descriptors: define script_code() for legacy outputs

This fixes the incorrect assumption made in
283676e that the `scriptCode` was
defined in bip-0143.

In fact, it just became convoluted with bip-0143.

Reported-by: Andrew Poelstra <apoelstra@wpsoftware.net>
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

src/descriptor/mod.rs

@@ -393,29 +393,28 @@ impl<Pk: MiniscriptKey + ToPublicKey> Descriptor<Pk> {
         }
     }
 
-    /// Get the `scriptCode` as [defined by
-    /// bip-0143](https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki#specification),
-    /// used to generate sighashes for spending Segwit outputs.
+    /// Get the `scriptCode` of a transaction output.
     ///
-    /// # Errors
-    /// - If the script descriptor type is non-segwit, as the script code is only defined for
-    /// segregated witness transaction outputs.
-    pub fn script_code(&self) -> Result<Script, Error> {
+    /// The `scriptCode` is the Script of the previous transaction output being serialized in the
+    /// sighash when evaluating a `CHECKSIG` & co. OP code.
+    pub fn script_code(&self) -> Script {
         match *self {
-            // Spending non-segwit outputs requires "legacy" sighash and don't use bip143's `scriptCode`.
+            // For "legacy" outputs, it is defined as the txo's scriptPubKey.
             Descriptor::Bare(..)
+            | Descriptor::Sh(..)
             | Descriptor::Pk(..)
-            | Descriptor::Pkh(..)
-            | Descriptor::Sh(..) => Err(Error::BadDescriptor),
+            | Descriptor::Pkh(..) => self.script_pubkey(),
+            // For SegWit outputs, it is defined by bip-0143 (quoted below) and is different from
+            // the previous txo's scriptPubKey.
             // The item 5:
             //     - For P2WPKH witness program, the scriptCode is `0x1976a914{20-byte-pubkey-hash}88ac`.
             Descriptor::Wpkh(ref pk) | Descriptor::ShWpkh(ref pk) => {
                 let addr = bitcoin::Address::p2pkh(&pk.to_public_key(), bitcoin::Network::Bitcoin);
-                Ok(addr.script_pubkey())
+                addr.script_pubkey()
             }
             //     - For P2WSH witness program, if the witnessScript does not contain any `OP_CODESEPARATOR`,
             //       the `scriptCode` is the `witnessScript` serialized as scripts inside CTxOut.
-            Descriptor::Wsh(ref d) | Descriptor::ShWsh(ref d) => Ok(d.encode()),
+            Descriptor::Wsh(ref d) | Descriptor::ShWsh(ref d) => d.encode(),
         }
     }
 }