leonardomso__rust-skills/rules/mem-avoid-format.md

mem-avoid-format

Avoid format!() when string literals work

Why It Matters

format!() always allocates a new String, even for constant text. In hot paths, these allocations add up. Use string literals, write!(), or pre-allocated buffers instead.

Bad

// Allocates every time, even for static text
fn get_error_message() -> String {
    format!("An error occurred")  // Unnecessary allocation!
}

// Allocates in a loop
for item in items {
    log::info!("{}", format!("Processing item: {}", item));  // Double work!
}

// format! in hot path
fn classify(n: i32) -> String {
    if n > 0 {
        format!("positive")  // Allocates!
    } else if n < 0 {
        format!("negative")  // Allocates!
    } else {
        format!("zero")      // Allocates!
    }
}

Good

// Return &'static str for constants
fn get_error_message() -> &'static str {
    "An error occurred"  // No allocation
}

// Use format args directly
for item in items {
    log::info!("Processing item: {}", item);  // No intermediate String
}

// Return Cow for mixed static/dynamic
use std::borrow::Cow;

fn classify(n: i32) -> Cow<'static, str> {
    if n > 0 {
        Cow::Borrowed("positive")  // No allocation
    } else if n < 0 {
        Cow::Borrowed("negative")  // No allocation
    } else {
        Cow::Borrowed("zero")      // No allocation
    }
}

// Or just &'static str if always static
fn classify_str(n: i32) -> &'static str {
    if n > 0 { "positive" }
    else if n < 0 { "negative" }
    else { "zero" }
}

Use write!() for Output

use std::io::Write;

// Bad: Allocate then write
fn bad_log(writer: &mut impl Write, msg: &str, code: u32) {
    let formatted = format!("[ERROR {}] {}", code, msg);  // Allocation!
    writer.write_all(formatted.as_bytes()).unwrap();
}

// Good: Write directly
fn good_log(writer: &mut impl Write, msg: &str, code: u32) {
    write!(writer, "[ERROR {}] {}", code, msg).unwrap();  // No allocation!
}

Pre-allocate for Multiple Appends

// Bad: Multiple allocations
fn build_message(parts: &[&str]) -> String {
    let mut result = String::new();
    for part in parts {
        result = format!("{}{}\n", result, part);  // Allocates each iteration!
    }
    result
}

// Good: Pre-allocate
fn build_message(parts: &[&str]) -> String {
    let total_len: usize = parts.iter().map(|p| p.len() + 1).sum();
    let mut result = String::with_capacity(total_len);
    for part in parts {
        result.push_str(part);
        result.push('\n');
    }
    result
}

// Good: Use join
fn build_message(parts: &[&str]) -> String {
    parts.join("\n")
}

CompactString for Small Strings

use compact_str::CompactString;

// Stack-allocated for strings <= 24 bytes
fn format_code(code: u32) -> CompactString {
    compact_str::format_compact!("ERR-{:04}", code)
    // Stack-allocated if result is small enough
}

When format!() Is Fine

// Rare/cold paths - clarity over micro-optimization
fn log_startup_message() {
    println!("{}", format!("Starting {} v{}", APP_NAME, VERSION));
}

// When you need an owned String anyway
fn create_user_greeting(name: &str) -> String {
    format!("Hello, {}!", name)  // Need owned String
}

// Error messages (already on error path)
return Err(format!("Invalid value: {}", value).into());

See Also

• mem-write-over-format - Use write!() instead of format!()
• mem-with-capacity - Pre-allocate strings
• own-cow-conditional - Use Cow for mixed static/dynamic