Rust Struct Update Syntax Reuses Fields from Another Instance

The ..existing syntax fills unspecified fields of a new struct instance from an existing instance of the same type. It still follows the ownership rules we learned last time: a non-Copy field can move into the new value in source

struct Profile {
    name: String,
    city: String,
    age: u8,
}
 
fn main() {
    let original = Profile {
        name: String::from("Asha"),
        city: String::from("Pune"),
        age: 24,
    };
 
    let updated = Profile {
        city: String::from("Bengaluru"),
        ..original
    };
 
    println!("{}, {}, {}", updated.name, updated.city, updated.age);
}

Here, we are creating a struct Profile with name, city and age as fields. Then in main, we are creating an instance of it called original with some values for its fields, then we are creating another instance of this struct called updated and in this we are specifying city field value but for the rest of the fields we are writing ..original, this moves the string from original.name to updated.name but for original.age, it gets copied to updated.age as its a u8.

If you want to confirm that move operation actually happened, try adding println!("{}", original.name); after creating the updated and run cargo check, you’ll get an error. Rust rejects that line because the name moved. Struct update syntax saves repetition; it does not clone the reused fields.