And parse-don't-validate is often very nice to work with in Rust, I can describe how to turn some UTF-8 text into my type Foo in a function:

  impl std::str::FromStr for Foo {
    type Err = ReasonsItIsNotAFoo;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        /* etc. */
    }
  }

And then whenever I've got a string which I know ought to be a Foo, I can:

  let foo: Foo = string.parse().expect("This {string:?} ought to be a Foo but it isn't");

Since we said foo is a Foo, by inference the parsing of string needs to either succeed with a Foo, or fail while trying, so it calls that FromStr implementation we wrote earlier to achieve that.