pub fn capacity(&self) -> usize {
        self.cap() - 1
    }
Tries to reserve the minimum capacity for exactly `additional` more elements to
be inserted in the given `VecDeque<T>`. After calling `try_reserve_exact`,
capacity will be greater than or equal to `self.len() + additional`.
Does nothing if the capacity is already sufficient.

Note that the allocator may give the collection more space than it
requests. Therefore, capacity can not be relied upon to be precisely
minimal. Prefer `reserve` if future insertions are expected.

# Errors

If the capacity overflows `usize`, or the allocator reports a failure, then an error
is returned.

# Examples

```
#![f
benchmarks/haystacks/code/rust-library.rs