How do I return a reference to a bool from a function?

Question

I was working on a bitboard implementation and was trying to implement the index trait but couldn't return value &bool because this creates a temp value which could not be returned. Is there any way I could return a &bool in another way?

use std::ops::Index;

pub struct Bitboard(usize);

impl Index<usize> for Bitboard {
    type Output = bool;

    fn index(&self, index: usize) -> &Self::Output {
        &(self.0 & (1 << index) != 0)
    }
}

fn main() {
    let board = Bitboard(0b000_000_000);
    // bit:                          ^
    println!("{}", board[0]);
    // false
}

Answer 1

Is there any way I could return a &bool in another way?

For arbitrary types returning a reference to a generated value would not be possible without leaking memory. But since there are only two distinct bool values, you could create them as static variables and return references to those:

static TRUE: bool = true;
static FALSE: bool = false;
// return &TRUE and &FALSE from index()

But it gets even easier because Rust treats a simple &true and &false as if you did just that, so this compiles:

fn index(&self, index: usize) -> &bool {
    if self.0 & (1 << index) != 0 {
        &true
    } else {
        &false
    }
}

Playground

The borrow checker allows returning &TRUE and &true because those expressions produce references with 'static lifetime. Such references can be coerced to references of any other lifetime, including the lifetime of &self as required by the signature of Index::index().