refactor: Remove Error::NoMoreWitnesses

We can remove an error path by using the respective zero value as
default witness value.

Author: uncomputable

src/lib.rs

@@ -88,8 +88,6 @@ pub enum Error {
     Type(types::Error),
     // Execution error
     Execution(bit_machine::ExecutionError),
-    /// Witness iterator ended early
-    NoMoreWitnesses,
     /// Tried to parse a jet but the name wasn't recognized
     InvalidJetName(String),
     /// Policy error
@@ -107,7 +105,6 @@ impl fmt::Display for Error {
             Error::Type(ref e) => fmt::Display::fmt(e, f),
             Error::Execution(ref e) => fmt::Display::fmt(e, f),
             Error::InvalidJetName(s) => write!(f, "unknown jet `{}`", s),
-            Error::NoMoreWitnesses => f.write_str("no more witness data available"),
             #[cfg(feature = "elements")]
             Error::Policy(ref e) => fmt::Display::fmt(e, f),
         }
@@ -121,7 +118,6 @@ impl std::error::Error for Error {
             Error::DisconnectRedeemTime => None,
             Error::Type(ref e) => Some(e),
             Error::Execution(ref e) => Some(e),
-            Error::NoMoreWitnesses => None,
             Error::InvalidJetName(..) => None,
             #[cfg(feature = "elements")]
             Error::Policy(ref e) => Some(e),

src/node/convert.rs

@@ -152,6 +152,10 @@ pub trait Converter<N: Marker, M: Marker> {
 /// Does not do any type-checking and may attach an invalid witness to a combinator.
 ///
 /// If it encounters a disconnect node, it simply returns an error.
+///
+/// **This finalizer should not be used in production.
+/// It introduces default values ([`Value::zero`]) to handle missing witness data,
+/// which might trigger unexpected spending paths.**
 // FIXME we should do type checking, but this would require a method to check
 // type compatibility between a Value and a type::Final.
 pub struct SimpleFinalizer<W: Iterator<Item = Value>> {
@@ -169,10 +173,13 @@ impl<W: Iterator<Item = Value>, J: Jet> Converter<Commit<J>, Redeem<J>> for Simp
 
     fn convert_witness(
         &mut self,
-        _: &PostOrderIterItem<&CommitNode<J>>,
+        data: &PostOrderIterItem<&CommitNode<J>>,
         _: &NoWitness,
     ) -> Result<Value, Self::Error> {
-        self.iter.next().ok_or(crate::Error::NoMoreWitnesses)
+        Ok(self
+            .iter
+            .next()
+            .unwrap_or_else(|| Value::zero(&data.node.arrow().target)))
     }
 
     fn convert_disconnect(