Revert "pre-expansion gate box_syntax"

This reverts commit e4ed8865786a787a7b0c045f7674569b6be0e9bc.

src/libsyntax/feature_gate/check.rs

@@ -153,6 +153,9 @@ fn leveled_feature_err<'a, S: Into<MultiSpan>>(
 
 }
 
+const EXPLAIN_BOX_SYNTAX: &str =
+    "box expression syntax is experimental; you can call `Box::new` instead";
+
 pub const EXPLAIN_STMT_ATTR_SYNTAX: &str =
     "attributes on expressions are experimental";
 
@@ -504,6 +507,9 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_expr(&mut self, e: &'a ast::Expr) {
         match e.kind {
+            ast::ExprKind::Box(_) => {
+                gate_feature_post!(&self, box_syntax, e.span, EXPLAIN_BOX_SYNTAX);
+            }
             ast::ExprKind::Type(..) => {
                 // To avoid noise about type ascription in common syntax errors, only emit if it
                 // is the *only* error.

src/test/ui/feature-gates/feature-gate-box_syntax.rs

@@ -1,9 +1,6 @@
 // Test that the use of the box syntax is gated by `box_syntax` feature gate.
 
-#[cfg(FALSE)]
-fn foo() {
+fn main() {
     let x = box 3;
     //~^ ERROR box expression syntax is experimental; you can call `Box::new` instead
 }
-
-fn main() {}

src/test/ui/feature-gates/feature-gate-box_syntax.stderr

@@ -1,5 +1,5 @@
 error[E0658]: box expression syntax is experimental; you can call `Box::new` instead
-  --> $DIR/feature-gate-box_syntax.rs:5:13
+  --> $DIR/feature-gate-box_syntax.rs:4:13
    |
 LL |     let x = box 3;
    |             ^^^^^