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: region.rs
    path of file: ./repos/panopticon/core/src
    
    the code of the file until where you have to start completion: /*
 * Panopticon - A libre disassembler
 * Copyright (C) 2014, 2015  Panopticon authors
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

//! A regions model continuous memory like RAM, flash memory or files.
//!
//! A region has an unique name that is used to reference it and a size. The
//! size is the number of `Cell`s in a region. A cell either has a value
//! between 0 an
    
    pub fn projection(&self) -> Vec<(Bound, RegionRef)> {
        let mut ret = Vec::<(Bound, RegionRef)>::new();
        let mut visited = HashSet::<RegionRef>::new();

        fn step(v: RegionRef, regs: &RegionGraph, ret: &mut Vec<(Bound, RegionRef)>, visited: &mut HashSet<RegionRef>) {
            let reg = regs.vertex_label(v).unwrap();
            let mut es = regs.out_edges(v).collect::<Vec<AdjacencyListEdgeDescriptor>>();
            let mut last = 0;

            es.sort_by(|&a, &b| regs.edge_label(a).unwrap().start.cmp(&regs.edge_label(b).unwrap().start));

            for e in es {
                let b = regs.edge_label(e).unwrap();
                let nx = regs.target(e);
                let free = Bound::new(last, b.start);

                if last < b.start {
                    ret.push((free, v));
                }
                last = b.end;

                if visited.insert(nx) {
                    step(nx, regs, ret, visited);
                }
            }

            if last < reg.size() {
                let free = Bound::new(last, reg.size());
                ret.push((free, v));
            }
        }

        if self.dependencies.num_vertices() > 0 {
            step(self.root, &self.dependencies, &mut ret, &mut visited);
        }
        ret
    }