Function toggle_sources 

pub fn toggle_sources(label: &str, states: &[&BoundState<bool>]) -> ViewHandle

Toggle(_:isOn: [Binding<Bool>]) — one toggle over several bindings.

The “select all” row at the top of a settings group: it reads mixed when the sources disagree, and setting it sets every one of them. There was no way to express that — a toggle took exactly one binding, so the mixed state had to be faked with a third value that nothing wrote back to.

sources is @in and generic, so the array travels by address; its elements are real Binding<Bool>s, each seventeen bytes in a twenty-four-byte stride, which is why they are placed by stride rather than packed.

Swift spells this Toggle("All", sources: items, isOn: \.self) — the isOn: [Binding<Bool>] overload exists and is what this calls, but the compiler cannot tell it from the single-binding one and rejects the array even with its type written out. Both spellings reach the same initialiser; the parity case uses the one that compiles.