CODEWITHSUNDEEP
rust
A H Bensiali
A H Bensiali

Reputation: 875

What are the curly braces in "&{ s }.borrow_self()" for?

I'm going through the statements and expressions chapter in the Rust book. What are the curly braces in &{ s }.borrow_self() for? Am I getting the pointer, and if so would it not make more sense to use &s.borrow_self()?

Just a little confused as to what purpose the curly braces have and the logic behind using them.

#![allow(unused)]
fn main() {
    struct Struct;

    impl Struct {
        fn consume_self(self) {}
        fn borrow_self(&self) {}
    }

    fn move_by_block_expression() {
        let s = Struct;

        // Move the value out of `s` in the block expression.
        (&{ s }).borrow_self();

        // Fails to execute because `s` is moved out of.
        s.consume_self();
    }
}

Upvotes: 0

Views: 634

Answers (2)

Cerberus
Cerberus

Reputation: 10237

Quoting the linked page from the reference:

This can be used to force moving a value if really needed. For example, the following example fails on the call to consume_self because the struct was moved out of s in the block expression.

So:

  • In general, curly braces can be used when something needs to be moved for the code to be correct.
  • In this example, their usage is explicitly stated to be incorrect. The valid code would contain just s.borrow_self(), as you've found out.
  • The value of this example is in providing the case when the move semantics are immediately obvious. It doesn't have to be useful as a code, since the quoted book is language reference, and it's more important for it to clearly show the expected semantics then to educate on "how the code should be written".

Upvotes: 3

Netwave
Netwave

Reputation: 42746

{} creates a new scope. Remember that the last statement in each scope is used as the return value. In this case {s} evaluates to s. So &{s} moves s into the scope, returns it, and then just references it.

Let's evaluate "manually":

  1. (&{s}).borrow_self() => Move s to the new scope and return it.
  2. (&s).borrow_self() => Reference s
  3. Struct::borrow_self(&s) => Evaluate method over &self (&s)

Upvotes: 4

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