Move for loop desugaring to lowering

src/libsyntax/ast.rs

@@ -375,6 +375,10 @@ pub const CRATE_NODE_ID: NodeId = 0;
 /// small, positive ids.
 pub const DUMMY_NODE_ID: NodeId = !0;
 
+pub trait NodeIdAssigner {
+    fn next_node_id(&self) -> NodeId;
+}
+
 /// The AST represents all type param bounds as types.
 /// typeck::collect::compute_bounds matches these against
 /// the "special" built-in traits (see middle::lang_items) and