fix: Perform occurs check before binding function types (#2027)

Fix binding function types without doing an occurs check

Author: jfecher

crates/noirc_frontend/src/hir/type_check/expr.rs

@@ -775,8 +775,10 @@ impl<'interner> TypeChecker<'interner> {
                 let ret = self.interner.next_type_variable();
                 let args = vecmap(args, |(arg, _)| arg);
                 let expected = Type::Function(args, Box::new(ret.clone()));
-                *binding.borrow_mut() = TypeBinding::Bound(expected);
 
+                if let Err(error) = binding.borrow_mut().bind_to(expected, span) {
+                    self.errors.push(error);
+                }
                 ret
             }
             Type::Function(parameters, ret) => {

crates/noirc_frontend/src/hir_def/types.rs

@@ -294,6 +294,20 @@ impl TypeBinding {
     pub fn is_unbound(&self) -> bool {
         matches!(self, TypeBinding::Unbound(_))
     }
+
+    pub fn bind_to(&mut self, binding: Type, span: Span) -> Result<(), TypeCheckError> {
+        match self {
+            TypeBinding::Bound(_) => panic!("Tried to bind an already bound type variable!"),
+            TypeBinding::Unbound(id) => {
+                if binding.occurs(*id) {
+                    Err(TypeCheckError::TypeAnnotationsNeeded { span })
+                } else {
+                    *self = TypeBinding::Bound(binding);
+                    Ok(())
+                }
+            }
+        }
+    }
 }
 
 /// A unique ID used to differentiate different type variables