This is a well-known limitation of closures: they cannot satisfy a for<'a> Trait<'a> bound where Trait<'a> requires Fn(&'a T). Here, the compiler can infer that the type of f implements Function<'a, String, (&'a str,)> for any particular lifetime 'a. This allows the explicit Function::call_func(&f, args) in main() to work with no issues. However, f cannot be passed to takes_any_function(), since f's type does not implement for<'a> Function<'a, String, (&'a str,)>. Currently, the only workaround is to use a lifetime-generic fn item instead of a closure. See also RFC 3216 for a proposed language solution to this issue.