Ignore digits when deriving BitMexPriceEventId's PartialOrd

This is important because deriving `PartialOrd` normally means that it
looks at all the struct's fields _in order_. For this type we want to
compare the `timestamp`, but do not care about the `digits`! More
importantly, someone could unintentionally change this behaviour by
reordering the fields.

We also have to ignore the `digits` field for other derived traits,
because inconsistencies there could cause trouble[1].

[1]: rust-lang/rust-clippy#1621.

Co-authored-by: Daniel Karzel <daniel.karzel@coblox.tech>

model/Cargo.toml

@@ -9,6 +9,7 @@ async-trait = "0.1"
 bdk = { version = "0.19.0", default-features = false }
 bdk-ext = { path = "../bdk-ext" }
 conquer-once = "0.3"
+derivative = "2"
 hex = "0.4"
 itertools = "0.10"
 libp2p-core = { version = "0.33", default-features = false, features = ["serde"] }

model/src/olivia.rs

@@ -1,6 +1,7 @@
 use anyhow::Context;
 use bdk::bitcoin::XOnlyPublicKey;
 use conquer_once::Lazy;
+use derivative::Derivative;
 use maia_core::secp256k1_zkp::SecretKey;
 use serde::Deserialize;
 use serde_with::DeserializeFromStr;
@@ -45,12 +46,17 @@ pub struct Attestation {
     pub scalars: Vec<SecretKey>,
 }
 
-#[derive(
-    Debug, Clone, Copy, SerializeDisplay, DeserializeFromStr, PartialEq, Eq, Hash, PartialOrd, Ord,
-)]
+#[derive(Derivative, Debug, Clone, Copy, SerializeDisplay, DeserializeFromStr)]
+#[derivative(PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct BitMexPriceEventId {
     /// The timestamp this price event refers to.
     timestamp: OffsetDateTime,
+    #[derivative(
+        PartialEq = "ignore",
+        PartialOrd = "ignore",
+        Ord = "ignore",
+        Hash = "ignore"
+    )]
     digits: usize,
 }