compile error for unreachable code from while condition

[andrewrk]

this gets past the compiler, but it should give a compile error because return null is unreachable code.

pub fn lastIndexOfScalarPos(comptime T: type, slice: []const T, start_index: usize, value: T) ?usize {
    var i: usize = start_index;

    while (i >= 0) : (i -= 1) {
        if (slice[i] == value) {
            return i;
        }
    }

     return null;
 }

[BraedonWooding]

Halting Problem however could restrict this? Since you could have cases where you have an infinite array that isn't easily seen, such as incrementing i through some other function and so on.

Also how is this code unreachable? This will only be unreachable if 'i < 0' else it could never find the value and i would decrement till it reached '0' then it would return null? Perhaps you mean i+=1?

[andrewrk]

zig already knows that while (i >= 0) when i is unsigned is equivalent to while (true). since there is no control flow to the end of the while, the compiler can be certain that return null is unreachable code.

it's a matter of making the return type of the while expression noreturn in this situation.

[BraedonWooding]

Oh I see what your saying, I didn't see that it was unsigned (just slipped by my mind).