### Rust r2d2 Connection Pool Example

Source: https://github.com/sfackler/r2d2/blob/master/README.md

Demonstrates how to create and use a connection pool with r2d2 and an imaginary 'foodb' database. It shows initializing the connection manager, building the pool with a maximum size, and spawning threads to acquire and use connections from the pool.

```Rust
use std::thread;

extern crate r2d2;
extern crate r2d2_foodb;

fn main() {
    let manager = r2d2_foodb::FooConnectionManager::new("localhost:1234");
    let pool = r2d2::Pool::builder()
        .max_size(15)
        .build(manager)
        .unwrap();

    for _ in 0..20 {
        let pool = pool.clone();
        thread::spawn(move || {
            let conn = pool.get().unwrap();
            // use the connection
            // it will be returned to the pool when it falls out of scope.
        })
    }
}
```

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