Merge #751: descriptor: introduce several Taproot accessors

86305f0 ci: clippy: remove a ton of now-elidable lifetimes (Andrew Poelstra)
dd3396e ci: fix new clippy lint related to doccomments (Andrew Poelstra)
998b423 descriptor: introduce several Taproot accessors (Andrew Poelstra)

Pull request description:

  When working with Taproot descriptors you typically need to do an annoying (and hard to discover) `match` statement to get the `Tr` out of the descriptor, and then call accessors on that to get the actual data out.

  Add two new methods to `Descriptor` that directly access the internal key and the taptree. Document that the actual leaves can be obtained by calling `.iter` on the taptree.

  Next, when a user is trying to sign a Taproot branch, they need to obtain a TapLeafHash. We have internal code which does this (which I have pulled into a helper function since there is some room to optimize it there..) but no exposed code, forcing the user to go digging through the rust-bitcoin docs to figure it out (including knowing the standard Taproot leaf version, which is an arcane detail of the sort that Miniscript otherwise hides).

  Add a new method `leaf_hash` on Taproot miniscripts, so that the user can directly obtain the leaf hashes.

  Now you can write e.g.

  ```rust
  for script in trdesc.tap_tree_iter() {
      let leaf_hash = script.leaf_hash();
      // Do whatever you want...
  }
  ```

  vs the previous code which was roughly

  ```rust
  let tr = match trdesc {
      Descriptor::Tr(ref tr) => tr,
      _ => unreachable!("I know this is a Taproot descriptor"),
  };
  // Or tr.tap_tree().unwrap().iter() in case you miss the weirdly-named // Tr::iter_scripts
  for script in tr.iter_scripts() {
      // Hope you know your rust-bitcoin docs by heart, and also that
      // .encode is the way to convert a Miniscript to a Script!
      let leaf_hash = TapLeafHash::from_script(
          LeafVersion::TapScript,
          script.encode(),
      );
  }
  ```

ACKs for top commit:
  sanket1729:
    ACK 86305f0

Tree-SHA512: a6d5ca4d63222e03520f08ab1835d5ad292f6fcd021f517427a56d7d53ecf28c975acd2c8bdd0e2d0b9741cf79d34d352cb775b59bd21600b0fb9daa06aced8d

Author: apoelstra

src/descriptor/checksum.rs

@@ -177,7 +177,7 @@ impl<'f, 'a> Formatter<'f, 'a> {
     }
 }
 
-impl<'f, 'a> fmt::Write for Formatter<'f, 'a> {
+impl fmt::Write for Formatter<'_, '_> {
     fn write_str(&mut self, s: &str) -> fmt::Result {
         self.fmt.write_str(s)?;
         self.eng.input(s).map_err(|_| fmt::Error)

src/descriptor/mod.rs

@@ -241,6 +241,40 @@ impl<Pk: MiniscriptKey> Descriptor<Pk> {
         Ok(Descriptor::Tr(Tr::new(key, script)?))
     }
 
+    /// For a Taproot descriptor, returns the internal key.
+    pub fn internal_key(&self) -> Option<&Pk> {
+        if let Descriptor::Tr(ref tr) = self {
+            Some(tr.internal_key())
+        } else {
+            None
+        }
+    }
+
+    /// For a Taproot descriptor, returns the [`TapTree`] describing the Taproot tree.
+    ///
+    /// To obtain the individual leaves of the tree, call [`TapTree::iter`] on the
+    /// returned value.
+    pub fn tap_tree(&self) -> Option<&TapTree<Pk>> {
+        if let Descriptor::Tr(ref tr) = self {
+            tr.tap_tree().as_ref()
+        } else {
+            None
+        }
+    }
+
+    /// For a Taproot descriptor, returns an iterator over the scripts in the Taptree.
+    ///
+    /// If the descriptor is not a Taproot descriptor, **or** if the descriptor is a
+    /// Taproot descriptor containing only a keyspend, returns an empty iterator.
+    pub fn tap_tree_iter(&self) -> tr::TapTreeIter<Pk> {
+        if let Descriptor::Tr(ref tr) = self {
+            if let Some(ref tree) = tr.tap_tree() {
+                return tree.iter();
+            }
+        }
+        tr::TapTreeIter::empty()
+    }
+
     /// Get the [DescriptorType] of [Descriptor]
     pub fn desc_type(&self) -> DescriptorType {
         match *self {
@@ -698,7 +732,7 @@ impl Descriptor<DescriptorPublicKey> {
 
         struct KeyMapWrapper<'a, C: secp256k1::Signing>(KeyMap, &'a secp256k1::Secp256k1<C>);
 
-        impl<'a, C: secp256k1::Signing> Translator<String> for KeyMapWrapper<'a, C> {
+        impl<C: secp256k1::Signing> Translator<String> for KeyMapWrapper<'_, C> {
             type TargetPk = DescriptorPublicKey;
             type Error = Error;
 
@@ -746,7 +780,7 @@ impl Descriptor<DescriptorPublicKey> {
     pub fn to_string_with_secret(&self, key_map: &KeyMap) -> String {
         struct KeyMapLookUp<'a>(&'a KeyMap);
 
-        impl<'a> Translator<DescriptorPublicKey> for KeyMapLookUp<'a> {
+        impl Translator<DescriptorPublicKey> for KeyMapLookUp<'_> {
             type TargetPk = String;
             type Error = core::convert::Infallible;
 
@@ -909,7 +943,7 @@ impl Descriptor<DefiniteDescriptorKey> {
     ) -> Result<Descriptor<bitcoin::PublicKey>, ConversionError> {
         struct Derivator<'a, C: secp256k1::Verification>(&'a secp256k1::Secp256k1<C>);
 
-        impl<'a, C: secp256k1::Verification> Translator<DefiniteDescriptorKey> for Derivator<'a, C> {
+        impl<C: secp256k1::Verification> Translator<DefiniteDescriptorKey> for Derivator<'_, C> {
             type TargetPk = bitcoin::PublicKey;
             type Error = ConversionError;
 

src/descriptor/tr.rs

@@ -465,6 +465,11 @@ pub struct TapTreeIter<'a, Pk: MiniscriptKey> {
     stack: Vec<(u8, &'a TapTree<Pk>)>,
 }
 
+impl<Pk: MiniscriptKey> TapTreeIter<'_, Pk> {
+    /// Helper function to return an empty iterator from Descriptor::tap_tree_iter.
+    pub(super) fn empty() -> Self { Self { stack: vec![] } }
+}
+
 impl<'a, Pk> Iterator for TapTreeIter<'a, Pk>
 where
     Pk: MiniscriptKey + 'a,

src/miniscript/iter.rs

@@ -176,7 +176,7 @@ impl<'a, Pk: MiniscriptKey, Ctx: ScriptContext> PkIter<'a, Pk, Ctx> {
     }
 }
 
-impl<'a, Pk: MiniscriptKey, Ctx: ScriptContext> Iterator for PkIter<'a, Pk, Ctx> {
+impl<Pk: MiniscriptKey, Ctx: ScriptContext> Iterator for PkIter<'_, Pk, Ctx> {
     type Item = Pk;
 
     fn next(&mut self) -> Option<Self::Item> {
@@ -199,22 +199,24 @@ impl<'a, Pk: MiniscriptKey, Ctx: ScriptContext> Iterator for PkIter<'a, Pk, Ctx>
     }
 }
 
-// Module is public since it export testcase generation which may be used in
-// dependent libraries for their own tasts based on Miniscript AST
+/// Module is public since it export testcase generation which may be used in
+/// dependent libraries for their own tasts based on Miniscript AST
 #[cfg(test)]
 pub mod test {
     use bitcoin::hashes::{hash160, ripemd160, sha256, sha256d, Hash};
 
     use super::Miniscript;
     use crate::miniscript::context::Segwitv0;
 
+    /// Test case.
     pub type TestData = (
         Miniscript<bitcoin::PublicKey, Segwitv0>,
         Vec<bitcoin::PublicKey>,
         Vec<hash160::Hash>,
         bool, // Indicates that the top-level contains public key or hashes
     );
 
+    /// Generate a deterministic list of public keys of the given length.
     pub fn gen_secp_pubkeys(n: usize) -> Vec<secp256k1::PublicKey> {
         let mut ret = Vec::with_capacity(n);
         let secp = secp256k1::Secp256k1::new();
@@ -233,13 +235,15 @@ pub mod test {
         ret
     }
 
+    /// Generate a deterministic list of Bitcoin public keys of the given length.
     pub fn gen_bitcoin_pubkeys(n: usize, compressed: bool) -> Vec<bitcoin::PublicKey> {
         gen_secp_pubkeys(n)
             .into_iter()
             .map(|inner| bitcoin::PublicKey { inner, compressed })
             .collect()
     }
 
+    /// Generate a deterministic list of test cases of the given length.
     pub fn gen_testcases() -> Vec<TestData> {
         let k = gen_bitcoin_pubkeys(10, true);
         let _h: Vec<hash160::Hash> = k

src/miniscript/lex.rs

@@ -50,7 +50,7 @@ pub enum Token<'s> {
     Bytes65(&'s [u8]),
 }
 
-impl<'s> fmt::Display for Token<'s> {
+impl fmt::Display for Token<'_> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
             Token::Num(n) => write!(f, "#{}", n),

src/miniscript/mod.rs

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: CC0-1.0
 
-//! # Abstract Syntax Tree
+//! Abstract Syntax Tree
 //!
 //! Defines a variety of data structures for describing Miniscript, a subset of
 //! Bitcoin Script which can be efficiently parsed and serialized from Script,
@@ -289,16 +289,27 @@ impl<Pk: MiniscriptKey, Ctx: ScriptContext> Miniscript<Pk, Ctx> {
         Ctx::max_satisfaction_size(self).ok_or(Error::ImpossibleSatisfaction)
     }
 
+    /// Helper function to produce Taproot leaf hashes
+    fn leaf_hash_internal(&self) -> TapLeafHash
+    where
+        Pk: ToPublicKey,
+    {
+        TapLeafHash::from_script(&self.encode(), LeafVersion::TapScript)
+    }
+
     /// Attempt to produce non-malleable satisfying witness for the
     /// witness script represented by the parse tree
     pub fn satisfy<S: satisfy::Satisfier<Pk>>(&self, satisfier: S) -> Result<Vec<Vec<u8>>, Error>
     where
         Pk: ToPublicKey,
     {
         // Only satisfactions for default versions (0xc0) are allowed.
-        let leaf_hash = TapLeafHash::from_script(&self.encode(), LeafVersion::TapScript);
-        let satisfaction =
-            satisfy::Satisfaction::satisfy(&self.node, &satisfier, self.ty.mall.safe, &leaf_hash);
+        let satisfaction = satisfy::Satisfaction::satisfy(
+            &self.node,
+            &satisfier,
+            self.ty.mall.safe,
+            &self.leaf_hash_internal(),
+        );
         self._satisfy(satisfaction)
     }
 
@@ -311,12 +322,11 @@ impl<Pk: MiniscriptKey, Ctx: ScriptContext> Miniscript<Pk, Ctx> {
     where
         Pk: ToPublicKey,
     {
-        let leaf_hash = TapLeafHash::from_script(&self.encode(), LeafVersion::TapScript);
         let satisfaction = satisfy::Satisfaction::satisfy_mall(
             &self.node,
             &satisfier,
             self.ty.mall.safe,
-            &leaf_hash,
+            &self.leaf_hash_internal(),
         );
         self._satisfy(satisfaction)
     }
@@ -344,8 +354,12 @@ impl<Pk: MiniscriptKey, Ctx: ScriptContext> Miniscript<Pk, Ctx> {
     where
         Pk: ToPublicKey,
     {
-        let leaf_hash = TapLeafHash::from_script(&self.encode(), LeafVersion::TapScript);
-        satisfy::Satisfaction::build_template(&self.node, provider, self.ty.mall.safe, &leaf_hash)
+        satisfy::Satisfaction::build_template(
+            &self.node,
+            provider,
+            self.ty.mall.safe,
+            &self.leaf_hash_internal(),
+        )
     }
 
     /// Attempt to produce a malleable witness template given the assets available
@@ -356,16 +370,22 @@ impl<Pk: MiniscriptKey, Ctx: ScriptContext> Miniscript<Pk, Ctx> {
     where
         Pk: ToPublicKey,
     {
-        let leaf_hash = TapLeafHash::from_script(&self.encode(), LeafVersion::TapScript);
         satisfy::Satisfaction::build_template_mall(
             &self.node,
             provider,
             self.ty.mall.safe,
-            &leaf_hash,
+            &self.leaf_hash_internal(),
         )
     }
 }
 
+impl Miniscript<<Tap as ScriptContext>::Key, Tap> {
+    /// Returns the leaf hash used within a Taproot signature for this script.
+    ///
+    /// Note that this method is only implemented for Taproot Miniscripts.
+    pub fn leaf_hash(&self) -> TapLeafHash { self.leaf_hash_internal() }
+}
+
 impl<Ctx: ScriptContext> Miniscript<Ctx::Key, Ctx> {
     /// Attempt to parse an insane(scripts don't clear sanity checks)
     /// script into a Miniscript representation.

src/miniscript/satisfy.rs

@@ -262,7 +262,7 @@ impl_satisfier_for_map_hash_tapleafhash_to_key_taproot_sig! {
     impl Satisfier<Pk> for HashMap<(hash160::Hash, TapLeafHash), (Pk, bitcoin::taproot::Signature)>
 }
 
-impl<'a, Pk: MiniscriptKey + ToPublicKey, S: Satisfier<Pk>> Satisfier<Pk> for &'a S {
+impl<Pk: MiniscriptKey + ToPublicKey, S: Satisfier<Pk>> Satisfier<Pk> for &S {
     fn lookup_ecdsa_sig(&self, p: &Pk) -> Option<bitcoin::ecdsa::Signature> {
         (**self).lookup_ecdsa_sig(p)
     }
@@ -322,7 +322,7 @@ impl<'a, Pk: MiniscriptKey + ToPublicKey, S: Satisfier<Pk>> Satisfier<Pk> for &'
     fn check_after(&self, n: absolute::LockTime) -> bool { (**self).check_after(n) }
 }
 
-impl<'a, Pk: MiniscriptKey + ToPublicKey, S: Satisfier<Pk>> Satisfier<Pk> for &'a mut S {
+impl<Pk: MiniscriptKey + ToPublicKey, S: Satisfier<Pk>> Satisfier<Pk> for &mut S {
     fn lookup_ecdsa_sig(&self, p: &Pk) -> Option<bitcoin::ecdsa::Signature> {
         (**self).lookup_ecdsa_sig(p)
     }

src/plan.rs

@@ -129,7 +129,7 @@ macro_rules! impl_log_method {
 }
 
 #[cfg(feature = "std")]
-impl<'a> AssetProvider<DefiniteDescriptorKey> for LoggerAssetProvider<'a> {
+impl AssetProvider<DefiniteDescriptorKey> for LoggerAssetProvider<'_> {
     impl_log_method!(provider_lookup_ecdsa_sig, pk: &DefiniteDescriptorKey, -> bool);
     impl_log_method!(provider_lookup_tap_key_spend_sig, pk: &DefiniteDescriptorKey, -> Option<usize>);
     impl_log_method!(provider_lookup_tap_leaf_script_sig, pk: &DefiniteDescriptorKey, leaf_hash: &TapLeafHash, -> Option<usize>);

src/primitives/threshold.rs

@@ -263,7 +263,7 @@ struct ThreshDisplay<'t, 's, T, const MAX: usize> {
     show_k: bool,
 }
 
-impl<'t, 's, T, const MAX: usize> fmt::Display for ThreshDisplay<'t, 's, T, MAX>
+impl<T, const MAX: usize> fmt::Display for ThreshDisplay<'_, '_, T, MAX>
 where
     T: fmt::Display,
 {
@@ -286,7 +286,7 @@ where
     }
 }
 
-impl<'t, 's, T, const MAX: usize> fmt::Debug for ThreshDisplay<'t, 's, T, MAX>
+impl<T, const MAX: usize> fmt::Debug for ThreshDisplay<'_, '_, T, MAX>
 where
     T: fmt::Debug,
 {

src/psbt/mod.rs

@@ -254,7 +254,7 @@ impl<'psbt> PsbtInputSatisfier<'psbt> {
     pub fn new(psbt: &'psbt Psbt, index: usize) -> Self { Self { psbt, index } }
 }
 
-impl<'psbt, Pk: MiniscriptKey + ToPublicKey> Satisfier<Pk> for PsbtInputSatisfier<'psbt> {
+impl<Pk: MiniscriptKey + ToPublicKey> Satisfier<Pk> for PsbtInputSatisfier<'_> {
     fn lookup_tap_key_spend_sig(&self) -> Option<bitcoin::taproot::Signature> {
         self.psbt.inputs[self.index].tap_key_sig
     }