feat: newer rules around casting Data

* you cannot cast FROM Data with a `let`
* you cannot cast FROM Data by passing
  Data to none Data when calling a function
* you MUST use `assert` to cast from data
* you can cast INTO Data with a `let`
* you can cast INTO Data by passing none Data
  to Data when calling a function
* You cannot assert cast Data without an
  annotation

crates/aiken-lang/src/ast.rs

@@ -769,6 +769,16 @@ pub enum AssignmentKind {
     Assert,
 }
 
+impl AssignmentKind {
+    pub fn is_let(&self) -> bool {
+        matches!(self, AssignmentKind::Let)
+    }
+
+    pub fn is_assert(&self) -> bool {
+        matches!(self, AssignmentKind::Assert)
+    }
+}
+
 pub type MultiPattern<PatternConstructor, Type> = Vec<Pattern<PatternConstructor, Type>>;
 
 pub type UntypedMultiPattern = MultiPattern<(), ()>;

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

@@ -1210,7 +1210,7 @@ impl<'a> Environment<'a> {
             return Ok(());
         }
 
-        if t2.is_data() {
+        if t1.is_data() || t2.is_data() {
             return Ok(());
         }
 

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

@@ -822,6 +822,16 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                 typed_value.type_defining_location(),
             )?;
 
+            if value_typ.is_data() && kind.is_let() && !ann_typ.is_data() {
+                return Err(Error::CouldNotUnify {
+                    location,
+                    expected: ann_typ,
+                    given: value_typ,
+                    situation: Some(UnifyErrorSituation::UnsafeCast),
+                    rigid_type_names: HashMap::new(),
+                });
+            }
+
             value_typ = ann_typ.clone();
 
             // Ensure the pattern matches the type of the value

examples/acceptance_tests/050/aiken.lock

@@ -0,0 +1,5 @@
+# This file was generated by Aiken
+# You typically do not need to edit this file
+
+requirements = []
+packages = []

examples/acceptance_tests/050/aiken.toml

@@ -0,0 +1,3 @@
+name = 'aiken-lang/acceptance_test_050'
+version = '0.0.0'
+description = ''

examples/acceptance_tests/050/lib/tests.ak

@@ -0,0 +1,40 @@
+use aiken/builtin
+
+pub type Thing {
+  wow: Int,
+}
+
+test assert_1() {
+  assert thing: Thing = builtin.constr_data(0, [builtin.i_data(1)])
+
+  thing.wow == 1
+}
+
+fn cast_to_thing(x: Data) -> Thing {
+  assert x: Thing = x
+
+  x
+}
+
+test assert_2() {
+  let thing = Thing { wow: 1 }
+
+  let still_thing = cast_to_thing(thing)
+
+  still_thing.wow == 1
+}
+// should not typecheck
+// test unlift_data_without_assert_1() {
+//   let thing: Thing = builtin.constr_data(0, [builtin.i_data(1)])
+//   thing.wow == 1
+// }
+
+// should not typecheck
+// fn bad_cast(x: Thing) -> Int {
+//   x.wow
+// }
+
+// test unlift_data_without_assert_2() {
+//   let thing = builtin.constr_data(0, [builtin.i_data(1)])
+//   bad_cast(thing) == 1
+// }