debug based on env var

rust/src/main.rs

@@ -1,3 +1,5 @@
+use std::env;
+
 use chunk::{Chunk, OpCode};
 use vm::VM;
 
@@ -10,8 +12,12 @@ mod value;
 mod vm;
 
 fn main() -> Result<()> {
+    let level = match env::var("DEBUG") {
+        Ok(_) => Level::DEBUG,
+        Err(_) => Level::ERROR,
+    };
     let subscriber = FmtSubscriber::builder()
-        .with_max_level(Level::DEBUG)
+        .with_max_level(level)
         .finish();
     tracing::subscriber::set_global_default(subscriber)
         .wrap_err("setting default subscriber failed")?;