dependencies for the build platform. The target platform is part of the name of the toolchain.

Originally posted by @ThyMYthOS in #728

Possibly related: #2048

[mosteo]

@ThyMYthOS when is relevant a different Host and Target platform? Or is it just a matter of nomenclature depending on whether we talk about the compiler or the program?

I guess this is clear as day for people doing embedded development for a living, so bear with me that is in my very distant past.

[ThyMYthOS]

Target is actually only relevant when building a compiler. I only mentioned it for completeness, as they are typically mentioned together.

See https://cmake.org/cmake/help/book/mastering-cmake/chapter/Cross%20Compiling%20With%20CMake.html how cmake deals with this topic.

Relevant quote: "The find_program command is typically used to find a program that will be executed during the build, so this should still search in the host file system, and not in the environment of the target platform. find_library is normally used to find a library that is then used for linking purposes, so this command should only search in the target environment."

This is because for embedded development, build dependencies are typically programs you want to execute on the build system (could be installed via alr install) while host dependencies are typically libraries you want to link into the final application for the target.