Add implementation of GhostBorrow and GhostBorrowMut for arrays of references

* Changes:

- Add implementation of GhostBorrow for arrays of references to GhostCell
- Add implementation of GhostBorrowMut for arrays of references to GhostCell.
- Various comments and documentation nits.

* Motivation:

More flexibility.

* Implementation Note:

Due to `mem::transmute` not working well with const generics (as per rust-lang/rust#61956), `ptr::read` is used instead of `mem::transmute`. This is expected to be safe, if verbose.

Author: konsumlamm

src/ghost_borrow.rs

@@ -4,6 +4,7 @@
 //! provides.
 
 use core::mem;
+use core::ptr;
 
 use crate::ghost_cell::*;
 
@@ -54,12 +55,24 @@ impl<'a, 'brand, T, const N: usize> GhostBorrow<'a, 'brand> for &'a [GhostCell<'
 
     fn borrow(self, _: &'a GhostToken<'brand>) -> Self::Result {
         //  Safety:
-        //  -   Exclusive access to the `GhostToken` ensures exclusive access to the cells' content.
+        //  -   Shared access to the `GhostToken` ensures shared access to the cells' content.
         //  -   `GhostCell` is `repr(transparent)`, hence `T` and `GhostCell<T>` have the same memory representation.
         unsafe { mem::transmute::<Self, Self::Result>(self) }
     }
 }
 
+impl<'a, 'brand, T, const N: usize> GhostBorrow<'a, 'brand> for [&'a GhostCell<'brand, T>; N] {
+    type Result = [&'a T; N];
+
+    fn borrow(self, _: &'a GhostToken<'brand>) -> Self::Result {
+        //  Safety:
+        //  -   `[&'a GhostCell<'brand, T>; N]` and `[&'a T; N]` have the same size.
+        //  -   `[&'a GhostCell<'brand, T>; N]` implements `Copy`, so no `mem::forget` is needed.
+        //  -   We can't use `mem::transmute`, because of https://github.com/rust-lang/rust/issues/61956.
+        unsafe { ptr::read(&self as *const _ as *const Self::Result) }
+    }
+}
+
 macro_rules! generate_public_instance {
     ( $($name:ident),* ; $($type_letter:ident),* ) => {
         impl<'a, 'brand, $($type_letter,)*> GhostBorrow<'a, 'brand> for

src/ghost_borrow_mut.rs

@@ -17,6 +17,7 @@
 //! The feature is experimental, to enable, use the feature "experimental-multiple-mutable-borrows".
 
 use core::mem;
+use core::ptr;
 
 use crate::ghost_cell::*;
 
@@ -147,6 +148,36 @@ impl<'a, 'brand, T, const N: usize> GhostBorrowMut<'a, 'brand> for &'a [GhostCel
     }
 }
 
+
+impl<'a, 'brand, T, const N: usize> GhostBorrowMut<'a, 'brand> for [&'a GhostCell<'brand, T>; N] {
+    type Result = [&'a mut T; N];
+    type Error = GhostAliasingError;
+
+    fn borrow_mut(self, token: &'a mut GhostToken<'brand>) -> Result<Self::Result, Self::Error> {
+        //  We require that the types are `Sized`, so no fat pointer problems.
+        //  Safety:
+        //  -   `[&'a GhostCell<'brand, T>; N]` and `[*const (); N]` have the same size.
+        //  -   `[&'a GhostCell<'brand, T>; N]` implements `Copy`, so no `mem::forget` is needed.
+        //  -   We can't use `mem::transmute`, because of https://github.com/rust-lang/rust/issues/61956.
+        check_distinct(unsafe { ptr::read(&self as *const _ as *const [*const (); N]) })?;
+
+        //  Safety:
+        //  -   The cells were checked to be distinct.
+        Ok(unsafe { self.borrow_mut_unchecked(token) })
+    }
+
+    unsafe fn borrow_mut_unchecked(self, _: &'a mut GhostToken<'brand>) -> Self::Result {
+        //  Safety:
+        //  -   Exclusive access to the `GhostToken` ensures exclusive access to the cells' content, if unaliased.
+        //  -   The caller guarantees the cells are not aliased.
+        //  -   `[&'a GhostCell<'brand, T>; N]` and `[&'a mut T; N]` have the same size.
+        //  -   `[&'a GhostCell<'brand, T>; N]` implements `Copy`, so no `mem::forget` is needed.
+        //  -   We can't use `mem::transmute`, because of https://github.com/rust-lang/rust/issues/61956.
+        ptr::read(&self as *const _ as *const Self::Result)
+    }
+}
+
+
 macro_rules! generate_public_instance {
     ( $($name:ident),* ; $($type_letter:ident),* ) => {
         impl<'a, 'brand, $($type_letter,)*> GhostBorrowMut<'a, 'brand> for
@@ -171,7 +202,6 @@ macro_rules! generate_public_instance {
 
                 //  Safety:
                 //  -   Exclusive access to the `GhostToken` ensures exclusive access to the cells' content, if unaliased.
-                //  -   `GhostCell` is `repr(transparent)`, hence `T` and `GhostCell<T>` have the same memory representation.
                 //  -   The caller guarantees the cells are not aliased.
                 ( $( &mut * $name.as_ptr(),)* )
             }