Replace indexing with get_unchecked calls

[GuillaumeGomez]

Results are not very impressive though:

bench name	current	new	diff
bench_i128	17.74 ns/iter (+/- 0.02)	17.5 ns/iter (+/- 0.24)	-1.4%
bench_i128_max	38.81 ns/iter (+/- 0.05)	37.65 ns/iter (+/- 0.51)	-3.0%
bench_i128_min	38.85 ns/iter (+/- 0.06)	37.47 ns/iter (+/- 0.75)	-3.6%
bench_i16	17.61 ns/iter (+/- 0.02)	17.55 ns/iter (+/- 0.64)	-0.3%
bench_i16_max	18.1 ns/iter (+/- 0.03)	18.07 ns/iter (+/- 0.46)	-0.2%
bench_i16_min	18.47 ns/iter (+/- 0.15)	18.34 ns/iter (+/- 0.47)	-0.7%
bench_i32	17.56 ns/iter (+/- 0.48)	17.64 ns/iter (+/- 0.06)	0.5%
bench_i32_max	21.04 ns/iter (+/- 0.12)	21.04 ns/iter (+/- 0.14)	0.0%
bench_i32_min	21.3 ns/iter (+/- 0.7)	21.12 ns/iter (+/- 0.06)	-0.8%
bench_i64	17.61 ns/iter (+/- 0.07)	17.54 ns/iter (+/- 0.06)	-0.4%
bench_i64_max	24.7 ns/iter (+/- 0.04)	24.72 ns/iter (+/- 0.29)	0.1%
bench_i64_min	25.28 ns/iter (+/- 0.04)	25.28 ns/iter (+/- 0.03)	0.0%
bench_i8	17.67 ns/iter (+/- 0.05)	17.62 ns/iter (+/- 0.02)	-0.3%
bench_i8_max	17.63 ns/iter (+/- 0.09)	17.52 ns/iter (+/- 0.3)	-0.6%
bench_i8_min	17.91 ns/iter (+/- 0.03)	17.79 ns/iter (+/- 0.06)	-0.7%
bench_isize	17.62 ns/iter (+/- 0.07)	17.58 ns/iter (+/- 0.05)	-0.2%
bench_isize_max	24.71 ns/iter (+/- 0.03)	24.72 ns/iter (+/- 0.03)	0.0%
bench_isize_min	25.28 ns/iter (+/- 0.04)	25.29 ns/iter (+/- 0.05)	0.0%
bench_u128	16.36 ns/iter (+/- 0.02)	16.05 ns/iter (+/- 0.03)	-1.9%
bench_u128_max	38.97 ns/iter (+/- 0.25)	37.08 ns/iter (+/- 0.06)	-4.8%
bench_u128_min	14.51 ns/iter (+/- 0.18)	13.8 ns/iter (+/- 0.27)	-4.9%
bench_u16	14.73 ns/iter (+/- 0.27)	15.18 ns/iter (+/- 0.03)	3.1%
bench_u16_max	14.96 ns/iter (+/- 0.02)	14.97 ns/iter (+/- 0.05)	0.1%
bench_u16_min	14.78 ns/iter (+/- 0.24)	15.02 ns/iter (+/- 0.13)	1.6%
bench_u32	14.64 ns/iter (+/- 0.27)	14.66 ns/iter (+/- 0.11)	0.1%
bench_u32_max	17.5 ns/iter (+/- 0.69)	17.81 ns/iter (+/- 0.31)	1.8%
bench_u32_min	14.67 ns/iter (+/- 0.11)	15.01 ns/iter (+/- 0.11)	2.3%
bench_u64	15.01 ns/iter (+/- 0.03)	14.96 ns/iter (+/- 0.16)	-0.3%
bench_u64_max	21.83 ns/iter (+/- 0.03)	21.84 ns/iter (+/- 0.17)	0.0%
bench_u64_min	14.63 ns/iter (+/- 0.04)	14.62 ns/iter (+/- 0.01)	-0.1%
bench_u8	14.41 ns/iter (+/- 0.04)	14.53 ns/iter (+/- 0.34)	0.8%
bench_u8_max	14.7 ns/iter (+/- 0.07)	14.58 ns/iter (+/- 0.12)	-0.8%
bench_u8_min	13.8 ns/iter (+/- 0.25)	14.43 ns/iter (+/- 0.31)	4.6%
bench_usize	14.8 ns/iter (+/- 0.04)	14.81 ns/iter (+/- 0.1)	0.1%
bench_usize_max	21.86 ns/iter (+/- 0.02)	21.83 ns/iter (+/- 0.02)	-0.1%
bench_usize_min	14.62 ns/iter (+/- 0.19)	15.52 ns/iter (+/- 0.23)	6.2%

Benchmark code

#![feature(test)]

extern crate test;

use test::{Bencher, black_box};

#[inline(always)]
fn convert_to_string<T: ToString>(n: T) -> String {
    n.to_string()
}

macro_rules! decl_benches {
    ($($name:ident $name_min:ident $name_max:ident: $ty:ident,)+) => {
        $(
            #[bench]
            fn $name(c: &mut Bencher) {
                c.iter(|| convert_to_string(black_box({ let nb: $ty = 20; nb })));
            }
            #[bench]
            fn $name_min(c: &mut Bencher) {
                c.iter(|| convert_to_string(black_box({ let nb: $ty = $ty::MIN; nb })));
            }
            #[bench]
            fn $name_max(c: &mut Bencher) {
                c.iter(|| convert_to_string(black_box({ let nb: $ty = $ty::MAX; nb })));
            }
        )+
    }
}

decl_benches! {
    bench_u8 bench_u8_min bench_u8_max: u8,
    bench_i8 bench_i8_min bench_i8_max: i8,
    bench_u16 bench_u16_min bench_u16_max: u16,
    bench_i16 bench_i16_min bench_i16_max: i16,
    bench_u32 bench_u32_min bench_u32_max: u32,
    bench_i32 bench_i32_min bench_i32_max: i32,
    bench_u64 bench_u64_min bench_u64_max: u64,
    bench_i64 bench_i64_min bench_i64_max: i64,
    bench_u128 bench_u128_min bench_u128_max: u128,
    bench_i128 bench_i128_min bench_i128_max: i128,
    bench_usize bench_usize_min bench_usize_max: usize,
    bench_isize bench_isize_min bench_isize_max: isize,
}

And there is no difference in the generated asm, so opening it and closing it right away to be able to link to it.

r? ghost