*RFL import: kernel::types::Bool

Commit reference: 3dfc5eb

Author: hoshinolina

rust/kernel/types.rs

@@ -445,3 +445,85 @@ pub enum Either<L, R> {
     /// Constructs an instance of [`Either`] containing a value of type `R`.
     Right(R),
 }
+
+/// A trait for boolean types.
+///
+/// This is meant to be used in type states to allow boolean constraints in implementation blocks.
+/// In the example below, the implementation containing `MyType::set_value` could _not_ be
+/// constrained to type states containing `Writable = true` if `Writable` were a constant instead
+/// of a type.
+///
+/// # Safety
+///
+/// No additional implementations of [`Bool`] should be provided, as [`True`] and [`False`] are
+/// already provided.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::{Bool, False, True};
+/// use core::marker::PhantomData;
+///
+/// // Type state specifies whether the type is writable.
+/// trait MyTypeState {
+///     type Writable: Bool;
+/// }
+///
+/// // In state S1, the type is writable.
+/// struct S1;
+/// impl MyTypeState for S1 {
+///     type Writable = True;
+/// }
+///
+/// // In state S2, the type is not writable.
+/// struct S2;
+/// impl MyTypeState for S2 {
+///     type Writable = False;
+/// }
+///
+/// struct MyType<T: MyTypeState> {
+///     value: u32,
+///     _p: PhantomData<T>,
+/// }
+///
+/// impl<T: MyTypeState> MyType<T> {
+///     fn new(value: u32) -> Self {
+///         Self {
+///             value,
+///             _p: PhantomData,
+///         }
+///     }
+/// }
+///
+/// // This implementation block only applies if the type state is writable.
+/// impl<T> MyType<T>
+/// where
+///     T: MyTypeState<Writable = True>,
+/// {
+///     fn set_value(&mut self, v: u32) {
+///         self.value = v;
+///     }
+/// }
+///
+/// let mut x = MyType::<S1>::new(10);
+/// let mut y = MyType::<S2>::new(20);
+///
+/// x.set_value(30);
+
+///
+/// // The code below fails to compile because `S2` is not writable.
+/// // y.set_value(40);
+/// ```
+pub unsafe trait Bool {}
+
+/// Represents the `true` value for types with [`Bool`] bound.
+pub struct True;
+
+// SAFETY: This is one of the only two implementations of `Bool`.
+unsafe impl Bool for True {}
+
+/// Represents the `false` value for types wth [`Bool`] bound.
+pub struct False;
+
+// SAFETY: This is one of the only two implementations of `Bool`.
+unsafe impl Bool for False {}