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src/libsyntax/ast.rs

@@ -429,6 +429,12 @@ pub struct Crate {
     pub module: Mod,
     pub attrs: Vec<Attribute>,
     pub span: Span,
+    /// The order of items in the HIR is unrelated to the order of
+    /// items in the AST. However, we generate proc macro harnesses
+    /// based on the AST order, and later refer to these harnesses
+    /// from the HIR. This field keeps track of the order in which
+    /// we generated proc macros harnesses, so that we can map
+    /// HIR proc macros items back to their harness items.
     pub proc_macros: Vec<NodeId>,
 }