matrix.rs

// works:
assert_eq!(10, (0..10).map(|i| m.columns[i][i]).sum());

let mut acc: i32 = 0;                                   
// works                                                
for i in 0..10 {                                        
    for j in 0..10 {                                    
        acc += m.columns[i][j];                         
    }                                                   
}                                                       
assert_eq!(10, acc);                                    
                                                        
// doesn't work without :i32                            
let acc: i32 = (0..10).map(                             
        |i| (0..10).map(|j| m.columns[i][j]).sum(): i32 
    ).sum();                                            
assert_eq!(10, acc);