Allow casting mut array ref to mut ptr

We now allow two new casts:

- mut array reference to mut ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *mut usize;

  We allow casting const array references to const pointers so not
  allowing mut references to mut pointers was inconsistent.

- mut array reference to const ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *const usize;

  This was similarly inconsistent as we allow casting mut references to
  const pointers.

Existing test 'vector-cast-weirdness' updated to test both cases.

Fixes #24151

src/test/ui/array-slice-vec/vector-cast-weirdness.rs

@@ -1,7 +1,11 @@
 // Issue #14893. Tests that casts from vectors don't behave strangely in the
 // presence of the `_` type shorthand notation.
+//
 // Update: after a change to the way casts are done, we have more type information
 // around and so the errors here are no longer exactly the same.
+//
+// Update: With PR #81479 some of the previously rejected cases are now allowed.
+// New test cases added.
 
 struct X {
     y: [u8; 2],
@@ -12,13 +16,19 @@ fn main() {
 
     // No longer a type mismatch - the `_` can be fully resolved by type inference.
     let p1: *const u8 = &x1.y as *const _;
+    let p1: *mut u8 = &x1.y as *mut _;
+    //~^ ERROR: casting `&[u8; 2]` as `*mut u8` is invalid
     let t1: *const [u8; 2] = &x1.y as *const _;
+    let t1: *mut [u8; 2] = &x1.y as *mut _;
+    //~^ ERROR: casting `&[u8; 2]` as `*mut [u8; 2]` is invalid
     let h1: *const [u8; 2] = &x1.y as *const [u8; 2];
+    let t1: *mut [u8; 2] = &x1.y as *mut [u8; 2];
+    //~^ ERROR: casting `&[u8; 2]` as `*mut [u8; 2]` is invalid
 
     let mut x1 = X { y: [0, 0] };
 
-    // This is still an error since we don't allow casts from &mut [T; n] to *mut T.
-    let p1: *mut u8 = &mut x1.y as *mut _;  //~ ERROR casting
+    let p1: *mut u8 = &mut x1.y as *mut _;
+    let p2: *const u8 = &mut x1.y as *const _;
     let t1: *mut [u8; 2] = &mut x1.y as *mut _;
     let h1: *mut [u8; 2] = &mut x1.y as *mut [u8; 2];
 }

src/test/ui/array-slice-vec/vector-cast-weirdness.stderr

@@ -1,9 +1,21 @@
-error[E0606]: casting `&mut [u8; 2]` as `*mut u8` is invalid
-  --> $DIR/vector-cast-weirdness.rs:21:23
+error[E0606]: casting `&[u8; 2]` as `*mut u8` is invalid
+  --> $DIR/vector-cast-weirdness.rs:19:23
    |
-LL |     let p1: *mut u8 = &mut x1.y as *mut _;
-   |                       ^^^^^^^^^^^^^^^^^^^
+LL |     let p1: *mut u8 = &x1.y as *mut _;
+   |                       ^^^^^^^^^^^^^^^
 
-error: aborting due to previous error
+error[E0606]: casting `&[u8; 2]` as `*mut [u8; 2]` is invalid
+  --> $DIR/vector-cast-weirdness.rs:22:28
+   |
+LL |     let t1: *mut [u8; 2] = &x1.y as *mut _;
+   |                            ^^^^^^^^^^^^^^^
+
+error[E0606]: casting `&[u8; 2]` as `*mut [u8; 2]` is invalid
+  --> $DIR/vector-cast-weirdness.rs:25:28
+   |
+LL |     let t1: *mut [u8; 2] = &x1.y as *mut [u8; 2];
+   |                            ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0606`.