Of course today in Rust we're not allowed to bless such types for ourselves, this is reserved to the standard library (core::num::NonZeroU32 is a 4-byte unsigned integer which can't be zero for example) because it's relying on a nasty permanently unstable compiler-only feature to say "Hey, not all possible representations the right size for this type are valid".
Rust has one very obvious set of types with such a niche, the references &T, but it also has not only the particular custom integers NonZero{U8,U16,U32,U64,I8,I16,I32,I64} and the Non-null pointers, but OS specific types (for Unix file descriptors, and Windows handles respectively) and of course char, it is 4 bytes but Unicode is explicitly limited to far fewer values, so that's a niche too.
But today making more is not allowed in stable Rust, so while my nook crate is an interesting curiosity (and some day I'll implement integer math traits etc.) it cannot be stabilised and I was persuaded that the path forward is a whole lot of work to deliver the Pattern Types feature which can do this stably.
Arbitrary niches aren't intended to be permanently unstable (at least not in the sense of explicit implementation details like lang items and intrinsics), people have expressed a desire to stabilize them for years (as discussed during the original nonzero int types RFC). If something has changed in that regard, I'd appreciate a link to a discussion.
The niche concept isn't permanently unstable, but the present mechanism is. Today the only way to make such a type is to use the magic attributes
#[rustc_nonnull_optimization_guaranteed]
#[rustc_layout_scalar_valid_range_start]
and
#[rustc_layout_scalar_valid_range_end]
As their names hint, these are permanently unstable compiler-only type hints.
I asked that some means be found to stabilize some equivalent hint (presumably with a name that doesn't look like a compiler-only feature) in one of the long series of tickets spawned after 3334 and I was persuaded that this is a terrible idea and won't happen.
Instead the proposed path forward is (at least was then) Pattern Types. Build at least an MVP of the full blown Pattern Types feature instead of a custom attribute.
Of course today in Rust we're not allowed to bless such types for ourselves, this is reserved to the standard library (core::num::NonZeroU32 is a 4-byte unsigned integer which can't be zero for example) because it's relying on a nasty permanently unstable compiler-only feature to say "Hey, not all possible representations the right size for this type are valid".
Rust has one very obvious set of types with such a niche, the references &T, but it also has not only the particular custom integers NonZero{U8,U16,U32,U64,I8,I16,I32,I64} and the Non-null pointers, but OS specific types (for Unix file descriptors, and Windows handles respectively) and of course char, it is 4 bytes but Unicode is explicitly limited to far fewer values, so that's a niche too.
But today making more is not allowed in stable Rust, so while my nook crate is an interesting curiosity (and some day I'll implement integer math traits etc.) it cannot be stabilised and I was persuaded that the path forward is a whole lot of work to deliver the Pattern Types feature which can do this stably.