docs: Update to reference host_platform instead of target_platform

[mgorny]

Update the documentation throughout to replace the references to target_platform with host_platform where appropriate. That said, unless I've missed some fine point, this means pretty much everywhere.

[wolfv]

Thanks for the changes! I am wondering which one is "clearer" to the user ... target_platform or host_platform?

[mgorny]

I think cross-builds are inherently "hard" difficulty, and all terms are hard. In Gentoo, I personally went for explicit CBUILD, CHOST, CTARGET to avoid ambiguity.

[mgorny]

This change was basically prompted by regro/cf-scripts#3832 (comment)

[h-vetinari]

Thanks for the changes! I am wondering which one is "clearer" to the user ... target_platform or host_platform?

@isuruf has been making the case for following build/host/target (and fixing conda's inconsistent usage of these terms). This matches autotools conventions, but not only. For example, I like meson's write-up on this topic, which comes to the same conclusion.

The "problem" is that the vast majority of cross-compiled cases (already not the most common concern itself) only care about two architectures - where it's built and where it's run. But if you use "target" for the latter, it becomes very confusing in those cases where the full generality is actually necessary (e.g. "canadian cross" compilers).

This is, for example, why our compiler feedstocks need to hack around this by using cross_target_platform, because conda-build uses target_platform for what should be host_platform.

In short: I agree that we should use v1 as an opportunity to clean up these inaccuracies, and +1 for using host_platform.

[isuruf]

I think the easiest way to explain to users is

1. build_platform is the platform for requirements/build
2. host_platform is the platform for requirements/host.
3. target_platform is for advanced users (compiler package authors) only.

[mgorny]

Rebased and updated more occurrences.

[wolfv]

I am fine with merging this, but should we explain host and target platform somewhere? Specifically target platform? I also still think that on the CLI, setting --target-platform linux-64 makes it intuitively very clear what you want (compile for the target of linux-64 ...)

[h-vetinari]

but should we explain host and target platform somewhere?

I have a page I started writing recently; still need to finish up some bits and pieces, but it's getting there: conda-forge/conda-forge.github.io#2738

Feedback welcome! You can copy stuff from there for the r-b docs, or maybe simply refer to that page then.

[mgorny]

Rebased. That said, I see that there's also some incorrect logic within rattler-build, so I guess I'll extend the PR to handle that as well.

[mgorny]

Or rather, I feel like I should do that. However, it seems that the codebase is almost universally using target_platform when it ought to use host_platform (and largely ignoring host_platform everywhere), so I'd like to get confirmation before I start on such a herculean task.

[mgorny]

I am fine with merging this, but should we explain host and target platform somewhere? Specifically target platform? I also still think that on the CLI, setting --target-platform linux-64 makes it intuitively very clear what you want (compile for the target of linux-64 ...)

Do you have a specific idea where to add this? IIRC the only relevant part of the docs is https://rattler-build.prefix.dev/latest/compilers/#cross-compilation which is updated here, or the command-line interface docs. I can expand either on the three platforms, though I suppose most of users won't need to know about "target" at all.