demonstration of zig's default unit testing allocator catching various mistakes

demo.zig

const std = @import("std");

// leak
test "example 1" {
    var array = std.ArrayList(i32).init(std.testing.allocator);

    try array.append(1234);
}

// double free
test "example 2" {
    const ally = std.testing.allocator;
    const slice1 = try ally.alloc(i32, 10);
    const slice2 = try ally.alloc(i32, 10);

    ally.free(slice1);
    ally.free(slice1); // oops!
}

// use after free
test "example 3" {
    const ally = std.testing.allocator;
    const slice1 = try ally.alloc(usize, 10);

    for (slice1) |*elem, i| {
        elem.* = i;
    }

    ally.free(slice1);

    const another_array = [5]i32{ 1, 2, 3, 4, 5 };

    const index = slice1[3]; // oops!

    const x = another_array[index];
}