> When would I use Into, when From

The standard library contains this code:

    // From implies Into
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T, U> Into<U> for T where U: From<T> {
        fn into(self) -> U {
            U::from(self)
        }
    }

So, you basically only ever implement From, and you get the equivalent Into for free.

More specifically, you would usually use `Into` to bound a generic function:

    fn is_hello<T: Into<Vec<u8>>>(s: T) {
        let bytes = b"hello".to_vec();
        assert_eq!(bytes, s.into());
    }
    
    let s = "hello".to_string();
    is_hello(s);

Whereas you'd use From to perform a conversion:

    let string = "hello".to_string();
    let other_string = String::from("hello");
    
    assert_eq!(string, other_string);

These APIs are pretty new; they landed _right_ before beta happened. We used to have FromFoo traits, and this generic system replaced them.