Lesser-known rust pattern-matching features

patterns.rs

// Destructuring assignment
fn fibonacci(n: u64) -> u64 {
    let mut a = 1;
    let mut b = 1;
    for _ in 0..n {
        (a, b) = (a + b, a);
    }
    b
}

// Slice patterns
fn print_slice(slice: &[u32]) {
    while let [x, rest @ ..] = slice {
        // rest: &[u32]
        
        println!("Found {x}, {} items remaining",
            rest.len());
            
        slice = rest;
    }
}

// Range patterns
match x {
    0..42 => ...,
    0..=42 => ...,
    ..42 => ...,
    42.. => ...,
}

// Constants in patterns
const BEST_VALUE: u32 = 42;

match x {
    BEST_VALUE => ...,
    0..BEST_VALUE => ...,
    _ => ...,
}

// Match ergonomics
fn as_ref(x: &Option<T>) -> Option<&T> {
    match x {
        Some(x) => Some(x), // x: &T
        None => None,
    }
}
  
// Or patterns
match ... {
    Some(true) => ...,
    Some(false) | None => ...,
}

match ... {
    Some(12 | 42) => ...,
    _ => ...,
}

// Let-else
fn do_something(vec: Vec<u32>) -> bool {
    let Some(x) = vec.first() else {
        return false
    };
    x >= 42
}
// same as:
fn do_something(vec: Vec<u32>) -> bool {
    if let Some(x) = vec.first() {
        x >= 42
    } else {
        return false
    }
}
  
// Let chains (unstable, see https://github.com/rust-lang/rust/issues/53667)
if let Some(foo) = vec.first()
    && let [x, y, ..] = f(foo)
    && x >= 1 {
    ...
}

// Guard patterns (future, see https://github.com/rust-lang/rust/issues/129967)
match x {
    Some(vec) if vec.is_empty()
    | None => ...,
    _ => ...,
}