Remove default trait method implementations

Implementors of `MiniscriptKey` must reason about the three trait
methods, this implies the trait methods are required, not provided.

We are using the default impls to remove code duplication, this is an
abuse of the default impls. It makes the docs read incorrectly, by implying
that implementors _do not_ need to think reason about these trait methods.

Remove default trait method implementations and manually implement the
trait methods for all current implementors.

Author: tcharding

fuzz/src/lib.rs

@@ -40,6 +40,10 @@ impl MiniscriptKey for FuzzPk {
     type Ripemd160 = u8;
     type Hash160 = u8;
     type Hash256 = u8;
+
+    fn is_uncompressed(&self) -> bool { false }
+    fn is_x_only_key(&self) -> bool { false }
+    fn num_der_paths(&self) -> usize { 0 }
 }
 
 impl ToPublicKey for FuzzPk {

src/interpreter/mod.rs

@@ -130,6 +130,8 @@ impl MiniscriptKey for BitcoinKey {
             BitcoinKey::XOnlyPublicKey(_) => false,
         }
     }
+    fn is_x_only_key(&self) -> bool { false }
+    fn num_der_paths(&self) -> usize { 0 }
 }
 
 impl<'txin> Interpreter<'txin> {

src/lib.rs

@@ -150,15 +150,15 @@ pub use crate::primitives::threshold::{Threshold, ThresholdError};
 /// Trait representing a key which can be converted to a hash type.
 pub trait MiniscriptKey: Clone + Eq + Ord + fmt::Debug + fmt::Display + hash::Hash {
     /// Returns true if the key is serialized uncompressed (defaults to `false`).
-    fn is_uncompressed(&self) -> bool { false }
+    fn is_uncompressed(&self) -> bool;
 
     /// Returns true if the key is an x-only pubkey (defaults to `false`).
-    fn is_x_only_key(&self) -> bool { false }
+    fn is_x_only_key(&self) -> bool;
 
     /// Returns the number of different derivation paths in this key (defaults to `0`).
     ///
     /// Only >1 for keys in BIP389 multipath descriptors.
-    fn num_der_paths(&self) -> usize { 0 }
+    fn num_der_paths(&self) -> usize;
 
     /// The type used in the sha256 fragment.
     type Sha256: Clone + Eq + Ord + fmt::Display + fmt::Debug + hash::Hash;
@@ -178,6 +178,10 @@ impl MiniscriptKey for bitcoin::secp256k1::PublicKey {
     type Hash256 = hash256::Hash;
     type Ripemd160 = ripemd160::Hash;
     type Hash160 = hash160::Hash;
+
+    fn is_uncompressed(&self) -> bool { false }
+    fn is_x_only_key(&self) -> bool { false }
+    fn num_der_paths(&self) -> usize { 0 }
 }
 
 impl MiniscriptKey for bitcoin::PublicKey {
@@ -187,6 +191,8 @@ impl MiniscriptKey for bitcoin::PublicKey {
     type Hash160 = hash160::Hash;
 
     fn is_uncompressed(&self) -> bool { !self.compressed }
+    fn is_x_only_key(&self) -> bool { false }
+    fn num_der_paths(&self) -> usize { 0 }
 }
 
 impl MiniscriptKey for bitcoin::secp256k1::XOnlyPublicKey {
@@ -195,14 +201,20 @@ impl MiniscriptKey for bitcoin::secp256k1::XOnlyPublicKey {
     type Ripemd160 = ripemd160::Hash;
     type Hash160 = hash160::Hash;
 
+    fn is_uncompressed(&self) -> bool { false }
     fn is_x_only_key(&self) -> bool { true }
+    fn num_der_paths(&self) -> usize { 0 }
 }
 
 impl MiniscriptKey for String {
     type Sha256 = String;
     type Hash256 = String;
     type Ripemd160 = String;
     type Hash160 = String;
+
+    fn is_uncompressed(&self) -> bool { false }
+    fn is_x_only_key(&self) -> bool { false }
+    fn num_der_paths(&self) -> usize { 0 }
 }
 
 /// Trait describing key types that can be converted to bitcoin public keys.