implement ptr::write without dedicated intrinsic

Author: RalfJung

library/core/src/intrinsics.rs

@@ -768,13 +768,6 @@ extern "rust-intrinsic" {
     #[rustc_const_stable(feature = "const_size_of", since = "1.40.0")]
     pub fn size_of<T>() -> usize;
 
-    /// Moves a value to an uninitialized memory location.
-    ///
-    /// Drop glue is not run on the destination.
-    ///
-    /// The stabilized version of this intrinsic is [`core::ptr::write`](crate::ptr::write).
-    pub fn move_val_init<T>(dst: *mut T, src: T);
-
     /// The minimum alignment of a type.
     ///
     /// The stabilized version of this intrinsic is [`core::mem::align_of`](crate::mem::align_of).

library/core/src/ptr/mod.rs

@@ -883,12 +883,18 @@ 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) {
-    if cfg!(debug_assertions) && !is_aligned_and_not_null(dst) {
-        // Not panicking to keep codegen impact smaller.
-        abort();
+    // We are calling the intrinsics directly to avoid function calls in the generated code.
+    extern "rust-intrinsic" {
+        fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
+    }
+
+    // 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, dst, 1);
+        intrinsics::forget(src);
     }
-    // SAFETY: the caller must uphold the safety contract for `move_val_init`.
-    unsafe { intrinsics::move_val_init(&mut *dst, src) }
 }
 
 /// Overwrites a memory location with the given value without reading or