Function rank 

pub fn rank() -> rank

Rank of current row within its partition, with gaps

Returns the rank of the current row, with gaps; that is, the row_number of the first row in its peer group.

This function must be used as window function. You need to call at least one of the methods WindowExpressionMethods from to use this function in your SELECT clause. It cannot be used outside of SELECT clauses.

For MySQL this function requires you to call .window_order()

let res = posts
    .select((
        title,
        user_id,
        rank().partition_by(user_id).window_order(user_id),
    ))
    .load::<(String, i32, i64)>(connection)?;
let expected = vec![
    ("My first post".to_owned(), 1, 1),
    ("About Rust".into(), 1, 1),
    ("My first post too".into(), 2, 1),
];
assert_eq!(expected, res);