Thanks! Generally LGTM!

Few notes:

Could you add a note documenting this behavior? Especially important since the behavior won't be what one expects from looking at the expression.

Actually, on that:
Is there a good way to inject a single line warning about this (in the build log, not an evaluation message)? Something like replacing checkPhase with "echo skipping tests in cross-build"? This way it's always clear something intercepted so that no one is left wondering why tests aren't running.
Will require rebuild for impacted cross derivations but that seems a small cost for long-term sanity.

Also: probably should extend this to cover the installcheck phase (doInstallCheck) as well?

Finally, we don't need it yet AFAIK but while brainstorming on it it may make sense to add a doCrossCheck or dontDisableCheckOnCross to override. Don't see any reason to add it until it's needed, though.

Sounds good to me. Also fully agree with @dtzWill's comments.

Probably can't so warning very well until mass rebuild, unless we do an eval time warning, but getting to the rest.

How does this look now?

Great! Bummer can't log without forcing a mass-rebuild :(.

I think the warning is absolutely worth the rebuild--although I'm happy if we tackle it separately and target staging with it or something (and not block this PR on it).

Do you know how such a warning might best be implemented?
Would it be a nix-level solution, modifying builder scripts, or blend of both?

I suppose if it's too much trouble re:complicating unrelated things, we don't "need" the warning. Would be nice though :).

Also, TIL:

nix-repl> let hasA = { a ? 1, ...} @ args: args ? a; in { X = hasA { }; Y = hasA { a = 2; }; }
{ X = false; Y = true; }

And less surprisingly:

nix-repl> :p let hasA = { a ? 1, ...} @ args: { contains = args ? a; inherit a; }; in { X = hasA { }; Y = hasA { a = 2; }; }
{ X = { a = 1; contains = false; }; Y = { a = 2; contains = true; }; }

Neat!

I love how this apparently causes "no" rebuilds?! (at least according to the rebuild-amount.sh script) 😁

@dtzWill yeah something in setup.sh, pretty sure.

Hmm, should this be comparing buildPlatform instead of targetPlatform?

@dtzWill haha got me! Indeed it should.

Would it be reasonable to add a trapdoor so one can still use a checkPhase when cross-compiling? Currently there's no way to check cross-compiled packages - you have to put the check in some other phase because doCheck is impossible to enable. I think there should be an "I know what I'm doing" flag so one can write an idiomatic check phase, which will at least run some cross-friendly sanity checks.

Sorry for commenting on old pr, but I agree with @r-burns, forcing doCheck can be useful in controlled environments like CI, where for example x86_64 system can run a x86 binary.