You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lib.rs
    path of file: ./repos/wasmtime/crates/slab/src
    
    the code of the file until where you have to start completion: //! A very simple, uniformly-typed slab arena that supports deallocation and
//! reusing deallocated entries' space.
//!
//! The free list of vacant 
    
    pub fn drain(&mut self) -> impl Iterator<Item = (Id, T)> + '_ {
        assert!(self.entries.len() <= Self::MAX_CAPACITY);
        self.len = 0;
        self.free = None;
        self.entries
            .drain(..)
            .enumerate()
            .filter_map(|(i, e)| match e {
                Entry::Occupied(x) => Some((Id(EntryIndex::new(i)), x)),
                Entry::Free { .. } => None,
            })
    }