Add hardcoded override to ElectionScore (#455)

* Add hardcoded override to ElectionScore so that we can give it the traits it needs

* cargo fmt + another comment

* copy over custom trait impls too

codegen/src/api/mod.rs

@@ -147,6 +147,12 @@ impl RuntimeGenerator {
                 "frame_support::traits::misc::WrapperKeepOpaque",
                 parse_quote!(::subxt::WrapperKeepOpaque),
             ),
+            // We override this because it's used as a key in a BTreeMap, and so we
+            // need to implement some extra derives for it for that to compile.
+            (
+                "sp_npos_elections::ElectionScore",
+                parse_quote!(::subxt::ElectionScore),
+            ),
         ]
         .iter()
         .map(|(path, substitute): &(&str, syn::TypePath)| {

subxt/src/lib.rs

@@ -239,3 +239,41 @@ impl<T> PhantomDataSendSync<T> {
 
 unsafe impl<T> Send for PhantomDataSendSync<T> {}
 unsafe impl<T> Sync for PhantomDataSendSync<T> {}
+
+/// [`ElectionScore`] overrides any generated instance of `sp_npos_elections::ElectionScore` found
+/// in the metadata, so that we can add some extra derives required for it to be used as the key
+/// in a [`std::collections::BTreeMap`].
+#[derive(Encode, Decode, Debug, PartialEq, Eq)]
+pub struct ElectionScore {
+    /// The minimal winner, in terms of total backing stake.
+    ///
+    /// This parameter should be maximized.
+    pub minimal_stake: u128,
+    /// The sum of the total backing of all winners.
+    ///
+    /// This parameter should maximized
+    pub sum_stake: u128,
+    /// The sum squared of the total backing of all winners, aka. the variance.
+    ///
+    /// Ths parameter should be minimized.
+    pub sum_stake_squared: u128,
+}
+
+// These are copied from the original impl; they don't affect encoding/decoding but help
+// to keep the ordering the same if we then work with the decoded results.
+impl Ord for ElectionScore {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        // we delegate this to the lexicographic cmp of slices`, and to incorporate that we want the
+        // third element to be minimized, we swap them.
+        [self.minimal_stake, self.sum_stake, other.sum_stake_squared].cmp(&[
+            other.minimal_stake,
+            other.sum_stake,
+            self.sum_stake_squared,
+        ])
+    }
+}
+impl PartialOrd for ElectionScore {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}