Change debug printlns to logs.

Addresses servo/servo#10012.

Cargo.toml

@@ -43,6 +43,7 @@ fnv="1.0"
 scoped_threadpool = "0.1.6"
 app_units = {version = "0.2.1", features = ["plugins"]}
 lazy_static = "0.1"
+log = "0.3"
 
 [target.x86_64-apple-darwin.dependencies]
 core-graphics = ">=0.2, <0.4"

src/aabbtree.rs

@@ -222,7 +222,7 @@ impl AABBTree {
             //           include transformed elements, however this isn't currently
             //           handled by the layout code! If it's not that, this is an
             //           unexpected condition and should be investigated!
-            println!("WARNING: insert rect {:?} outside bounds, dropped.", rect);
+            debug!("WARNING: insert rect {:?} outside bounds, dropped.", rect);
         } else {
             for node_index in self.work_node_indices.drain(..) {
                 let NodeIndex(node_index) = node_index;

src/lib.rs

@@ -6,6 +6,8 @@
 
 #[macro_use]
 extern crate lazy_static;
+#[macro_use]
+extern crate log;
 
 mod aabbtree;
 mod batch;