Parameter Type Validation Helper: Added

validate_parameter_type(&syn::Type):

Verifies the type is a reference (syn::Type::Reference).
Ensures the reference is mutable (type_ref.mutability.is_some()),
reporting: #[turtle_main] parameter must be a mutable reference: '&mut
TurtlePlan'.
Checks that the target type (type_ref.elem) is a path whose trailing
identifier is TurtlePlan, allowing both &mut TurtlePlan and qualified
paths like &mut turtle_lib::TurtlePlan.
Rejects other types (like value: i32 or owned t: TurtlePlan), reporting:
Integration into validate_input: Called
validate_parameter_type(&pat_type.ty)?; directly within
syn::FnArg::Typed(pat_type).

Unit Tests Added:

test_valid_qualified_type: verifies &mut turtle_lib::TurtlePlan is
accepted.
test_rejects_wrong_type: verifies value: i32 is rejected with an
informative error.
test_rejects_immutable_reference: verifies t: &TurtlePlan is rejected.
test_rejects_owned_type: verifies t: TurtlePlan is rejected.
test_rejects_wrong_reference_type: verifies t: &mut i32 is rejected.

turtle-lib-macros/src/lib.rs

@@ -102,6 +102,40 @@ use syn::ItemFn;
 ///     });
 /// }
 /// ```
+fn validate_parameter_type(ty: &syn::Type) -> Result<(), syn::Error> {
+    match ty {
+        syn::Type::Reference(type_ref) => {
+            if type_ref.mutability.is_none() {
+                return Err(syn::Error::new_spanned(
+                    type_ref,
+                    "#[turtle_main] parameter must be a mutable reference: `&mut TurtlePlan`",
+                ));
+            }
+
+            if let syn::Type::Path(type_path) = &*type_ref.elem {
+                let is_turtle_plan = type_path
+                    .path
+                    .segments
+                    .last()
+                    .is_some_and(|seg| seg.ident == "TurtlePlan");
+
+                if is_turtle_plan {
+                    return Ok(());
+                }
+            }
+
+            Err(syn::Error::new_spanned(
+                &type_ref.elem,
+                "#[turtle_main] expected reference to `TurtlePlan`, e.g. `&mut TurtlePlan`",
+            ))
+        }
+        _ => Err(syn::Error::new_spanned(
+            ty,
+            "#[turtle_main] parameter must be of type `&mut TurtlePlan`",
+        )),
+    }
+}
+
 fn validate_input(input_fn: &ItemFn) -> Result<(), syn::Error> {
     if input_fn.sig.asyncness.is_some() {
         return Err(syn::Error::new_spanned(
@@ -137,6 +171,8 @@ fn validate_input(input_fn: &ItemFn) -> Result<(), syn::Error> {
                         ));
                     }
                 }
+
+                validate_parameter_type(&pat_type.ty)?;
             }
         }
     }
@@ -356,6 +392,50 @@ mod tests {
         assert!(err.to_string().contains("cannot have a return type"));
     }
 
+    #[test]
+    fn test_valid_qualified_type() {
+        let input: ItemFn = parse_quote! {
+            fn my_draw(t: &mut turtle_lib::TurtlePlan) {}
+        };
+        assert!(validate_input(&input).is_ok());
+    }
+
+    #[test]
+    fn test_rejects_wrong_type() {
+        let input: ItemFn = parse_quote! {
+            fn my_draw(value: i32) {}
+        };
+        let err = validate_input(&input).unwrap_err();
+        assert!(err.to_string().contains("parameter must be of type `&mut TurtlePlan`"));
+    }
+
+    #[test]
+    fn test_rejects_immutable_reference() {
+        let input: ItemFn = parse_quote! {
+            fn my_draw(t: &TurtlePlan) {}
+        };
+        let err = validate_input(&input).unwrap_err();
+        assert!(err.to_string().contains("parameter must be a mutable reference: `&mut TurtlePlan`"));
+    }
+
+    #[test]
+    fn test_rejects_owned_type() {
+        let input: ItemFn = parse_quote! {
+            fn my_draw(t: TurtlePlan) {}
+        };
+        let err = validate_input(&input).unwrap_err();
+        assert!(err.to_string().contains("parameter must be of type `&mut TurtlePlan`"));
+    }
+
+    #[test]
+    fn test_rejects_wrong_reference_type() {
+        let input: ItemFn = parse_quote! {
+            fn my_draw(t: &mut i32) {}
+        };
+        let err = validate_input(&input).unwrap_err();
+        assert!(err.to_string().contains("expected reference to `TurtlePlan`"));
+    }
+
     #[test]
     fn test_expansion_preserves_custom_param_name() {
         let input = quote! {