Add comment about the lack of ExpnData serialization for proc-macro crates

src/librustc_metadata/rmeta/encoder.rs

@@ -280,6 +280,10 @@ impl<'a, 'tcx> SpecializedEncoder<Span> for EncodeContext<'a, 'tcx> {
         // cross-crate inconsistencies (getting one behavior in the same
         // crate, and a different behavior in another crate) due to the
         // limited surface that proc-macros can expose.
+        //
+        // IMPORTANT: If this is ever changed, be sure to update
+        // `rustc_span::hygiene::raw_encode_expn_id` to handle
+        // encoding `ExpnData` for proc-macro crates.
         if self.is_proc_macro {
             SyntaxContext::root().encode(self)?;
         } else {

src/librustc_span/hygiene.rs

@@ -1188,6 +1188,9 @@ pub fn raw_encode_expn_id<E: Encoder>(
             let data = expn.expn_data();
             // We only need to serialize the ExpnData
             // if it comes from this crate.
+            // We currently don't serialize any hygiene information data for
+            // proc-macro crates: see the `SpecializedEncoder<Span>` impl
+            // for crate metadata.
             if data.krate == LOCAL_CRATE {
                 needs_data();
             }