Add Contiguous trait

src/contiguous.rs

@@ -0,0 +1,203 @@
+use super::*;
+use core::mem::{size_of, transmute_copy};
+
+/// A trait indicating that:
+///
+/// 1. A type has an equivalent representation to some known integral type.
+/// 2. All instances of this type fall in a fixed range of values.
+/// 3. Within that range, there are no gaps.
+///
+/// This is generally useful for fieldless enums (aka "c-style" enums), however
+/// it's important that it only be used for those with an explicit `#[repr]`, as
+/// `#[repr(Rust)]` fieldess enums have an unspecified layout.
+///
+/// Additionally, you shouldn't assume that all implementations are enums. Any
+/// type which meets the requirements above while following the rules under
+/// "Safety" below is valid.
+///
+/// # Example
+///
+/// ```
+/// # use bytemuck::Contiguous;
+/// #[repr(u8)]
+/// #[derive(Debug, Copy, Clone, PartialEq)]
+/// enum Foo {
+///   A = 0,
+///   B = 1,
+///   C = 2,
+///   D = 3,
+///   E = 4,
+/// }
+/// unsafe impl Contiguous for Foo {
+///   type Int = u8;
+///   const MIN_VALUE: u8 = Foo::A as u8;
+///   const MAX_VALUE: u8 = Foo::E as u8;
+/// }
+/// assert_eq!(Foo::from_integer(3).unwrap(), Foo::D);
+/// assert_eq!(Foo::from_integer(8), None);
+/// assert_eq!(Foo::C.into_integer(), 2);
+/// ```
+/// # Safety
+///
+/// This is an unsafe trait, and incorrectly implementing it is undefined
+/// behavior.
+///
+/// Informally, by implementing it, you're asserting that `C` is identical to
+/// the integral type `C::Int`, and that every `C` falls between `C::MIN_VALUE`
+/// and `C::MAX_VALUE` exactly once, without any gaps.
+///
+/// Precisely, the guarantees you must uphold when implementing `Contiguous` for
+/// some type `C` are:
+///
+/// 1. The size of `C` and `C::Int` must be the same, and neither may be a ZST.
+///    (Note: alignment is explicitly allowed to differ)
+///
+/// 2. `C::Int` must be a primitive integer, and not a wrapper type. In the
+///    future, this may be lifted to include cases where the behavior is
+///    identical for a relevant set of traits (Ord, arithmetic, ...).
+///
+/// 3. All `C::Int`s which are in the *inclusive* range between `C::MIN_VALUE`
+///    and `C::MAX_VALUE` are bitwise identical to unique valid instances of
+///    `C`.
+///
+/// 4. There exist no instances of `C` such that their bitpatterns, when
+///    interpreted as instances of `C::Int`, fall outside of the `MAX_VALUE` /
+///    `MIN_VALUE` range -- It is legal for unsafe code to assume that if it
+///    gets a `C` that implements `Contiguous`, it is in the appropriate range.
+///
+/// 5. Finally, you promise not to provide overridden implementations of
+///    `Contiguous::from_integer` and `Contiguous::into_integer`.
+///
+/// For clarity, the following rules could be derived from the above, but are
+/// listed explicitly:
+///
+/// - `C::MAX_VALUE` must be greater or equal to `C::MIN_VALUE` (therefore, `C`
+///   must be an inhabited type).
+///
+/// - There exist no two values between `MIN_VALUE` and `MAX_VALUE` such that
+///   when interpreted as a `C` they are considered identical (by, say, match).
+pub unsafe trait Contiguous: Copy + 'static {
+  /// The primitive integer type with an identical representation to this
+  /// type.
+  ///
+  /// Contiguous is broadly intended for use with fieldless enums, and for
+  /// these the correct integer type is easy: The enum should have a
+  /// `#[repr(Int)]` or `#[repr(C)]` attribute, (if it does not, it is
+  /// *unsound* to implement `Contiguous`!).
+  ///
+  /// - For `#[repr(Int)]`, use the listed `Int`. e.g. `#[repr(u8)]` should
+  ///   use `type Int = u8`.
+  ///
+  /// - For `#[repr(C)]`, use whichever type the C compiler will use to
+  ///   represent the given enum. This is usually `c_int` (from `std::os::raw`
+  ///   or `libc`), but it's up to you to make the determination as the
+  ///   implementer of the unsafe trait.
+  ///
+  /// For precise rules, see the list under "Safety" above.
+  type Int: Copy + Ord;
+
+  /// The upper *inclusive* bound for valid instances of this type.
+  const MAX_VALUE: Self::Int;
+
+  /// The lower *inclusive* bound for valid instances of this type.
+  const MIN_VALUE: Self::Int;
+
+  /// If `value` is within the range for valid instances of this type,
+  /// returns `Some(converted_value)`, otherwise, returns `None`.
+  ///
+  /// This is a trait method so that you can write `value.into_integer()` in
+  /// your code. It is a contract of this trait that if you implement
+  /// `Contiguous` on your type you **must not** override this method.
+  ///
+  /// # Panics
+  ///
+  /// We will not panic for any correct implementation of `Contiguous`, but
+  /// *may* panic if we detect an incorrect one.
+  ///
+  /// This is undefined behavior regardless, so it could have been the nasal
+  /// demons at that point anyway ;).
+  #[inline]
+  fn from_integer(value: Self::Int) -> Option<Self> {
+    // Guard against an illegal implementation of Contiguous. Annoyingly we
+    // can't rely on `transmute` to do this for us (see below), but
+    // whatever, this gets compiled into nothing in release.
+    assert!(size_of::<Self>() == size_of::<Self::Int>());
+    if Self::MIN_VALUE <= value && value <= Self::MAX_VALUE {
+      // SAFETY: We've checked their bounds (and their size, even though
+      // they've sworn under the Oath Of Unsafe Rust that that already
+      // matched) so this is allowed by `Contiguous`'s unsafe contract.
+      //
+      // So, the `transmute_copy`. ideally we'd use transmute here, which
+      // is more obviously safe. Sadly, we can't, as these types still
+      // have unspecified sizes.
+      Some(unsafe { transmute_copy::<Self::Int, Self>(&value) })
+    } else {
+      None
+    }
+  }
+
+  /// Perform the conversion from `C` into the underlying integral type. This
+  /// mostly exists otherwise generic code would need unsafe for the `value as
+  /// integer`
+  ///
+  /// This is a trait method so that you can write `value.into_integer()` in
+  /// your code. It is a contract of this trait that if you implement
+  /// `Contiguous` on your type you **must not** override this method.
+  ///
+  /// # Panics
+  ///
+  /// We will not panic for any correct implementation of `Contiguous`, but
+  /// *may* panic if we detect an incorrect one.
+  ///
+  /// This is undefined behavior regardless, so it could have been the nasal
+  /// demons at that point anyway ;).
+  #[inline]
+  fn into_integer(self) -> Self::Int {
+    // Guard against an illegal implementation of Contiguous. Annoyingly we
+    // can't rely on `transmute` to do the size check for us (see
+    // `from_integer's comment`), but whatever, this gets compiled into
+    // nothing in release. Note that we don't check the result of cast
+    assert!(size_of::<Self>() == size_of::<Self::Int>());
+
+    // SAFETY: The unsafe contract requires that these have identical
+    // representations, and that the range be entirely valid. Using
+    // transmute_copy instead of transmute here is annoying, but is required
+    // as `Self` and `Self::Int` have unspecified sizes still.
+    unsafe { transmute_copy::<Self, Self::Int>(&self) }
+  }
+}
+
+macro_rules! impl_contiguous {
+  ($($src:ty as $repr:ident in [$min:expr, $max:expr];)*) => {$(
+    unsafe impl Contiguous for $src {
+      type Int = $repr;
+      const MAX_VALUE: $repr = $max;
+      const MIN_VALUE: $repr = $min;
+    }
+  )*};
+}
+
+impl_contiguous! {
+  bool as u8 in [0, 1];
+
+  u8 as u8 in [0, u8::max_value()];
+  u16 as u16 in [0, u16::max_value()];
+  u32 as u32 in [0, u32::max_value()];
+  u64 as u64 in [0, u64::max_value()];
+  u128 as u128 in [0, u128::max_value()];
+  usize as usize in [0, usize::max_value()];
+
+  i8 as i8 in [i8::min_value(), i8::max_value()];
+  i16 as i16 in [i16::min_value(), i16::max_value()];
+  i32 as i32 in [i32::min_value(), i32::max_value()];
+  i64 as i64 in [i64::min_value(), i64::max_value()];
+  i128 as i128 in [i128::min_value(), i128::max_value()];
+  isize as isize in [isize::min_value(), isize::max_value()];
+
+  NonZeroU8 as u8 in [1, u8::max_value()];
+  NonZeroU16 as u16 in [1, u16::max_value()];
+  NonZeroU32 as u32 in [1, u32::max_value()];
+  NonZeroU64 as u64 in [1, u64::max_value()];
+  NonZeroU128 as u128 in [1, u128::max_value()];
+  NonZeroUsize as usize in [1, usize::max_value()];
+}

src/lib.rs

@@ -63,6 +63,9 @@ pub use zeroable::*;
 mod pod;
 pub use pod::*;
 
+mod contiguous;
+pub use contiguous::*;
+
 // Used from macros to ensure we aren't using some locally defined name and
 // actually are referencing libcore. This also would allow pre-2018 edition
 // crates to use our macros, but I'm not sure how important that is.