replace box_new in Box::new with write_via_move

Author: RalfJung

library/alloc/src/alloc.rs

@@ -343,11 +343,15 @@ unsafe impl Allocator for Global {
 }
 
 /// The allocator for `Box`.
+///
+/// # Safety
+///
+/// `size` and `align` must satisfy the conditions in [`Layout::from_size_align`].
 #[cfg(not(no_global_oom_handling))]
 #[lang = "exchange_malloc"]
 #[inline]
 #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
-unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
+pub(crate) unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
     let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
     match Global.allocate(layout) {
         Ok(ptr) => ptr.as_mut_ptr(),

library/alloc/src/boxed.rs

@@ -203,7 +203,7 @@ use core::task::{Context, Poll};
 
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::handle_alloc_error;
-use crate::alloc::{AllocError, Allocator, Global, Layout};
+use crate::alloc::{AllocError, Allocator, Global, Layout, exchange_malloc};
 use crate::raw_vec::RawVec;
 #[cfg(not(no_global_oom_handling))]
 use crate::str::from_boxed_utf8_unchecked;
@@ -259,7 +259,15 @@ 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);
+        // SAFETY: the size and align of a valid type `T` are always valid for `Layout`.
+        let ptr = unsafe {
+            exchange_malloc(<T as SizedTypeProperties>::SIZE, <T as SizedTypeProperties>::ALIGN)
+        } as *mut T;
+        // Nothing below can panic so we do not have to worry about deallocating `ptr`.
+        // SAFETY: we just allocated the box to store `x`.
+        unsafe { core::intrinsics::write_via_move(ptr, x) };
+        // SAFETY: we just initialized `b`.
+        unsafe { mem::transmute(ptr) }
     }
 
     /// Constructs a new box with uninitialized contents.

tests/mir-opt/inline_coroutine_body.run2-{closure#0}.Inline.panic-unwind.diff

@@ -213,7 +213,7 @@
 +         StorageLive(_42);
 +         _42 = Option::<()>::None;
 +         _35 = copy ((*_37).0: std::option::Option<()>);
-+         ((*_37).0: std::option::Option<()>) = copy _42;
++         ((*_37).0: std::option::Option<()>) = move _42;
 +         StorageDead(_42);
 +         StorageLive(_43);
 +         _43 = discriminant(_35);

tests/ui/limits/issue-17913.32bit.stderr

@@ -1,19 +1,24 @@
 error[E0080]: values of the type `[&usize; usize::MAX]` are too big for the target architecture
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
    |
-   = note: evaluation of `<[&usize; usize::MAX] as std::mem::SizedTypeProperties>::SIZE` failed here
+   = note: evaluation of `<std::mem::MaybeUninit<[&usize; usize::MAX]> as std::mem::SizedTypeProperties>::SIZE` failed here
 
 error[E0080]: values of the type `[&usize; usize::MAX]` are too big for the target architecture
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
    |
-   = note: evaluation of `<[&usize; usize::MAX] as std::mem::SizedTypeProperties>::ALIGN` failed here
+   = note: evaluation of `<std::mem::MaybeUninit<[&usize; usize::MAX]> as std::mem::SizedTypeProperties>::ALIGN` failed here
+
+note: the above error was encountered while instantiating `fn Box::<[&usize; usize::MAX]>::new_uninit_in`
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
 
-note: the above error was encountered while instantiating `fn Box::<[&usize; usize::MAX]>::new`
-  --> $DIR/issue-17913.rs:16:21
+error[E0080]: values of the type `[&usize; usize::MAX]` are too big for the target architecture
+  --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
    |
-LL |     let a: Box<_> = Box::new([&n; SIZE]);
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: evaluation of `<[&usize; usize::MAX] as std::mem::SizedTypeProperties>::SIZE` failed here
+
+note: the above error was encountered while instantiating `fn Box::<[&usize; usize::MAX]>::try_new_uninit_in`
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0080`.