Rust-clippy: Suggest binding to a tuple if possible in the `useless_let_if_seq` lint

[josephrocca]

I could be missing something here, but it seems like clippy (0.0.212 (e3cb40e 2019-06-25)) is giving a false positive for this code (useless_let_if_seq)?

fn main() {

    let i = 1;

    let mut arr0 = vec![0,0,0];
    let mut more_arrs = vec![ vec![1,1,1], vec![2,2,2], vec![3,3,3] ];

    let first;
    let second;
    if i == 0 {
        first = &mut arr0;
        second = &mut more_arrs[0];
    } else {
        let (start, end) = more_arrs.split_at_mut(i);
        first = &mut start[start.len()-1]; 
        second = &mut end[0];
    }

    println!("{:?}", first);
    println!("{:?}", second);

}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=0cbbd72da8e9a800131ca458f1e76016

Might be similar to #2176 and #2918?

[flip1995]

Couldn't this be rewritten to

    let (first, second) = if i == 0 {
        (&mut arr0, &mut more_arrs[0])
    } else {
        let (start, end) = more_arrs.split_at_mut(i);
        (&mut start[start.len()-1], &mut end[0])
    };

?

[josephrocca]

@flip1995 Good point! Since Clippy's suggestion is let second = if ..., maybe this issue should remain open so that it can be corrected?

warning: `if _ { .. } else { .. }` is an expression
  --> src/main.rs:9:5
   |
9  | /     let second;
10 | |     if i == 0 {
11 | |         first = &mut arr0;
12 | |         second = &mut more_arrs[0];
...  |
16 | |         second = &mut end[0];
17 | |     }
   | |_____^ help: it is more idiomatic to write: `let second = if i == 0 { ..; &mut more_arrs[0] } else { ..; &mut end[0] };`

[flip1995]

I mean the suggested code also works, but it can still be improved. So yeah, the suggestion should account for multiple bindings.

[josephrocca]

I also noticed that if you switch the order of the second = ... and first = ... lines in the if, or the if else, then Clippy's suggestion goes away (playground). But I imagine that will be fixed as a by-product of the above improvements.