Can anyone explain rust pattern binding mode

1

image
image2048×410 121 KB

I do not know why the code is error?
let [mut x] = &[()]; //~ ERROR
let [ref x] = &[()]; //~ ERROR
let [ref mut x] = &mut [()]; //~ ERROR

2

Before the 2024 edition, using ref and mut works in patterns with ref and ref mut binding modes. But in the 2024 edition, you can only use these keywords in move binding mode:

You can convert your patterns to move binding mode by adding the leading reference:

fn main() {
    let &[mut x] = &[()];
    let &[ref x] = &[()];
    let &mut [ref mut x] = &mut [()];
}

Playground.

6 Likes
3

The reason why the rules were restricted in this way is so that the meaning of each one is clearer. Previously, you could, for example, add mut and find that you lost a reference, because it overwrote the default binding mode, and this confused people. Now, it is always the case that:

  • mut adds mutability to an otherwise immutable binding.
  • ref or ref mut makes the binding reference instead of move the input.
  • All 3 are always different from if you wrote nothing.
6 Likes