Why is there a borrow of a moved value when calling a method that takes self by value with an argument that also calls a method?

问题

I ran into an issue that forces me to split a nice oneliner into a {} block with an intermediate let. The reason for this isn't clear to me at all. I was able to isolate the issue in this minimal example:

struct AB {
    a: u8,
    b: u8,
}

impl AB {
    fn foo(&self) -> String {
        String::from("foo")
    }
    fn bar(self, x: String) -> String {
        format!("{} - {} - {}!", x, self.a, self.b)
    }
}

fn main() {
    let x = AB { a: 3, b: 5 };
    let result = x.bar(x.foo());
    println!("{}", result);
}

I was under the impression that the arguments to the bar function would be evaluated before calling bar. foo borrows x during its execution, but when it returns its String, the borrow is finished, as String is not a reference bearing x's lifetime. When bar gets called, foo's borrow should be over.

However, the compiler disagrees:

error[E0382]: borrow of moved value: `x`
  --> src/main.rs:17:24
   |
17 |     let result = x.bar(x.foo());
   |                  -     ^ value borrowed here after move
   |                  |
   |                  value moved here
   |
   = note: move occurs because `x` has type `AB`, which does not implement the `Copy` trait

I'm not disagreeing with the fact that bar indeed moves x. My issue is with the statement that foo borrows x after the move takes place.

A simple (but ugly) fix:

struct AB {
    a: u8,
    b: u8,
}

impl AB {
    fn foo(&self) -> String {
        String::from("foo")
    }
    fn bar(self, x: String) -> String {
        format!("{} - {} - {}!", x, self.a, self.b)
    }
}

fn main() {
    let x = AB { a: 3, b: 5 };
    let y = x.foo();
    let result = x.bar(y);
    println!("{}", result);
}

separating the assignment of x.foo() to an intermediate variable y compiles fine, confirming my expectation that the borrow is indeed over once foo returns, but why does this work? Is there something I don't understand about evaluation order? Why can't I get rid of the intermediate let y ?

回答1:

Evaluation order, for the purpose of borrows, is from left to right.

This means that the subject of the bar call, the "move-out" mention of x, is considered before the "borrow" mention of x in the foo call, and therefore, the compiler considers the variable to have been moved from.

For the similar case where the outer mention is a mutable borrow, RFC 2025 has been accepted as a solution, but hasn't been implemented yet. Unfortunately this RFC doesn't appear to cover your case, where the outer use is a move.

来源：https://stackoverflow.com/questions/55117392/why-is-there-a-borrow-of-a-moved-value-when-calling-a-method-that-takes-self-by

标签

rust

borrow-checker