Niches

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+- Feature Name: `niche`
+- Start Date: 2022-10-16
+- RFC PR: [rust-lang/rfcs#3334](https://github.com/rust-lang/rfcs/pull/3334)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Provide a stable attribute to define "niche" values of a type. The type cannot
+store these values, allowing the compiler to use them to optimize the
+representation of containing types such as `Option<Type>`.
+
+# Motivation
+[motivation]: #motivation
+
+Rust makes extensive use of types like `Option`, and many programs benefit from
+the efficient storage of such types. Many programs also interface with others
+via FFI, via interfaces that provide data and a sentinel value (such as for
+errors or missing data) within the same bits.
+
+The Rust compiler already provides support for this via "niche" optimizations,
+and various types providing guarantees of such optimizations, including
+references, `bool`, `char`, and the `NonZero` family of types. However, Rust
+does not provide any stable means of defining new types with niches, reserving
+this mechanism for the standard library. This puts pressure on the standard
+library to provide additional families of types with niches, while preventing
+the broader crate ecosystem from experimenting with such types.
+
+Past efforts to define a stable niche mechanism stalled out due to scope creep:
+alignment niches, null-page niches, multiple niches, structures with multiple
+fields, and many other valid but challenging ideas (documented in the "Future
+possibilities" section). This RFC defines a *simple* mechanism for defining one
+common type of niche, while leaving room for future extension.
+
+Defining a niche mechanism allows libraries to build arbitrary types containing
+niches, and simplifies handling of space-efficient data structures in Rust
+without manual bit-twiddling.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+When defining a structure containing exactly one field of a non-zero-sized type
+(non-ZST), you can attach a `niche` attribute on it to declare a specific value
+or range of values for that field as invalid. This promises the compiler that
+you will never store those values in that field, which allows the compiler to
+use those in-memory representations for different purposes, such as the
+representation of `None` in a containing `Option`.
+
+```rust
+use std::mem::size_of;
+
+#[niche(value = 42)]
+struct MeaninglessNumber(u64);
+
+assert_eq!(size_of::<MeaninglessNumber>(), 8);
+assert_eq!(size_of::<Option<MeaninglessNumber>>(), 8);
+
+#[niche(range = 2..)]
+struct Bit(u8);
+
+assert_eq!(size_of::<Bit>(), 1);
+assert_eq!(size_of::<Option<Option<Option<Bit>>>>(), 1);
+```
+
+Constructing a structure with a niche value, or writing to the non-ZST field of
+such a structure, or obtaining a mutable reference to such a field, requires
+`unsafe` code. Causing a type with a niche to contain an invalid value (whther
+by construction, writing, or transmuting) results in undefined behavior.
+
+If a type `T` contains only a single niche value, `Option<T>` (and other enums
+isomorphic to it, with one variant containing `T` and one nullary variant) will
+use that value to represent `None` (the nullary variant). If such a `T` is
+additionally `repr(transparent)` or `repr(C)` or otherwise permitted in FFI,
+`Option<T>` will likewise be permitted in FFI, with the niche value mapping
+bidirectionally to `None` across the FFI boundary.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The attribute `#[niche]` may only appear on a struct declaration. The struct
+must contain exactly one field of a non-zero-sized type (non-ZST). The struct
+may contain zero or more ZST fields, such as `PhantomData`.
+
+Declaring a niche on any item other than a struct declaration results in an
+error.
+
+Declaring multiple `niche` attributes on a single item, or multiple key-value
+pairs within a single `niche` attribute, results in an error.
+
+Declaring a niche on a struct containing more or less than one non-zero-sized
+field results in an error.
+
+Declaring a range niche with an empty range (e.g. `0..0`) results in a
+warn-by-default lint. As with many lints, this lint should be automatically
+suppressed for code expanded from a macro.
+
+Declaring a range niche with an invalid range (e.g. `5..0`) results in an
+error.
+
+Declaring a niche using a negative value or a negative range endpoint results
+in an error. The text of the error should suggest the appropriate unsigned
+equivalent to use. The compiler may support this in the future.
+
+Declaring a range niche with an open start (`..3`) results in an error, for
+forwards-compatibility with support for negative values.
+
+Declaring a niche using a non-literal value (e.g. `usize::MAX`) results in an
+error. Constants can use compile-time evaluation, and compile-time evaluation
+does not occur early enough for attributes such as niche declarations.
+
+If a type `T` contains multiple niche values (e.g. `#[niche(range = 8..16)]`),
+the compiler does not define the representation of types containing `T`, except
+that multiple instances of the same identical type (e.g. `Option<T>` and
+`Option<T>`) will use an identical representation (permitting a round-trip
+`transmute` of such a value via bytes). In particular, the compiler does not
+commit to making use of all the invalid values of the niche, even if it
+otherwise could have.
+
+If a type `T` contains niches and uses `repr(C)` or `repr(transparent)`, the
+compiler guarantees to use the same storage size for the type as it would
+without the niche, even if the niche might allow storing fewer bytes. If a type
+`T` contains niches and uses the default (`Rust`) `repr`, the compiler may
+choose to represent the type using fewer bytes if the niche would allow doing
+so. For instance:
+
+```rust
+#[niche(range = 4..)]
+struct S {
+    field: u16,
+}
+
+// `size_of::<S>()` may return less than 2
+```
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+We could allow defining *either* valid or invalid ranges. For instance,
+`niche(invalid_range(0..=3))` or `niche(valid_range(4..))`. Different types
+could use whichever of the two proved simpler for a given use case. However, in
+addition to adding gratuitous complexity and requiring longer names
+(`invalid_range` vs `range`), this would double the number of cases when
+defining other kinds of niches in the future. For instance, a future syntax for
+bit-pattern niches would need to provide both `valid` and `invalid` variants as
+well. We could introduce another level of nesting to make this orthogonal, such
+as `niche(invalid(range(...)))` and `niche(invalid(range(...)))`, but that
+further increases complexity.
+
+Rather than defining the range of *invalid* values, the attribute could define
+the range of *valid* values. Different types may find one or the other case
+simpler. This RFC chooses to define the range of *invalid* values for three
+reasons:
+- As an arbitrary choice, because we need to pick one or the other (see above).
+- The most common case will be a single invalid value, for which defining
+  invalid values results in simpler code.
+- This mechanism commonly goes by the name `niche`, and `niche` also refers to
+  the invalid value. So, an attribute defining the niche of a type most
+  naturally refers to the invalid value.
+
+Note that the compiler already supports having a niche in the middle of a
+type's possible values; internally, the compiler represents this by defining a
+valid range that wraps around the type's possible values. For instance,
+`#[niche(value = 42)]` gets represented internally in the compiler as a valid
+range starting at 43 and ending at 41.
+
+We could define *only* single-value niches, not ranges. However, the compiler
+already supports ranges internally, and the standard library already makes use
+of multi-value ranges, so this seems like an artificial limitation.
+
+We could define only ranges, not single-value niches, and users could express
+single-value niches via ranges, such as `0..=0`. However, that makes
+single-value niches more verbose to define, and makes mistakes such as `0..0`
+more likely. (This RFC suggests a lint to catch such cases, but the syntax
+should still attempt to guide users away from that mistake.)
+
+We could guarantee more usage of niches than just a single value; however, this
+would constrain the compiler in areas that still see active development.
+
+We could avoid guaranteeing the use of a single-value niche for `Option`;
+however, this would eliminate one of the primary user goals for such niches.
+
+We could require types to opt into the guaranteed use of a niche, separately
+from declaring a niche. This seems unnecessarily verbose, as well as limiting:
+we can't yet provide a full guarantee of all *future* uses we may want to
+guarantee, only of the limited single-value uses.
+
+We could implement niches using a lang-item type that uses const generics (e.g.
+`Niche<T, const RANGE: std::ops::Range<T>>`. This type would be useful
+regardless, and we should likely provide it if we can. However, this RFC
+advocates (eventually) building such a type on an underlying language-level
+building block like `niche`, and providing the underlying building blocks to
+the ecosystem as well.
+
+We could implement niches using a trait `Niche` implemented for a type, with
+associated consts for invalid values. If we chose to do this in the future, the
+`#[niche(...)]` attribute could become forward-compatible with this, by
+generating the trait impl.
+
+# Prior art
+[prior-art]: #prior-art
+
+The Rust compiler has supported niches for types like `Option` in various forms
+since versions prior to Rust 1.0. In particular, Rust 1.0 already guaranteed
+that `Option<&T>` has the same size as `&T`. Rust has had many additional
+niche-related optimizations since then.
+
+The Rust compiler already supports user-defined niches via the unstable
+attributes `rustc_layout_scalar_valid_range_start` and
+`rustc_layout_scalar_valid_range_end`.
+
+Bit-twiddling tricks to store information compactly have seen widespread use
+and innovation since computing antiquity.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+Does the compiler support niches on structs containing ZST fields such as
+`PhantomData`? If it doesn't, then initially, having a limitation to only
+structs containing a single field would be fine, and would not substantially
+reduce the usefulness of stabilizing this feature.
+
+Could we support niches on generic types? For instance, could we support
+declaring a niche of `0` on a generic structure with a single field?
+
+Could we support negative numbers in a niche attribute, at least for fields of
+concrete primitive type? That would provide a much more friendly interface, but
+would require the compiler to better understand the type and its size.
+
+Will something go wrong if applying a niche to a struct whose non-ZST field is
+itself a struct containing multiple fields? Do we need to restrict niches to
+structs containing primitive types, or similar?
+
+Do we need to make `niche` mutually exclusive with `packed`? What about other
+attributes?
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Niches offer possibilities as vast, rich, clever, and depraved as the
+collective ingenuity of bit-twiddlers everywhere. This section includes many
+possibilities that have come up in the past. This RFC deliberately excludes all
+of these possibilities from the scope of the initial version, choosing to
+specify only behavior that the Rust compiler already implements.
+
+New types of niches can use the same `niche` attribute, adding new key-values
+within the attribute.
+
+- **Signed values**: This RFC requires the use of unsigned values when defining
+  niches. A future version could permit the use of signed values, to avoid
+  having to manually perform the twos-complement conversion. This may
+  require either making the compiler's implementation smarter, or using a
+  syntax that defines the size of the integer type (e.g. `-1isize`).
+- **Limited constant evaluation**: This RFC excludes the possibility of using
+  constants in the range expression, because doing so simplifies the
+  implementation. Ideally, a future version would allow ranges to use at least
+  *simple* numeric constants, such as `usize::MAX`. Full constant evaluation
+  may be much harder to support.
+- **Alignment niches**: If a pointer requires a certain alignment, any bit pattern
+  corresponding to an unaligned pointer could serve as a niche. This provides
+  an automatic mechanism for handling "tagged pointers" using the low bits.
+- **Null-page niches**: If a target treats the entire null page as invalid,
+  pointers on that target could have a niche corresponding to that entire page,
+  rather than just the null value. This would allow defining niches spanning a
+  large swath of the value space. However, this would either require extensive
+  use of `cfg_attr` for various targets, or a new mechanism for obtaining the
+  valid range from the compiler. In addition, for some targets the valid range
+  may vary based on environment, even for the same target; in such cases, the
+  compiler would need to provide a mechanism for the user to supply the valid
+  range *to* the compiler.
+- **Invalid-pointer niches**: On targets where certain pointer values cannot
+  represent a valid pointer in a given context (such as on x86-64 where the
+  upper half of the address space represents kernel-space address and the lower
+  half represents userspace addresses), types containing such pointers could use
+  a large swathe of values as a niche.
+- **Pointer high-bit niches**: On targets that don't permit addresses with some of
+  the high bits set (such as implicitly on historical x86 or ARM platforms, or
+  explicitly defined via ARM's "top-byte ignore" or AMD's "upper address
+  ignore" or Intel's "Linear Address Masking"), types containing pointers could
+  potentially use values with those high bits set as a niche. This would likely
+  require compile-time configuration.
+- **Multiple niches**: A type could define multiple niches, rather than just a
+  single range.
+- **Other bit-pattern niches**: A type could define niches via a bit pattern,
+  rather than a range.
+- **Per-field niches**: A structure containing multiple fields could have a
+  niche on a specific field, rather than the whole structure.
+- **Whole-structure niches**: A structure containing multiple non-zero-sized
+  fields could have a niche of invalid values for the whole structure.
+- **Union niches**: A union could have a niche.
+- **Enum niches**: An enum or an enum variant could have a niche.
+- **Specified mappings into niches**: Users may want to rely on mappings of
+  multiple values into a multi-value niche. For instance, users could define a
+  type with a niche containing a range of integer values, and a range of
+  integer error codes, and rely on `Result<T, E>` assigning specific niche
+  values to specific error codes, in order to match a specific ABI (such as the
+  Linux kernel's `ERR_PTR`).
+- **Safety**: The attribute specified in this RFC requires an unsafe block to
+  set the field. Future extensions could allow safely setting the field, after
+  verifying in a compiler-visible manner that the value works. For instance:
+- **`derive(TryInto)`**: Rust could support deriving `TryInto` from the
+  contained type to the structure. The implementation could explicitly check
+  the range, and return an error if not in-range. This would avoid the need to
+  write explicit `unsafe` code, and many uses may be able to elide or coalesce
+  the check if the compiler can prove the range of a value at compile time.
+- **Lints**: Multiple lints may help users define niches, or detect usages of
+  niches that may be better expressed via other means. For instance, a lint
+  could detect a newtype whose constructor maintains a range invariant, and
+  suggest adding a niche.
+- **Range types**: Rust (or libraries built atop Rust) could provide integer
+  types with associated valid ranges, along with operations that
+  expand/contract/propagate those ranges as appropriate.
+- **`unsafe` fields**: If in the future Rust introduces `unsafe` fields,
+  declaring a niche could internally mark the field as unsafe, taking advantage
+  of the same machinery.
+- **Move types, or types that don't support references**: Rust currently
+  requires that all values of a given type have the same representation no
+  matter where they get stored, to allow taking references to such types and
+  passing them to contexts that don't know about any relevant storage quirks
+  such as niches. Given a mechanism for disallowing references to a type and
+  requiring users to copy or move it rather than referencing it in-place, Rust
+  could more aggressively optimize storage layout, such as by renumbering enum
+  values and translating them back when read.