# 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](https://bevyengine.org/) 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`:
```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
```rust
#[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.
~~~bash
cargo run --features slow_function_warning/enabled
~~~
## Custom Message Example
```rust
// 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:
```rust
// 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.
```rust
// 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:
```rust
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
}
```