rust string to &'static str

how-to-convert-a-string-into-a-static-str

copy form here(how-to-convert-a-string-into-a-static-str)

cannot return reference to temporary value

usage:

```rust use staticstr::tostr;

fn main() { let optionvalue = Some(123); let v = optionvalue.mapor("", |v| tostr(v.tostring())); asserteq!(v, "123"); }

```