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In practice, this is a strictly stronger condition than target != build as we never have build = target != host. Really, the attribute should be removed altogether, but for now we make it work for plain libraries, which do not care about the target platform. In the few cases where the compilers use this and actually care about the target platform, I'll manually change them to use `targetPlatform` instead.
The previous commit redefines `stdenv.cross` for the sake of normal libaries, the most common use-case of that attribute. Some compilers however relied on the old definition so we have them use `targetPlatform` instead. This special casing is fine because we eventually want to remove `stdenv.cross` and use either `hostPlatform` or `targetPlatform` instead.
Before all overrides were also pruned in the previous stage, now only gcc and binutils are, because they alone care about about the target platform. The rest of the overrides don't, so it's better to preserve them in order to avoid spurious rebuilds.
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6.topic: cross-compilation
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Motivation for this change
stdenv.crossis a silly attribute that needs to go leaving the well-definedhostPlatformandtargetPlatform. This PR doesn't remove it, but changes its definition: before it tracked the target platform which is sometimes more useful for compilers, and now it tracks the host platform which is more useful for everything else. Most usages are libraries, falling in the "everything else" category, so changing the definition makes sense to appease the majority. The few compiler (gcc in particular) uses that exist I remove to usetargetPlatform--- preserving correctness and becoming more explicit in the process.I would also update the documentation aside mentioning
stdenv.crossas deprecated, but the definition given actually erroneously assumes this PR is already merged!This is split off from #25047
Things done
I commandeered and old jobset of mine to create http://hydra.nixos.org/jobset/nixpkgs/pr-25194 to test, because I couldn't figure out how to run a hydra job locally. http://hydra.nixos.org/eval/1353037?compare=cross-trunk shows no regressions. Yay!