Why I can not use u8 as an index value of a Rust array?

Question

I am new to Rust, and I am trying to write simple bitwise replacer.

I have this code:

const TABLE: [u64; 8] = [
    0xC462A5B9E8D703F1,
    0x68239A5C1E47BD0F,
    0xB3582FADE174C960,
    0xC821D4F670A53E9B,
    0x7F5A816D093EB42C,
    0x5DF692CAB78143E0,
    0x8E25691CF4B0DA37,
    0x17ED05834FA69CB2,
];

fn get_part(u: u64, i: u8) -> u8 {
    ((u & (0xFu64 << (16 - i))) >> (16 - i)) as u8
}

fn process(o: u8, i1: u8, i2: u8) -> u8 {
    let left: u8 = o >> 4;
    let right: u8 = o & 0xF;
    (get_part(TABLE[left], left) << 4) + get_part(TABLE[right], right)
}

I got errors like this one:

error[E0277]: the trait bound `u8: std::slice::SliceIndex<[u64]>` is not satisfied
  --> src/main.rs:19:15
   |
19 |     (get_part(TABLE[left], left) << 4) + get_part(TABLE[right], right)
   |               ^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `std::slice::SliceIndex<[u64]>` is not implemented for `u8`
   = note: required because of the requirements on the impl of `std::ops::Index<u8>` for `[u64]`

error[E0277]: the trait bound `u8: std::slice::SliceIndex<[u64]>` is not satisfied
  --> src/main.rs:19:51
   |
19 |     (get_part(TABLE[left], left) << 4) + get_part(TABLE[right], right)
   |                                                   ^^^^^^^^^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `std::slice::SliceIndex<[u64]>` is not implemented for `u8`
   = note: required because of the requirements on the impl of `std::ops::Index<u8>` for `[u64]`

I don't understand why it's illegal to use u8 as the index value. How can I convert u8 to a compatible type? I don't even know which type is compatible.

Answer 1

You can look at the documentation for SliceIndex by searching the Rust standard library. The list of implementations of this trait at the bottom of the documentation page indicates that this trait is implemented for usize and various usize ranges.

This should answer both of your questions: indexing is not implemented for u8 type and you need to cast u8 to usize.

(get_part(TABLE[left as usize], left) << 4) + get_part(TABLE[right as usize], right)