Merge #752: Document an error case for at_derivation_index()

3d9acd2 Document an error case for at_derivation_index() (Nadav Ivgi)

Pull request description:

ACKs for top commit:
  apoelstra:
    ACK 3d9acd2 successfully ran local tests; thanks!

Tree-SHA512: 8b4958e40342dd1a8955a4aeedb424c326afc7f01b086662f9e11c57258e28e8e6dd5bde1f846f7bd72f6ebab8fdb053ca3305bb4a99524a3bf66064c0e82419

Author: apoelstra

src/descriptor/key.rs

@@ -627,6 +627,7 @@ impl DescriptorPublicKey {
     /// # Errors
     ///
     /// - If `index` is hardened.
+    /// - If the key contains multi-path derivations
     pub fn at_derivation_index(self, index: u32) -> Result<DefiniteDescriptorKey, ConversionError> {
         let definite = match self {
             DescriptorPublicKey::Single(_) => self,

src/descriptor/mod.rs

@@ -596,6 +596,7 @@ impl Descriptor<DescriptorPublicKey> {
     ///
     /// # Errors
     /// - If index ≥ 2^31
+    /// - If the descriptor contains multi-path derivations
     pub fn at_derivation_index(
         &self,
         index: u32,
@@ -650,7 +651,8 @@ impl Descriptor<DescriptorPublicKey> {
     ///
     /// # Errors
     ///
-    /// This function will return an error if hardened derivation is attempted.
+    /// This function will return an error for multi-path descriptors
+    /// or if hardened derivation is attempted,
     pub fn derived_descriptor<C: secp256k1::Verification>(
         &self,
         secp: &secp256k1::Secp256k1<C>,