Why does this tuple move its string?

Putting an existing variable into a tuple transfers that variable's ownership to the tuple. In addition, the order in which those things happen matters in some situations, and your example is one of them. Your example won't compile because the tuple takes the value before you access it again (the message.len() call happens after the move). However, write it this way instead:

let message = String::from("hello");

let pair = (message.len(), message);

and compilation succeeds, since message.len() runs before the tuple takes ownership.

I'd suggest going through the rustbook ownership chapter (https://doc.rust-lang.org/book/ch04-01-what-is-ownership.html) as well, which should help make this clearer.

and to preserve that order (message followed by length), introduce another variable and work out the lenght before you move message.

let message = String::from("hello");

let size = message.len();
let pair = (message, size);