zig/doc/langref/test_pointer_arithmetic.zig

const expect = @import("std").testing.expect;

test "pointer arithmetic with many-item pointer" {
    const array = [_]i32{ 1, 2, 3, 4 };
    var ptr: [*]const i32 = &array;

    try expect(ptr[0] == 1);
    ptr += 1;
    try expect(ptr[0] == 2);

    // slicing a many-item pointer without an end is equivalent to
    // pointer arithmetic: `ptr[start..] == ptr + start`
    try expect(ptr[1..] == ptr + 1);

    // subtraction between any two pointers except slices based on element size is supported
    try expect(&ptr[1] - &ptr[0] == 1);
}

test "pointer arithmetic with slices" {
    var array = [_]i32{ 1, 2, 3, 4 };
    var length: usize = 0; // var to make it runtime-known
    _ = &length; // suppress 'var is never mutated' error
    var slice = array[length..array.len];

    try expect(slice[0] == 1);
    try expect(slice.len == 4);

    slice.ptr += 1;
    // now the slice is in an bad state since len has not been updated

    try expect(slice[0] == 2);
    try expect(slice.len == 4);
}

// test