miniscript: make display prefer 'u' over 'l' in the fragment l:0

When fuzzing my new non-recursive tree parsing logic, I noticed that we
were deviating from the released 12.0 in the way that we display l:0.
This is an ambiguous fragment which can be displayed either as l:0 or
u:0. In our released code we use u:0 so stick with that.

This was unintentially changed as part of #722. Change it back.

While we are at it add a regression test for #735

Author: apoelstra

src/descriptor/mod.rs

@@ -1036,6 +1036,21 @@ mod tests {
         );
     }
 
+    #[test]
+    fn display_prefers_u() {
+        // The fragments u:0 and l:0 are identical in terms of Script and
+        // in terms of the in-memory representation -- OrI(False, False).
+        // Test that the way we display the ambiguous fragment doesn't
+        // change, in case somebody somehow is depending on it.
+        let desc = StdDescriptor::from_str("sh(u:0)").unwrap();
+        assert_eq!("sh(u:0)#ncq3yf9h", desc.to_string());
+
+        // This is a regression test for https://github.com/rust-bitcoin/rust-miniscript/pull/735
+        // which was found at the same time. It's just a bug plain and simple.
+        let desc = StdDescriptor::from_str("sh(and_n(u:0,1))").unwrap();
+        assert_eq!("sh(and_n(u:0,1))#5j5tw8nm", desc.to_string());
+    }
+
     #[test]
     fn desc_rtt_tests() {
         roundtrip_descriptor("c:pk_k()");

src/miniscript/display.rs

@@ -260,14 +260,14 @@ impl<Pk: MiniscriptKey, Ctx: ScriptContext> Terminal<Pk, Ctx> {
             Terminal::ZeroNotEqual(..) => "n",
             Terminal::AndV(_, ref r) if matches!(r.as_inner(), Terminal::True) => "t",
             Terminal::AndV(..) => "and_v",
-            Terminal::AndB(..) => "and_b",
             Terminal::AndOr(_, _, ref c) if matches!(c.as_inner(), Terminal::False) => "and_n",
+            Terminal::AndB(..) => "and_b",
             Terminal::AndOr(..) => "andor",
             Terminal::OrB(..) => "or_b",
             Terminal::OrD(..) => "or_d",
             Terminal::OrC(..) => "or_c",
-            Terminal::OrI(ref l, _) if matches!(l.as_inner(), Terminal::False) => "l",
             Terminal::OrI(_, ref r) if matches!(r.as_inner(), Terminal::False) => "u",
+            Terminal::OrI(ref l, _) if matches!(l.as_inner(), Terminal::False) => "l",
             Terminal::OrI(..) => "or_i",
             Terminal::Thresh(..) => "thresh",
             Terminal::Multi(..) => "multi",