Rust 1.29.2

# if let

There is a new edition of the book and this is an old link.

> The `if let` syntax lets you combine `if` and `let` into a less verbose way to handle values that match one pattern and ignore the rest.

    let some_u8_value = Some(3u8);
    if let Some(3) = some_u8_value {
        println!("three");
    }Run

Here are the relevant sections in the new and old books:

- In the second edition: Ch 6.03 — Concise Control Flow with `if let`
- In the first edition: Ch 3.21 — if let