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Move语义6: exercises/move_semantics/move_semantics6.rs
题目
rust
// move_semantics6.rs
//
// You can't change anything except adding or removing references.
//
// Execute `rustlings hint move_semantics6` or use the `hint` watch subcommand
// for a hint.
// I AM NOT DONE
fn main() {
let data = "Rust is great!".to_string();
get_char(data);
string_uppercase(&data);
}
// Should not take ownership
fn get_char(data: String) -> char {
data.chars().last().unwrap()
}
// Should take ownership
fn string_uppercase(mut data: &String) {
data = &data.to_uppercase();
println!("{}", data);
}题目要求：只能添加或删除借用，其他的不能做任何修改。
题目解析
编译器提供的信息如下：
txt
⚠️ Compiling of exercises/move_semantics/move_semantics6.rs failed! Please try again. Here's the output:
error[E0382]: borrow of moved value: `data`
--> exercises/move_semantics/move_semantics6.rs:15:22
|
11 | let data = "Rust is great!".to_string();
| ---- move occurs because `data` has type `String`, which does not implement the `Copy` trait
12 |
13 | get_char(data);
| ---- value moved here
14 |
15 | string_uppercase(&data);
| ^^^^^ value borrowed here after move
|
note: consider changing this parameter type in function `get_char` to borrow instead if owning the value isn't necessary
--> exercises/move_semantics/move_semantics6.rs:19:19
|
19 | fn get_char(data: String) -> char {
| -------- ^^^^^^ this parameter takes ownership of the value
| |
| in this function
help: consider cloning the value if the performance cost is acceptable
|
13 | get_char(data.clone());
| ++++++++
error[E0716]: temporary value dropped while borrowed
--> exercises/move_semantics/move_semantics6.rs:25:13
|
24 | fn string_uppercase(mut data: &String) {
| - let's call the lifetime of this reference `'1`
25 | data = &data.to_uppercase();
| --------^^^^^^^^^^^^^^^^^^^- temporary value is freed at the end of this statement
| | |
| | creates a temporary value which is freed while still in use
| assignment requires that borrow lasts for `'1`
error: aborting due to 2 previous errors
Some errors have detailed explanations: E0382, E0716.
For more information about an error, try `rustc --explain E0382`.获取到的错误信息如下：
get_char(data)函数时data已经转移了所有权- 后面
string_uppercase(&data)又对data进行借用，因为上一步所有权已经转移，此处报错。
我们将其调换以下即可：
rust
fn main() {
let data = "Rust is great!".to_string();
get_char(data);
get_char(&data);
string_uppercase(&data);
string_uppercase(data);
}
// Should not take ownership
fn get_char(data: String) -> char {
fn get_char(data: &String) -> char {
data.chars().last().unwrap()
}
// Should take ownership
fn string_uppercase(mut data: &String) {
fn string_uppercase(mut data: String) {
data = &data.to_uppercase();
data = data.to_uppercase();
println!("{}", data);
}