Owned iterator: into_ones()

[sitegui]

Hi,

I'm building a tiny abstraction over this lib for my use case (a set of usizes with an API very similar to BTreeSet) and I'm stuck on the implementation of my IntoIterator because there is no owning equivalent of ones(). I mean, One<'a> iterates over a borrow of the FixedBitSet and I cannot easily keep the FixedBitSet and the One<'_> around on the same struct:

struct IntoIter<'a> {
    bit_set: FixedBitSet,
    ones: Ones<'a>,
}

I'm currently using rental as a workaround, but that is quite complex.

Does it make sense to implement a method like pub fn into_ones(&self) -> IntoOnes? I can work on a PR if you are interested.

Best regards,

[bluss]

I think it makes sense. My review time is a bit limited, but hopefully it can be solved

[jrraymond]

@sitegui do you mind clarifying how into_ones allows writing struct IntoIter?

[sitegui]

Hi @jrraymond , of course :)

This is a reduced example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=aadc6f54122373d6724ead8fa8a8e828

The main problem is that Ones borrows the FixedBitSet, so I cannot create an owning struct. See that IntoIter ends up borrowing as well:

struct IntoIter<'a>(Ones<'a>);

impl<'a> IntoIterator for MySet {
    type Item = usize;
    type IntoIter = IntoIter<'a>;
    
    fn into_iter(self) -> Self::IntoIter {
        IntoIter(self.0.ones())
    }
}

The example is very reduced and seems very artificial, but in my use-case I have a complex type T that I convert into usize to store into the bitset and then I'd like to code like this to work seamlessly:

let my_set: MySet = todo!();
for el in my_set {
  // an owned `el`
}

but that requires MySet: IntoIterator. My type T is Copy, so I can work around with:

for &el in &my_set {
  // an owned `el`
}

However, I've ended up using a Vec<bool> instead of FixedBitSet in my project to implement the API I wanted.

Hope it clarifies ;)