improve debug logging to use an env var

It used to be "always on" whenever it was compiled in, which was
annoying, since sometimes you would want to quickly test to see if your
new code worked without the full overhead of logging.

Author: nikomatsakis

Cargo.toml

@@ -15,6 +15,7 @@ rand = "0.3"
 num_cpus = "1.0"
 deque = "0.3.1"
 libc = "0.2.16"
+lazy_static = "0.2.2"
 
 [dev-dependencies]
 compiletest_rs = "0.2.1"

src/lib.rs

@@ -2,6 +2,8 @@
 #![cfg_attr(test, feature(conservative_impl_trait))]
 
 extern crate deque;
+#[macro_use]
+extern crate lazy_static;
 extern crate libc;
 extern crate num_cpus;
 extern crate rand;

src/log.rs

@@ -1,5 +1,10 @@
 //! Debug Logging
+//!
+//! To use in a debug build, set the env var `RAYON_RS_LOG=1`.  In a
+//! release build, logs are compiled out. You will have to change
+//! `DUMP_LOGS` to be `true`.
 
+use std::env;
 use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
 
 #[derive(Debug)]
@@ -23,11 +28,15 @@ pub enum Event {
     LostJob { worker: usize },
 }
 
-pub const DUMP_LOGS: bool = false;
+pub const DUMP_LOGS: bool = cfg!(debug_assertions);
+
+lazy_static! {
+    pub static ref LOG_ENV: bool = env::var("RAYON_RS_LOG").is_ok();
+}
 
 macro_rules! log {
     ($event:expr) => {
-        if ::log::DUMP_LOGS { println!("{:?}", $event); }
+        if ::log::DUMP_LOGS { if *::log::LOG_ENV { println!("{:?}", $event); } }
     }
 }