Pattern matching on slices

问题

I did something like this, which works:

let s = " \"".as_bytes();
let (space, quote) = (s[0], s[1]);

I wanted to do something like this

&[space, quote] = " \"".as_bytes();

But it gives me the error

slice pattern syntax is experimental (see issue #23121)

Is it possible to do something similar?

回答1:

As the error tells you, slice pattern syntax is experimental. This means either the semantics are not clear or the syntax might change in the future. As a result, you need a nightly version of the compiler and to explicitly request that feature:

#![feature(slice_patterns)]

fn main() {
    match " \"".as_bytes() {
        &[space, quote] => println!("space: {:?}, quote: {:?}", space, quote),
        _ => println!("the slice lenght is not 2!"),
    }
}

Also note that you can't just write &[space, quote] = whatever anyway because there is a possibility that whatever is not the right length. To make the pattern matching exhaustive, you need a _ case or a case with ... What you tried would give another error:

error[E0005]: refutable pattern in local binding: `&[]`, `&[_]` and `&[_, _, _, ..]` not covered
 --> src/main.rs:4:9
  |
4 |     let &[space, quote] = " \"".as_bytes();
  |         ^^^^^^^^^^^^^^^ patterns `&[]`, `&[_]` and `&[_, _, _, ..]` not covered
  |
  = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
  = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
help: you might want to use `if let` to ignore the variant that isn't matched
  |
4 |     if let &[space, quote] = " \"".as_bytes() { /* */ }

As of Rust 1.26 you can pattern match on an array instead of a slice. If you convert the slice to an array, you can then match on it:

use std::convert::TryInto;

fn main() {
    let bytes = " \"".as_bytes();

    let bytes: &[_; 2] = bytes.try_into().expect("Must have exactly two bytes");
    let &[space, quote] = bytes;

    println!("space: {:?}, quote: {:?}", space, quote);
}

来源：https://stackoverflow.com/questions/38545609/pattern-matching-on-slices

标签

rust

matching