feat(core): Add TxUpdate::missing

This is a set of txids missing from the mempool.

Author: evanlinjin

crates/core/src/tx_update.rs

@@ -1,4 +1,4 @@
-use crate::collections::{BTreeMap, BTreeSet, HashMap};
+use crate::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use alloc::{sync::Arc, vec::Vec};
 use bitcoin::{OutPoint, Transaction, TxOut, Txid};
 
@@ -34,6 +34,8 @@ pub struct TxUpdate<A = ()> {
     /// Seen at times for transactions. This records when a transaction was most recently seen in
     /// the user's mempool for the sake of tie-breaking other conflicting transactions.
     pub seen_ats: HashMap<Txid, u64>,
+    /// A set of txids missing from the mempool.
+    pub missing: HashSet<Txid>,
 }
 
 impl<A> Default for TxUpdate<A> {
@@ -43,6 +45,7 @@ impl<A> Default for TxUpdate<A> {
             txouts: Default::default(),
             anchors: Default::default(),
             seen_ats: Default::default(),
+            missing: Default::default(),
         }
     }
 }
@@ -62,6 +65,7 @@ impl<A: Ord> TxUpdate<A> {
                 .map(|(a, txid)| (map(a), txid))
                 .collect(),
             seen_ats: self.seen_ats,
+            missing: self.missing,
         }
     }