Function formal_names 

pub(crate) fn formal_names<'a>(
    signatures: impl IntoIterator<Item = (&'a Punctuated<FnArg, Comma>, Formals)>,
) -> BTreeSet<String>

The R formals of every wrapper built from signatures, each a parameter list with the formals it declares: the union of each signature’s r_formal_names.

An impl block or a trait impl is finalized as one text, so its formals are pooled across its methods. A method whose sibling has a colliding formal is then qualified too, which calls the same function.