Merge rust-bitcoin/rust-bitcoin#987: Implement `std::error::Error` for the new MSRV

97a5bb1439 Implement std::error::source codebase wide (Tobin C. Harding)
0a9191b429 Add parenthesis around left hand side of companion (Tobin C. Harding)
7cf8af2f86 Put Error impl block below Display (Tobin C. Harding)
2384712364 Re-order Display match arms (Tobin C. Harding)

Pull request description:

  Now that we have MSRV of 1.41.1 we should use `source` instead of `cause`. Audit the whole codebase and implement `source` for _every_ error type we have.

  The first three patches are preparatory cleanup, patch 3 is particularly shameful (adds parenthesis to make my editor work).

  CC @Kixunil because he is championing the error stuff.

ACKs for top commit:
  apoelstra:
    ACK 97a5bb1439

Tree-SHA512: 46313a28929445f32e01e30ca3b0246b30bc9d5e43db5754d4b441e9c30d3e427efaf247100eb6b452f98beec5a4fcde1daba7943a772114aa34f78ab52cbc60

src/blockdata/block.rs

@@ -355,7 +355,16 @@ impl fmt::Display for Bip34Error {
 }
 
 #[cfg(feature = "std")]
-impl ::std::error::Error for Bip34Error {}
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Bip34Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Bip34Error::*;
+
+        match self {
+            Unsupported | NotPresent | UnexpectedPush(_) => None,
+        }
+    }
+}
 
 #[cfg(test)]
 mod tests {

src/blockdata/script.rs

@@ -188,7 +188,20 @@ impl fmt::Display for Error {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for Error {}
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            NonMinimalPush
+            | EarlyEndOfScript
+            | NumericOverflow
+            | BitcoinConsensus(_) // TODO: This should return `Some` but bitcoinconsensus::Error does not implement Error.
+            | UnknownSpentOutput(_)
+            | SerializationError => None,
+        }
+    }
+}
 
 // Our internal error proves that we only return these two cases from `read_uint_iter`.
 // Since it's private we don't bother with trait impls besides From.

src/blockdata/transaction.rs

@@ -27,7 +27,6 @@ use crate::prelude::*;
 
 use crate::io;
 use core::{fmt, str, default::Default};
-#[cfg(feature = "std")] use std::error;
 
 use crate::hashes::{self, Hash, sha256d};
 use crate::hashes::hex::FromHex;
@@ -142,12 +141,14 @@ impl fmt::Display for ParseOutPointError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl error::Error for ParseOutPointError {
-    fn cause(&self) -> Option<&dyn  error::Error> {
-        match *self {
-            ParseOutPointError::Txid(ref e) => Some(e),
-            ParseOutPointError::Vout(ref e) => Some(e),
-            _ => None,
+impl std::error::Error for ParseOutPointError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::ParseOutPointError::*;
+
+        match self {
+            Txid(e) => Some(e),
+            Vout(e) => Some(e),
+            Format | TooLong | VoutNotCanonical => None,
         }
     }
 }
@@ -733,7 +734,11 @@ impl fmt::Display for NonStandardSighashType {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl error::Error for NonStandardSighashType {}
+impl std::error::Error for NonStandardSighashType {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        None
+    }
+}
 
 /// Legacy Hashtype of an input's signature
 #[deprecated(since = "0.28.0", note = "Please use [`EcdsaSighashType`] instead")]
@@ -886,9 +891,13 @@ impl fmt::Display for SighashTypeParseError {
     }
 }
 
-#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 #[cfg(feature = "std")]
-impl ::std::error::Error for SighashTypeParseError {}
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for SighashTypeParseError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        None
+    }
+}
 
 #[cfg(test)]
 mod tests {

src/consensus/encode.rs

@@ -30,7 +30,6 @@
 use crate::prelude::*;
 
 use core::{fmt, mem, u32, convert::From};
-#[cfg(feature = "std")] use std::error;
 
 use crate::hashes::{sha256d, Hash, sha256};
 use crate::hash_types::{BlockHash, FilterHash, TxMerkleNode, FilterHeader};
@@ -106,18 +105,20 @@ impl fmt::Display for Error {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for Error {
-    fn cause(&self) -> Option<&dyn  error::Error> {
-        match *self {
-            Error::Io(ref e) => Some(e),
-            Error::Psbt(ref e) => Some(e),
-            Error::UnexpectedNetworkMagic { .. }
-            | Error::OversizedVectorAllocation { .. }
-            | Error::InvalidChecksum { .. }
-            | Error::NonMinimalVarInt
-            | Error::UnknownNetworkMagic(..)
-            | Error::ParseFailed(..)
-            | Error::UnsupportedSegwitFlag(..) => None,
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            Io(e) => Some(e),
+            Psbt(e) => Some(e),
+            UnexpectedNetworkMagic { .. }
+            | OversizedVectorAllocation { .. }
+            | InvalidChecksum { .. }
+            | NonMinimalVarInt
+            | UnknownNetworkMagic(_)
+            | ParseFailed(_)
+            | UnsupportedSegwitFlag(_) => None,
         }
     }
 }

src/network/message.rs

@@ -111,9 +111,13 @@ impl fmt::Display for CommandStringError {
     }
 }
 
-#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 #[cfg(feature = "std")]
-impl ::std::error::Error for CommandStringError {}
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for CommandStringError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        None
+    }
+}
 
 /// A Network message
 #[derive(Clone, Debug, PartialEq, Eq)]

src/network/mod.rs

@@ -20,7 +20,6 @@
 
 use crate::io;
 use core::fmt;
-#[cfg(feature = "std")] use std::error;
 
 pub mod constants;
 
@@ -79,11 +78,13 @@ impl From<io::Error> for Error {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl error::Error for Error {
-    fn cause(&self) -> Option<&dyn error::Error> {
-        match *self {
-            Error::Io(ref e) => Some(e),
-            Error::SocketMutexPoisoned | Error::SocketNotConnectedToPeer => None,
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            Io(e) => Some(e),
+            SocketMutexPoisoned | SocketNotConnectedToPeer => None,
         }
     }
 }

src/util/address.rs

@@ -37,7 +37,6 @@ use crate::prelude::*;
 use core::fmt;
 use core::num::ParseIntError;
 use core::str::FromStr;
-#[cfg(feature = "std")] use std::error;
 
 use secp256k1::{Secp256k1, Verification, XOnlyPublicKey};
 use bech32;
@@ -104,13 +103,22 @@ impl fmt::Display for Error {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for Error {
-    fn cause(&self) -> Option<&dyn  error::Error> {
-        match *self {
-            Error::Base58(ref e) => Some(e),
-            Error::Bech32(ref e) => Some(e),
-            Error::UnparsableWitnessVersion(ref e) => Some(e),
-            _ => None,
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            Base58(e) => Some(e),
+            Bech32(e) => Some(e),
+            UnparsableWitnessVersion(e) => Some(e),
+            EmptyBech32Payload
+            | InvalidBech32Variant { .. }
+            | InvalidWitnessVersion(_)
+            | MalformedWitnessVersion
+            | InvalidWitnessProgramLength(_)
+            | InvalidSegwitV0ProgramLength(_)
+            | UncompressedPubkey
+            | ExcessiveScriptSize => None,
         }
     }
 }

src/util/amount.rs

@@ -192,7 +192,7 @@ impl fmt::Display for ParseAmountError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for ParseAmountError {}
+impl std::error::Error for ParseAmountError {}
 
 fn is_too_precise(s: &str, precision: usize) -> bool {
     s.contains('.') || precision >= s.len() || s.chars().rev().take(precision).any(|d| d != '0')

src/util/base58.rs

@@ -57,8 +57,8 @@ impl fmt::Display for Error {
             Error::BadByte(b) => write!(f, "invalid base58 character 0x{:x}", b),
             Error::BadChecksum(exp, actual) => write!(f, "base58ck checksum 0x{:x} does not match expected 0x{:x}", actual, exp),
             Error::InvalidLength(ell) => write!(f, "length {} invalid for this base58 type", ell),
-            Error::InvalidAddressVersion(ref v) => write!(f, "address version {} is invalid for this base58 type", v),
             Error::InvalidExtendedKeyVersion(ref v) => write!(f, "extended key version {:#04x?} is invalid for this base58 type", v),
+            Error::InvalidAddressVersion(ref v) => write!(f, "address version {} is invalid for this base58 type", v),
             Error::TooShort(_) => write!(f, "base58ck data not even long enough for a checksum"),
             Error::Secp256k1(ref e) => fmt::Display::fmt(&e, f),
             Error::Hex(ref e) => write!(f, "Hexadecimal decoding error: {}", e)
@@ -68,7 +68,22 @@ impl fmt::Display for Error {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for Error {}
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            BadByte(_)
+            | BadChecksum(_, _)
+            | InvalidLength(_)
+            | InvalidExtendedKeyVersion(_)
+            | InvalidAddressVersion(_)
+            | TooShort(_) => None,
+            Secp256k1(e) => Some(e),
+            Hex(e) => Some(e),
+        }
+    }
+}
 
 /// Vector-like object that holds the first 100 elements on the stack. If more space is needed it
 /// will be allocated on the heap.

src/util/bip158.rs

@@ -74,10 +74,6 @@ pub enum Error {
     Io(io::Error),
 }
 
-#[cfg(feature = "std")]
-#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for Error {}
-
 impl Display for Error {
     fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
         match *self {
@@ -87,6 +83,20 @@ impl Display for Error {
     }
 }
 
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            UtxoMissing(_) => None,
+            Io(e) => Some(e),
+        }
+    }
+}
+
+
 impl From<io::Error> for Error {
     fn from(io: io::Error) -> Self {
         Error::Io(io)

src/util/bip32.rs

@@ -22,7 +22,6 @@ use crate::prelude::*;
 use crate::io::Write;
 use core::{fmt, str::FromStr, default::Default};
 use core::ops::Index;
-#[cfg(feature = "std")] use std::error;
 #[cfg(feature = "serde")] use serde;
 
 use crate::hash_types::XpubIdentifier;
@@ -497,12 +496,20 @@ impl fmt::Display for Error {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl error::Error for Error {
-    fn cause(&self) -> Option<&dyn error::Error> {
-        if let Error::Secp256k1(ref e) = *self {
-            Some(e)
-        } else {
-            None
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            Secp256k1(e) => Some(e),
+            Base58(e) => Some(e),
+            Hex(e) => Some(e),
+            CannotDeriveFromHardenedKey
+            | InvalidChildNumber(_)
+            | InvalidChildNumberFormat
+            | InvalidDerivationPathFormat
+            | UnknownVersion(_)
+            | WrongExtendedKeyLength(_) => None,
         }
     }
 }

src/util/ecdsa.rs

@@ -102,20 +102,31 @@ pub enum EcdsaSigError {
 impl fmt::Display for EcdsaSigError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
+            EcdsaSigError::HexEncoding(e) =>
+                write!(f, "EcdsaSig hex encoding error: {}", e),
             EcdsaSigError::NonStandardSighashType(hash_ty) =>
                 write!(f, "Non standard signature hash type {}", hash_ty),
-            EcdsaSigError::Secp256k1(ref e) =>
-                write!(f, "Invalid Ecdsa signature: {}", e),
             EcdsaSigError::EmptySignature =>
                 write!(f, "Empty ECDSA signature"),
-            EcdsaSigError::HexEncoding(e) => write!(f, "EcdsaSig hex encoding error: {}", e)
+            EcdsaSigError::Secp256k1(ref e) =>
+                write!(f, "Invalid Ecdsa signature: {}", e),
         }
     }
 }
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for EcdsaSigError {}
+impl std::error::Error for EcdsaSigError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::EcdsaSigError::*;
+
+        match self {
+            HexEncoding(e) => Some(e),
+            Secp256k1(e) => Some(e),
+            NonStandardSighashType(_) | EmptySignature => None,
+        }
+    }
+}
 
 impl From<secp256k1::Error> for EcdsaSigError {
     fn from(e: secp256k1::Error) -> EcdsaSigError {

src/util/key.rs

@@ -16,16 +16,14 @@
 //! This module provides keys used in Bitcoin that can be roundtrip
 //! (de)serialized.
 
-pub use secp256k1::{XOnlyPublicKey, KeyPair};
-
 use crate::prelude::*;
 
 use core::{ops, str::FromStr};
 use core::fmt::{self, Write};
-use crate::io;
-#[cfg(feature = "std")] use std::error;
 
-use secp256k1::{self, Secp256k1};
+pub use secp256k1::{self, Secp256k1, XOnlyPublicKey, KeyPair};
+
+use crate::io;
 use crate::network::constants::Network;
 use crate::hashes::{Hash, hash160, hex, hex::FromHex};
 use crate::hash_types::{PubkeyHash, WPubkeyHash};
@@ -57,13 +55,15 @@ impl fmt::Display for Error {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for Error {
-    fn cause(&self) -> Option<&dyn error::Error> {
-        match *self {
-            Error::Base58(ref e) => Some(e),
-            Error::Secp256k1(ref e) => Some(e),
-            Error::InvalidKeyPrefix(_) => None,
-            Error::Hex(ref e) => Some(e)
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            Base58(e) => Some(e),
+            Secp256k1(e) => Some(e),
+            InvalidKeyPrefix(_) => None,
+            Hex(e) => Some(e),
         }
     }
 }

src/util/misc.rs

@@ -37,7 +37,6 @@ mod message_signing {
     #[cfg(feature = "base64")] use crate::prelude::*;
 
     use core::fmt;
-    #[cfg(feature = "std")] use std::error;
 
     use crate::hashes::sha256d;
     use secp256k1;
@@ -73,11 +72,13 @@ mod message_signing {
 
     #[cfg(feature = "std")]
     #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-    impl error::Error for MessageSignatureError {
-        fn cause(&self) -> Option<&dyn error::Error> {
-            match *self {
-                MessageSignatureError::InvalidEncoding(ref e) => Some(e),
-                _ => None,
+    impl std::error::Error for MessageSignatureError {
+        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+            use self::MessageSignatureError::*;
+
+            match self {
+                InvalidEncoding(e) => Some(e),
+                InvalidLength | InvalidBase64 | UnsupportedAddressType(_) => None,
             }
         }
     }

src/util/mod.rs

@@ -39,7 +39,6 @@ pub(crate) mod endian;
 use crate::prelude::*;
 use crate::io;
 use core::fmt;
-#[cfg(feature = "std")] use std::error;
 
 use crate::network;
 use crate::consensus::encode;
@@ -91,12 +90,16 @@ impl fmt::Display for Error {
 }
 
 #[cfg(feature = "std")]
-impl ::std::error::Error for Error {
-    fn cause(&self) -> Option<&dyn  error::Error> {
-        match *self {
-            Error::Encode(ref e) => Some(e),
-            Error::Network(ref e) => Some(e),
-            Error::BlockBadProofOfWork | Error::BlockBadTarget => None
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            Encode(e) => Some(e),
+            Network(e) => Some(e),
+            BlockBadProofOfWork
+            | BlockBadTarget => None
         }
     }
 }

src/util/psbt/error.rs

@@ -88,34 +88,59 @@ pub enum Error {
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
+            Error::InvalidMagic => f.write_str("invalid magic"),
+            Error::MissingUtxo => f.write_str("UTXO information is not present in PSBT"),
+            Error::InvalidSeparator => f.write_str("invalid separator"),
+            Error::PsbtUtxoOutOfbounds => f.write_str("output index is out of bounds of non witness script output array"),
             Error::InvalidKey(ref rkey) => write!(f, "invalid key: {}", rkey),
             Error::InvalidProprietaryKey => write!(f, "non-proprietary key type found when proprietary key was expected"),
             Error::DuplicateKey(ref rkey) => write!(f, "duplicate key: {}", rkey),
-            Error::UnexpectedUnsignedTx { expected: ref e, actual: ref a } => write!(f, "different unsigned transaction: expected {}, actual {}", e.txid(), a.txid()),
-            Error::NonStandardSighashType(ref sht) => write!(f, "non-standard sighash type: {}", sht),
-            Error::InvalidMagic => f.write_str("invalid magic"),
-            Error::InvalidSeparator => f.write_str("invalid separator"),
             Error::UnsignedTxHasScriptSigs => f.write_str("the unsigned transaction has script sigs"),
             Error::UnsignedTxHasScriptWitnesses => f.write_str("the unsigned transaction has script witnesses"),
             Error::MustHaveUnsignedTx => {
                 f.write_str("partially signed transactions must have an unsigned transaction")
             }
             Error::NoMorePairs => f.write_str("no more key-value pairs for this psbt map"),
+            Error::UnexpectedUnsignedTx { expected: ref e, actual: ref a } => write!(f, "different unsigned transaction: expected {}, actual {}", e.txid(), a.txid()),
+            Error::NonStandardSighashType(ref sht) => write!(f, "non-standard sighash type: {}", sht),
             Error::HashParseError(e) => write!(f, "Hash Parse Error: {}", e),
-            Error::MissingUtxo => f.write_str("UTXO information is not present in PSBT"),
-            Error::PsbtUtxoOutOfbounds => f.write_str("output index is out of bounds of non witness script output array"),
             Error::InvalidPreimageHashPair{ref preimage, ref hash, ref hash_type} => {
                 // directly using debug forms of psbthash enums
                 write!(f, "Preimage {:?} does not match {:?} hash {:?}", preimage, hash_type, hash )
-            }
-            Error::CombineInconsistentKeySources(ref s) => { write!(f, "combine conflict: {}", s) }
+            },
+            Error::CombineInconsistentKeySources(ref s) => { write!(f, "combine conflict: {}", s) },
             Error::ConsensusEncoding => f.write_str("bitcoin consensus or BIP-174 encoding error"),
         }
     }
 }
 
 #[cfg(feature = "std")]
-impl ::std::error::Error for Error {}
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            HashParseError(e) => Some(e),
+            | InvalidMagic
+            | MissingUtxo
+            | InvalidSeparator
+            | PsbtUtxoOutOfbounds
+            | InvalidKey(_)
+            | InvalidProprietaryKey
+            | DuplicateKey(_)
+            | UnsignedTxHasScriptSigs
+            | UnsignedTxHasScriptWitnesses
+            | MustHaveUnsignedTx
+            | NoMorePairs
+            | UnexpectedUnsignedTx { .. }
+            | NonStandardSighashType(_)
+            | InvalidPreimageHashPair{ .. }
+            | CombineInconsistentKeySources(_)
+            | ConsensusEncoding => None,
+        }
+    }
+}
 
 #[doc(hidden)]
 impl From<hashes::Error> for Error {

src/util/psbt/map/output.rs

@@ -110,7 +110,15 @@ impl core::fmt::Display for IncompleteTapTree {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for IncompleteTapTree {}
+impl std::error::Error for IncompleteTapTree {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::IncompleteTapTree::*;
+
+        match self {
+            NotFinalized(_) | HiddenParts(_) => None,
+        }
+    }
+}
 
 /// Taproot Tree representing a finalized [`TaprootBuilder`] (a complete binary tree).
 #[derive(Clone, Debug)]

src/util/psbt/mod.rs

@@ -241,7 +241,16 @@ mod display_from_str {
 
     #[cfg(feature = "std")]
     #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-    impl ::std::error::Error for PsbtParseError {}
+    impl std::error::Error for PsbtParseError {
+        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+            use self::PsbtParseError::*;
+
+            match self {
+                PsbtEncoding(e) => Some(e),
+                Base64Encoding(e) => Some(e),
+            }
+        }
+    }
 
     #[cfg_attr(docsrs, doc(cfg(feature = "base64")))]
     impl Display for PartiallySignedTransaction {

src/util/schnorr.rs

@@ -286,7 +286,16 @@ impl fmt::Display for SchnorrSigError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for SchnorrSigError {}
+impl std::error::Error for SchnorrSigError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::SchnorrSigError::*;
+
+        match self {
+            Secp256k1(e) => Some(e),
+            InvalidSighashType(_) | InvalidSchnorrSigSize(_) => None,
+        }
+    }
+}
 
 impl From<secp256k1::Error> for SchnorrSigError {
 

src/util/sighash.rs

@@ -224,7 +224,23 @@ impl fmt::Display for Error {
 }
 
 #[cfg(feature = "std")]
-impl ::std::error::Error for Error {}
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match self {
+            Io(_)
+            | IndexOutOfInputsBounds { .. }
+            | SingleWithoutCorrespondingOutput { .. }
+            | PrevoutsSize
+            | PrevoutIndex
+            | PrevoutKind
+            | WrongAnnex
+            | InvalidSighashType(_) => None,
+        }
+    }
+}
 
 impl<'u, T> Prevouts<'u, T> where T: Borrow<TxOut> {
     fn check_all(&self, tx: &Transaction) -> Result<(), Error> {

src/util/taproot.rs

@@ -22,8 +22,6 @@ use secp256k1::{self, Secp256k1};
 
 use core::fmt;
 use core::cmp::Reverse;
-#[cfg(feature = "std")]
-use std::error;
 
 use crate::hashes::{sha256, Hash, HashEngine};
 use crate::schnorr::{TweakedPublicKey, UntweakedPublicKey, TapTweak};
@@ -486,7 +484,7 @@ impl TaprootBuilder {
         // We cannot insert a leaf at a lower depth while a deeper branch is unfinished. Doing
         // so would mean the add_leaf/add_hidden invocations do not correspond to a DFS traversal of a
         // binary tree.
-        if depth as usize + 1 < self.branch.len() {
+        if (depth as usize + 1) < self.branch.len() {
             return Err(TaprootBuilderError::NodeNotInDfsOrder);
         }
 
@@ -989,6 +987,9 @@ pub enum TaprootBuilderError {
 impl fmt::Display for TaprootBuilderError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
+            TaprootBuilderError::InvalidMerkleTreeDepth(d) => {
+                write!(f, "Merkle Tree depth({}) must be less than {}", d, TAPROOT_CONTROL_MAX_NODE_COUNT)
+            }
             TaprootBuilderError::NodeNotInDfsOrder => {
                 write!(f, "add_leaf/add_hidden must be called in DFS walk order",)
             }
@@ -997,9 +998,6 @@ impl fmt::Display for TaprootBuilderError {
                 "Attempted to create a tree with two nodes at depth 0. There must\
                 only be a exactly one node at depth 0",
             ),
-            TaprootBuilderError::InvalidMerkleTreeDepth(d) => {
-                write!(f, "Merkle Tree depth({}) must be less than {}", d, TAPROOT_CONTROL_MAX_NODE_COUNT)
-            }
             TaprootBuilderError::InvalidInternalKey(e) => {
                 write!(f, "Invalid Internal XOnly key : {}", e)
             }
@@ -1015,7 +1013,20 @@ impl fmt::Display for TaprootBuilderError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl error::Error for TaprootBuilderError {}
+impl std::error::Error for TaprootBuilderError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::TaprootBuilderError::*;
+
+        match self {
+            InvalidInternalKey(e) => Some(e),
+            InvalidMerkleTreeDepth(_)
+                | NodeNotInDfsOrder
+                | OverCompleteTree
+                | IncompleteTree
+                | EmptyTree => None
+        }
+    }
+}
 
 /// Detailed error type for taproot utilities.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -1069,7 +1080,23 @@ impl fmt::Display for TaprootError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl error::Error for TaprootError {}
+impl std::error::Error for TaprootError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::TaprootError::*;
+
+        match self {
+            InvalidInternalKey(e) => Some(e),
+            InvalidMerkleBranchSize(_)
+            | InvalidMerkleTreeDepth(_)
+            | InvalidTaprootLeafVersion(_)
+            | InvalidControlBlockSize(_)
+            | InvalidParity(_)
+            | EmptyTree => None,
+        }
+    }
+}
+
+
 #[cfg(test)]
 mod test {
     use crate::{Address, Network};

src/util/uint.rs

@@ -528,7 +528,11 @@ impl ::core::fmt::Display for ParseLengthError {
 
 #[cfg(feature = "std")]
 #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
-impl ::std::error::Error for ParseLengthError {}
+impl std::error::Error for ParseLengthError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        None
+    }
+}
 
 impl Uint256 {
     /// Decay to a uint128