Implement TryFrom<TaprootBuilder> for TapTree

TryFrom` became available in Rust 1.34 so we can use it now we have
bumped our MSRV.

Implement `TryFrom<TaprootBuilder>` for `TapTree` and deprecate the
`from_builder` method.

src/util/psbt/map/output.rs

@@ -14,6 +14,7 @@
 
 use crate::prelude::*;
 use core;
+use core::convert::TryFrom;
 
 use crate::io;
 
@@ -158,16 +159,12 @@ impl TapTree {
     /// # Returns
     /// A [`TapTree`] iff the `builder` is complete, otherwise return [`IncompleteTapTree`]
     /// error with the content of incomplete `builder` instance.
+    #[deprecated(since = "0.29.0", note = "use try_from instead")]
     pub fn from_builder(builder: TaprootBuilder) -> Result<Self, IncompleteTapTree> {
-        if !builder.is_finalized() {
-            Err(IncompleteTapTree::NotFinalized(builder))
-        } else if builder.has_hidden_nodes() {
-            Err(IncompleteTapTree::HiddenParts(builder))
-        } else {
-            Ok(TapTree(builder))
-        }
+        Self::try_from(builder)
     }
 
+
     /// Converts self into builder [`TaprootBuilder`]. The builder is guaranteed to be finalized.
     pub fn into_builder(self) -> TaprootBuilder {
         self.0
@@ -194,6 +191,25 @@ impl TapTree {
     }
 }
 
+impl TryFrom<TaprootBuilder> for TapTree {
+    type Error = IncompleteTapTree;
+
+    /// Constructs [`TapTree`] from a [`TaprootBuilder`] if it is complete binary tree.
+    ///
+    /// # Returns
+    /// A [`TapTree`] iff the `builder` is complete, otherwise return [`IncompleteTapTree`]
+    /// error with the content of incomplete `builder` instance.
+    fn try_from(builder: TaprootBuilder) -> Result<Self, Self::Error> {
+        if !builder.is_finalized() {
+            Err(IncompleteTapTree::NotFinalized(builder))
+        } else if builder.has_hidden_nodes() {
+            Err(IncompleteTapTree::HiddenParts(builder))
+        } else {
+            Ok(TapTree(builder))
+        }
+    }
+}
+
 /// Iterator for a taproot script tree, operating in DFS order over leaf depth and
 /// leaf script pairs.
 pub struct TapTreeIter<'tree> {

src/util/psbt/serialize.rs

@@ -370,6 +370,8 @@ fn key_source_len(key_source: &KeySource) -> usize {
 
 #[cfg(test)]
 mod tests {
+    use core::convert::TryFrom;
+
     use crate::hashes::hex::FromHex;
     use super::*;
 
@@ -393,14 +395,14 @@ mod tests {
         let mut builder = compose_taproot_builder(0x51, &[2, 2, 2]);
         builder = builder.add_leaf_with_ver(3, Script::from_hex("b9").unwrap(), LeafVersion::from_consensus(0xC2).unwrap()).unwrap();
         builder = builder.add_hidden_node(3, sha256::Hash::default()).unwrap();
-        assert!(TapTree::from_builder(builder).is_err());
+        assert!(TapTree::try_from(builder).is_err());
     }
 
     #[test]
     fn taptree_roundtrip() {
         let mut builder = compose_taproot_builder(0x51, &[2, 2, 2, 3]);
         builder = builder.add_leaf_with_ver(3, Script::from_hex("b9").unwrap(), LeafVersion::from_consensus(0xC2).unwrap()).unwrap();
-        let tree = TapTree::from_builder(builder).unwrap();
+        let tree = TapTree::try_from(builder).unwrap();
         let tree_prime = TapTree::deserialize(&tree.serialize()).unwrap();
         assert_eq!(tree, tree_prime);
     }