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: peg.rs
    path of file: ./repos/xi-editor/rust/experimental/lang/src
    
    the code of the file until where you have to start completion: // Copyright 2017 The xi-editor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Simple parser expression generator

use std::char::from_u32;
use std::ops;

pub trait Peg {
    fn p(&self, s: &[u8]) -> Option<usize>;
}

impl<F: Fn(&[u8]) -> Option<usize>> Peg for F {
    #[inline(always)]
    fn p(&self, s: &[u8]) -> Option<usize> {
        self(s)
    }
}

pub struct OneByte<F>(pub F);

impl<F: Fn(u8) -> bool> Peg for OneByte<F> {
    #[inline(always)]
    fn p(&self, s: &[u8]) -> Option<usize> {
        if s.is_empty() || !self.0(s[0]) {
            None
        } el
    
fn decode_utf8(s: &[u8]) -> Option<(char, usize)> {
    if s.is_empty() {
        return None;
    }
    let b = s[0];
    if b < 0x80 {
        return Some((b as char, 1));
    } else if (0xc2..=0xe0).contains(&b) && s.len() >= 2 {
        let b2 = s[1];
        if (b2 as i8) > -0x40 {
            return None;
        }
        let cp = (u32::from(b) << 6) + u32::from(b2) - 0x3080;
        return from_u32(cp).map(|ch| (ch, 2));
    } else if (0xe0..=0xf0).contains(&b) && s.len() >= 3 {
        let b2 = s[1];
        let b3 = s[2];
        if (b2 as i8) > -0x40 || (b3 as i8) > -0x40 {
            return None;
        }
        let cp = (u32::from(b) << 12) + (u32::from(b2) << 6) + u32::from(b3) - 0xe2080;
        if cp < 0x800 {
            return None;
        } // overlong encoding
        return from_u32(cp).map(|ch| (ch, 3));
    } else if (0xf0..=0xf5).contains(&b) && s.len() >= 4 {
        let b2 = s[1];
        let b3 = s[2];
        let b4 = s[3];
        if (b2 as i8) > -0x40 || (b3 as i8) > -0x40 || (b4 as i8) > -0x40 {
            return None;
        }
        let cp =
            (u32::from(b) << 18) + (u32::from(b2) << 12) + (u32::from(b3) << 6) + u32::from(b4)
                - 0x03c8_2080;
        if cp < 0x10000 {
            return None;
        } // overlong encoding
        return from_u32(cp).map(|ch| (ch, 4));
    }
    None
}