Update tests for desugaring box and placement-in.

Namely:

* Update run-pass/new-box-syntax
* Fix doc-embedded test for `alloc::boxed` to reflect new syntax.
* Fix test/debuginfo/box.rs to reflect new syntax.

Part of what lands with Issue 22181.

Author: pnkfelix

src/liballoc/boxed.rs

@@ -63,18 +63,19 @@ use core::ops::{Placer, Boxed, Place, InPlace, BoxPlace};
 use core::ptr::Unique;
 use core::raw::TraitObject;
 
-/// A value that represents the heap. This is the default place that the `box`
-/// keyword allocates into when no place is supplied.
+/// A value that represents the heap. This is the place that the `box`
+/// keyword allocates into.
 ///
 /// The following two examples are equivalent:
 ///
 /// ```rust
 /// #![feature(box_syntax)]
+/// #![feature(placement_in_syntax)]
 /// use std::boxed::HEAP;
 ///
 /// fn main() {
-///     let foo = box(HEAP) 5;
-///     let foo = box 5;
+///     let foo = in HEAP { 5 };
+///     let foo: Box<_> = box 5;
 /// }
 /// ```
 #[lang = "exchange_heap"]

src/test/debuginfo/box.rs

@@ -32,11 +32,14 @@
 
 #![allow(unused_variables)]
 #![feature(box_syntax)]
+#![feature(placement_in_syntax)]
 #![omit_gdb_pretty_printer_section]
 
+use std::boxed::HEAP;
+
 fn main() {
-    let a = box 1;
-    let b = box() (2, 3.5f64);
+    let a: Box<_> = box 1;
+    let b = in HEAP { (2, 3.5f64) };
 
     zzz(); // #break
 }

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

@@ -13,6 +13,7 @@
 
 #![allow(unknown_features)]
 #![feature(box_syntax)]
+#![feature(placement_in_syntax)]
 
 // Tests that the new `box` syntax works with unique pointers.
 
@@ -24,8 +25,8 @@ struct Structure {
 }
 
 pub fn main() {
-    let x: Box<int> = box(HEAP) 2;
+    let x: Box<int> = in HEAP { 2 };
     let y: Box<int> = box 2;
-    let b: Box<int> = box()(1 + 2);
-    let c = box()(3 + 4);
+    let b: Box<int> = box () (1 + 2);
+    let c: Box<_> = box () (3 + 4);
 }