key: replace ConversionError with NonDefiniteKeyError

The new error type is now more descriptive and a strict superset
of the old one. Furthermore, the new name is more appropriate since
this error typically arises during construction of a DefiniteDescriptorKey
rather than during conversion between different key types.

Author: apoelstra

src/descriptor/key.rs

@@ -689,33 +689,6 @@ impl From<PublicKey> for DescriptorPublicKey {
     }
 }
 
-/// Descriptor key conversion error
-#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy)]
-pub enum ConversionError {
-    /// Attempted to convert a key with hardened derivations to a bitcoin public key
-    HardenedChild,
-    /// Attempted to convert a key with multiple derivation paths to a bitcoin public key
-    MultiKey,
-}
-
-impl fmt::Display for ConversionError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.write_str(match *self {
-            Self::HardenedChild => "hardened child step in bip32 path",
-            Self::MultiKey => "multiple existing keys",
-        })
-    }
-}
-
-#[cfg(feature = "std")]
-impl error::Error for ConversionError {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        match self {
-            Self::HardenedChild | Self::MultiKey => None,
-        }
-    }
-}
-
 impl DescriptorPublicKey {
     /// The fingerprint of the master key associated with this key, `0x00000000` if none.
     pub fn master_fingerprint(&self) -> bip32::Fingerprint {
@@ -849,7 +822,10 @@ impl DescriptorPublicKey {
     ///
     /// - If `index` is hardened.
     /// - If the key contains multi-path derivations
-    pub fn at_derivation_index(self, index: u32) -> Result<DefiniteDescriptorKey, ConversionError> {
+    pub fn at_derivation_index(
+        self,
+        index: u32,
+    ) -> Result<DefiniteDescriptorKey, NonDefiniteKeyError> {
         let definite = match self {
             DescriptorPublicKey::Single(_) => self,
             DescriptorPublicKey::XPub(xpub) => {
@@ -858,12 +834,12 @@ impl DescriptorPublicKey {
                     Wildcard::Unhardened => xpub.derivation_path.into_child(
                         bip32::ChildNumber::from_normal_idx(index)
                             .ok()
-                            .ok_or(ConversionError::HardenedChild)?,
+                            .ok_or(NonDefiniteKeyError::HardenedStep)?,
                     ),
                     Wildcard::Hardened => xpub.derivation_path.into_child(
                         bip32::ChildNumber::from_hardened_idx(index)
                             .ok()
-                            .ok_or(ConversionError::HardenedChild)?,
+                            .ok_or(NonDefiniteKeyError::HardenedStep)?,
                     ),
                 };
                 DescriptorPublicKey::XPub(DescriptorXKey {
@@ -873,7 +849,7 @@ impl DescriptorPublicKey {
                     wildcard: Wildcard::None,
                 })
             }
-            DescriptorPublicKey::MultiXPub(_) => return Err(ConversionError::MultiKey),
+            DescriptorPublicKey::MultiXPub(_) => return Err(NonDefiniteKeyError::Multipath),
         };
 
         Ok(DefiniteDescriptorKey::new(definite)

src/descriptor/mod.rs

@@ -53,9 +53,9 @@ pub mod checksum;
 mod key;
 
 pub use self::key::{
-    ConversionError, DefiniteDescriptorKey, DerivPaths, DescriptorKeyParseError,
-    DescriptorMultiXKey, DescriptorPublicKey, DescriptorSecretKey, DescriptorXKey, InnerXKey,
-    MalformedKeyDataKind, SinglePriv, SinglePub, SinglePubKey, Wildcard,
+    DefiniteDescriptorKey, DerivPaths, DescriptorKeyParseError, DescriptorMultiXKey,
+    DescriptorPublicKey, DescriptorSecretKey, DescriptorXKey, InnerXKey, MalformedKeyDataKind,
+    NonDefiniteKeyError, SinglePriv, SinglePub, SinglePubKey, Wildcard,
 };
 
 /// Alias type for a map of public key to secret key
@@ -660,17 +660,14 @@ impl Descriptor<DescriptorPublicKey> {
     pub fn at_derivation_index(
         &self,
         index: u32,
-    ) -> Result<Descriptor<DefiniteDescriptorKey>, ConversionError> {
+    ) -> Result<Descriptor<DefiniteDescriptorKey>, NonDefiniteKeyError> {
         struct Derivator(u32);
 
         impl Translator<DescriptorPublicKey> for Derivator {
             type TargetPk = DefiniteDescriptorKey;
-            type Error = ConversionError;
+            type Error = NonDefiniteKeyError;
 
-            fn pk(
-                &mut self,
-                pk: &DescriptorPublicKey,
-            ) -> Result<DefiniteDescriptorKey, ConversionError> {
+            fn pk(&mut self, pk: &DescriptorPublicKey) -> Result<Self::TargetPk, Self::Error> {
                 pk.clone().at_derivation_index(self.0)
             }
 
@@ -711,7 +708,7 @@ impl Descriptor<DescriptorPublicKey> {
         &self,
         secp: &secp256k1::Secp256k1<C>,
         index: u32,
-    ) -> Result<Descriptor<bitcoin::PublicKey>, ConversionError> {
+    ) -> Result<Descriptor<bitcoin::PublicKey>, NonDefiniteKeyError> {
         Ok(self.at_derivation_index(index)?.derived_descriptor(secp))
     }
 
@@ -852,7 +849,7 @@ impl Descriptor<DescriptorPublicKey> {
         secp: &secp256k1::Secp256k1<C>,
         script_pubkey: &Script,
         range: Range<u32>,
-    ) -> Result<Option<(u32, Descriptor<bitcoin::PublicKey>)>, ConversionError> {
+    ) -> Result<Option<(u32, Descriptor<bitcoin::PublicKey>)>, NonDefiniteKeyError> {
         let range = if self.has_wildcard() { range } else { 0..1 };
 
         for i in range {

src/psbt/mod.rs

@@ -925,14 +925,14 @@ pub trait PsbtInputExt {
     fn update_with_descriptor_unchecked(
         &mut self,
         descriptor: &Descriptor<DefiniteDescriptorKey>,
-    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::ConversionError>;
+    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::NonDefiniteKeyError>;
 }
 
 impl PsbtInputExt for psbt::Input {
     fn update_with_descriptor_unchecked(
         &mut self,
         descriptor: &Descriptor<DefiniteDescriptorKey>,
-    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::ConversionError> {
+    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::NonDefiniteKeyError> {
         let (derived, _) = update_item_with_descriptor_helper(self, descriptor, None)?;
         Ok(derived)
     }
@@ -959,14 +959,14 @@ pub trait PsbtOutputExt {
     fn update_with_descriptor_unchecked(
         &mut self,
         descriptor: &Descriptor<DefiniteDescriptorKey>,
-    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::ConversionError>;
+    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::NonDefiniteKeyError>;
 }
 
 impl PsbtOutputExt for psbt::Output {
     fn update_with_descriptor_unchecked(
         &mut self,
         descriptor: &Descriptor<DefiniteDescriptorKey>,
-    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::ConversionError> {
+    ) -> Result<Descriptor<bitcoin::PublicKey>, descriptor::NonDefiniteKeyError> {
         let (derived, _) = update_item_with_descriptor_helper(self, descriptor, None)?;
         Ok(derived)
     }
@@ -1083,10 +1083,10 @@ fn update_item_with_descriptor_helper<F: PsbtFields>(
     check_script: Option<&Script>,
     // We return an extra boolean here to indicate an error with `check_script`. We do this
     // because the error is "morally" a UtxoUpdateError::MismatchedScriptPubkey, but some
-    // callers expect a `descriptor::ConversionError`, which cannot be produced from a
+    // callers expect a `descriptor::NonDefiniteKeyError`, which cannot be produced from a
     // `UtxoUpdateError`, and those callers can't get this error anyway because they pass
     // `None` for `check_script`.
-) -> Result<(Descriptor<bitcoin::PublicKey>, bool), descriptor::ConversionError> {
+) -> Result<(Descriptor<bitcoin::PublicKey>, bool), descriptor::NonDefiniteKeyError> {
     let secp = Secp256k1::verification_only();
 
     // 1. Derive the descriptor, recording each key derivation in a map from xpubs
@@ -1182,7 +1182,7 @@ pub enum UtxoUpdateError {
     /// The unsigned transaction didn't have an input at that index
     MissingInputUtxo,
     /// Derivation error
-    DerivationError(descriptor::ConversionError),
+    DerivationError(descriptor::NonDefiniteKeyError),
     /// The PSBT's `witness_utxo` and/or `non_witness_utxo` were invalid or missing
     UtxoCheck,
     /// The PSBT's `witness_utxo` and/or `non_witness_utxo` had a script_pubkey that did not match
@@ -1231,7 +1231,7 @@ pub enum OutputUpdateError {
     /// The raw unsigned transaction didn't have an output at that index
     MissingTxOut,
     /// Derivation error
-    DerivationError(descriptor::ConversionError),
+    DerivationError(descriptor::NonDefiniteKeyError),
     /// The output's script_pubkey did not match the descriptor
     MismatchedScriptPubkey,
 }