String Manipulation

Build and transform strings with push, push_str, +, format!, and capacity control.

Recipe

fn join_parts(parts: &[&str]) -> String {
    let mut out = String::new();
    for (i, p) in parts.iter().enumerate() {
        if i > 0 { out.push(' '); }
        out.push_str(p);
    }
    out
}

When to reach for this: Assembling output, normalizing text, builder patterns.

Working Example

fn with_capacity_join(items: &[&str]) -> String {
    let len: usize = items.iter().map(|s| s.len()).sum::<usize>() + items.len();
    let mut s = String::with_capacity(len);
    for (i, item) in items.iter().enumerate() {
        if i > 0 { s.push(','); }
        s.push_str(item);
    }
    s
}

What this demonstrates:

• with_capacity reduces realloc
• In-place building vs format! in loop
• + consumes left String - prefer push_str

Deep Dive

insert, insert_str, replace, replacen, drain, clear, truncate (byte-based!). split, lines, trim.

Gotchas

• s1 + &s2 moves s1 - Left operand consumed. Fix: format!("{s1}{s2}") or push_str.
• truncate splits codepoints - Invalid if mid-char. Fix: Char-aware truncation.
• Repeated format! in loop - Allocations. Fix: Single buffer write! or push_str.
• replace allocates - New string each call. Fix: In-place if pattern rare.
• clear retains capacity - Good for reuse buffers.

Alternatives

FAQs

Related

• Strings Basics
• Formatting
• Cow
• Collections 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+.