Function placeholder_type_error

pub(crate) fn placeholder_type_error<'tcx>(
    cx: &dyn HirTyLowerer<'tcx>,
    generics: Option<&Generics<'_>>,
    placeholder_types: Vec<Span>,
    suggest: bool,
    hir_ty: Option<&Ty<'_>>,
    kind: &'static str,
)

If there are any placeholder types (_), emit an error explaining that this is not allowed and suggest adding type parameters in the appropriate place, taking into consideration any and all already existing generic type parameters to avoid suggesting a name that is already in use.