Add a comment for arena_types!

src/librustc/arena.rs

@@ -6,6 +6,15 @@ use std::cell::RefCell;
 use std::marker::PhantomData;
 use smallvec::SmallVec;
 
+/// This declares a list of types which can be allocated by `Arena`.
+///
+/// The `few` modifier will cause allocation to use the shared arena and recording the destructor.
+/// This is faster and more memory efficient if there's only a few allocations of the type.
+/// Leaving `few` out will cause the type to get its own dedicated `TypedArena` which is
+/// faster and more memory efficient if there is lots of allocations.
+///
+/// Specifying the `decode` modifier will add decode impls for &T and &[T] where T is the type
+/// listed. These impls will appear in the implement_ty_decoder! macro.
 #[macro_export]
 macro_rules! arena_types {
     ($macro:path, $args:tt, $tcx:lifetime) => (