allocate before calling T::default in <Box<T>>::default()

The `Box<T: Default>` impl currently calls `T::default()` before allocating
the `Box`.

Most `Default` impls are trivial, which should in theory allow
LLVM to construct `T: Default` directly in the `Box` allocation when calling
`<Box<T>>::default()`.

However, the allocation may fail, which necessitates calling `T's` destructor if it has one.
If the destructor is non-trivial, then LLVM has a hard time proving that it's
sound to elide, which makes it construct `T` on the stack first, and then copy it into the allocation.

Create an uninit `Box` first, and then write `T::default` into it, so that LLVM now only needs to prove
that the `T::default` can't panic, which should be trivial for most `Default` impls.

library/alloc/src/boxed.rs

@@ -1688,7 +1688,7 @@ impl<T: Default> Default for Box<T> {
     /// Creates a `Box<T>`, with the `Default` value for T.
     #[inline]
     fn default() -> Self {
-        Box::new(T::default())
+        Box::write(Box::new_uninit(), T::default())
     }
 }
 

tests/codegen/placement-new.rs

@@ -9,9 +9,9 @@
 // CHECK-LABEL: @box_default_inplace
 #[no_mangle]
 pub fn box_default_inplace() -> Box<(String, String)> {
-    // CHECK: [[ALLOCA:%.*]] = alloca
+    // CHECK-NOT: alloca
     // CHECK: [[BOX:%.*]] = {{.*}}call {{.*}}__rust_alloc(
-    // CHECK: call void @llvm.memcpy{{.*}}(ptr {{.*}}[[BOX]], ptr {{.*}}[[ALLOCA]]
+    // CHECK-NOT: call void @llvm.memcpy
     // CHECK: ret ptr [[BOX]]
     Box::default()
 }