nom the parser performance comparison between spaces and tabs https://github.com/Geal/nom

main.rs

use std::time::Instant;

use nom::character::complete::*;
use nom::error::ParseError;
use nom::{AsChar, InputIter, InputTakeAtPosition, Slice};

fn parse_spaces<T, E: ParseError<T>>(mut input: T) -> i32
where
    T: InputTakeAtPosition + InputIter + Slice<std::ops::RangeFrom<usize>>,
    <T as InputTakeAtPosition>::Item: AsChar + Clone,
    <T as InputIter>::Item: AsChar,
{
    let mut count = 0;
    while let Ok((s, _)) = space1::<T, E>(input) {
        match newline::<T, E>(s) {
            Ok((s, _)) => {
                input = s;
                count += 1;
            }
            _ => break,
        }
    }
    count
}

fn parse_tabs<T, E: ParseError<T>>(mut input: T) -> i32
where
    T: InputTakeAtPosition + InputIter + Slice<std::ops::RangeFrom<usize>>,
    <T as InputTakeAtPosition>::Item: AsChar + Clone,
    <T as InputIter>::Item: AsChar,
{
    let mut count = 0;
    while let Ok((s, _)) = tab::<T, E>(input) {
        match newline::<T, E>(s) {
            Ok((s, _)) => {
                input = s;
                count += 1;
            }
            _ => break,
        }
    }
    count
}

fn main() {
    let times = 22;
    let space_str = " \n".repeat(2usize.pow(times));
    let two_spaces_str = "  \n".repeat(2usize.pow(times));
    let three_spaces_str = "   \n".repeat(2usize.pow(times));
    let four_spaces_str = "    \n".repeat(2usize.pow(times));
    let tabs_str = "\t\n".repeat(2usize.pow(times));

    let now = Instant::now();
    parse_spaces::<_, ()>(space_str.as_str());
    println!("spaces_str: {}ms", now.elapsed().as_millis());

    let now = Instant::now();
    parse_spaces::<_, ()>(two_spaces_str.as_str());
    println!("two_spaces_str: {}ms", now.elapsed().as_millis());

    let now = Instant::now();
    parse_spaces::<_, ()>(three_spaces_str.as_str());
    println!("three_spaces_str: {}ms", now.elapsed().as_millis());

    let now = Instant::now();
    parse_spaces::<_, ()>(four_spaces_str.as_str());
    println!("four_spaces_str: {}ms", now.elapsed().as_millis());

    let now = Instant::now();
    parse_tabs::<_, ()>(tabs_str.as_str());
    println!("tabs_str: {}ms", now.elapsed().as_millis());
}

stdout

spaces_str: 8798ms
two_spaces_str: 9693ms
three_spaces_str: 11965ms
four_spaces_str: 13677ms
tabs_str: 5345ms