I have never written a line of rust in my life but I assume that a statement that terminates with a value can be used as a rvalue in an assignment or other expression.

So (I guess) both examples set the variable to 1.

Rust is an expression-based language, so a loop block is just an expression. Break exits the loop and returns the expression following the keyword, which is the integer 1 in this example, but it could also be nothing if the loop returns nothing (aka unit "()", aka void in some languages), or any other expression as well.