Stabilize const_raw_ptr_deref for *const T

This stabilizes dereferencing immutable raw pointers in const contexts.
It does not stabilize `*mut T` dereferencing. This is placed behind the
`const_raw_mut_ptr_deref` feature gate.

Author: jhpratt

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -725,7 +725,7 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
         match elem {
             ProjectionElem::Deref => {
                 let base_ty = Place::ty_from(place_local, proj_base, self.body, self.tcx).ty;
-                if let ty::RawPtr(_) = base_ty.kind() {
+                if base_ty.is_unsafe_ptr() {
                     if proj_base.is_empty() {
                         let decl = &self.body.local_decls[place_local];
                         if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
@@ -734,7 +734,13 @@ impl Visitor<'tcx> for Checker<'mir, 'tcx> {
                             return;
                         }
                     }
-                    self.check_op(ops::RawPtrDeref);
+
+                    // `*const T` is stable, `*mut T` is not
+                    if !base_ty.is_mutable_ptr() {
+                        return;
+                    }
+
+                    self.check_op(ops::RawMutPtrDeref);
                 }
 
                 if context.is_mutating_use() {

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -400,18 +400,18 @@ impl NonConstOp for RawPtrComparison {
 }
 
 #[derive(Debug)]
-pub struct RawPtrDeref;
-impl NonConstOp for RawPtrDeref {
+pub struct RawMutPtrDeref;
+impl NonConstOp for RawMutPtrDeref {
     fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
-        Status::Unstable(sym::const_raw_ptr_deref)
+        Status::Unstable(sym::const_mut_refs)
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
         feature_err(
             &ccx.tcx.sess.parse_sess,
-            sym::const_raw_ptr_deref,
+            sym::const_mut_refs,
             span,
-            &format!("dereferencing raw pointers in {}s is unstable", ccx.const_kind(),),
+            &format!("dereferencing raw mutable pointers in {}s is unstable", ccx.const_kind(),),
         )
     }
 }