revert back to the old syntax while landing the protocol alone.

src/liballoc/boxed.rs

@@ -68,19 +68,21 @@ use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
 use core::ptr::{Unique};
 use core::raw::{TraitObject};
 
+// FIXME (#22181): Put in the new placement-in syntax once that lands.
+
 /// A value that represents the heap. This is the place that the `box`
 /// keyword allocates into.
 ///
 /// The following two examples are equivalent:
 ///
 /// ```
-/// # #![feature(box_heap)]
+/// # #![feature(box_heap, core)]
 /// #![feature(box_syntax)]
 /// #![feature(placement_in_syntax)]
 /// use std::boxed::HEAP;
 ///
 /// fn main() {
-///     let foo = in HEAP { 5 };
+///     let foo = box (HEAP) { 5 };
 ///     let foo: Box<_> = box 5;
 /// }
 /// ```

src/test/compile-fail/feature-gate-box-expr.rs

@@ -20,7 +20,7 @@ fn main() {
     let x: Box<_> = box (HEAP) 'c'; //~ ERROR placement-in expression syntax is experimental
     println!("x: {}", x);
 
-    // XXX
+    // FIXME (#22181) put back when new placement-in syntax is supported
     // let x = in HEAP { 'c' };
     // println!("x: {}", x);
 }

src/test/debuginfo/box.rs

@@ -33,13 +33,16 @@
 #![allow(unused_variables)]
 #![feature(box_syntax)]
 #![feature(placement_in_syntax)]
+// both needed for HEAP use for some reason
+#![feature(core, alloc)]
 #![omit_gdb_pretty_printer_section]
 
 use std::boxed::HEAP;
 
 fn main() {
     let a: Box<_> = box 1;
-    let b = in HEAP { (2, 3.5f64) };
+    // FIXME (#22181): Put in the new placement-in syntax once that lands.
+    let b = box (HEAP) { (2, 3.5f64) };
 
     zzz(); // #break
 }

src/test/run-pass/new-box-syntax.rs

@@ -27,7 +27,7 @@ struct Structure {
 }
 
 pub fn main() {
-    // XXX
+    // FIXME (#22181) put back when new placement-in syntax is supported
     // let x: Box<isize> = in HEAP { 2 };
     let y: Box<isize> = box 2;
     let b: Box<isize> = box () (1 + 2);