What's wrong with this line and why?

help
1 
fn main() {
   
   let name = String::from("hello");

   let (name, len) = calc_len(name);

   println!("{name} : {len}");
    
}
fn calc_len(name:String)->(String,usize) {
    (name,name.len()) // this one ????
}
2

you're transfering ownership of the 'name' and in the same time you want to access it

3

but i think tuple should be evaluted first then return /transfer

or in some diff way process is going on??

4

...and in this process of evaluation, name will be moved into its first element, before evaluation comes to the second one. Swapping the order of the fields works.

5

There's no "build the tuple, then transfer" step. Building (a, b) is evaluating a and moving it into slot 0, then evaluating b and moving it into slot 1 - left to right. So name is already moved by the time name.len() runs, and you get E0382.

Proof it's purely ordering: (name.len(), name) compiles fine.

6

In other words, (<some-expr-x>, <some-expr-y>) is evaluated as:

let tuple_0 = <some-expr-x>;
let tuple_1 = <some-expr-y>;
(tuple_0, tuple_1)

So (name, name.len()) is equivalent to:

let tuple_0 = name; // moves `name`
let tuple_1 = name.len(); // `name` already moved
(tuple_0, tuple_1)
7

For the sake of completion, it's worth mentioning that in Rust you'll usually create temporary ad-hoc variables particularly to get around this problem:

fn calc_len(name:String)->(String,usize) {
    let len = name.len();

    (name,len)
}