Constify ptr::write and the write[_unaligned] methods on *mut T

Constify intrinsics::forget

library/core/src/intrinsics.rs

@@ -833,6 +833,7 @@ extern "rust-intrinsic" {
     ///
     /// This exists solely for [`mem::forget_unsized`]; normal `forget` uses
     /// `ManuallyDrop` instead.
+    #[rustc_const_unstable(feature = "const_intrinsic_forget", issue = "none")]
     pub fn forget<T: ?Sized>(_: T);
 
     /// Reinterprets the bits of a value of one type as another type.

library/core/src/lib.rs

@@ -73,10 +73,12 @@
 #![feature(const_discriminant)]
 #![feature(const_cell_into_inner)]
 #![feature(const_intrinsic_copy)]
+#![feature(const_intrinsic_forget)]
 #![feature(const_float_classify)]
 #![feature(const_float_bits_conv)]
 #![feature(const_int_unchecked_arith)]
 #![feature(const_mut_refs)]
+#![feature(const_refs_to_cell)]
 #![feature(const_cttz)]
 #![feature(const_panic)]
 #![feature(const_pin)]
@@ -90,6 +92,7 @@
 #![feature(const_ptr_offset)]
 #![feature(const_ptr_offset_from)]
 #![feature(const_ptr_read)]
+#![feature(const_ptr_write)]
 #![feature(const_raw_ptr_comparison)]
 #![feature(const_raw_ptr_deref)]
 #![feature(const_slice_from_raw_parts)]

library/core/src/ptr/mod.rs

@@ -902,7 +902,8 @@ pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
 /// ```
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub unsafe fn write<T>(dst: *mut T, src: T) {
+#[rustc_const_unstable(feature = "const_ptr_write", issue = "none")]
+pub const unsafe fn write<T>(dst: *mut T, src: T) {
     // SAFETY: the caller must guarantee that `dst` is valid for writes.
     // `dst` cannot overlap `src` because the caller has mutable access
     // to `dst` while `src` is owned by this function.
@@ -998,14 +999,16 @@ pub unsafe fn write<T>(dst: *mut T, src: T) {
 /// ```
 #[inline]
 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
-pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
+#[rustc_const_unstable(feature = "const_ptr_write", issue = "none")]
+pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
     // SAFETY: the caller must guarantee that `dst` is valid for writes.
     // `dst` cannot overlap `src` because the caller has mutable access
     // to `dst` while `src` is owned by this function.
     unsafe {
         copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
+        // We are calling the intrinsic directly to avoid function calls in the generated code.
+        intrinsics::forget(src);
     }
-    mem::forget(src);
 }
 
 /// Performs a volatile read of the value from `src` without moving it. This

library/core/src/ptr/mut_ptr.rs

@@ -1003,8 +1003,9 @@ impl<T: ?Sized> *mut T {
     ///
     /// [`ptr::write`]: crate::ptr::write()
     #[stable(feature = "pointer_methods", since = "1.26.0")]
+    #[rustc_const_unstable(feature = "const_ptr_write", issue = "none")]
     #[inline]
-    pub unsafe fn write(self, val: T)
+    pub const unsafe fn write(self, val: T)
     where
         T: Sized,
     {
@@ -1057,8 +1058,9 @@ impl<T: ?Sized> *mut T {
     ///
     /// [`ptr::write_unaligned`]: crate::ptr::write_unaligned()
     #[stable(feature = "pointer_methods", since = "1.26.0")]
+    #[rustc_const_unstable(feature = "const_ptr_write", issue = "none")]
     #[inline]
-    pub unsafe fn write_unaligned(self, val: T)
+    pub const unsafe fn write_unaligned(self, val: T)
     where
         T: Sized,
     {

library/core/tests/const_ptr.rs

@@ -49,3 +49,53 @@ fn mut_ptr_read() {
     const UNALIGNED: u16 = unsafe { UNALIGNED_PTR.read_unaligned() };
     assert_eq!(UNALIGNED, u16::from_ne_bytes([0x23, 0x45]));
 }
+
+#[test]
+fn write() {
+    use core::ptr;
+
+    const fn write_aligned() -> i32 {
+        let mut res = 0;
+        unsafe {
+            ptr::write(&mut res as *mut _, 42);
+        }
+        res
+    }
+    const ALIGNED: i32 = write_aligned();
+    assert_eq!(ALIGNED, 42);
+
+    const fn write_unaligned() -> [u16; 2] {
+        let mut two_aligned = [0u16; 2];
+        unsafe {
+            let unaligned_ptr = (two_aligned.as_mut_ptr() as *mut u8).add(1) as *mut u16;
+            ptr::write_unaligned(unaligned_ptr, u16::from_ne_bytes([0x23, 0x45]));
+        }
+        two_aligned
+    }
+    const UNALIGNED: [u16; 2] = write_unaligned();
+    assert_eq!(UNALIGNED, [u16::from_ne_bytes([0x00, 0x23]), u16::from_ne_bytes([0x45, 0x00])]);
+}
+
+#[test]
+fn mut_ptr_write() {
+    const fn aligned() -> i32 {
+        let mut res = 0;
+        unsafe {
+            (&mut res as *mut i32).write(42);
+        }
+        res
+    }
+    const ALIGNED: i32 = aligned();
+    assert_eq!(ALIGNED, 42);
+
+    const fn write_unaligned() -> [u16; 2] {
+        let mut two_aligned = [0u16; 2];
+        unsafe {
+            let unaligned_ptr = (two_aligned.as_mut_ptr() as *mut u8).add(1) as *mut u16;
+            unaligned_ptr.write_unaligned(u16::from_ne_bytes([0x23, 0x45]));
+        }
+        two_aligned
+    }
+    const UNALIGNED: [u16; 2] = write_unaligned();
+    assert_eq!(UNALIGNED, [u16::from_ne_bytes([0x00, 0x23]), u16::from_ne_bytes([0x45, 0x00])]);
+}

library/core/tests/lib.rs

@@ -14,6 +14,7 @@
 #![feature(const_cell_into_inner)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_ptr_read)]
+#![feature(const_ptr_write)]
 #![feature(const_ptr_offset)]
 #![feature(control_flow_enum)]
 #![feature(core_intrinsics)]