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

compiler/rustc_mir/src/borrow_check/type_check/mod.rs

@@ -2191,19 +2191,18 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     CastKind::Pointer(PointerCast::ArrayToPointer) => {
                         let ty_from = op.ty(body, tcx);
 
-                        let opt_ty_elem = match ty_from.kind() {
-                            ty::RawPtr(ty::TypeAndMut {
-                                mutbl: hir::Mutability::Not,
-                                ty: array_ty,
-                            }) => match array_ty.kind() {
-                                ty::Array(ty_elem, _) => Some(ty_elem),
-                                _ => None,
-                            },
+                        let opt_ty_elem_mut = match ty_from.kind() {
+                            ty::RawPtr(ty::TypeAndMut { mutbl: array_mut, ty: array_ty }) => {
+                                match array_ty.kind() {
+                                    ty::Array(ty_elem, _) => Some((ty_elem, *array_mut)),
+                                    _ => None,
+                                }
+                            }
                             _ => None,
                         };
 
-                        let ty_elem = match opt_ty_elem {
-                            Some(ty_elem) => ty_elem,
+                        let (ty_elem, ty_mut) = match opt_ty_elem_mut {
+                            Some(ty_elem_mut) => ty_elem_mut,
                             None => {
                                 span_mirbug!(
                                     self,
@@ -2215,11 +2214,10 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             }
                         };
 
-                        let ty_to = match ty.kind() {
-                            ty::RawPtr(ty::TypeAndMut {
-                                mutbl: hir::Mutability::Not,
-                                ty: ty_to,
-                            }) => ty_to,
+                        let (ty_to, ty_to_mut) = match ty.kind() {
+                            ty::RawPtr(ty::TypeAndMut { mutbl: ty_to_mut, ty: ty_to }) => {
+                                (ty_to, *ty_to_mut)
+                            }
                             _ => {
                                 span_mirbug!(
                                     self,
@@ -2231,6 +2229,17 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                             }
                         };
 
+                        if ty_to_mut == Mutability::Mut && ty_mut == Mutability::Not {
+                            span_mirbug!(
+                                self,
+                                rvalue,
+                                "ArrayToPointer cast from const {:?} to mut {:?}",
+                                ty,
+                                ty_to
+                            );
+                            return;
+                        }
+
                         if let Err(terr) = self.sub_types(
                             ty_elem,
                             ty_to,

compiler/rustc_typeck/src/check/cast.rs

@@ -765,9 +765,8 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         m_expr: ty::TypeAndMut<'tcx>,
         m_cast: ty::TypeAndMut<'tcx>,
     ) -> Result<CastKind, CastError> {
-        // array-ptr-cast.
-
-        if m_expr.mutbl == hir::Mutability::Not && m_cast.mutbl == hir::Mutability::Not {
+        // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
+        if m_expr.mutbl == hir::Mutability::Mut || m_cast.mutbl == hir::Mutability::Not {
             if let ty::Array(ety, _) = m_expr.ty.kind() {
                 // Due to the limitations of LLVM global constants,
                 // region pointers end up pointing at copies of

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`.