Rollup merge of #130966 - RalfJung:ptr-metadata-const-stable, r=scottmcm

make ptr metadata functions callable from stable const fn

So far this was done with a bunch of `rustc_allow_const_fn_unstable`. But those should be the exception, not the norm. If we are confident we can expose the ptr metadata APIs *indirectly* in stable const fn, we should just mark them as `rustc_const_stable`. And we better be confident we can do that since it's already been done a while ago. ;)

In particular this marks two intrinsics as const-stable: `aggregate_raw_ptr`, `ptr_metadata`. This should be uncontroversial, they are trivial to implement in the interpreter.
Cc `@rust-lang/wg-const-eval` `@rust-lang/lang`

library/core/src/intrinsics.rs

@@ -3165,7 +3165,7 @@ pub const fn type_id<T: ?Sized + 'static>() -> u128 {
 /// change the possible layouts of pointers.
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
 pub const fn aggregate_raw_ptr<P: AggregateRawPtr<D, Metadata = M>, D, M>(_data: D, _meta: M) -> P {
@@ -3190,7 +3190,7 @@ impl<P: ?Sized, T: ptr::Thin> AggregateRawPtr<*mut T> for *mut P {
 /// This is used to implement functions like `ptr::metadata`.
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[rustc_intrinsic]
 #[rustc_intrinsic_must_be_overridden]
 pub const fn ptr_metadata<P: ptr::Pointee<Metadata = M> + ?Sized, M>(_ptr: *const P) -> M {

library/core/src/ptr/const_ptr.rs

@@ -92,7 +92,7 @@ impl<T: ?Sized> *const T {
     /// }
     /// ```
     #[unstable(feature = "set_ptr_value", issue = "75091")]
-    #[rustc_const_unstable(feature = "set_ptr_value", issue = "75091")]
+    #[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[inline]
     pub const fn with_metadata_of<U>(self, meta: *const U) -> *const U
@@ -412,7 +412,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset(self, count: isize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `offset`.
@@ -495,7 +494,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_offset(self, count: isize) -> Self {
         self.cast::<u8>().wrapping_offset(count).with_metadata_of(self)
     }
@@ -645,7 +643,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize {
         // SAFETY: the caller must uphold the safety contract for `offset_from`.
@@ -873,7 +870,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_add(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `add`.
@@ -956,7 +952,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_sub(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `sub`.
@@ -1039,7 +1034,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_add(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_add(count).with_metadata_of(self)
     }
@@ -1120,7 +1114,6 @@ impl<T: ?Sized> *const T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_sub(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_sub(count).with_metadata_of(self)
     }
@@ -1554,7 +1547,6 @@ impl<T> *const [T] {
     #[inline]
     #[stable(feature = "slice_ptr_len", since = "1.79.0")]
     #[rustc_const_stable(feature = "const_slice_ptr_len", since = "1.79.0")]
-    #[rustc_allow_const_fn_unstable(ptr_metadata)]
     pub const fn len(self) -> usize {
         metadata(self)
     }

library/core/src/ptr/metadata.rs

@@ -92,7 +92,7 @@ pub trait Thin = Pointee<Metadata = ()>;
 ///
 /// assert_eq!(std::ptr::metadata("foo"), 3_usize);
 /// ```
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[inline]
 pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
     ptr_metadata(ptr)
@@ -106,7 +106,7 @@ pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
 ///
 /// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
 #[unstable(feature = "ptr_metadata", issue = "81513")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[inline]
 pub const fn from_raw_parts<T: ?Sized>(
     data_pointer: *const impl Thin,
@@ -120,7 +120,7 @@ pub const fn from_raw_parts<T: ?Sized>(
 ///
 /// See the documentation of [`from_raw_parts`] for more details.
 #[unstable(feature = "ptr_metadata", issue = "81513")]
-#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+#[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
 #[inline]
 pub const fn from_raw_parts_mut<T: ?Sized>(
     data_pointer: *mut impl Thin,

library/core/src/ptr/mod.rs

@@ -599,7 +599,6 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_promotable]
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
-#[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_null"]
 pub const fn null<T: ?Sized + Thin>() -> *const T {
     from_raw_parts(without_provenance::<()>(0), ())
@@ -625,7 +624,6 @@ pub const fn null<T: ?Sized + Thin>() -> *const T {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_promotable]
 #[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
-#[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_null_mut"]
 pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {
     from_raw_parts_mut(without_provenance_mut::<()>(0), ())
@@ -949,7 +947,6 @@ pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T {
 #[inline]
 #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
 #[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
-#[rustc_allow_const_fn_unstable(ptr_metadata)]
 #[rustc_diagnostic_item = "ptr_slice_from_raw_parts"]
 pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
     from_raw_parts(data, len)

library/core/src/ptr/mut_ptr.rs

@@ -74,7 +74,7 @@ impl<T: ?Sized> *mut T {
     /// }
     /// ```
     #[unstable(feature = "set_ptr_value", issue = "75091")]
-    #[rustc_const_unstable(feature = "set_ptr_value", issue = "75091")]
+    #[rustc_const_stable(feature = "ptr_metadata_const", since = "CURRENT_RUSTC_VERSION")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[inline]
     pub const fn with_metadata_of<U>(self, meta: *const U) -> *mut U
@@ -412,7 +412,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset(self, count: isize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `offset`.
@@ -492,7 +491,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_offset(self, count: isize) -> Self {
         self.cast::<u8>().wrapping_offset(count).with_metadata_of(self)
     }
@@ -808,7 +806,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize {
         // SAFETY: the caller must uphold the safety contract for `offset_from`.
@@ -954,7 +951,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_add(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `add`.
@@ -1037,7 +1033,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn byte_sub(self, count: usize) -> Self {
         // SAFETY: the caller must uphold the safety contract for `sub`.
@@ -1118,7 +1113,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_add(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_add(count).with_metadata_of(self)
     }
@@ -1197,7 +1191,6 @@ impl<T: ?Sized> *mut T {
     #[inline(always)]
     #[stable(feature = "pointer_byte_offsets", since = "1.75.0")]
     #[rustc_const_stable(feature = "const_pointer_byte_offsets", since = "1.75.0")]
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     pub const fn wrapping_byte_sub(self, count: usize) -> Self {
         self.cast::<u8>().wrapping_sub(count).with_metadata_of(self)
     }
@@ -1804,7 +1797,6 @@ impl<T> *mut [T] {
     #[inline(always)]
     #[stable(feature = "slice_ptr_len", since = "1.79.0")]
     #[rustc_const_stable(feature = "const_slice_ptr_len", since = "1.79.0")]
-    #[rustc_allow_const_fn_unstable(ptr_metadata)]
     pub const fn len(self) -> usize {
         metadata(self)
     }

library/core/src/ptr/non_null.rs

@@ -567,7 +567,6 @@ impl<T: ?Sized> NonNull<T> {
     #[must_use]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
     #[rustc_const_stable(feature = "non_null_convenience", since = "1.80.0")]
     pub const unsafe fn byte_add(self, count: usize) -> Self {
@@ -651,7 +650,6 @@ impl<T: ?Sized> NonNull<T> {
     #[must_use]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-    #[rustc_allow_const_fn_unstable(set_ptr_value)]
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
     #[rustc_const_stable(feature = "non_null_convenience", since = "1.80.0")]
     pub const unsafe fn byte_sub(self, count: usize) -> Self {

library/core/src/slice/mod.rs

@@ -111,7 +111,6 @@ impl<T> [T] {
     #[lang = "slice_len_fn"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_slice_len", since = "1.39.0")]
-    #[rustc_allow_const_fn_unstable(ptr_metadata)]
     #[inline]
     #[must_use]
     pub const fn len(&self) -> usize {

tests/ui/consts/auxiliary/unstable_but_const_stable.rs

@@ -0,0 +1,13 @@
+#![feature(staged_api, rustc_attrs, intrinsics)]
+#![stable(since="1.0.0", feature = "stable")]
+
+extern "rust-intrinsic" {
+    #[unstable(feature = "unstable", issue = "42")]
+    #[rustc_const_stable(feature = "stable", since = "1.0.0")]
+    #[rustc_nounwind]
+    pub fn write_bytes<T>(dst: *mut T, val: u8, count: usize);
+}
+
+#[unstable(feature = "unstable", issue = "42")]
+#[rustc_const_stable(feature = "stable", since = "1.0.0")]
+pub const fn some_unstable_fn() {}

tests/ui/consts/unstable-const-stable.rs

@@ -0,0 +1,14 @@
+//@ aux-build:unstable_but_const_stable.rs
+
+extern crate unstable_but_const_stable;
+use unstable_but_const_stable::*;
+
+fn main() {
+    some_unstable_fn(); //~ERROR use of unstable library feature
+    unsafe { write_bytes(4 as *mut u8, 0, 0) }; //~ERROR use of unstable library feature
+}
+
+const fn const_main() {
+    some_unstable_fn(); //~ERROR use of unstable library feature
+    unsafe { write_bytes(4 as *mut u8, 0, 0) }; //~ERROR use of unstable library feature
+}

tests/ui/consts/unstable-const-stable.stderr

@@ -0,0 +1,43 @@
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:7:5
+   |
+LL |     some_unstable_fn();
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:8:14
+   |
+LL |     unsafe { write_bytes(4 as *mut u8, 0, 0) };
+   |              ^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:12:5
+   |
+LL |     some_unstable_fn();
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'unstable'
+  --> $DIR/unstable-const-stable.rs:13:14
+   |
+LL |     unsafe { write_bytes(4 as *mut u8, 0, 0) };
+   |              ^^^^^^^^^^^
+   |
+   = note: see issue #42 <https://github.com/rust-lang/rust/issues/42> for more information
+   = help: add `#![feature(unstable)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.