rdr: elaborate on how EII-s should be included in the public api hash

Author: susitsm

compiler/rustc_metadata/src/rmeta/encoder/public_api_hasher.rs

@@ -266,7 +266,12 @@ pub(crate) fn hash_crate_root_public_api(
     // FIXME do we need this?
     hasher.digest(has_default_lib_allocator, hcx);
 
-    // FIXME do we need this?
+    // Changing externally implementable items should cause recompiles in all downstream crates.
+    // FIXME EiiDecl and EiiImpl contain spans. Should changing the span of these items cause
+    // recompiles?
+    // FIXME eii-s are collected in `rustc_metadata::eii::collect`. We should probably stable sort
+    // that there to make the query result more stable (but sorting might be useless if this
+    // should be sensitive to span changes)
     let _ = externally_implementable_items;
     hasher.digest(tcx.externally_implementable_items(LOCAL_CRATE), hcx);