feat: introduce RestrictAccess<To> and generalize restrict to all access types

src/access.rs

@@ -1,61 +1,84 @@
 //! Marker types for limiting access.
 
-/// Sealed trait that is implemented for the types in this module.
-pub trait Access: Copy + Default + private::Sealed {
-    /// Reduced access level to safely share the corresponding value.
-    type RestrictShared: Access;
+/// A trait for restricting one [`Access`] type to another [`Access`] type.
+///
+/// Restricting `Self` to `To` results in [`Self::Restricted`].
+///
+/// Restriction is a symmetric operation which is denoted by ∩, as it is the intersection of permissions.
+/// The following table holds:
+///
+/// | `Self`        | `To`          | `Self` ∩ `To` |
+/// | ------------- | ------------- | ------------- |
+/// | `T`           | `T`           | `T`           |
+/// | [`ReadWrite`] | `T`           | `T`           |
+/// | [`NoAccess`]  | `T`           | [`NoAccess`]  |
+/// | [`ReadOnly`]  | [`WriteOnly`] | [`NoAccess`]  |
+pub trait RestrictAccess<To>: Access {
+    /// The resulting [`Access`] type of `Self` restricted to `To`.
+    type Restricted: Access;
 }
 
-/// Helper trait that is implemented by [`ReadWrite`] and [`ReadOnly`].
-pub trait Readable: Copy + Default + private::Sealed {
-    /// Reduced access level to safely share the corresponding value.
-    type RestrictShared: Readable + Access;
+impl<To: Access> RestrictAccess<To> for ReadWrite {
+    type Restricted = To;
 }
 
+impl<To> RestrictAccess<To> for NoAccess {
+    type Restricted = Self;
+}
+
+// Sadly, we cannot provide more generic implementations, since they would overlap.
+macro_rules! restrict_impl {
+    ($SelfT:ty, $To:ty, $Restricted:ty) => {
+        impl RestrictAccess<$To> for $SelfT {
+            type Restricted = $Restricted;
+        }
+    };
+}
+
+restrict_impl!(ReadOnly, ReadWrite, ReadOnly);
+restrict_impl!(ReadOnly, ReadOnly, ReadOnly);
+restrict_impl!(ReadOnly, WriteOnly, NoAccess);
+restrict_impl!(ReadOnly, NoAccess, NoAccess);
+
+restrict_impl!(WriteOnly, ReadWrite, WriteOnly);
+restrict_impl!(WriteOnly, ReadOnly, NoAccess);
+restrict_impl!(WriteOnly, WriteOnly, WriteOnly);
+restrict_impl!(WriteOnly, NoAccess, NoAccess);
+
+/// Sealed trait that is implemented for the types in this module.
+pub trait Access: Copy + Default + private::Sealed {}
+
+/// Helper trait that is implemented by [`ReadWrite`] and [`ReadOnly`].
+pub trait Readable: Access {}
+impl<A: RestrictAccess<ReadOnly, Restricted = ReadOnly>> Readable for A {}
+
 /// Helper trait that is implemented by [`ReadWrite`] and [`WriteOnly`].
-pub trait Writable: Access + private::Sealed {}
+pub trait Writable: Access {}
+impl<A: RestrictAccess<WriteOnly, Restricted = WriteOnly>> Writable for A {}
 
 /// Implemented for access types that permit copying of `VolatileRef`.
-pub trait Copyable: private::Sealed {}
-
-impl<T> Access for T
-where
-    T: Readable + Default + Copy,
-{
-    type RestrictShared = <T as Readable>::RestrictShared;
-}
+pub trait Copyable: Access {}
+impl<A: RestrictAccess<ReadOnly, Restricted = Self>> Copyable for A {}
 
 /// Zero-sized marker type for allowing both read and write access.
 #[derive(Debug, Default, Copy, Clone)]
 pub struct ReadWrite;
-impl Readable for ReadWrite {
-    type RestrictShared = ReadOnly;
-}
-impl Writable for ReadWrite {}
+impl Access for ReadWrite {}
 
 /// Zero-sized marker type for allowing only read access.
 #[derive(Debug, Default, Copy, Clone)]
 pub struct ReadOnly;
-impl Readable for ReadOnly {
-    type RestrictShared = ReadOnly;
-}
-impl Copyable for ReadOnly {}
+impl Access for ReadOnly {}
 
 /// Zero-sized marker type for allowing only write access.
 #[derive(Debug, Default, Copy, Clone)]
 pub struct WriteOnly;
-impl Access for WriteOnly {
-    type RestrictShared = NoAccess;
-}
-impl Writable for WriteOnly {}
+impl Access for WriteOnly {}
 
 /// Zero-sized marker type that grants no access.
 #[derive(Debug, Default, Copy, Clone)]
 pub struct NoAccess;
-impl Access for NoAccess {
-    type RestrictShared = NoAccess;
-}
-impl Copyable for NoAccess {}
+impl Access for NoAccess {}
 
 mod private {
     pub trait Sealed {}

src/volatile_ptr/operations.rs

@@ -4,7 +4,7 @@ use core::{
 };
 
 use crate::{
-    access::{Access, ReadOnly, ReadWrite, Readable, Writable, WriteOnly},
+    access::{Access, ReadOnly, ReadWrite, Readable, RestrictAccess, Writable, WriteOnly},
     VolatilePtr,
 };
 
@@ -211,7 +211,7 @@ where
 }
 
 /// Methods for restricting access.
-impl<'a, T> VolatilePtr<'a, T, ReadWrite>
+impl<'a, T, A> VolatilePtr<'a, T, A>
 where
     T: ?Sized,
 {
@@ -220,7 +220,7 @@ where
     /// ## Example
     ///
     /// ```
-    /// use volatile::access::ReadOnly;
+    /// use volatile::access::{ReadOnly, WriteOnly};
     /// use volatile::VolatilePtr;
     ///
     /// let mut value: i16 = -4;
@@ -229,14 +229,24 @@ where
     /// let read_only = volatile.restrict::<ReadOnly>();
     /// assert_eq!(read_only.read(), -4);
     /// // read_only.write(10); // compile-time error
+    ///
+    /// let no_access = read_only.restrict::<WriteOnly>();
+    /// // no_access.read(); // compile-time error
+    /// // no_access.write(10); // compile-time error
     /// ```
-    pub fn restrict<A>(self) -> VolatilePtr<'a, T, A>
+    pub fn restrict<To>(self) -> VolatilePtr<'a, T, A::Restricted>
     where
-        A: Access,
+        A: RestrictAccess<To>,
     {
         unsafe { VolatilePtr::new_restricted(Default::default(), self.pointer) }
     }
+}
 
+/// Methods for restricting access.
+impl<'a, T> VolatilePtr<'a, T, ReadWrite>
+where
+    T: ?Sized,
+{
     /// Restricts access permissions to read-only.
     ///
     /// ## Example

src/volatile_ref.rs

@@ -1,5 +1,5 @@
 use crate::{
-    access::{Access, Copyable, ReadOnly, ReadWrite, WriteOnly},
+    access::{Access, Copyable, ReadOnly, ReadWrite, RestrictAccess, WriteOnly},
     volatile_ptr::VolatilePtr,
 };
 use core::{cmp::Ordering, fmt, hash, marker::PhantomData, ptr::NonNull};
@@ -139,9 +139,9 @@ where
     /// This method creates a `VolatileRef` tied to the lifetime of the `&VolatileRef` it is created from.
     /// This is useful for providing a volatile reference without moving the original `VolatileRef`.
     /// In comparison with creating a `&VolatileRef<'a, T>`, this avoids the additional indirection and lifetime.
-    pub fn borrow(&self) -> VolatileRef<'_, T, A::RestrictShared>
+    pub fn borrow(&self) -> VolatileRef<'_, T, A::Restricted>
     where
-        A: Access,
+        A: RestrictAccess<ReadOnly>,
     {
         unsafe { VolatileRef::new_restricted(Default::default(), self.pointer) }
     }
@@ -161,9 +161,9 @@ where
     /// Borrows this `VolatileRef` as a read-only [`VolatilePtr`].
     ///
     /// Use this method to do (partial) volatile reads of the referenced data.
-    pub fn as_ptr(&self) -> VolatilePtr<'_, T, A::RestrictShared>
+    pub fn as_ptr(&self) -> VolatilePtr<'_, T, A::Restricted>
     where
-        A: Access,
+        A: RestrictAccess<ReadOnly>,
     {
         unsafe { VolatilePtr::new_restricted(Default::default(), self.pointer) }
     }
@@ -194,7 +194,7 @@ where
 }
 
 /// Methods for restricting access.
-impl<'a, T> VolatileRef<'a, T, ReadWrite>
+impl<'a, T, A> VolatileRef<'a, T, A>
 where
     T: ?Sized,
 {
@@ -203,7 +203,7 @@ where
     /// ## Example
     ///
     /// ```
-    /// use volatile::access::ReadOnly;
+    /// use volatile::access::{ReadOnly, WriteOnly};
     /// use volatile::VolatileRef;
     ///
     /// let mut value: i16 = -4;
@@ -212,14 +212,24 @@ where
     /// let read_only = volatile.restrict::<ReadOnly>();
     /// assert_eq!(read_only.as_ptr().read(), -4);
     /// // read_only.as_ptr().write(10); // compile-time error
+    ///
+    /// let no_access = read_only.restrict::<WriteOnly>();
+    /// // no_access.read(); // compile-time error
+    /// // no_access.write(10); // compile-time error
     /// ```
-    pub fn restrict<A>(self) -> VolatileRef<'a, T, A>
+    pub fn restrict<To>(self) -> VolatileRef<'a, T, A::Restricted>
     where
-        A: Access,
+        A: RestrictAccess<To>,
     {
         unsafe { VolatileRef::new_restricted(Default::default(), self.pointer) }
     }
+}
 
+/// Methods for restricting access.
+impl<'a, T> VolatileRef<'a, T, ReadWrite>
+where
+    T: ?Sized,
+{
     /// Restricts access permissions to read-only.
     ///
     /// ## Example