Compiler

Rust’s #[must_use] attribute turns silent bugs into compile-time warnings — making sure important return values never get accidentally ignored.

The Problem: Silently Ignoring Results

Here’s a classic bug that can haunt any codebase:

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fn remove_expired_tokens(tokens: &mut Vec<String>) -> usize {
    let before = tokens.len();
    tokens.retain(|t| !t.starts_with("exp_"));
    before - tokens.len()
}

fn main() {
    let mut tokens = vec![
        "exp_abc".to_string(),
        "valid_xyz".to_string(),
        "exp_def".to_string(),
    ];

    // Bug: we call the function but ignore the count!
    remove_expired_tokens(&mut tokens);

    // No warning, no error — the return value just vanishes
}

The function works fine, but the caller threw away useful information without even a whisper from the compiler.

The Fix: #[must_use]

Add #[must_use] to the function and the compiler has your back:

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#[must_use = "returns the number of removed tokens"]
fn remove_expired_tokens(tokens: &mut Vec<String>) -> usize {
    let before = tokens.len();
    tokens.retain(|t| !t.starts_with("exp_"));
    before - tokens.len()
}

Now if someone calls remove_expired_tokens(&mut tokens); without using the result, the compiler emits:

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warning: unused return value of `remove_expired_tokens` that must be used
  --> src/main.rs:14:5
   |
   = note: returns the number of removed tokens

Works on Types Too

#[must_use] isn’t just for functions — it shines on types:

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#[must_use = "this Result may contain an error that should be handled"]
enum DatabaseResult<T> {
    Ok(T),
    Err(String),
}

This is exactly why calling .map() on an iterator without collecting produces a warning — Map is marked #[must_use] in std.

Already in the Standard Library

Rust’s standard library uses #[must_use] extensively. Result, Option, MutexGuard, and many iterator adapters are all marked with it. That’s why you get a warning for:

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vec![1, 2, 3].iter().map(|x| x * 2);  // warning: unused `Map`

The iterator does nothing until consumed — and #[must_use] makes sure you don’t forget.

Quick Rules

Use #[must_use] when:

• A function returns a Result or error indicator — callers should handle failures
• A function is pure (no side effects) — ignoring the return means the call was pointless
• A type is lazy (like iterators) — it does nothing until consumed
• The return value carries critical information the caller likely needs

The custom message string is optional but highly recommended — it tells the developer why they shouldn’t ignore the value.