Fix subtle bug in pattern rendering

When rendering missing or redundant patterns, linked-list would
  wrongly suggest the last nil constructor as a pattern on non-empty
  list.

  For example, before this commit, the exhaustivness checker would yield:

  ```
  [(_, True), []]
  ```

  as a suggestion, for being the result of being a list pattern with a
  single argument being `(_, True) :: Nil`. Blindly following the
  compiler suggestion here would cause a type unification error (since
  `[]` doesn't unify with a 2-tuple).

  Indeed, we mustn't render the Nil constructor when rendering non-empty
  lists! So the correct suggestion should be:

  ```
  [(_, True)]
  ```

crates/aiken-lang/src/tests/check.rs

@@ -497,6 +497,120 @@ fn exhaustiveness_complex() {
     ))
 }
 
+#[test]
+fn exhaustiveness_tuple() {
+    let source_code = r#"
+        fn foo() {
+            when (14, True) is {
+                (14, True) -> Void
+            }
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((
+            _,
+            Error::NotExhaustivePatternMatch {
+                unmatched,
+                ..
+            }
+        )) if unmatched[0] == "(_, _)"
+    ))
+}
+
+#[test]
+fn exhaustiveness_nested_list_and_tuples() {
+    fn assert_step(step: &str, expected: &str) {
+        let result = check(parse(step));
+        println!("{result:#?}");
+        assert!(matches!(
+            result,
+            Err((
+                _,
+                Error::NotExhaustivePatternMatch {
+                    unmatched,
+                    ..
+                }
+            )) if unmatched[0] == expected
+        ));
+    }
+
+    assert_step(
+        r#"
+        fn foo() {
+            let xs : List<(List<(Int, Bool)>, Int)> = [([(14, True)], 42)]
+            when xs is {
+                [                      ] -> Void
+                [([(14, True)], 42), ..] -> Void
+            }
+        }
+        "#,
+        "[([], _), ..]",
+    );
+
+    assert_step(
+        r#"
+        fn foo() {
+            let xs : List<(List<(Int, Bool)>, Int)> = [([(14, True)], 42)]
+            when xs is {
+                [                     ] -> Void
+                [([(_, True)], 42), ..] -> Void
+                [([         ],  _), ..] -> Void
+            }
+        }
+        "#,
+        "[([(_, False), ..], _), ..]",
+    );
+
+    assert_step(
+        r#"
+        fn foo() {
+            let xs : List<(List<(Int, Bool)>, Int)> = [([(14, True)], 42)]
+            when xs is {
+                [                          ] -> Void
+                [([(_, True )    ], 42), ..] -> Void
+                [([              ],  _), ..] -> Void
+                [([(_, False), ..],  _), ..] -> Void
+            }
+        }
+        "#,
+        "[([(_, True), _, ..], _), ..]",
+    );
+
+    assert_step(
+        r#"
+        fn foo() {
+            let xs : List<(List<(Int, Bool)>, Int)> = [([(14, True)], 42)]
+            when xs is {
+                [                             ] -> Void
+                [([(_, True )       ], 42), ..] -> Void
+                [([                 ],  _), ..] -> Void
+                [([(_, False)   , ..],  _), ..] -> Void
+                [([(_, True ), _, ..],  _), ..] -> Void
+            }
+        }
+        "#,
+        "[([(_, True)], _), ..]",
+    );
+
+    let source_code = r#"
+        fn foo() {
+            let xs : List<(List<(Int, Bool)>, Int)> = [([(14, True)], 42)]
+            when xs is {
+                [                             ] -> Void
+                [([(_, True )       ], 42), ..] -> Void
+                [([                 ],  _), ..] -> Void
+                [([(_, False)   , ..],  _), ..] -> Void
+                [([(_, True ), _, ..],  _), ..] -> Void
+                [([(_, True )       ],  _), ..] -> Void
+            }
+        }
+    "#;
+
+    assert!(matches!(check(parse(source_code)), Ok(_)))
+}
+
 #[test]
 fn expect_sugar_correct_type() {
     let source_code = r#"

crates/aiken-lang/src/tipo/exhaustive.rs

@@ -405,11 +405,16 @@ impl Pattern {
                 let args = args
                     .into_iter()
                     .enumerate()
-                    .map(|(index, p)| {
+                    .filter_map(|(index, p)| {
                         if index == 1 {
-                            pretty_tail(p)
+                            let tail = pretty_tail(p);
+                            if tail == "[]" {
+                                None
                             } else {
-                            p.pretty()
+                                Some(tail)
+                            }
+                        } else {
+                            Some(p.pretty())
                         }
                     })
                     .join(", ");