Function arg_borrows 

pub(crate) fn arg_borrows(
    inputs: &Punctuated<FnArg, Comma>,
    unborrowed: &[String],
    self_ty: Option<&Type>,
) -> TokenStream

The borrows of the borrowed external parameters of inputs, except those unborrowed names (a parameter marker rebinds them to another type). Each binds the parameter to its reference or view, holding the value’s RefCell until the wrapper’s closure ends, after return conversion.