Auto merge of rust-lang#137371 - matthiaskrgr:rollup-3qkdqar, r=matth…

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#128080 (Specify scope in `out_of_scope_macro_calls` lint)
 - rust-lang#135630 (add more `s390x` target features)
 - rust-lang#136089 (Reduce `Box::default` stack copies in debug mode)
 - rust-lang#137204 (Clarify MIR dialects and phases)
 - rust-lang#137299 (Simplify `Postorder` customization.)
 - rust-lang#137302 (Use a probe to avoid registering stray region obligations when re-checking drops in MIR typeck)
 - rust-lang#137305 (Tweaks in and around `rustc_middle`)
 - rust-lang#137313 (Some codegen_llvm cleanups)

r? `@ghost`
`@rustbot` modify labels: rollup

alloc/src/boxed.rs

@@ -1689,7 +1689,20 @@ impl<T: Default> Default for Box<T> {
     /// Creates a `Box<T>`, with the `Default` value for T.
     #[inline]
     fn default() -> Self {
-        Box::write(Box::new_uninit(), T::default())
+        let mut x: Box<mem::MaybeUninit<T>> = Box::new_uninit();
+        unsafe {
+            // SAFETY: `x` is valid for writing and has the same layout as `T`.
+            // If `T::default()` panics, dropping `x` will just deallocate the Box as `MaybeUninit<T>`
+            // does not have a destructor.
+            //
+            // We use `ptr::write` as `MaybeUninit::write` creates
+            // extra stack copies of `T` in debug mode.
+            //
+            // See https://github.com/rust-lang/rust/issues/136043 for more context.
+            ptr::write(&raw mut *x as *mut T, T::default());
+            // SAFETY: `x` was just initialized above.
+            x.assume_init()
+        }
     }
 }