Function simplify_fn_constraint

fn simplify_fn_constraint<'a, 'tcx>(
    self_: Option<&'a Type>,
    generics: &Generics,
    constraint: &'a AssocItemConstraint,
    tcx: TyCtxt<'tcx>,
    recurse: usize,
    res: &mut Vec<(RenderTypeId, Vec<RenderType>)>,
    rgen: &mut FxIndexMap<SimplifiedParam, (isize, Vec<RenderType>)>,
    is_return: bool,
    cache: &Cache,
)