改变存储在 Vec 中的枚举中的值

Question

The following code fails to compile with the error below:

enum Test {
    C(i32),
}

fn main() {
    let mut v = Vec::new();

    v.push(Test::C(0));

    if let Some(Test::C(ref mut c)) = v.last_mut() {
        *c = *c + 1;
    }
}

 error[E0308]: mismatched types --> src/main.rs:10:17 | 10 | if let Some(Test::C(ref mut c)) = v.last_mut() { | ^^^^^^^^^^^^^^^^^^ expected &mut Test, found enum `Test` | = note: expected type `&mut Test` found type `Test`

last_mut() returns a mutable reference, and I'm taking the i32 as a mutable reference. I've tried making the mutability even more clear as follows, but I get the same compiler error.

if let Some(ref mut e) = v.last_mut() {
    if let Test::C(ref mut c) = e {
        *c = *c + 1;
    }
}

Why doesn't this work?

Answer 1

You just need to match exactly what the error message says. It expects a &mut Test , so you should match on that:

if let Some(&mut Test::C(ref mut c)) = v.last_mut() {
    //     ^^^^^^
    *c = *c + 1;
}

Here it is running in the playground .

As of Rust 1.26 , your original code works as-is and the explicit ref and ref mut keywords are no longer required:

if let Some(Test::C(c)) = v.last_mut() {
    *c = *c + 1;
}