This is a semantic bit-index within or one bit after an R register. It is the index of the first “dead” bit after a “live” region, and corresponds to the similar half-open range concept in the Rust Range type or the LLVM memory model, pointer values include the address one object past the end of a region.
It is a counter in the ring 0 ..= R::BITS (note the inclusive high end). Like BitIdx, this is a virtual semantic index with no bearing on real memory effects; unlike BitIdx, it can never be translated to real memory because it does not describe real memory.
This type is necessary in order to preserve the distinction between a dead memory address that is not part of a region and a live memory address that is within a region. Additionally, it makes computation of region extension or offsets easy. BitIdx is insufficient to this task, and produces off-by-one errors when used in its stead.
R: The register element that this dead-bit index governs.Values of this type are required to be in the range 0 ..= R::BITS. Any value greater than R::BITS makes the program invalid and will likely cause either a crash or incorrect memory access.
This type cannot be publicly constructed except by using the iterators provided for testing.