slint-ui/slint · error · syn::Error

VTable pointer need to be the first

Error message

VTable pointer need to be the first

What it means

Error "VTable pointer need to be the first" thrown in slint-ui/slint.

Source

Thrown at helper_crates/vtable/macro/macro.rs:318

                        Some((matches!(mutability, PointerMutability::Mut(_)), elem))
                    }
                    Type::Reference(TypeReference { mutability, elem, .. }) => {
                        Some((mutability.is_some(), elem))
                    }
                    _ => None,
                };
                if let Some((is_mut, elem)) = ptr_target
                    && let Type::Path(p) = &**elem
                    && let Some(pointer_to) = p.path.get_ident()
                    && pointer_to == &vtable_name
                {
                    if is_mut {
                        return Error::new(p.span(), "VTable cannot be mutable")
                            .to_compile_error()
                            .into();
                    }
                    if call_code.is_some() || !sig.inputs.is_empty() {
                        return Error::new(p.span(), "VTable pointer need to be the first")
                            .to_compile_error()
                            .into();
                    }
                    call_code = Some(quote!(vtable as _,));
                    continue;
                }

                let (is_pin, self_ty) = match is_pin(&param.ty) {
                    Some(t) => (true, t),
                    None => (false, &param.ty),
                };

                // check for self
                if let (true, mutability) = if match_generic_type(self_ty, "VRef", &vtable_name) {
                    (true, None)
                } else if match_generic_type(self_ty, "VRefMut", &vtable_name) {
                    (true, Some(Default::default()))
                } else {

View on GitHub (pinned to 3fd8f2ec03)

Solutions

1. Place the vtable pointer as the first field of the struct.
2. Reorder the struct fields accordingly.

When it happens

Trigger: Thrown at helper_crates/vtable/macro/macro.rs:318 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/fe37704215716218. Report an issue: GitHub.