slint-ui/slint · error

Called a not-implemented method

Error message

Called a not-implemented method

What it means

The vtable crate's `vtable!` macro generates safe accessors for trait methods that call through an optional function pointer; when the vtable slot is None (method not implemented by the backing object), the generated code panics with 'Called a not-implemented method'. It means a caller invoked a trait method that the concrete object never registered in its vtable.

Solutions

1. Fill in the missing method implementation in the vtable construction instead of None
2. Regenerate/compile all vtable definitions after changing the trait so slots are populated
3. For optional methods, check the slot (or an `has_*` helper) before calling instead of relying on the panic path
4. Review unsafe code that builds vtables manually to ensure every required fn pointer is set

Example fix

// before
DropVTable { drop_in_place: Some(drop_in_place), ..Default::default() } // missing new method
// after
DropVTable { drop_in_place: Some(drop_in_place), new_method: Some(my_impl), ..Default::default() }

// before calling through the trait object, ensure the slot is implemented
// check an availability helper or the vtable in debug
assert!(vtable.some_method.is_some(), "some_method not implemented in vtable");

// use std::panic::catch_unwind around experimental dynamic dispatch paths
let ok = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| obj.some_method()));

When it happens

Trigger: Accessing a trait object whose vtable left a method slot None and then calling that method — typically a partially implemented vtable or calling a method from a newer trait revision against an older/default vtable.

Common situations: Implementing a vtable struct with `None` (or Rust default) for a required method; adding a new method to a vtable trait without updating implementations; unsafe/direct construction of vtables in FFI code.

Related errors

• a setter belongs to a field
• an identifier
• attempt to multiply with overflow
• binding was of the wrong type
• Cannot get property

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/9594973703124ad5. Report an issue: GitHub.

Appendix: source

Thrown at helper_crates/vtable/macro/macro.rs:499

                default: None,
                semi_token: Some(Default::default()),
            }));

            generated_to_fn_trait.push(ImplItemFn {
                attrs: field.attrs.clone(),
                vis: Visibility::Public(Default::default()),
                modifiers: Default::default(),
                sig: sig.clone(),
                block: if has_self {
                    parse_quote!({
                        // Safety: this rely on the vtable being valid, and the ptr being a valid instance for this vtable
                        #[allow(unsafe_code)]
                        unsafe {
                            let vtable = self.vtable.as_ref();
                            if let #some(func) = vtable.#ident {
                                func (#call_code)
                            } else {
                                panic!("Called a not-implemented method")
                            }
                        }
                    })
                } else {
                    // This should never happen: nobody should be able to access the Trait Object directly.
                    parse_quote!({ panic!("Calling Sized method on a Trait Object") })
                },
            });

            if !has_self {
                sig.inputs.insert(
                    0,
                    FnArg::Receiver(Receiver {
                        attrs: Default::default(),
                        mutability: None,
                        self_token: Default::default(),
                        kind: ReceiverKind::Reference(Default::default(), None, None),
                    }),

View on GitHub (pinned to bb937076de)