Memory Safety's Hardest Problem

Memory Safety’s Hardest Problem Jul 20, 2026 Uplifting a lobsters comment for easier reference. The central memory safety counter example, the hardest case to solve, doesn’t have anything to do with destructors or heap: const std = @import("std"); const E = union(enum) { a: u128, b: []const u8, }; pub fn main() void { const bad_addr: u128 = @intFromPtr(&main); var e: E = .{ .b = "hello" }; const oh_no_pointer: *const []const u8 = switch (e) { .a => unreachable, .b => |*p| p, }; e = .{ .a = (16 << 64) + bad_addr }; const oh_no: []const u8 = oh_no_pointer.*; std.debug.print("{s}\n", .{oh_no}); } $ zig run main.zig ��C�� � This sort of example also breaks Ada: https://www.enyo.de/fw/notes/ada-type-safety.html We have a tagged union, which can hold either A or B. We initialize the union as A, take a pointer to its internals, overwrite the original with B, and then use the pointer. The pointer is still typed as A, but the bytes it points to now belong to B: a type confusion. This being…