Auto merge of rust-lang#117905 - RalfJung:no-const-mut, r=lcnr

revert stabilization of const_intrinsic_copy

`@rust-lang/wg-const-eval`  I don't know what we were thinking when we approved rust-lang#97276... const-eval isn't supposed to be able to mutate anything yet! It's also near impossible to actually call `copy` in const on stable since `&mut` expressions are generally unstable. However, there's one exception...

```rust
static mut INT: i32 = unsafe {
    let val = &mut [1]; // `&mut` on arrays is allowed in `static mut`
    (val as *mut [i32; 1]).copy_from(&[42], 1);
    val[0]
};

fn main() { unsafe {
    dbg!(INT);
} }
```

Inside `static mut`, we accept some `&mut` since ~forever, to make `static mut FOO: &mut [T] = &mut [...];` work. We reject any attempt to actually write to that mutable reference though... except for the `copy` functions.

I think we should revert stabilizing these functions that take `*mut`, and then re-stabilize them together with `ptr.write` once mutable references are stable.

(This will likely fail on PowerPC until rust-lang/stdarch#1497 lands. But we'll need a crater run first anyway.)

Author: bors

library/core/src/intrinsics.rs

@@ -2749,13 +2749,13 @@ pub(crate) fn is_nonoverlapping<T>(src: *const T, dst: *const T, count: usize) -
 #[doc(alias = "memcpy")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_allowed_through_unstable_modules]
-#[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+#[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 #[rustc_diagnostic_item = "ptr_copy_nonoverlapping"]
 pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize) {
     extern "rust-intrinsic" {
-        #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+        #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
         #[rustc_nounwind]
         pub fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
     }
@@ -2845,13 +2845,13 @@ pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: us
 #[doc(alias = "memmove")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_allowed_through_unstable_modules]
-#[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+#[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
 #[inline(always)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 #[rustc_diagnostic_item = "ptr_copy"]
 pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
     extern "rust-intrinsic" {
-        #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+        #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
         #[rustc_nounwind]
         fn copy<T>(src: *const T, dst: *mut T, count: usize);
     }

library/core/src/lib.rs

@@ -135,6 +135,7 @@
 #![feature(const_hint_assert_unchecked)]
 #![feature(const_index_range_slice_index)]
 #![feature(const_int_unchecked_arith)]
+#![feature(const_intrinsic_copy)]
 #![feature(const_intrinsic_forget)]
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]

library/core/src/ptr/const_ptr.rs

@@ -1284,7 +1284,7 @@ impl<T: ?Sized> *const T {
     /// See [`ptr::copy`] for safety concerns and examples.
     ///
     /// [`ptr::copy`]: crate::ptr::copy()
-    #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+    #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[inline]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
@@ -1304,7 +1304,7 @@ impl<T: ?Sized> *const T {
     /// See [`ptr::copy_nonoverlapping`] for safety concerns and examples.
     ///
     /// [`ptr::copy_nonoverlapping`]: crate::ptr::copy_nonoverlapping()
-    #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+    #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[inline]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces

library/core/src/ptr/mod.rs

@@ -1176,7 +1176,6 @@ pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "const_ptr_read", since = "1.71.0")]
-#[rustc_allow_const_fn_unstable(const_mut_refs, const_maybe_uninit_as_mut_ptr)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 #[rustc_diagnostic_item = "ptr_read"]
 pub const unsafe fn read<T>(src: *const T) -> T {
@@ -1294,7 +1293,11 @@ pub const unsafe fn read<T>(src: *const T) -> T {
 #[inline]
 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
 #[rustc_const_stable(feature = "const_ptr_read", since = "1.71.0")]
-#[rustc_allow_const_fn_unstable(const_mut_refs, const_maybe_uninit_as_mut_ptr)]
+#[rustc_allow_const_fn_unstable(
+    const_mut_refs,
+    const_maybe_uninit_as_mut_ptr,
+    const_intrinsic_copy
+)]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 #[rustc_diagnostic_item = "ptr_read_unaligned"]
 pub const unsafe fn read_unaligned<T>(src: *const T) -> T {

library/core/src/ptr/mut_ptr.rs

@@ -1385,7 +1385,7 @@ impl<T: ?Sized> *mut T {
     /// See [`ptr::copy`] for safety concerns and examples.
     ///
     /// [`ptr::copy`]: crate::ptr::copy()
-    #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+    #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
@@ -1405,7 +1405,7 @@ impl<T: ?Sized> *mut T {
     /// See [`ptr::copy_nonoverlapping`] for safety concerns and examples.
     ///
     /// [`ptr::copy_nonoverlapping`]: crate::ptr::copy_nonoverlapping()
-    #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+    #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
@@ -1425,7 +1425,7 @@ impl<T: ?Sized> *mut T {
     /// See [`ptr::copy`] for safety concerns and examples.
     ///
     /// [`ptr::copy`]: crate::ptr::copy()
-    #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+    #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
@@ -1445,7 +1445,7 @@ impl<T: ?Sized> *mut T {
     /// See [`ptr::copy_nonoverlapping`] for safety concerns and examples.
     ///
     /// [`ptr::copy_nonoverlapping`]: crate::ptr::copy_nonoverlapping()
-    #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
+    #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces

library/core/tests/lib.rs

@@ -14,6 +14,7 @@
 #![feature(const_cell_into_inner)]
 #![feature(const_hash)]
 #![feature(const_heap)]
+#![feature(const_intrinsic_copy)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_nonnull_new)]
 #![feature(const_pointer_is_aligned)]

tests/ui/consts/const-eval/raw-pointer-ub.rs

@@ -1,4 +1,4 @@
-#![feature(const_mut_refs)]
+#![feature(const_mut_refs, const_intrinsic_copy)]
 
 
 const MISALIGNED_LOAD: () = unsafe {