False positive when swapping elements of a matrix

[krishna-veerareddy]

Clippy(clippy 0.0.212 (3aea860 2019-09-03)) incorrectly suggests to use std::mem::swap when trying to swap elements of a matrix across different rows. The lint is clippy::manual_swap.

Here's a minimal example:

fn swap(a: &mut[Vec<u64>]) {
    let tmp = a[0][1];

    a[0][1] = a[1][0];
    a[1][0] = tmp;
}

[flip1995]

It seems like #981 wasn't fixed completely.