Rust編譯器的「精神分裂症」

代碼在此

不可變量 immutable

// 編譯成功
fn main() {
    let num = 1;
    let num = 2;
    println!("num: {}", num);
}
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編譯失敗

fn main() {
    let num = 1;
    num = 2;
    println!("num: {}", num);
}
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error[E0384]: cannot assign twice to immutable variable `num`
30 |     let num = 1;
   |         ---
   |         |
   |         first assignment to `num`
   |         help: make this binding mutable: `mut num`
31 |     num = 2;
   |     ^^^^^^^ cannot assign twice to immutable variable

error: aborting due to previous error
For more information about this error, try `rustc --explain E0384`.
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// 編譯成功
fn main() {
    let mut num = 1;
    num = 2;

    println!("num: {}", num);
}
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全部權移動

// 編譯成功
fn main() {
    // let s1 = Box::new("word");
    let s1 = String::from("word");

    println!("s1: {}", s1);
}
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編譯失敗

fn main() {
    // let s1 = Box::new("word");
    let s1 = String::from("word");
    let s2 = s1;
    println!("s1: {}", s1);
}
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error[E0382]: borrow of moved value: `s1`
25 |     let s1 = String::from("word");
   |         -- move occurs because `s1` has type `std::string::String`, which does not implement the `Copy` trait
26 |     let s2 = s1;
   |              -- value moved here
27 |
28 |     println!("s1: {}", s1);
   |                        ^^ value borrowed here after move

error: aborting due to previous error
For more information about this error, try `rustc --explain E0382`.
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struct

// 編譯成功
#[derive(Debug)]
struct Gps {
    x: i32,
    y: i32,
}

fn main() {
    let a = Gps { x: 3, y: 7 };
    println!("a struct: {:#?}", a);
}
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// 編譯失敗

#[derive(Debug)]
struct Gps {
    x: i32,
    y: i32,
}

fn main() {
    let a = Gps { x: 3, y: 7 };
    let b = a;
    println!("a struct: {:#?}", a);
}
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error[E0382]: borrow of moved value: `a`
9  |     let a = Gps { x: 3, y: 7 };
   |         - move occurs because `a` has type `Gps`, which does not implement the `Copy` trait
10 |     let b = a;
   |             - value moved here
11 |     println!("a struct: {:#?}", a);
   |                                 ^ value borrowed here after move

error: aborting due to previous error
For more information about this error, try `rustc --explain E0382`.
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// 編譯成功
#[derive(Debug, Copy, Clone)]
struct Gps {
    x: i32,
    y: i32,
}

fn main() {
    let a = Gps { x: 3, y: 7 };
    let b = a;
    println!("a struct: {:#?}", a);
}
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