slint-ui/slint · error

invalid argument for callback {}::{}

Error message

invalid argument for callback {}::{}

What it means

Error "invalid argument for callback {}::{}" thrown in slint-ui/slint.

Source

Thrown at internal/compiler/generator/rust_live_preview.rs:121

            let return_type = rust_primitive_type(&callback.return_type).unwrap();
            let args_name =
                (0..callback.args.len()).map(|i| format_ident!("arg_{}", i)).collect::<Vec<_>>();
            let caller_ident = accessor_names::rust_accessor_ident(name, AccessorKind::Invoker);
            property_and_callback_accessors.push(quote!(
                #[allow(dead_code)]
                pub fn #caller_ident(&self, #(#args_name : #callback_args,)*) -> #return_type {
                    self.0.borrow().invoke(#prop_name, &[#(#args_name.into(),)*])
                        .try_into().unwrap_or_else(|_| panic!("Invalid return type for callback {}::{}", #component_name, #prop_name))
                }
            ));
            let on_ident = accessor_names::rust_accessor_ident(name, AccessorKind::Handler);
            property_and_callback_accessors.push(quote!(
                #[allow(dead_code)]
                pub fn #on_ident(&self, f: impl FnMut(#(#callback_args),*) -> #return_type + 'static) {
                    let f = ::core::cell::RefCell::new(f);
                    self.0.borrow().set_callback(#prop_name, sp::Rc::new(move |values| {
                        let [#(#args_name,)*] = values else { panic!("invalid number of argument for callback {}::{}", #component_name, #prop_name) };
                        (*f.borrow_mut())(#(#args_name.clone().try_into().unwrap_or_else(|_| panic!("invalid argument for callback {}::{}", #component_name, #prop_name)),)*).into()
                    }))
                }
            ));
        } else if let Type::Function(function) = &p.ty {
            let callback_args =
                function.args.iter().map(|a| rust_primitive_type(a).unwrap()).collect::<Vec<_>>();
            let return_type = rust_primitive_type(&function.return_type).unwrap();
            let args_name =
                (0..function.args.len()).map(|i| format_ident!("arg_{}", i)).collect::<Vec<_>>();
            let caller_ident = accessor_names::rust_accessor_ident(name, AccessorKind::Invoker);
            property_and_callback_accessors.push(quote!(
                #[allow(dead_code)]
                pub fn #caller_ident(&self, #(#args_name : #callback_args,)*) -> #return_type {
                    self.0.borrow().invoke(#prop_name, &[#(#args_name.into(),)*])
                        .try_into().unwrap_or_else(|_| panic!("Invalid return type for function {}::{}", #component_name, #prop_name))
                }
            ));
        } else {

View on GitHub (pinned to 3fd8f2ec03)

Solutions

1. Adjust the Rust callback argument types to match the .slint callback declaration.
2. Check for mismatched argument order or types.

When it happens

Trigger: Thrown at internal/compiler/generator/rust_live_preview.rs:121 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of slint-ui/slint@3fd8f2ec03 (2026-08-19). Data as JSON: /api/errors/a16edef70b563893. Report an issue: GitHub.