Transform ...rest parameters of generator functions more efficiently

[benjamn]

As @maxnordlund points out in https://github.com/facebook/regenerator/issues/131, aliasing the arguments object has negative performance consequences.

If you happen to transform ...rest parameters into ES5 arguments-style code earlier in your transformation pipeline than the Regenerator transform, Regenerator will currently capture the arguments object and generate inefficient code that aliases the arguments object in the outer function, only to turn it into an array in the inner function.

If Regenerator handled ...rest parameters itself, then it could be smart about where the array conversion happened, and potentially avoid aliasing the arguments object at all.

Regenerator should continue supporting all the exotic behavior of the arguments object when you need it, but it would be great if the standard recommendation for improving arguments performance was "just use a ...rest parameter if possible!"

[benjamn]

Hmm… but if Regenerator handles ...rest parameters only for generator functions, then you'd still need to have a separate transform for all other ...rest parameters, and you might still make the mistake of running that transform before Regenerator.

[ghost]

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