rust 10 lines · 1 tab

Box<T> for heap allocation and recursive types

Marcus Chen Jan 2026

1 tab

enum List {
    Cons(i32, Box<List>),
    Nil,
}

use List::{Cons, Nil};

fn main() {
    let list = Cons(1, Box::new(Cons(2, Box::new(Nil))));
}

1 file · rust Explain with highlit

Box<T> is Rust's simplest smart pointer: it allocates T on the heap and gives you ownership. When the Box goes out of scope, the heap memory is freed. I use Box when I need indirection (like recursive types), when moving large structs is expensive, or when I want trait objects (Box<dyn Trait>). The size of a Box is one pointer, making it cheap to move. For recursive data structures like trees or linked lists, Box is essential because the compiler needs a known size. It's also useful for abstracting over types with Box<dyn Error> or Box<dyn Iterator>. Unlike reference counting, Box has zero runtime overhead beyond the allocation itself.

Related snips

rust

struct Config<'a> {
    name: &'a str,
    value: &'a str,
}

fn parse_config(line: &str) -> Config {

Lifetime annotations for flexible borrowing in structs

rust lifetimes borrowing

by Marcus Chen 1 tab

rust

use crossbeam::channel::unbounded;
use std::thread;

fn main() {
    let (tx, rx) = unbounded();

Crossbeam for advanced concurrent data structures

rust concurrency lock-free

by Marcus Chen 1 tab

rust

use tracing::{info, instrument};

#[instrument]
fn process_request(user_id: u64) {
    info!(user_id, "Processing request");
    // Work happens here

tracing for structured logging and distributed tracing

rust observability tracing

by Marcus Chen 1 tab

rust

use std::sync::mpsc;
use std::thread;

fn main() {
    let (tx, rx) = mpsc::channel();

Channels (mpsc) for message passing between threads

rust concurrency channels

by Marcus Chen 1 tab

rust

use clap::Parser;

#[derive(Parser, Debug)]
#[command(author, version, about)]
struct Args {
    #[arg(short, long)]

clap for CLI argument parsing with derive macros

rust cli clap

by Marcus Chen 1 tab

rust

macro_rules! create_function {
    ($func_name:ident) => {
        fn $func_name() {
            println!("Called {}", stringify!($func_name));
        }
    };

Declarative macros (macro_rules!) for code generation

rust macros metaprogramming

by Marcus Chen 1 tab

Share this code

Here's the card — post it anywhere.

Box<T> for heap allocation and recursive types — share card

Link copied