[pointer][transmute] Support generic TransmuteFrom

This commit replaces the `TransparentWrapper` trait with a more generic,
bidirectional `TransmuteFrom<T>` trait. `U: TransmuteFrom<T>` indicates
that `T` and `U` have the same sizes and that `T` can be transmuted into
`U` given certain alignment and validity invariant mappings.

Makes progress on #1122

gherrit-pr-id: I7af68cdaa5ea47ab245a942d1cbd8bc5e6407223

Author: joshlf

src/impls.rs

@@ -443,13 +443,13 @@ mod atomics {
     use super::*;
 
     macro_rules! impl_traits_for_atomics {
-        ($($atomics:ident),* $(,)?) => {
+        ($($atomics:ident[$repr:ty]),* $(,)?) => {
             $(
                 impl_known_layout!($atomics);
-                impl_for_transparent_wrapper!(=> TryFromBytes for $atomics);
-                impl_for_transparent_wrapper!(=> FromZeros for $atomics);
-                impl_for_transparent_wrapper!(=> FromBytes for $atomics);
-                impl_for_transparent_wrapper!(=> IntoBytes for $atomics);
+                impl_for_transmute_from!(=> TryFromBytes for $atomics[UnsafeCell<$repr>]);
+                impl_for_transmute_from!(=> FromZeros for $atomics[UnsafeCell<$repr>]);
+                impl_for_transmute_from!(=> FromBytes for $atomics[UnsafeCell<$repr>]);
+                impl_for_transmute_from!(=> IntoBytes for $atomics[UnsafeCell<$repr>]);
             )*
         };
     }
@@ -461,13 +461,13 @@ mod atomics {
 
         use super::*;
 
-        impl_traits_for_atomics!(AtomicU8, AtomicI8);
+        impl_traits_for_atomics!(AtomicU8[u8], AtomicI8[i8]);
 
         impl_known_layout!(AtomicBool);
 
-        impl_for_transparent_wrapper!(=> TryFromBytes for AtomicBool);
-        impl_for_transparent_wrapper!(=> FromZeros for AtomicBool);
-        impl_for_transparent_wrapper!(=> IntoBytes for AtomicBool);
+        impl_for_transmute_from!(=> TryFromBytes for AtomicBool[UnsafeCell<bool>]);
+        impl_for_transmute_from!(=> FromZeros for AtomicBool[UnsafeCell<bool>]);
+        impl_for_transmute_from!(=> IntoBytes for AtomicBool[UnsafeCell<bool>]);
 
         safety_comment! {
             /// SAFETY:
@@ -497,7 +497,7 @@ mod atomics {
             /// SAFETY:
             /// All of these pass an atomic type and that type's native equivalent, as
             /// required by the macro safety preconditions.
-            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU8 [u8], AtomicI8 [i8], AtomicBool [bool]);
+            unsafe_impl_transmute_from_for_atomic!(AtomicU8 [u8], AtomicI8 [i8], AtomicBool [bool]);
         }
     }
 
@@ -508,13 +508,13 @@ mod atomics {
 
         use super::*;
 
-        impl_traits_for_atomics!(AtomicU16, AtomicI16);
+        impl_traits_for_atomics!(AtomicU16[u16], AtomicI16[i16]);
 
         safety_comment! {
             /// SAFETY:
             /// All of these pass an atomic type and that type's native equivalent, as
             /// required by the macro safety preconditions.
-            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU16 [u16], AtomicI16 [i16]);
+            unsafe_impl_transmute_from_for_atomic!(AtomicU16 [u16], AtomicI16 [i16]);
         }
     }
 
@@ -525,13 +525,13 @@ mod atomics {
 
         use super::*;
 
-        impl_traits_for_atomics!(AtomicU32, AtomicI32);
+        impl_traits_for_atomics!(AtomicU32[u32], AtomicI32[i32]);
 
         safety_comment! {
             /// SAFETY:
             /// All of these pass an atomic type and that type's native equivalent, as
             /// required by the macro safety preconditions.
-            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU32 [u32], AtomicI32 [i32]);
+            unsafe_impl_transmute_from_for_atomic!(AtomicU32 [u32], AtomicI32 [i32]);
         }
     }
 
@@ -542,13 +542,13 @@ mod atomics {
 
         use super::*;
 
-        impl_traits_for_atomics!(AtomicU64, AtomicI64);
+        impl_traits_for_atomics!(AtomicU64[u64], AtomicI64[i64]);
 
         safety_comment! {
             /// SAFETY:
             /// All of these pass an atomic type and that type's native equivalent, as
             /// required by the macro safety preconditions.
-            unsafe_impl_transparent_wrapper_for_atomic!(AtomicU64 [u64], AtomicI64 [i64]);
+            unsafe_impl_transmute_from_for_atomic!(AtomicU64 [u64], AtomicI64 [i64]);
         }
     }
 
@@ -559,21 +559,21 @@ mod atomics {
 
         use super::*;
 
-        impl_traits_for_atomics!(AtomicUsize, AtomicIsize);
+        impl_traits_for_atomics!(AtomicUsize[usize], AtomicIsize[isize]);
 
         impl_known_layout!(T => AtomicPtr<T>);
 
         // TODO(#170): Implement `FromBytes` and `IntoBytes` once we implement
         // those traits for `*mut T`.
-        impl_for_transparent_wrapper!(T => TryFromBytes for AtomicPtr<T>);
-        impl_for_transparent_wrapper!(T => FromZeros for AtomicPtr<T>);
+        impl_for_transmute_from!(T => TryFromBytes for AtomicPtr<T>[UnsafeCell<*mut T>]);
+        impl_for_transmute_from!(T => FromZeros for AtomicPtr<T>[UnsafeCell<*mut T>]);
 
         safety_comment! {
             /// SAFETY:
             /// This passes an atomic type and that type's native equivalent, as
             /// required by the macro safety preconditions.
-            unsafe_impl_transparent_wrapper_for_atomic!(AtomicUsize [usize], AtomicIsize [isize]);
-            unsafe_impl_transparent_wrapper_for_atomic!(T => AtomicPtr<T> [*mut T]);
+            unsafe_impl_transmute_from_for_atomic!(AtomicUsize [usize], AtomicIsize [isize]);
+            unsafe_impl_transmute_from_for_atomic!(T => AtomicPtr<T> [*mut T]);
         }
     }
 }
@@ -603,12 +603,12 @@ safety_comment! {
     assert_unaligned!(PhantomData<()>, PhantomData<u8>, PhantomData<u64>);
 }
 
-impl_for_transparent_wrapper!(T: Immutable => Immutable for Wrapping<T>);
-impl_for_transparent_wrapper!(T: TryFromBytes => TryFromBytes for Wrapping<T>);
-impl_for_transparent_wrapper!(T: FromZeros => FromZeros for Wrapping<T>);
-impl_for_transparent_wrapper!(T: FromBytes => FromBytes for Wrapping<T>);
-impl_for_transparent_wrapper!(T: IntoBytes => IntoBytes for Wrapping<T>);
-impl_for_transparent_wrapper!(T: Unaligned => Unaligned for Wrapping<T>);
+impl_for_transmute_from!(T: Immutable => Immutable for Wrapping<T>[T]);
+impl_for_transmute_from!(T: TryFromBytes => TryFromBytes for Wrapping<T>[T]);
+impl_for_transmute_from!(T: FromZeros => FromZeros for Wrapping<T>[T]);
+impl_for_transmute_from!(T: FromBytes => FromBytes for Wrapping<T>[T]);
+impl_for_transmute_from!(T: IntoBytes => IntoBytes for Wrapping<T>[T]);
+impl_for_transmute_from!(T: Unaligned => Unaligned for Wrapping<T>[T]);
 assert_unaligned!(Wrapping<()>, Wrapping<u8>);
 
 safety_comment! {
@@ -620,22 +620,22 @@ safety_comment! {
     unsafe_impl!(T => FromBytes for MaybeUninit<T>);
 }
 
-impl_for_transparent_wrapper!(T: Immutable => Immutable for MaybeUninit<T>);
-impl_for_transparent_wrapper!(T: Unaligned => Unaligned for MaybeUninit<T>);
+impl_for_transmute_from!(T: Immutable => Immutable for MaybeUninit<T>[T]);
+impl_for_transmute_from!(T: Unaligned => Unaligned for MaybeUninit<T>[T]);
 assert_unaligned!(MaybeUninit<()>, MaybeUninit<u8>);
 
-impl_for_transparent_wrapper!(T: ?Sized + Immutable => Immutable for ManuallyDrop<T>);
-impl_for_transparent_wrapper!(T: ?Sized + TryFromBytes => TryFromBytes for ManuallyDrop<T>);
-impl_for_transparent_wrapper!(T: ?Sized + FromZeros => FromZeros for ManuallyDrop<T>);
-impl_for_transparent_wrapper!(T: ?Sized + FromBytes => FromBytes for ManuallyDrop<T>);
-impl_for_transparent_wrapper!(T: ?Sized + IntoBytes => IntoBytes for ManuallyDrop<T>);
-impl_for_transparent_wrapper!(T: ?Sized + Unaligned => Unaligned for ManuallyDrop<T>);
+impl_for_transmute_from!(T: ?Sized + Immutable => Immutable for ManuallyDrop<T>[T]);
+impl_for_transmute_from!(T: ?Sized + TryFromBytes => TryFromBytes for ManuallyDrop<T>[T]);
+impl_for_transmute_from!(T: ?Sized + FromZeros => FromZeros for ManuallyDrop<T>[T]);
+impl_for_transmute_from!(T: ?Sized + FromBytes => FromBytes for ManuallyDrop<T>[T]);
+impl_for_transmute_from!(T: ?Sized + IntoBytes => IntoBytes for ManuallyDrop<T>[T]);
+impl_for_transmute_from!(T: ?Sized + Unaligned => Unaligned for ManuallyDrop<T>[T]);
 assert_unaligned!(ManuallyDrop<()>, ManuallyDrop<u8>);
 
-impl_for_transparent_wrapper!(T: ?Sized + FromZeros => FromZeros for UnsafeCell<T>);
-impl_for_transparent_wrapper!(T: ?Sized + FromBytes => FromBytes for UnsafeCell<T>);
-impl_for_transparent_wrapper!(T: ?Sized + IntoBytes => IntoBytes for UnsafeCell<T>);
-impl_for_transparent_wrapper!(T: ?Sized + Unaligned => Unaligned for UnsafeCell<T>);
+impl_for_transmute_from!(T: ?Sized + FromZeros => FromZeros for UnsafeCell<T>[T]);
+impl_for_transmute_from!(T: ?Sized + FromBytes => FromBytes for UnsafeCell<T>[T]);
+impl_for_transmute_from!(T: ?Sized + IntoBytes => IntoBytes for UnsafeCell<T>[T]);
+impl_for_transmute_from!(T: ?Sized + Unaligned => Unaligned for UnsafeCell<T>[T]);
 assert_unaligned!(UnsafeCell<()>, UnsafeCell<u8>);
 
 // SAFETY: See safety comment in `is_bit_valid` impl.

src/lib.rs

@@ -2720,7 +2720,7 @@ unsafe fn try_read_from<S, T: TryFromBytes>(
     // We use `from_mut` despite not mutating via `c_ptr` so that we don't need
     // to add a `T: Immutable` bound.
     let c_ptr = Ptr::from_mut(&mut candidate);
-    let c_ptr = c_ptr.transparent_wrapper_into_inner();
+    let c_ptr = c_ptr.transmute::<_, pointer::transmute::BecauseBidirectional>();
     // SAFETY: `c_ptr` has no uninitialized sub-ranges because it derived from
     // `candidate`, which the caller promises is entirely initialized.
     let c_ptr = unsafe { c_ptr.assume_validity::<invariant::Initialized>() };

src/pointer/invariant.rs

@@ -57,7 +57,7 @@ impl<A: Aliasing, AA: Alignment, V: Validity> Invariants for (A, AA, V) {
 /// The aliasing invariant of a [`Ptr`][super::Ptr].
 ///
 /// All aliasing invariants must permit reading from the bytes of a pointer's
-/// referent which are not covered by [`UnsafeCell`]s.
+/// referent which are not covered by [`UnsafeCell`](core::cell::UnsafeCell)s.
 pub trait Aliasing: Sealed {
     /// Is `Self` [`Exclusive`]?
     #[doc(hidden)]
@@ -70,18 +70,56 @@ pub trait Aliasing: Sealed {
     type Variance<'a, T: 'a + ?Sized>;
 }
 
-/// The alignment invariant of a [`Ptr`][super::Ptr].
-pub trait Alignment: Sealed {
-    #[doc(hidden)]
+// NOTE: The `AlignmentInner`/`Alignment` distinction is required so that we can
+// add a `MappedTo<Preserved> = Self` bound. For an explanation of the design
+// space (and prior attempts), see:
+// https://users.rust-lang.org/t/how-to-implement-a-type-level-map-with-an-identity-function/119745
+#[doc(hidden)]
+pub trait AlignmentInner: Sealed {
     type MappedTo<M: AlignmentMapping>: Alignment;
 }
 
-/// The validity invariant of a [`Ptr`][super::Ptr].
-pub trait Validity: Sealed {
-    #[doc(hidden)]
+/// The alignment invariant of a [`Ptr`][super::Ptr].
+pub trait Alignment:
+    AlignmentInner<MappedTo<Preserved> = Self>
+    + AlignmentInner<MappedTo<Unknown> = Unknown>
+    + AlignmentInner<MappedTo<Aligned> = Aligned>
+    + Sealed
+{
+}
+impl<
+        A: AlignmentInner<MappedTo<Preserved> = Self>
+            + AlignmentInner<MappedTo<Unknown> = Unknown>
+            + AlignmentInner<MappedTo<Aligned> = Aligned>,
+    > Alignment for A
+{
+}
+
+#[doc(hidden)]
+pub trait ValidityInner: Sealed {
     type MappedTo<M: ValidityMapping>: Validity;
 }
 
+/// The validity invariant of a [`Ptr`][super::Ptr].
+pub trait Validity:
+    ValidityInner<MappedTo<Preserved> = Self>
+    + ValidityInner<MappedTo<Unknown> = Unknown>
+    + ValidityInner<MappedTo<AsInitialized> = AsInitialized>
+    + ValidityInner<MappedTo<Initialized> = Initialized>
+    + ValidityInner<MappedTo<Valid> = Valid>
+    + Sealed
+{
+}
+impl<
+        V: ValidityInner<MappedTo<Preserved> = Self>
+            + ValidityInner<MappedTo<Unknown> = Unknown>
+            + ValidityInner<MappedTo<AsInitialized> = AsInitialized>
+            + ValidityInner<MappedTo<Initialized> = Initialized>
+            + ValidityInner<MappedTo<Valid> = Valid>,
+    > Validity for V
+{
+}
+
 /// An [`Aliasing`] invariant which is either [`Shared`] or [`Exclusive`].
 ///
 /// # Safety
@@ -93,10 +131,10 @@ pub trait Reference: Aliasing + Sealed {}
 /// It is unknown whether any invariant holds.
 pub enum Unknown {}
 
-impl Alignment for Unknown {
+impl AlignmentInner for Unknown {
     type MappedTo<M: AlignmentMapping> = M::FromUnknown;
 }
-impl Validity for Unknown {
+impl ValidityInner for Unknown {
     type MappedTo<M: ValidityMapping> = M::FromUnknown;
 }
 
@@ -127,10 +165,10 @@ impl Aliasing for Exclusive {
 }
 impl Reference for Exclusive {}
 
-/// The referent is aligned: for `Ptr<T>`, the referent's address is a
-/// multiple of the `T`'s alignment.
+/// The referent is aligned: for `Ptr<T>`, the referent's address is a multiple
+/// of the `T`'s alignment.
 pub enum Aligned {}
-impl Alignment for Aligned {
+impl AlignmentInner for Aligned {
     type MappedTo<M: AlignmentMapping> = M::FromAligned;
 }
 
@@ -161,20 +199,20 @@ impl Alignment for Aligned {
 ///   enum type, in which case the same rules apply depending on the state of
 ///   its discriminant, and so on recursively).
 pub enum AsInitialized {}
-impl Validity for AsInitialized {
+impl ValidityInner for AsInitialized {
     type MappedTo<M: ValidityMapping> = M::FromAsInitialized;
 }
 
-/// The byte ranges in the referent are fully initialized. In other words, if
-/// the referent is `N` bytes long, then it contains a bit-valid `[u8; N]`.
+/// The byte ranges in the referent are fully initialized. In other words,
+/// if the referent is `N` bytes long, then it contains a bit-valid `[u8; N]`.
 pub enum Initialized {}
-impl Validity for Initialized {
+impl ValidityInner for Initialized {
     type MappedTo<M: ValidityMapping> = M::FromInitialized;
 }
 
 /// The referent is bit-valid for `T`.
 pub enum Valid {}
-impl Validity for Valid {
+impl ValidityInner for Valid {
     type MappedTo<M: ValidityMapping> = M::FromValid;
 }
 
@@ -272,6 +310,12 @@ mod mapping {
         type FromValid: Validity;
     }
 
+    /// A mapping which preserves all invariants as-is.
+    ///
+    /// `Preserved` is a valid type for any mapping trait ([`AlignmentMapping`]
+    /// and [`ValidityMapping`]).
+    pub enum Preserved {}
+
     /// The application of the [`AlignmentMapping`] `M` to the [`Alignment`] `A`.
     #[allow(type_alias_bounds)]
     pub type MappedAlignment<A: Alignment, M: AlignmentMapping> = A::MappedTo<M>;
@@ -281,12 +325,27 @@ mod mapping {
     pub type MappedValidity<V: Validity, M: ValidityMapping> = V::MappedTo<M>;
 
     impl<FromUnknown: Alignment, FromAligned: Alignment> AlignmentMapping
-        for ((Unknown, FromUnknown), (Shared, FromAligned))
+        for ((Unknown, FromUnknown), (Aligned, FromAligned))
     {
         type FromUnknown = FromUnknown;
         type FromAligned = FromAligned;
     }
 
+    impl AlignmentMapping for Unknown {
+        type FromUnknown = Unknown;
+        type FromAligned = Unknown;
+    }
+
+    impl AlignmentMapping for Preserved {
+        type FromUnknown = Unknown;
+        type FromAligned = Aligned;
+    }
+
+    impl AlignmentMapping for Aligned {
+        type FromUnknown = Aligned;
+        type FromAligned = Aligned;
+    }
+
     impl<
             FromUnknown: Validity,
             FromAsInitialized: Validity,
@@ -312,4 +371,39 @@ mod mapping {
         type FromInitialized = FromInitialized;
         type FromValid = Unknown;
     }
+
+    impl ValidityMapping for Unknown {
+        type FromUnknown = Unknown;
+        type FromAsInitialized = Unknown;
+        type FromInitialized = Unknown;
+        type FromValid = Unknown;
+    }
+
+    impl ValidityMapping for Preserved {
+        type FromUnknown = Unknown;
+        type FromAsInitialized = AsInitialized;
+        type FromInitialized = Initialized;
+        type FromValid = Valid;
+    }
+
+    impl ValidityMapping for AsInitialized {
+        type FromUnknown = AsInitialized;
+        type FromAsInitialized = AsInitialized;
+        type FromInitialized = AsInitialized;
+        type FromValid = AsInitialized;
+    }
+
+    impl ValidityMapping for Initialized {
+        type FromUnknown = Initialized;
+        type FromAsInitialized = Initialized;
+        type FromInitialized = Initialized;
+        type FromValid = Initialized;
+    }
+
+    impl ValidityMapping for Valid {
+        type FromUnknown = Valid;
+        type FromAsInitialized = Valid;
+        type FromInitialized = Valid;
+        type FromValid = Valid;
+    }
 }