Back out "Revert D91571264"

Summary:
Original commit changeset: 9a083f95c9c2

Original Phabricator Diff: D91780401

Reviewed By: christolliday

Differential Revision: D91840580

fbshipit-source-id: 23fd4c266dbf9a769aa804a031dce901dc126869

Author: cjhopman

pagable/src/impls/collections.rs

@@ -135,6 +135,31 @@ impl<'de, K: Ord + PagableDeserialize<'de>, V: PagableDeserialize<'de>> PagableD
     }
 }
 
+impl<V: PagableSerialize> PagableSerialize for std::collections::BTreeSet<V> {
+    fn pagable_serialize(&self, serializer: &mut dyn PagableSerializer) -> crate::Result<()> {
+        usize::serialize(&self.len(), serializer.serde())?;
+        for v in self {
+            v.pagable_serialize(serializer)?;
+        }
+        Ok(())
+    }
+}
+impl<'de, V: Ord + PagableDeserialize<'de>> PagableDeserialize<'de>
+    for std::collections::BTreeSet<V>
+{
+    fn pagable_deserialize<D: PagableDeserializer<'de> + ?Sized>(
+        deserializer: &mut D,
+    ) -> crate::Result<Self> {
+        let items = usize::deserialize(deserializer.serde())?;
+        let mut set = std::collections::BTreeSet::new();
+        for _ in 0..items {
+            let v = V::pagable_deserialize(deserializer)?;
+            set.insert(v);
+        }
+        Ok(set)
+    }
+}
+
 impl<T: PagableSerialize> PagableSerialize for Vec<T> {
     fn pagable_serialize(&self, serializer: &mut dyn PagableSerializer) -> crate::Result<()> {
         usize::serialize(&self.len(), serializer.serde())?;
@@ -304,4 +329,24 @@ mod tests {
         assert_eq!(map, restored);
         Ok(())
     }
+
+    #[test]
+    fn test_btreeset_roundtrip() -> crate::Result<()> {
+        use std::collections::BTreeSet;
+
+        let mut set: BTreeSet<String> = BTreeSet::new();
+        set.insert("one".to_owned());
+        set.insert("two".to_owned());
+        set.insert("three".to_owned());
+
+        let mut serializer = TestingSerializer::new();
+        set.pagable_serialize(&mut serializer)?;
+        let bytes = serializer.finish();
+
+        let mut deserializer = TestingDeserializer::new(&bytes);
+        let restored: BTreeSet<String> = BTreeSet::pagable_deserialize(&mut deserializer)?;
+
+        assert_eq!(set, restored);
+        Ok(())
+    }
 }