tr: add conversion from TrSpendInfo to bitcoin::TapTree

Moves a bit of ugly logic out of the PSBT module into the spendinfo
module so that it's available for other users. We can convert from a
TrSpendInfo to a bitcoin::TapTree but we can't do the opposite
conversion since TrSpendInfo expects to have a Miniscript for each leaf.

Author: apoelstra

src/descriptor/tr/spend_info.rs

@@ -189,6 +189,28 @@ impl<Pk: ToPublicKey> TrSpendInfo<Pk> {
             done_left_stack: BitStack128::default(),
         }
     }
+
+    /// If the Taproot tree is not keyspend-only, converts it to a [`bitcoin::taproot::TapTree`] structure.
+    ///
+    /// This conversion is not particularly efficient but the resulting data structure is
+    /// useful for interacting with PSBTs.
+    pub fn to_tap_tree(&self) -> Option<bitcoin::taproot::TapTree> {
+        if self.nodes.is_empty() {
+            return None;
+        }
+
+        let mut builder = bitcoin::taproot::TaprootBuilder::new();
+        for leaf in self.leaves() {
+            builder = builder
+                .add_leaf_with_ver(
+                    leaf.depth(),
+                    ScriptBuf::from(leaf.script()),
+                    leaf.leaf_version(),
+                )
+                .expect("iterating through tree in correct DFS order")
+        }
+        Some(bitcoin::taproot::TapTree::try_from(builder).expect("tree is complete"))
+    }
 }
 
 /// An internal node of the spend

src/psbt/mod.rs

@@ -1109,13 +1109,9 @@ fn update_item_with_descriptor_helper<F: PsbtFields>(
             *merkle_root = spend_info.merkle_root();
         }
 
-        let mut builder = taproot::TaprootBuilder::new();
         for leaf_derived in spend_info.leaves() {
             let leaf_script = (ScriptBuf::from(leaf_derived.script()), leaf_derived.leaf_version());
             let tapleaf_hash = leaf_derived.leaf_hash();
-            builder = builder
-                .add_leaf(leaf_derived.depth(), leaf_script.0.clone())
-                .expect("Computing spend data on a valid tree should always succeed");
             if let Some(tap_scripts) = item.tap_scripts() {
                 let control_block = leaf_derived.control_block().clone();
                 tap_scripts.insert(control_block, leaf_script);
@@ -1141,15 +1137,10 @@ fn update_item_with_descriptor_helper<F: PsbtFields>(
             tapleaf_hashes.dedup();
         }
 
-        match item.tap_tree() {
-            // Only set the tap_tree if the item supports it (it's an output) and the descriptor actually
-            // contains one, otherwise it'll just be empty
-            Some(tap_tree) if tr_derived.tap_tree().is_some() => {
-                *tap_tree = Some(
-                    taproot::TapTree::try_from(builder).expect("The tree should always be valid"),
-                );
-            }
-            _ => {}
+        // Only set the tap_tree if the item supports it (it's an output) and the descriptor actually
+        // contains one, otherwise it'll just be empty
+        if let Some(tap_tree) = item.tap_tree() {
+            *tap_tree = spend_info.to_tap_tree();
         }
     } else {
         item.bip32_derivation().append(&mut bip32_derivation.0);