Incorrect slicing suggestion for pattern matching against a `Vec` inside a `Result` or `Option`

[edmorley]

When trying to pattern match against a Vec inside a Result or Option, an incorrect suggestion is given, that says to append [..] to the variable to slice it, which won't work since the variable is of type Result / Option rather than being a Vec itself.

Instead one option is to use .as_deref() on the Result:
https://doc.rust-lang.org/std/result/enum.Result.html#method.as_deref

Given the following code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1e834c61673228c36f08e8c294efc68d

fn main() {
    let s = "1:2";
    let result: Result<Vec<u64>, _> = s.split(':').map(str::parse).collect();

    match result {
        Ok([first, second]) => {
            println!("first: {}, second: {}", first, second);
        }
        _ => {}
    }
}

The current output is:

error[E0529]: expected an array or slice, found `Vec<u64>`
 --> src/main.rs:6:12
  |
5 |     match result {
  |           ------ help: consider slicing here: `result[..]`
6 |         Ok([first, second]) => {
  |            ^^^^^^^^^^^^^^^ pattern cannot match with input type `Vec<u64>`

Ideally the output should look like:

error[E0529]: expected an array or slice, found `Vec<u64>`
 --> src/main.rs:6:12
  |
5 |     match result {
  |           ------ help: consider converting the contents of the Result to a slice using: `result.as_deref()`
6 |         Ok([first, second]) => {
  |            ^^^^^^^^^^^^^^^ pattern cannot match with input type `Vec<u64>`