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#[allow(unused_imports)]
use crate::TypeEq;
use core::marker::PhantomData;
/// Gets a [type witness](crate#what-are-type-witnesses) for `Self`.
///
#[doc = explain_type_witness!()]
///
/// This trait is a helper to write [`W: MakeTypeWitness<Arg = T>`](MakeTypeWitness)
/// with the `T` and `W` type parameters flipped,
/// most useful in supertrait bounds.
///
/// This trait can't be implemented outside of `typewit`.
///
/// # Example
///
/// This example shows how one can make a `const fn` that converts both
/// `&str` and `&[u8]` to `&str`
///
/// (this example requires Rust 1.64.0)
///
#[cfg_attr(not(feature = "rust_stable"), doc = "```ignore")]
#[cfg_attr(feature = "rust_stable", doc = "```rust")]
/// use typewit::{HasTypeWitness, TypeWitnessTypeArg, MakeTypeWitness, TypeEq};
///
/// fn main() {
/// assert_eq!(str_try_from("hello"), Ok("hello"));
///
/// assert_eq!(str_try_from(&[b'w', b'o', b'r', b'l', b'd']), Ok("world"));
///
/// assert_eq!(str_try_from(b"foo bar" as &[_]), Ok("foo bar"));
/// }
///
/// pub const fn str_try_from<'a, T, const L: usize>(
/// input: T
/// ) -> Result<&'a str, std::str::Utf8Error>
/// where
/// T: StrTryFrom<'a, L>
/// {
/// // `T::WITNESS` expands to
/// // `<T as HasTypeWitness<StrTryFromWitness<'a, L, T>>::WITNESS`
/// match T::WITNESS {
/// StrTryFromWitness::Str(te) => {
/// // `te` (a `TypeEq<T, &'a str>`) allows coercing between `T` and `&'a str`.
/// let string: &str = te.to_right(input);
/// Ok(string)
/// }
/// StrTryFromWitness::Bytes(te) => {
/// let bytes: &[u8] = te.to_right(input);
/// std::str::from_utf8(bytes)
/// }
/// StrTryFromWitness::Array(te) => {
/// let slice: &[u8] = te.to_right(input);
/// str_try_from(slice)
/// }
/// }
/// }
///
///
/// // trait alias pattern
/// pub trait StrTryFrom<'a, const L: usize>:
/// Copy + HasTypeWitness<StrTryFromWitness<'a, L, Self>>
/// {}
///
/// impl<'a, T, const L: usize> StrTryFrom<'a, L> for T
/// where
/// T: Copy + HasTypeWitness<StrTryFromWitness<'a, L, T>>
/// {}
///
/// // This macro declares a type witness enum
/// typewit::simple_type_witness! {
/// // Declares `enum StrTryFromWitness<'a, const L: usize, __Wit>`
/// // (the `__Wit` type parameter is implicitly added after all generics)
/// // `#[non_exhausitve]` allows adding more supported types.
/// #[non_exhaustive]
/// pub enum StrTryFromWitness<'a, const L: usize> {
/// // This variant requires `__Wit == &'a str`
/// //
/// // The `<'a, 0>` here changes this macro from generating
/// // `impl<'a, const L: usize> MakeTypeWitness for StrTryFromWitness<'a, L, &'a [u8]>`
/// // to
/// // `impl<'a> MakeTypeWitness for StrTryFromWitness<'a, 0, &'a [u8]>`
/// // which allows the compiler to infer generic arguments when
/// // using the latter `MakeTypeWitness` impl`
/// Str<'a, 0> = &'a str,
///
/// // This variant requires `__Wit == &'a [u8]`
/// Bytes<'a, 0> = &'a [u8],
///
/// // This variant requires `__Wit == &'a [u8; L]`
/// Array = &'a [u8; L],
/// }
/// }
/// ```
pub trait HasTypeWitness<W: TypeWitnessTypeArg<Arg = Self>> {
/// A constant of the type witness
const WITNESS: W;
// prevents dependencies from implementing this trait
#[doc(hidden)]
const __PRIV_KO9Y329U2U: priv_::__Priv<Self, W>;
}
impl<T, W> HasTypeWitness<W> for T
where
T: ?Sized,
W: MakeTypeWitness<Arg = T>,
{
const WITNESS: W = W::MAKE;
#[doc(hidden)]
const __PRIV_KO9Y329U2U: priv_::__Priv<Self, W> = priv_::__Priv(PhantomData, PhantomData);
}
mod priv_ {
use core::marker::PhantomData;
#[doc(hidden)]
pub struct __Priv<T: ?Sized, W>(
pub(super) PhantomData<fn() -> PhantomData<W>>,
pub(super) PhantomData<fn() -> PhantomData<T>>,
);
}
////////////////////////////////////////////////
/// Gets the type argument that this [type witness](crate#what-are-type-witnesses) witnesses.
///
/// [**example shared with `MakeTypeWitness`**](MakeTypeWitness#example)
///
#[doc = explain_type_witness!()]
///
/// This trait should be implemented generically,
/// as generic as the type definition of the implementor,
/// doing so will help type inference.
///
pub trait TypeWitnessTypeArg {
/// The type parameter used for type witnesses.
///
/// Usually, enums that implement this trait have
/// variants with [`TypeEq`]`<`[`Self::Arg`]`, SomeType>` fields.
type Arg: ?Sized;
}
/// Constructs this [type witness](crate#what-are-type-witnesses).
///
#[doc = explain_type_witness!()]
///
/// This trait can be automatically implemented for simple type witnesses
/// by declaring the type witness with the [`simple_type_witness`] macro.
///
/// # Example
///
/// (this example requires Rust 1.61.0)
#[cfg_attr(not(feature = "rust_1_61"), doc = "```ignore")]
#[cfg_attr(feature = "rust_1_61", doc = "```rust")]
/// use typewit::{TypeWitnessTypeArg, MakeTypeWitness, TypeEq};
///
/// const fn default<'a, T, const L: usize>() -> T
/// where
/// Defaultable<'a, L, T>: MakeTypeWitness
/// {
/// match MakeTypeWitness::MAKE {
/// // `te` is a `TypeEq<T, i32>`, which allows coercing between `T` and `i32`.
/// // `te.to_left(...)` goes from `i32` to `T`.
/// Defaultable::I32(te) => te.to_left(3),
///
/// // `te` is a `TypeEq<T, bool>`
/// Defaultable::Bool(te) => te.to_left(true),
///
/// // `te` is a `TypeEq<T, &'a str>`
/// Defaultable::Str(te) => te.to_left("empty"),
///
/// // `te` is a `TypeEq<T, [u32; L]>`
/// Defaultable::Array(te) => te.to_left([5; L]),
/// }
/// }
///
/// let number: i32 = default();
/// assert_eq!(number, 3);
///
/// let boolean: bool = default();
/// assert_eq!(boolean, true);
///
/// let string: &str = default();
/// assert_eq!(string, "empty");
///
/// let array: [u32; 3] = default();
/// assert_eq!(array, [5, 5, 5]);
///
///
/// // This enum is a type witness (documented in the root module)
/// #[non_exhaustive]
/// enum Defaultable<'a, const L: usize, T> {
/// // This variant requires `T == i32`
/// I32(TypeEq<T, i32>),
///
/// // This variant requires `T == bool`
/// Bool(TypeEq<T, bool>),
///
/// // This variant requires `T == &'a str`
/// Str(TypeEq<T, &'a str>),
///
/// // This variant requires `T == [u32; L]`
/// Array(TypeEq<T, [u32; L]>),
/// }
///
/// impl<T, const L: usize> TypeWitnessTypeArg for Defaultable<'_, L, T> {
/// // this aids type inference for what type parameter is witnessed
/// type Arg = T;
/// }
///
/// // Specifying dummy values for the generics that the `I32` variant doesn't use,
/// // so that they don't have to be specified when this impl is used.
/// impl MakeTypeWitness for Defaultable<'_, 0, i32> {
/// // The `TypeEq<T, i32>` field can be constructed because `T == i32` here.
/// const MAKE: Self = Self::I32(TypeEq::NEW);
/// }
///
/// impl MakeTypeWitness for Defaultable<'_, 0, bool> {
/// const MAKE: Self = Self::Bool(TypeEq::NEW);
/// }
///
/// impl<'a> MakeTypeWitness for Defaultable<'a, 0, &'a str> {
/// const MAKE: Self = Self::Str(TypeEq::NEW);
/// }
///
/// impl<const L: usize> MakeTypeWitness for Defaultable<'_, L, [u32; L]> {
/// const MAKE: Self = Self::Array(TypeEq::NEW);
/// }
///
/// ```
///
/// The `Defaultable` type definition and its impls can also be written using
/// the [`simple_type_witness`] macro:
#[cfg_attr(not(feature = "rust_1_61"), doc = "```ignore")]
#[cfg_attr(feature = "rust_1_61", doc = "```rust")]
/// typewit::simple_type_witness!{
/// // Declares `enum Defaultable<'a, const L: usize, __Wit>`
/// // The `__Wit` type parameter is implicit and always the last generic parameter.
/// #[non_exhaustive]
/// enum Defaultable<'a, const L: usize> {
/// // `<'a, 0>` is necessary to have
/// // `impl MakeTypeWitness for Defaultable<'_, 0, i32>` instead of
/// // `impl<'a, const L: u32> MakeTypeWitness for Defaultable<'a, L, i32>`,
/// // which allows the generic arguments to be inferred.
/// I32<'a, 0> = i32,
///
/// Bool<'a, 0> = bool,
///
/// Str<'a, 0> = &'a str,
///
/// Array = [u32; L],
/// }
/// }
/// ```
/// note that [`simple_type_witness`] can't replace enums whose
/// witnessed type parameter is not the last,
/// or have variants with anything but one [`TypeEq`] field each.
///
///
/// [`simple_type_witness`]: crate::simple_type_witness
pub trait MakeTypeWitness: TypeWitnessTypeArg {
/// A constant with the type witness
const MAKE: Self;
}