Slow Function Warning

This crate provides a procedural macro to inject timers into functions and print a warning if it takes longer than expected. It can be particularly useful for debugging performance issues during development.

This is not meant to be a benchmarking tool, but rather a way to detect potential performance issues in your code.

For example my use case was for developing a game in Bevy and I've added this to all my systems to detect if any game system function takes longer than a 1ms.

Usage

Installation

Add the following to your Cargo.toml:

[features]
slow_function_warning = ["dep:slow_function_warning"]

[dependencies]
# Add as a feature to avoid affecting the LSP.
slow_function_warning = { version = "0.5.0", optional = true }

# For wasm targets
[target.'cfg(target_family = "wasm")'.dependencies]
web-time = "1"

Basic Example

#[cfg_attr(feature = "slow_function_warning", slow_function_warning(1000ms))] // Warn if the function takes longer than 1000 milliseconds
fn example_function() {
    // Function implementation
}

The warning is not on by default and is only recommended for debugging purposes. To enable it use the slow_function_warning feature.

cargo run --features slow_function_warning/enabled

Custom Message Example

// Warn if the function takes longer than a second with a custom message
#[cfg_attr(feature = "slow_function_warning", slow_function_warning(1ms, println!("Function {function} took too long!")))]
fn example_function() {
    // Function implementation
}

You can also use the function parameters in your message:

// Warn if the function takes longer than a second with a custom message
#[cfg_attr(feature = "slow_function_warning", slow_function_warning(1s, println!("Function {function} took {millis} for {} values!", values.len())))]
fn sort(values: &Vec<u32>) {
    // Function implementation
}

Duration Syntax

You can specify the duration using numeric literals followed by a suffix:

• ns for nanoseconds
• ms for milliseconds
• s for seconds
• m for minutes
• h for hours
• d for days

Available Variables

• module: String - The name of the module
• function: String - The name of the function
• elapsed: Duration - The elapsed time
• nanos: u64 - The elapsed time in nanoseconds
• ns: u64 - The elapsed time in nanoseconds
• millis: u64 - The elapsed time in milliseconds
• ms: u64 - The elapsed time in milliseconds
• secs: u64 - The elapsed time in seconds
• s: u64 - The elapsed time in seconds

How it works

This is a procedural macro that takes the content of a function and places it in a closure, executes it and times how long it took.

// Warn if the function takes longer than a second with a custom message
#[cfg_attr(feature = "slow_function_warning", slow_function_warning(1s, println!("Function {function} took too long!")))]
fn example_function() {
    let x = 10;
}

Becomes:

fn example_function() {
    let closure = || {
        let x = 10;
    };
    #[cfg(not(target_family = "wasm"))]
    let start = std::time::Instant::now();
    #[cfg(target_family = "wasm")]
    let start = web_time::Instant::now();
    let result = closure();
    if start.elapsed().as_nanos() > 1000000 {
        let module = "module name";
        let function = "example_function";
        let elapsed = start.elapsed();
        let ns = elapsed.as_nanos();
        let nanos = ns;
        let ms = elapsed.as_millis();
        let millis = ms;
        let s = elapsed.as_secs();
        let secs = s;
        println!("Warning: {module}::{function}: ran for {millis}ms")
    }
    result
}