Basic instrumentation profiler for Rust

profiler.rs

// Based on https://gist.github.com/TheCherno/31f135eea6ee729ab5f26a6908eb3a5e
// Usage:
// 1. Call init before doing anything else with this file/snippet.
// 2. Put `let _timer = Timer::new()` at the begining of any scope you want to measure (DO NOT name the variable `_`. Because it will be dropped instantly after creation.)
// 3. Drag result.json into chrome://tracing (If you have a chromium-based browser) or https://ui.perfetto.dev for visualization

static mut TIMER: Option<std::time::Instant> = None;
static mut OUT_STREAM: String = String::new();
static mut NOT_FIRST: bool = false;

pub struct Timer {
    name: &'static str,
    start: std::time::Duration
}

impl Timer {
    pub fn new(name: &'static str) -> Timer {
        Timer {
            name,
            start: unsafe { TIMER.unwrap().elapsed() }
        }
    }
}

impl Drop for Timer {
    fn drop(&mut self) {
        unsafe {
//             OUT_STREAM.push_str("{");
//             OUT_STREAM.push_str("\"cat\":\"function\",");
//             OUT_STREAM.push_str(&format!("\"dur\":{},", (TIMER.unwrap().elapsed() - self.start).as_micros()));
//             OUT_STREAM.push_str(&format!("\"name\":\"{}\",", self.name));
//             OUT_STREAM.push_str("\"ph\":\"X\",");
//             OUT_STREAM.push_str("\"pid\":0,");
//             OUT_STREAM.push_str("\"tid\":0,");
//             OUT_STREAM.push_str(&format!("\"ts\":{}", self.start.as_micros()));
//             OUT_STREAM.push_str("}");
            
            if NOT_FIRST {
                OUT_STREAM.push_str(",");
            }
            NOT_FIRST = true;

            OUT_STREAM.push_str(&format!("{{\"cat\":\"function\",\"dur\":{},\"name\":\"{}\",\"ph\":\"X\",\"pid\":0,\"tid\":0,\"ts\":{}}}", (TIMER.unwrap().elapsed() - self.start).as_micros(), self.name, self.start.as_micros()));
        }
    }
}

pub fn init() {
    unsafe {
        TIMER = Some(std::time::Instant::now());
        OUT_STREAM = "{\"otherData\": {},\"traceEvents\":[".to_string();
    }
}

pub fn shutdown() {
    unsafe {
        OUT_STREAM.push_str("]}");
        std::fs::write("result.json", OUT_STREAM.as_str()).unwrap();
    }
}