Use the size_hint method of Arbitrary to more intelligently pick the input size.

[brson]

Most users are going to use the default arb function, with an input size of 256; and in cases that is insufficent, will silently get low-quality test inputs because many/most Arbitrary impls allow short inputs, treating them as 0-padded up to the needed length (I think I recall this).

And picking the correct byte length to fully satisfy any particular Arbitrary impl, calling arb-sized is not really reasonable for most people.

The default method could more intellegently use the Arbitrary::size_hint method to pick an input size.

[leighmcculloch]

+1 this seems like a good improvement. I don't see a reason not to do this.

@graydon wdyt?

[graydon]

Oh, sure, seems plausible!