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(),),
         )
     }
 }

compiler/rustc_feature/src/accepted.rs

@@ -299,6 +299,8 @@ declare_features! (
     (accepted, const_panic, "1.57.0", Some(51999), None),
     /// Lessens the requirements for structs to implement `Unsize`.
     (accepted, relaxed_struct_unsize, "1.58.0", Some(81793), None),
+    /// Allows dereferencing raw pointers during const eval.
+    (accepted, const_raw_ptr_deref, "1.58.0", Some(51911), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features

compiler/rustc_feature/src/active.rs

@@ -408,9 +408,6 @@ declare_features! (
     /// Allows inferring `'static` outlives requirements (RFC 2093).
     (active, infer_static_outlives_requirements, "1.26.0", Some(54185), None),
 
-    /// Allows dereferencing raw pointers during const eval.
-    (active, const_raw_ptr_deref, "1.27.0", Some(51911), None),
-
     /// Allows inconsistent bounds in where clauses.
     (active, trivial_bounds, "1.28.0", Some(48214), None),
 

library/core/src/lib.rs

@@ -156,7 +156,7 @@
 #![feature(const_impl_trait)]
 #![feature(const_mut_refs)]
 #![feature(const_precise_live_drops)]
-#![feature(const_raw_ptr_deref)]
+#![cfg_attr(bootstrap, feature(const_raw_ptr_deref))]
 #![feature(const_refs_to_cell)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

library/core/tests/lib.rs

@@ -56,7 +56,7 @@
 #![feature(const_mut_refs)]
 #![feature(const_pin)]
 #![feature(const_slice_from_raw_parts)]
-#![feature(const_raw_ptr_deref)]
+#![cfg_attr(bootstrap, feature(const_raw_ptr_deref))]
 #![feature(never_type)]
 #![feature(unwrap_infallible)]
 #![feature(result_into_ok_or_err)]

library/std/src/lib.rs

@@ -264,7 +264,8 @@
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_option)]
-#![feature(const_raw_ptr_deref)]
+#![cfg_attr(bootstrap, feature(const_raw_ptr_deref))]
+#![cfg_attr(not(bootstrap), feature(const_mut_refs))]
 #![feature(const_socketaddr)]
 #![feature(const_trait_impl)]
 #![feature(container_error_extra)]

src/test/ui/consts/const-deref-ptr.rs

@@ -1,7 +1,7 @@
-// Check that you can't dereference raw pointers in constants.
+// Check that you can't dereference invalid raw pointers in constants.
 
 fn main() {
     static C: u64 = unsafe {*(0xdeadbeef as *const u64)};
-    //~^ ERROR dereferencing raw pointers in statics is unstable
+    //~^ ERROR could not evaluate static initializer
     println!("{}", C);
 }

src/test/ui/consts/const-deref-ptr.stderr

@@ -1,12 +1,9 @@
-error[E0658]: dereferencing raw pointers in statics is unstable
+error[E0080]: could not evaluate static initializer
   --> $DIR/const-deref-ptr.rs:4:29
    |
 LL |     static C: u64 = unsafe {*(0xdeadbeef as *const u64)};
-   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: see issue #51911 <https://github.com/rust-lang/rust/issues/51911> for more information
-   = help: add `#![feature(const_raw_ptr_deref)]` to the crate attributes to enable
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^ 0xdeadbeef is not a valid pointer
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0658`.
+For more information about this error, try `rustc --explain E0080`.