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