pub struct FlatSet<Key> {
    values: Vec<Key>
}

impl<Key> FlatSet<Key> where Key: std::cmp::PartialOrd {
    pub fn new() -> FlatSet<Key> {
        FlatSet { values: Vec::new() }
    }

    pub fn with_capacity(capacity: usize) -> FlatSet<Key> {
        FlatSet { values: Vec::with_capacity(capacity) }
    }

    pub fn capacity(&self) -> usize {
        self.values.capacity()
    }

    pub fn reserve(&mut self, additional: usize) {
        self.values.reserve(additional)
    }

    pub fn reserve_exact(&mut self, additional: usize) {
        self.values.reserve_exact(additional)
    }

    pub fn into_boxed_slice(self) -> Box<[Key]> {
        self.values.into_boxed_slice()
    }

    pub fn as_slice(&self) -> &[Key] {
        self.values.as_slice()
    }

    pub fn add(&mut self, k: Key) -> bool {
        if self.values.is_empty() {
            self.values.push(k);
            return true
        } else {
            let mut a = 0;
            let mut b = self.values.len();
            let mut idx;
            while a < b {
                idx = (a + b) / 2;
                if k < self.values[idx] {
                    b = idx;
                } else if k > self.values[idx] {
                    a = idx + 1;
                } else {
                    return false
                }
            }
            self.values.insert(a, k);
            return true
        }
    }

//    pub fn remove(&mut self, index: usize) -> Key {
//    }

    pub fn clear(&mut self) {
        self.clear()
    }

    pub fn len(&self) -> usize {
        self.len()
    }

    pub fn is_empty(&self) -> bool {
        self.is_empty()
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        assert_eq!(2 + 2, 4);
    }
}

$ cargo build
   Compiling akarui v0.1.0 (/home/phdp/Dropbox/work/akarui)
warning: function cannot return without recursing
  --> src/lib.rs:60:5
   |
60 |     pub fn clear(&mut self) {
   |     ^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
61 |         self.clear()
   |         ------------ recursive call site
   |
   = note: #[warn(unconditional_recursion)] on by default
   = help: a `loop` may express intention better if this is on purpose

warning: function cannot return without recursing
  --> src/lib.rs:64:5
   |
64 |     pub fn len(&self) -> usize {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
65 |         self.len()
   |         ---------- recursive call site
   |
   = help: a `loop` may express intention better if this is on purpose

warning: function cannot return without recursing
  --> src/lib.rs:68:5
   |
68 |     pub fn is_empty(&self) -> bool {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot return without recursing
69 |         self.is_empty()
   |         --------------- recursive call site
   |
   = help: a `loop` may express intention better if this is on purpose

    Finished dev [unoptimized + debuginfo] target(s) in 0.11s