got some basics working!

src/backend/error.rs

@@ -43,6 +43,11 @@ pub enum BackendError {
     InvalidTypeCast { from: PrimitiveType, to: Type },
     #[error("Unknown string constant '{0}")]
     UnknownString(ArcIntern<String>),
+    #[error("Compiler doesn't currently support function arguments")]
+    NoFunctionArguments {
+        function_name: String,
+        argument_name: String,
+    },
 }
 
 impl From<BackendError> for Diagnostic<usize> {
@@ -73,6 +78,13 @@ impl From<BackendError> for Diagnostic<usize> {
             ),
             BackendError::UnknownString(str) => Diagnostic::error()
                 .with_message(format!("Unknown string found trying to compile: '{}'", str)),
+            BackendError::NoFunctionArguments {
+                function_name,
+                argument_name,
+            } => Diagnostic::error().with_message(format!(
+                "Function {} takes a function argument ({}), which is not supported",
+                function_name, argument_name
+            )),
         }
     }
 }
@@ -128,6 +140,17 @@ impl PartialEq for BackendError {
                 BackendError::UnknownString(b) => a == b,
                 _ => false,
             },
+
+            BackendError::NoFunctionArguments {
+                function_name: f1,
+                argument_name: a1,
+            } => match other {
+                BackendError::NoFunctionArguments {
+                    function_name: f2,
+                    argument_name: a2,
+                } => f1 == f2 && a1 == a2,
+                _ => false,
+            },
         }
     }
 }