Function pack 

pub unsafe fn pack<T: ?Sized>(ptr: *mut T, a: u8, s: bool, v: u8) -> *mut T

Packs a pointer into the bottom sizeof(usize) - (a + s + v) bits of a usize.

Safety

• T’s alignment must enable stealing A bits.
• The high bits (sign upwards) must match a stack pointer’s high bits.
• If compiling for a 64bit arch, V must be at most 25.
• If compiling for a non-64bit arch, V must be 0.

These invariants are checked with debug_assert only, hence unsafe. The usual caveats of pointers apply.