Add some comments

Author: Zoxc

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -1341,7 +1341,8 @@ impl DepNodeColorMap {
     }
 
     /// This tries to atomically mark a node green and assign `index` as the new
-    /// index.
+    /// index. This returns `Ok` if `index` gets assigned, otherwise it returns
+    /// the alreadly allocated index in `Err`.
     #[inline]
     pub(super) fn try_mark_green(
         &self,

compiler/rustc_query_system/src/dep_graph/serialized.rs

@@ -585,6 +585,7 @@ impl<D: Deps> EncoderState<D> {
         DepNodeIndex::from_u32(local.next_node_index)
     }
 
+    /// Marks the index previously returned by `next_index` as used.
     #[inline]
     fn bump_index(&self, local: &mut LocalEncoderState) {
         local.remaining_node_index -= 1;