Move BitSet constructors from impl<u32> to impl<B>

[polazarus]

The way things are, BitSet<B> for any B except u32 is almost unusable. The only constructor is pretty much hidden (default()).
I propose to move the constructors around from the specialized implementation to the generic one.
It may impact the performances for some use cases. I'm prepared to look into it.
But first, I would love to hear your thought on this.

[bluss]

This is by design for now, to make BitSet easy to use for the usual case (where you don't care about which B is used).

There is no real nice way to provide a default B. When rust-lang/rust#27336 is turned on, this will be much easier.

[polazarus]

A default seems nice indeed. But I think functionality trumps convenience.

As it is, BitSet<B> is rather useless. I cannot create a full set for instance. The worst is for from_bit_vec which does not make sense: B could be inferred from the bit_vec itself.

[bluss]

BitSet::default() isn't really hidden. Only that the documentation doesn't point it out. Or anything else either ;)

[bluss]

As it is now, changing for example BitSet::new() to be generic over B will break compilation for some users, where B can not inferred. That's the kind of drawback waiting for the above linked feature will remove.

[polazarus]

As a middle ground, I would like to just move from_bit_vec. This way it would be possible, if a bit complicated, to create a set with capacity or with initialization.

[bluss]

Moving from_bit_vec looks simple enough. Note that BitVec has the same u32 specialization for its constructors as BitSet, for the same reasons.

[polazarus]

😢 yes i didn't see that one. So there is no way to efficiently initialize a bit set (or a bit vector)...

[Gankra]

BitVec has these reverse dependencies (15): https://crates.io/crates/bit-vec/reverse_dependencies

BitSet has these reverse dependencies (9): https://crates.io/crates/bit-set/reverse_dependencies

If we do a 0.x version bump, it's arguable a sound "reasonable breakage". I had hoped that the default functionality would have been flipped on by now, but it seems there's been some technical difficulties.

[pczarn]

Waiting for a larger overhaul of bit-vec.