Cannot use Rng methods on `Gen` when implementing `Arbitrary` about quickcheck

[BurntSushi]

Yes. The plan is to make a seed available, and then you should be able to materialize any RNG from that.

[nbraud]

Seems related to #267

[audunska]

This really hurts the ability to write good Arbitrary impls for complicated data. I'm sure there are lots of examples, but my use case can be reduced to just a vector of vectors, Vec<Vec<u32>>. To make sure I sample the data properly I want to generate both tall skinny arrays and short wide ones. This means that the size parameter must be divided randomly into two factors in the arbitrary method, which requires a bounded random float.

If the issue with gen_range is that it exposes rand types, could I propose simply adding a public method Gen::gen_uniform(&mut self) -> f32 which generates a uniformly distributed float in the unit interval? That should satisfy most needs.

[divagant-martian]

Is there a known plan to make this work that would be followed by a first time contributor?

[bergmark]

Perhaps a bit hacky, but the workaround I ended up using was:

pub fn range(gen: &mut Gen, range: Range<u64>) -> u64 {
    u64::arbitrary(gen) % (range.end - range.start) + range.start
}