Merge rust-bitcoin/rust-miniscript#501: Remove unreachable statement

fb4c33b5d200dbf8522c4e57a2ed974c4767fc95 Use if let and panic (Tobin C. Harding)

Pull request description:

  Use if let and panic

  Clippy emits:
  ```
  warning: you seem to be trying to use `match` for destructuring a single pattern. Consider using `if let`
  ```
  The code in question is functionally asserting that the `witness` argument does not contain variant `Threshold`, the same can be achieved using an `if let` and a `panic`.

ACKs for top commit:
  apoelstra:
    ACK fb4c33b5d200dbf8522c4e57a2ed974c4767fc95

Tree-SHA512: 8fc07b7ee7eb7eb47fe13f37f7f39a440eefc9620f8e573d5de535bf90dd5e9f6bb4f404698d2290fe4eafa8be8476a51208fe068df752892b36bf1e55acd09f

Author: heap-coder

src/policy/semantic.rs

@@ -223,11 +223,11 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
     // a normalized policy
     pub(crate) fn satisfy_constraint(self, witness: &Policy<Pk>, available: bool) -> Policy<Pk> {
         debug_assert!(self.clone().normalized() == self);
-        match *witness {
-            // only for internal purposes, safe to use unreachable!
-            Policy::Threshold(..) => unreachable!(),
-            _ => {}
-        };
+        if let Policy::Threshold { .. } = *witness {
+            // We can't debug_assert on Policy::Threshold.
+            panic!("should be unreachable")
+        }
+
         let ret = match self {
             Policy::Threshold(k, subs) => {
                 let mut ret_subs = vec![];