rust - 类型 `&[u8]` 不能被 `usize` 索引？

Question

下面是测试代码:

pub fn reverse_complement_seq_u8<T>(seq: T, len: usize) -> Vec<u8> 
    where T: std::ops::Index<usize, Output = u8>
{
    (0..len).rev().map(|i| match seq[i] {
            65 | 97  => 84, // 'A' | 'a' => 'T'
            84 | 116 => 65, // 'T' | 't' => 'A'
            71 | 103 => 67, // 'G' | 'g' => 'C'
            67 | 99  => 71, // 'C' | 'c' => 'G'
            n => n,
        } 
    ).collect()
}


fn main() {
    let seqs = "ACGATGCTACGA".as_bytes();//generated by another function
    let revcom_seq = reverse_complement_seq_u8(seqs, seqs.len());
    println!("{:?}", revcom_seq);
}

因为调用 seqs.to_owend() 很昂贵，所以我只想将它的引用传递给 reverse_complement_seq_u8，但是这会导致以下错误:

error[E0277]: the type `&[u8]` cannot be indexed by `usize`
  --> src/main.rs:17:48
   |
17 |     let revcom_seq = reverse_complement_seq_u8(seqs, seqs.len());
   |                      ------------------------- ^^^^ `&[u8]` cannot be indexed by `usize`
   |                      |
   |                      required by a bound introduced by this call
   |
   = help: the trait `Index<usize>` is not implemented for `&[u8]`
   = help: the following other types implement trait `Index<Idx>`:
             [T; N]
             [T]
note: required by a bound in `reverse_complement_seq_u8`

Answer 1

问题在于，虽然切片本身 [u8] 可以被索引，但对切片 &[u8] 的引用不能。大多数时候，索引切片引用是有效的，因为 Rust 会根据需要自动解除引用，但是当使用泛型时，您需要更严格地告诉编译器。

一个可能的解决方法是在参数中使用seq: &T 这样T 将是可以索引的[u8]。这需要一个额外的 + ?Sized 绑定(bind)，否则编译器会添加一个隐式的 Sized 绑定(bind)，而切片不会实现:

pub fn reverse_complement_seq_u8<T>(seq: &T, len: usize) -> Vec<u8>
where
    T: std::ops::Index<usize, Output = u8> + ?Sized,
{
    (0..len)
        .rev()
        .map(|i| match seq[i] {
            65 | 97 => 84,  // 'A' | 'a' => 'T'
            84 | 116 => 65, // 'T' | 't' => 'A'
            71 | 103 => 67, // 'G' | 'g' => 'C'
            67 | 99 => 71,  // 'C' | 'c' => 'G'
            n => n,
        })
        .collect()
}

fn main() {
    let seqs = "ACGATGCTACGA".as_bytes(); //generated by another function
    let revcom_seq = reverse_complement_seq_u8(seqs, seqs.len());
    println!("{:?}", revcom_seq);
}

Playground

另一个也适用于盒装切片的选项是指定 T 必须取消引用某些可索引类型:

use std::ops::Deref;
pub fn reverse_complement_seq_u8<T, U>(seq: T, len: usize) -> Vec<u8>
where
    T: Deref<Target = U>,
    U: std::ops::Index<usize, Output = u8> + ?Sized,
{
    (0..len)
        .rev()
        .map(|i| match seq[i] {
            65 | 97 => 84,  // 'A' | 'a' => 'T'
            84 | 116 => 65, // 'T' | 't' => 'A'
            71 | 103 => 67, // 'G' | 'g' => 'C'
            67 | 99 => 71,  // 'C' | 'c' => 'G'
            n => n,
        })
        .collect()
}

fn main() {
    let seqs = "ACGATGCTACGA".as_bytes(); //generated by another function
    let revcom_seq = reverse_complement_seq_u8(seqs, seqs.len());
    println!("{:?}", revcom_seq);
}

Playground