descriptor: fix Taproot tree descriptor formatting

fuzz/fuzz_targets/roundtrip_descriptor.rs

@@ -32,4 +32,9 @@ mod tests {
         let v = hex::decode_to_vec("abcd").unwrap();
         super::do_test(&v);
     }
+
+    #[test]
+    fn taproot_tree_format_regression() {
+        super::do_test(b"tr(acc0,{{multi_a(3,acc10,acc11,acc12),and_v(v:multi_a(2,acc10,acc11,acc12),after(10))},and_v(v:multi_a(1,acc10,acc11,ac12),after(100))})");
+    }
 }

src/descriptor/tr/mod.rs

@@ -519,20 +519,30 @@ mod tests {
 
     use super::*;
 
+    fn strip_ws(desc: &str) -> String { desc.replace(&[' ', '\n'][..], "") }
+
     fn descriptor() -> String {
         let desc = "tr(acc0, {
-            multi_a(3, acc10, acc11, acc12), {
+            {
+              multi_a(3, acc10, acc11, acc12),
               and_v(
                 v:multi_a(2, acc10, acc11, acc12),
                 after(10)
-              ),
-              and_v(
-                v:multi_a(1, acc10, acc11, ac12),
-                after(100)
               )
-            }
+            },
+            and_v(
+              v:multi_a(1, acc10, acc11, ac12),
+              after(100)
+            )
          })";
-        desc.replace(&[' ', '\n'][..], "")
+        strip_ws(desc)
+    }
+
+    fn assert_display_roundtrip(desc: &str) {
+        let desc = strip_ws(desc);
+        let tr = Tr::<String>::from_str(&desc).unwrap();
+        assert_eq!(format!("{:#}", tr), desc);
+        Tr::<String>::from_str(&tr.to_string()).unwrap();
     }
 
     #[test]
@@ -543,6 +553,13 @@ mod tests {
         assert!(!tr.for_each_key(|k| k.starts_with("acc")));
     }
 
+    #[test]
+    fn display_roundtrips_unbalanced_taptree() {
+        assert_display_roundtrip(&descriptor());
+        assert_display_roundtrip("tr(acc0,{pk(acc1),{pk(acc2),pk(acc3)}})");
+        assert_display_roundtrip("tr(acc0,{{pk(acc1),pk(acc2)},{pk(acc3),pk(acc4)}})");
+    }
+
     #[test]
     fn tr_maximum_depth() {
         // Copied from integration tests

src/descriptor/tr/taptree.rs

@@ -89,27 +89,34 @@ fn fmt_helper<Pk: MiniscriptKey>(
     f: &mut fmt::Formatter,
     mut fmt_ms: impl FnMut(&mut fmt::Formatter, &Miniscript<Pk, Tap>) -> fmt::Result,
 ) -> fmt::Result {
-    let mut last_depth = 0;
+    let mut child_counts = Vec::<u8>::with_capacity(TAPROOT_CONTROL_MAX_NODE_COUNT);
+
     for item in view.leaves() {
-        if last_depth > 0 {
+        let depth = usize::from(item.depth());
+
+        if !child_counts.is_empty() {
             f.write_str(",")?;
         }
 
-        while last_depth < item.depth() {
+        while child_counts.len() < depth {
             f.write_str("{")?;
-            last_depth += 1;
+            child_counts.push(0);
         }
+
         fmt_ms(f, item.miniscript())?;
-        while last_depth > item.depth() {
+
+        if let Some(child_count) = child_counts.last_mut() {
+            *child_count += 1;
+        }
+        while let Some(2) = child_counts.last() {
             f.write_str("}")?;
-            last_depth -= 1;
+            child_counts.pop();
+            if let Some(child_count) = child_counts.last_mut() {
+                *child_count += 1;
+            }
         }
     }
 
-    while last_depth > 0 {
-        f.write_str("}")?;
-        last_depth -= 1;
-    }
     Ok(())
 }
 
@@ -284,3 +291,28 @@ impl<Pk: MiniscriptKey> TapTreeBuilder<Pk> {
     #[inline]
     pub(super) fn finalize(self) -> TapTree<Pk> { TapTree { depths_leaves: self.depths_leaves } }
 }
+
+#[cfg(test)]
+mod tests {
+    use core::str::FromStr;
+
+    use super::*;
+
+    fn leaf(ms: &str) -> TapTree<String> {
+        TapTree::leaf(Arc::new(Miniscript::<String, Tap>::from_str(ms).unwrap()))
+    }
+
+    #[test]
+    fn display_single_leaf_taptree() {
+        let tree = leaf("pk(A)");
+
+        assert_eq!(format!("{}", tree), "pk(A)");
+    }
+
+    #[test]
+    fn debug_binary_taptree() {
+        let tree = TapTree::combine(leaf("pk(A)"), leaf("pk(B)")).unwrap();
+
+        assert_eq!(format!("{:?}", tree), "{[B/onduesm]pk(\"A\"),[B/onduesm]pk(\"B\")}");
+    }
+}