executing method on variable pointer moves the variable itself?-CodePudding

I'm using the rusqlite crate and am doing some queries, basically i'm just trying to check the length of the query results before trying to proceed, however I'm running into error[E0382]: use of moved value: key_rows whilst trying to compile, I don't understand since i'm borrowing a reference to the variable so it wouldn't move it's local in memory?

Maybe it's due to the method i'm calling on the variable's pointer?

Full compiler error:

error[E0382]: use of moved value: `key_rows`
   --> src/handle.rs:126:16
    |
108 |     let key_rows = key_stmt.query_map(&[(":key", key.as_str())], |row| {
    |         -------- move occurs because `key_rows` has type `MappedRows<'_, [closure@src/handle.rs:108:66: 113:6]>`, which does not implement the `Copy` trait
...
117 |     if(&key_rows.count() == &0){
    |                  ------- `key_rows` moved due to this method call
...
126 |     for row in key_rows {
    |                ^^^^^^^^ value used here after move
    |
note: this function takes ownership of the receiver `self`, which moves `key_rows`

Erroneous code:

    let mut key_stmt = conn.prepare("SELECT id , key FROM key_table WHERE key = :key;").unwrap();
    
    let key_rows = key_stmt.query_map(&[(":key", key.as_str())], |row| {
        Ok(Table {
            id: row.get(0)?,
            payload: row.get(1)?,
        })
    }).unwrap();

    //Checking that the key exists:

    if(&key_rows.count() == &0){
        panic!("Can't find the key...")
    }
    //Putting in a default value since the compiler is worried.
    let mut reference_id : i32 = 0;

    //For loop is nessessary since MappedRow type cannot be indexed regularly (weird)
    for row in key_rows {

        reference_id = row.unwrap().id;
        println!("{:?}", reference_id.to_string());
    }

CodePudding user response：

You aren't borrowing key_rows then checking the count, but rather calling key_rows.count(), then borrowing the result.

Iterator#count consumes the entire iterator, returning how many elements were traversed.

let mut reference_id = key_rows.next().unwrap_or_else(|| panic!("Can't find the key..."));

// Remove the code past this point if you are only expecting one, or the first value.

for row in key_rows {
    reference_id = row.unwrap().id;
    println!("{:?}", reference_id.to_string());
}