Rust

Table of Contents

Concepts

Ownership

let s1 = String::from("hello");
let s2 = s1;

println!("{}", s1);
error[E0382]: use of moved value: `s1`
 --> src/main.rs:4:27
  |
3 |     let s2 = s1;
  |         -- value moved here
4 |     println!("{}, world!",s1);
  |                           ^^ value used here after move
  |
  = note: move occurs because `s1` has type `std::string::String`,
which does not implement the `Copy` trait


fn main() {
    let s1 = gives_ownership();         // gives_ownership moves its return
                                        // value into s1.

    let s2 = String::from("hello");     // s2 comes into scope.

    let s3 = takes_and_gives_back(s2);  // s2 is moved into
                                        // takes_and_gives_back, which also
                                        // moves its return value into s3.
} // Here, s3 goes out of scope and is dropped. s2 goes out of scope but was
  // moved, so nothing happens. s1 goes out of scope and is dropped.

fn gives_ownership() -> String {             // gives_ownership will move its
                                             // return value into the function
                                             // that calls it.

    let some_string = String::from("hello"); // some_string comes into scope.

    some_string                              // some_string is returned and
                                             // moves out to the calling
                                             // function.
}

// takes_and_gives_back will take a String and return one.
fn takes_and_gives_back(a_string: String) -> String { // a_string comes into
                                                      // scope.

    a_string  // a_string is returned and moves out to the calling function.
}
fn main() {
    let s = String::from("hello");

    change(&s);
}

fn change(some_string: &String) {
    some_string.push_str(", world");
}
error: cannot borrow immutable borrowed content `*some_string` as mutable
 --> error.rs:8:5
  |
8 |     some_string.push_str(", world");
  |     ^^^^^^^^^^^

fn main() {
    let mut s = String::from("hello");

    change(&mut s);
}

fn change(some_string: &mut String) {
    some_string.push_str(", world");
}
let mut s = String::from("hello");

let r1 = &mut s;
let r2 = &mut s;
error[E0499]: cannot borrow `s` as mutable more than once at a time
 --> borrow_twice.rs:5:19
  |
4 |     let r1 = &mut s;
  |                   - first mutable borrow occurs here
5 |     let r2 = &mut s;
  |                   ^ second mutable borrow occurs here
6 | }
  | - first borrow ends here

At any given time, you can have either but not both of:

fn main() {
    let mut s = String::from("hello world");

    let word = first_word(&s); // word will get the value 5.

    s.clear(); // This empties the String, making it equal to "".

    // word still has the value 5 here, but there's no more string that
    // we could meaningfully use the value 5 with. word is now totally invalid!
}
let s = String::from("hello");

let len = s.len();

let slice = &s[0..len];
let slice = &s[..];
fn first_word(s: &String) -> &str {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return &s[0..i];
        }
    }

    &s[..]
}
fn main() {
    let mut s = String::from("hello world");

    let word = first_word(&s);

    s.clear(); // Error!
}
17:6 error: cannot borrow `s` as mutable because it is also borrowed as
            immutable [E0502]
    s.clear(); // Error!
    ^
15:29 note: previous borrow of `s` occurs here; the immutable borrow prevents
            subsequent moves or mutable borrows of `s` until the borrow ends
    let word = first_word(&s);
                           ^
18:2 note: previous borrow ends here
fn main() {

}
^