Formatting & Printing

Rust formats text with macros (println!, format!, write!) backed by the std::fmt traits Display and Debug. Format specifiers control width, precision, and alignment.

Recipe

#[derive(Debug)]
struct User { id: u64, name: String }
 
fn main() {
    let u = User { id: 1, name: "Ada".into() };
    println!("{}", u.name);
    println!("{:?}", u);
    println!("{:#?}", u);
    eprintln!("log: id={}", u.id);
}

When to reach for this: CLI output, logs, test failures, and implementing user-visible string forms of your types.

Working Example

use std::fmt;
 
struct Point { x: i32, y: i32 }
 
impl fmt::Display for Point {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "({}, {})", self.x, self.y)
    }
}
 
fn main() {
    let p = Point { x: 3, y: 4 };
    println!("point={p}");
    println!("{:>10}", p);
    let s = format!("moved {p}");
    assert_eq!(s, "moved (3, 4)");
}

What this demonstrates:

• Manual Display impl for custom formatting
• Alignment width {:>10} right-aligns in 10 columns
• format! allocates a String
• eprintln! writes to stderr

Deep Dive

Common Macros

Format Specifiers

println!("{:.2}", 3.14159);      // 3.14
println!("{:08x}", 255);         // 000000ff
println!("{name}", name = "Rust"); // named args

Debug vs Display

• Debug: developer-oriented, often derived
• Display: user-facing, manual impl required for custom types
• #? pretty-prints Debug with indentation

Gotchas

• println!(my_struct) without Display - Compile error. Fix: Derive Debug and use {:?}, or impl Display.
• Formatting non-UTF-8 for users - Debug may escape strings oddly. Fix: Implement Display for human text.
• Using format! in hot loops - Allocates every call. Fix: Use write! to a buffer or reuse String with clear().
• Wrong brace escaping - {{ and }} produce literal braces. Fix: Double braces in format strings.
• Panicking in Display::fmt - Should return fmt::Error, not panic. Fix: Propagate fmt::Result.

Alternatives

FAQs

Related

• Strings Basics - String vs &str
• Formatting - advanced format patterns
• Deriving Traits - #[derive(Debug)]
• Common Standard Traits - Display, Debug

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.