Module self_desc_pointer 

SelfDescPointer<T> - pointer carrying type ID + layout shape in stolen high bits.

Layout: a single u64 with the high 16 bits stolen:

• bits 56..=63: type_id (u8)
• bits 53..=55: layout shape (3 bits, LayoutShape)
• bits 0..=52: the address (53-bit virtual; ample on x86_64 / Apple Silicon)

The architectural win: a heterogeneous container of SelfDescPointers can dispatch on type WITHOUT a vtable lookup. When the type universe is small enough to fit in 8 bits (256 distinct types), the dispatch is a switch on the high byte; the compiler can compile this to a jump table inline at the call site.

Comparison vs Rust’s existing options

Mechanism	Per-call cost	Type universe
dyn Trait vtable	1 indirect call	unbounded
enum with variants	tag-match	bounded
SelfDescPointer<T>	switch on byte	<= 256

The architectural shape mirrors JVM compressed-klass pointers (where the klass is encoded in the top bits of an object ref) but at a lighter weight: only 8 bits + 3 layout-shape bits stolen, vs JVM’s full 32-bit compressed klass.

Bit budget

• 8 bits for type ID: 256 distinct types in the universe. For wider universes use Box<dyn Trait> or an enum.
• 3 bits for layout shape: 8 shapes covered by LayoutShape.
• 53 bits for address: 8 PiB virtual memory; well above any current process.

Structs

SelfDescPointer

8-byte pointer with (type_id, layout_shape, address) packed.

Enums

LayoutShape

Layout shape encoded in 3 bits.

Constants

ADDR_MASK

Address mask: low 53 bits.

SHAPE_MASK

SHAPE_SHIFT

Shape field: 3 bits at positions 53..56.

TYPE_MASK

TYPE_SHIFT

Type ID field: 8 bits at positions 56..64.