Largest product in a grid: Project Euler 11
Code review:
1. You don't need to use u64, since the input data is all u8, so use Vec&lt;u8&gt; like the following code and parse all the input data in one line:

fn main() {
    let s = "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08 49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00 81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65 52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91 22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80 24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50 32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70 67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21 24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72 21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95 78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92 16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57 86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58 19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40 04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66 88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69 04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36 20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16 20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54 01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48";
    let grid: Vec&lt;u8&gt; = s.split_whitespace().map(|v| v.parse().unwrap()).collect();
    let mut max = 0;
    for row in 0..20 {
        for col in 0..20 {
            for direction in &amp;[(0, 1), (1, 0), (1, -1), (1, 1)] {
                max = std::cmp::max(max, mul_4_adj(&amp;grid, row, col, direction));
            }
        }
    }
    println!("max={}", max); // 70600674
}
// Calculate the product of four adjacent numbers in the same direction:
fn mul_4_adj(grid: &amp;[u8], mut r: usize, mut c: usize, direction: &amp;(i8, i8)) -&gt; u32 {
    let mut f = 1;
    for _ in 0..4 {
        f *= grid[20 * r + c] as u32;
        let (row, col) = (r as i8 + direction.0, c as i8 + direction.1);
        if row &lt; 0 || row &gt;= 20 || col &lt; 0 || col &gt;= 20 {
            return 0;
        }
        r = row as usize;
        c = col as usize;
    }
    f
}



Using chunks with the CHUNK_SIZE of 20, instead of grid: &amp;[Vec&lt;u64&gt;] use grid: &amp;[&amp;[u8]]:


fn main() {
    let s = "08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08 49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00 81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65 52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91 22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80 24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50 32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70 67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21 24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72 21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95 78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92 16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57 86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58 19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40 04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66 88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69 04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36 20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16 20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54 01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48";
    let v: Vec&lt;u8&gt; = s.split_whitespace().map(|v| v.parse().unwrap()).collect();
    let grid: Vec&lt;_&gt; = v.chunks(20).collect();
    let mut max = 0;
    for row in 0..grid.len() {
        for col in 0..grid[row].len() {
            for direction in &amp;[(0, 1), (1, 0), (1, -1), (1, 1)] {
                max = std::cmp::max(max, mul_4_adj(&amp;grid, row, col, direction));
            }
        }
    }
    println!("max={}", max); // 70600674
}
// Calculate the product of four adjacent numbers in the same direction:
fn mul_4_adj(grid: &amp;[&amp;[u8]], mut r: usize, mut c: usize, direction: &amp;(i8, i8)) -&gt; u32 {
    let mut f = 1;
    for _ in 0..4 {
        f *= grid[r][c] as u32;
        let (row, col) = (r as i8 + direction.0, c as i8 + direction.1);
        r = row as usize;
        c = col as usize;
        if row &lt; 0 || col &lt; 0 || r &gt;= grid.len() || c &gt;= grid[r].len() {
            return 0;
        }
    }
    f
}






Also instead of Vec&lt;Vec&lt;u64&gt;&gt; you may use a two-dimensional array in Rust:


let mut grid = [[0_u8; 20]; 20];


Then simply find and print the max:

fn main() {
    let gridstr = r#"
08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00
81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65
52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91
22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80
24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50
32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70
67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21
24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95
78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92
16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57
86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58
19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40
04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66
88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69
04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36
20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16
20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54
01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48
"#;
    let mut grid = [[0_u8; 20]; 20];
    let lines: Vec&lt;_&gt; = gridstr.trim().split("\n").collect();
    for row in 0..20 {
        let line: Vec&lt;_&gt; = lines[row].trim().split_whitespace().collect();
        for col in 0..20 {
            grid[row][col] = line[col].parse().unwrap();
        }
    }

    let mut max = 0;
    for row in 0..20 {
        for col in 0..20 {
            for direction in &amp;[(0, 1), (1, 0), (1, -1), (1, 1)] {
                max = std::cmp::max(max, mul_4_adj(&amp;grid, row, col, direction));
            }
        }
    }
    println!("max={}", max); // 70600674
}
// Calculate the product of four adjacent numbers in the same direction:
fn mul_4_adj(grid: &amp;[[u8; 20]; 20], mut r: usize, mut c: usize, direction: &amp;(i8, i8)) -&gt; u32 {
    let mut f = 1;
    for _ in 0..4 {
        f *= grid[r][c] as u32;
        let (row, col) = (r as i8 + direction.0, c as i8 + direction.1);
        if row &lt; 0 || row &gt;= 20 || col &lt; 0 || col &gt;= 20 {
            return 0;
        }
        r = row as usize;
        c = col as usize;
    }
    f
}




Also you may use grid directly:


let grid = [ [ 08, 02, 22, 97, 38, 15, 00, 40, 00, 75, 04, 05, 07, 78, 52, 12, 50, 77, 91, 08, ], [ 49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 04, 56, 62, 00, ], [ 81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 03, 49, 13, 36, 65, ], [ 52, 70, 95, 23, 04, 60, 11, 42, 69, 24, 68, 56, 01, 32, 56, 71, 37, 02, 36, 91, ], [ 22, 31, 16, 71, 51, 67, 63, 89, 41, 92, 36, 54, 22, 40, 40, 28, 66, 33, 13, 80, ], [ 24, 47, 32, 60, 99, 03, 45, 02, 44, 75, 33, 53, 78, 36, 84, 20, 35, 17, 12, 50, ], [ 32, 98, 81, 28, 64, 23, 67, 10, 26, 38, 40, 67, 59, 54, 70, 66, 18, 38, 64, 70, ], [ 67, 26, 20, 68, 02, 62, 12, 20, 95, 63, 94, 39, 63, 08, 40, 91, 66, 49, 94, 21, ], [ 24, 55, 58, 05, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72, ], [ 21, 36, 23, 09, 75, 00, 76, 44, 20, 45, 35, 14, 00, 61, 33, 97, 34, 31, 33, 95, ], [ 78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 03, 80, 04, 62, 16, 14, 09, 53, 56, 92, ], [ 16, 39, 05, 42, 96, 35, 31, 47, 55, 58, 88, 24, 00, 17, 54, 24, 36, 29, 85, 57, ], [ 86, 56, 00, 48, 35, 71, 89, 07, 05, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58, ], [ 19, 80, 81, 68, 05, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 04, 89, 55, 40, ], [ 04, 52, 08, 83, 97, 35, 99, 16, 07, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66, ], [ 88, 36, 68, 87, 57, 62, 20, 72, 03, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69, ], [ 04, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18, 08, 46, 29, 32, 40, 62, 76, 36, ], [ 20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74, 04, 36, 16, ], [ 20, 73, 35, 29, 78, 31, 90, 01, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 05, 54, ], [ 01, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 01, 89, 19, 67, 48, ], ];



You don't need extra parenthesis and : i64 here:


let ret1: i64 = (lft as i64) + (rgt as i64);


This works:

let ret1 = lft as i64 + rgt as i64;



Use for _ in 0..4 instead of four find_elem(...) as the above examples.
You may use u32 instead of u64, since 255 * 255 * 255 * 255 = 4_228_250_625 = 0xfc05_fc01 for the product of four adjacent numbers.