Auto merge of #108476 - saethlin:remove-library-rustc-box, r=thomcc

Remove or document uses of #[rustc_box] in library

r? `@thomcc`

Only one of these uses is tested for in the rustc-perf benchmark suite. The impact there on compile time is somewhat dramatic, but I am inclined to make this change as a simplification to the library and wait for people to complain if it explodes their compilation time. I think in the absence of data or reports from users about what code paths really matter, if we are optimizing for compilation time, it's hard to argue against using `#[rustc_box]` everywhere we currently call `Box::new`.

library/alloc/src/boxed.rs

@@ -283,9 +283,7 @@ impl<T> Box<T> {
     #[must_use]
     #[inline(always)]
     pub fn pin(x: T) -> Pin<Box<T>> {
-        (#[rustc_box]
-        Box::new(x))
-        .into()
+        Box::new(x).into()
     }
 
     /// Allocates memory on the heap then places `x` into it,
@@ -1242,8 +1240,8 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: Default> Default for Box<T> {
     /// Creates a `Box<T>`, with the `Default` value for T.
+    #[inline]
     fn default() -> Self {
-        #[rustc_box]
         Box::new(T::default())
     }
 }
@@ -1252,6 +1250,7 @@ impl<T: Default> Default for Box<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 impl<T> const Default for Box<[T]> {
+    #[inline]
     fn default() -> Self {
         let ptr: Unique<[T]> = Unique::<[T; 0]>::dangling();
         Box(ptr, Global)
@@ -1262,6 +1261,7 @@ impl<T> const Default for Box<[T]> {
 #[stable(feature = "default_box_extra", since = "1.17.0")]
 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
 impl const Default for Box<str> {
+    #[inline]
     fn default() -> Self {
         // SAFETY: This is the same as `Unique::cast<U>` but with an unsized `U = str`.
         let ptr: Unique<str> = unsafe {
@@ -1616,7 +1616,6 @@ impl<T, const N: usize> From<[T; N]> for Box<[T]> {
     /// println!("{boxed:?}");
     /// ```
     fn from(array: [T; N]) -> Box<[T]> {
-        #[rustc_box]
         Box::new(array)
     }
 }

library/alloc/src/macros.rs

@@ -48,6 +48,8 @@ macro_rules! vec {
     );
     ($($x:expr),+ $(,)?) => (
         $crate::__rust_force_expr!(<[_]>::into_vec(
+            // This rustc_box is not required, but it produces a dramatic improvement in compile
+            // time when constructing arrays with many elements.
             #[rustc_box]
             $crate::boxed::Box::new([$($x),+])
         ))

library/alloc/src/vec/mod.rs

@@ -3131,10 +3131,7 @@ impl<T, const N: usize> From<[T; N]> for Vec<T> {
     /// ```
     #[cfg(not(test))]
     fn from(s: [T; N]) -> Vec<T> {
-        <[T]>::into_vec(
-            #[rustc_box]
-            Box::new(s),
-        )
+        <[T]>::into_vec(Box::new(s))
     }
 
     #[cfg(test)]