Add #[rustc_box] and use it inside alloc

compiler/rustc_ast_lowering/src/expr.rs

@@ -41,7 +41,22 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
                 ExprKind::Call(ref f, ref args) => {
-                    if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
+                    if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
+                        if let [inner] = &args[..] && e.attrs.len() == 1 {
+                            let kind = hir::ExprKind::Box(self.lower_expr(&inner));
+                            let hir_id = self.lower_node_id(e.id);
+                            return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
+                        } else {
+                            self.sess
+                                .struct_span_err(
+                                    e.span,
+                                    "#[rustc_box] requires precisely one argument \
+                                    and no other attributes are allowed",
+                                )
+                                .emit();
+                            hir::ExprKind::Err
+                        }
+                    } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
                     } else {
                         let f = self.lower_expr(f);

compiler/rustc_feature/src/builtin_attrs.rs

@@ -675,6 +675,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "#[rustc_has_incoherent_inherent_impls] allows the addition of incoherent inherent impls for \
          the given type by annotating all impl items with #[rustc_allow_incoherent_impl]."
     ),
+    rustc_attr!(
+        rustc_box, AttributeType::Normal, template!(Word), ErrorFollowing,
+        "#[rustc_box] allows creating boxes \
+        and it is only intended to be used in `alloc`."
+    ),
+
     BuiltinAttribute {
         name: sym::rustc_diagnostic_item,
         // FIXME: This can be `true` once we always use `tcx.is_diagnostic_item`.

compiler/rustc_span/src/symbol.rs

@@ -1173,6 +1173,7 @@ symbols! {
         rustc_allow_const_fn_unstable,
         rustc_allow_incoherent_impl,
         rustc_attrs,
+        rustc_box,
         rustc_builtin_macro,
         rustc_capture_analysis,
         rustc_clean,

library/alloc/src/boxed.rs

@@ -192,7 +192,25 @@ impl<T> Box<T> {
     /// ```
     /// let five = Box::new(5);
     /// ```
-    #[cfg(not(no_global_oom_handling))]
+    #[cfg(all(not(no_global_oom_handling), not(bootstrap)))]
+    #[inline(always)]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
+    pub fn new(x: T) -> Self {
+        #[rustc_box]
+        Box::new(x)
+    }
+
+    /// Allocates memory on the heap and then places `x` into it.
+    ///
+    /// This doesn't actually allocate if `T` is zero-sized.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let five = Box::new(5);
+    /// ```
+    #[cfg(all(not(no_global_oom_handling), bootstrap))]
     #[inline(always)]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use]
@@ -259,7 +277,9 @@ impl<T> Box<T> {
     #[must_use]
     #[inline(always)]
     pub fn pin(x: T) -> Pin<Box<T>> {
-        (box x).into()
+        (#[cfg_attr(not(bootstrap), rustc_box)]
+        Box::new(x))
+        .into()
     }
 
     /// Allocates memory on the heap then places `x` into it,
@@ -1186,7 +1206,8 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
 impl<T: Default> Default for Box<T> {
     /// Creates a `Box<T>`, with the `Default` value for T.
     fn default() -> Self {
-        box T::default()
+        #[cfg_attr(not(bootstrap), rustc_box)]
+        Box::new(T::default())
     }
 }
 
@@ -1550,7 +1571,8 @@ impl<T, const N: usize> From<[T; N]> for Box<[T]> {
     /// println!("{boxed:?}");
     /// ```
     fn from(array: [T; N]) -> Box<[T]> {
-        box array
+        #[cfg_attr(not(bootstrap), rustc_box)]
+        Box::new(array)
     }
 }
 

library/alloc/src/lib.rs

@@ -147,7 +147,7 @@
 #![feature(allocator_internals)]
 #![feature(allow_internal_unstable)]
 #![feature(associated_type_bounds)]
-#![feature(box_syntax)]
+#![cfg_attr(bootstrap, feature(box_syntax))]
 #![feature(cfg_sanitize)]
 #![feature(const_deref)]
 #![feature(const_mut_refs)]
@@ -170,6 +170,7 @@
 #![feature(rustc_attrs)]
 #![feature(slice_internals)]
 #![feature(staged_api)]
+#![feature(stmt_expr_attributes)]
 #![cfg_attr(test, feature(test))]
 #![feature(unboxed_closures)]
 #![feature(unsized_fn_params)]

library/alloc/src/macros.rs

@@ -34,7 +34,28 @@
 /// be mindful of side effects.
 ///
 /// [`Vec`]: crate::vec::Vec
-#[cfg(all(not(no_global_oom_handling), not(test)))]
+#[cfg(all(not(no_global_oom_handling), not(test), not(bootstrap)))]
+#[macro_export]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "vec_macro"]
+#[allow_internal_unstable(rustc_attrs, liballoc_internals)]
+macro_rules! vec {
+    () => (
+        $crate::__rust_force_expr!($crate::vec::Vec::new())
+    );
+    ($elem:expr; $n:expr) => (
+        $crate::__rust_force_expr!($crate::vec::from_elem($elem, $n))
+    );
+    ($($x:expr),+ $(,)?) => (
+        $crate::__rust_force_expr!(<[_]>::into_vec(
+            #[rustc_box]
+            $crate::boxed::Box::new([$($x),+])
+        ))
+    );
+}
+
+/// Creates a `Vec` containing the arguments (bootstrap version).
+#[cfg(all(not(no_global_oom_handling), not(test), bootstrap))]
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_diagnostic_item = "vec_macro"]
@@ -65,7 +86,7 @@ macro_rules! vec {
         $crate::vec::from_elem($elem, $n)
     );
     ($($x:expr),*) => (
-        $crate::slice::into_vec(box [$($x),*])
+        $crate::slice::into_vec($crate::boxed::Box::new([$($x),*]))
     );
     ($($x:expr,)*) => (vec![$($x),*])
 }

library/alloc/src/vec/mod.rs

@@ -2983,12 +2983,15 @@ impl<T, const N: usize> From<[T; N]> for Vec<T> {
     /// ```
     #[cfg(not(test))]
     fn from(s: [T; N]) -> Vec<T> {
-        <[T]>::into_vec(box s)
+        <[T]>::into_vec(
+            #[cfg_attr(not(bootstrap), rustc_box)]
+            Box::new(s),
+        )
     }
 
     #[cfg(test)]
     fn from(s: [T; N]) -> Vec<T> {
-        crate::slice::into_vec(box s)
+        crate::slice::into_vec(Box::new(s))
     }
 }
 

src/test/ui/type/ascription/issue-47666.stderr

@@ -1,4 +1,4 @@
-error: expected type, found `<[_]>::into_vec(box [0, 1])`
+error: expected type, found `<[_]>::into_vec(#[rustc_box] ::alloc::boxed::Box::new([0, 1]))`
   --> $DIR/issue-47666.rs:3:25
    |
 LL |     let _ = Option:Some(vec![0, 1]);