I'm excited about this one. This connects a lot of dots and is part of the unofficial Make The Safe Build Modes More Safe project (#2301).

Here are some of the features of Zig this depends on:

• there is no such thing as zero initialization
• all values must be explicitly initialized (but initialization to undefined is OK)
• most types (e.g. non-packed non-extern structs) intentionally have no well-defined in-memory layout

The proposal is to add a secret safety field to types which have no well-defined in-memory layout, similar to how unions have a secret safety tag field. The secret safety field has an integer which denotes the type id. A unique integer id will be generated for every type across an entire compilation.

Next, augment the rules about undefined values (see #1947) with this: in safe build modes, the bit pattern of undefined shall be 0xaa (repeating) across the store size of the type and for types which have no well-defined in-memory layout, the bit pattern 0xaa repeated across the store size shall not match a valid state.

This makes it possible to add safety checks to @ptrCast, @intToPtr, and @fieldParentPtr. It will be detectable illegal behavior (see #2402) if the actual element type does not match the target type specified in the cast, or if the memory has an undefined value.

Sometimes it is desired to @ptrCast or @intToPtr when you know the memory is undefined. For these cases we introduce @ptrCastUndef and @intToPtrUndef which simultaneously cast and assign undefined to the memory. These functions allow the programmer to change the type of memory in a legal way.