Trait Contiguous 

pub unsafe trait Contiguous: Copy + 'static {
    type Int: Copy + Ord;

    const MAX_VALUE: Self::Int;
    const MIN_VALUE: Self::Int;

    // Provided methods
    fn from_integer(value: Self::Int) -> Option<Self> { ... }
    fn into_integer(self) -> Self::Int { ... }
}

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

#[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::Ints 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).

Required Associated Constants

const MAX_VALUE: Self::Int

The upper inclusive bound for valid instances of this type.

const MIN_VALUE: Self::Int

The lower inclusive bound for valid instances of this type.

Required Associated Types

type Int: Copy + Ord

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.

Provided Methods

fn from_integer(value: Self::Int) -> Option<Self>

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 ;).

fn into_integer(self) -> Self::Int

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 ;).

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementations on Foreign Types

impl Contiguous for bool

const MAX_VALUE: u8 = 1

const MIN_VALUE: u8 = 0

type Int = u8

impl Contiguous for i8

const MAX_VALUE: i8

const MIN_VALUE: i8

type Int = i8

impl Contiguous for i16

const MAX_VALUE: i16

const MIN_VALUE: i16

type Int = i16

impl Contiguous for i32

const MAX_VALUE: i32

const MIN_VALUE: i32

type Int = i32

impl Contiguous for i64

const MAX_VALUE: i64

const MIN_VALUE: i64

type Int = i64

impl Contiguous for i128

const MAX_VALUE: i128

const MIN_VALUE: i128

type Int = i128

impl Contiguous for isize

const MAX_VALUE: isize

const MIN_VALUE: isize

type Int = isize

impl Contiguous for u8

const MAX_VALUE: u8

const MIN_VALUE: u8 = 0

type Int = u8

impl Contiguous for u16

const MAX_VALUE: u16

const MIN_VALUE: u16 = 0

type Int = u16

impl Contiguous for u32

const MAX_VALUE: u32

const MIN_VALUE: u32 = 0

type Int = u32

impl Contiguous for u64

const MAX_VALUE: u64

const MIN_VALUE: u64 = 0

type Int = u64

impl Contiguous for u128

const MAX_VALUE: u128

const MIN_VALUE: u128 = 0

type Int = u128

impl Contiguous for usize

const MAX_VALUE: usize

const MIN_VALUE: usize = 0

type Int = usize

impl Contiguous for NonZeroU8

const MAX_VALUE: u8

const MIN_VALUE: u8 = 1

type Int = u8

impl Contiguous for NonZeroU16

const MAX_VALUE: u16

const MIN_VALUE: u16 = 1

type Int = u16

impl Contiguous for NonZeroU32

const MAX_VALUE: u32

const MIN_VALUE: u32 = 1

type Int = u32

impl Contiguous for NonZeroU64

const MAX_VALUE: u64

const MIN_VALUE: u64 = 1

type Int = u64

impl Contiguous for NonZeroU128

const MAX_VALUE: u128

const MIN_VALUE: u128 = 1

type Int = u128

impl Contiguous for NonZeroUsize

const MAX_VALUE: usize

const MIN_VALUE: usize = 1

type Int = usize

Implementors