perf experiment: try to replace box_new with write_via_move

Author: RalfJung

library/alloc/src/boxed.rs

@@ -259,7 +259,12 @@ impl<T> Box<T> {
     #[rustc_diagnostic_item = "box_new"]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub fn new(x: T) -> Self {
-        return box_new(x);
+        let mut b = Box::new_uninit();
+        let ptr = mem::MaybeUninit::as_mut_ptr(&mut *b);
+        // SAFETY: we just allocated the box to store `x`.
+        unsafe { core::intrinsics::write_via_move(ptr, x) };
+        // SAFETY: we just initialized `b`.
+        unsafe { b.assume_init() }
     }
 
     /// Constructs a new box with uninitialized contents.