Formatting

Use format!, write!, and std::fmt traits with precision, width, alignment, and custom Display/Debug.

Recipe

fn main() {
    let pi = 3.14159;
    println!("{pi:.2}");
    let s = format!("{:>8}", "right");
    println!("{s}");
}

When to reach for this: User-facing strings, logs, tables, padded output.

Working Example

use std::fmt;
 
struct Percent(f64);
impl fmt::Display for Percent {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{:.1}%", self.0 * 100.0)
    }
}
 
fn main() {
    println!("{}", Percent(0.856));
}

What this demonstrates:

• Custom Display for domain formatting
• Formatter precision control
• write! macro inside impl

Deep Dive

Specifiers: hex, binary, {:?} Debug, {:#?} pretty Debug, {name} named args, {:08} pad zeros.

Gotchas

• Debug for users - Ugly escapes. Fix: Display.
• format in hot loop - Alloc each time. Fix: reuse buffer write!.
• Wrong precision on integers - Still works - width vs precision differs.
• Display not derivable - Manual impl required.
• Unicode width vs chars - Padding uses char width mostly - terminal nuances.

Alternatives

FAQs

Related

• Formatting & Printing
• Deriving Traits
• Parsing & Conversion
• Strings Best Practices

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+.