starlark: check for duplicate kwarg keys in dict

[josharian]

Fixes #55

[josharian]

Oops. Do not review. This fix is in the wrong place.

[josharian]

PTAL

[josharian]

Note that doing bazelbuild/starlark#21 will render this unnecessary.

[alandonovan]

I don't think it's unnecessary in all cases. A call to dict or dict.update should not have repeated keys either statically (as in bazelbuild/starlark#21) or dynamically, as in this PR. Two separate checks are required. All other built-ins either don't accept keyword args, or call UnpackArgs, which performs the dynamic check. All Starlark defined functions have the check in setArgs.

dict and dict.update are unusual in they are built-ins that accept keyword args but don't use UnpackArgs.

[josharian]

Oh right, I forgot about the mixed static and dynamic kwargs case.

[alandonovan]

Thanks for your diligence here. I think this change will be necessary exactly as it is even if we implement a static check for repeated keyword args in a call.

[josharian]

Done

[josharian]

All done; thanks for the review.