feat: divide logging into stdout and stderr by verbosity

src/main.rs

@@ -14,7 +14,11 @@ use std::{collections::BTreeMap, fmt::Debug, str::FromStr, sync::Arc};
 use strum::EnumString;
 use tokio::{select, signal::ctrl_c, task::JoinSet};
 use tokio_util::{sync::CancellationToken, time::FutureExt as _};
-use tracing_subscriber::{EnvFilter, fmt::format::FmtSpan};
+use tracing::Level;
+use tracing_subscriber::{
+    EnvFilter,
+    fmt::{format::FmtSpan, writer::MakeWriterExt},
+};
 use twilight_gateway::{Event, EventTypeFlags, Intents, Shard, StreamExt};
 use twilight_model::{
     application::interaction::InteractionData,
@@ -173,13 +177,16 @@ async fn main() -> Result<(), MainError> {
 
     let LoggingArgs { env_filter } = logging_args;
 
-    let (non_blocking_writer, _guard) = tracing_appender::non_blocking(std::io::stdout());
+    let (stdout, _stdout_guard) = tracing_appender::non_blocking(std::io::stdout());
+    let (stderr, _stderr_guard) = tracing_appender::non_blocking(std::io::stderr());
+
+    let writer = stderr.with_max_level(Level::WARN).or_else(stdout);
 
     tracing_subscriber::fmt()
         .pretty()
         .with_env_filter(env_filter)
         .with_span_events(FmtSpan::NEW | FmtSpan::CLOSE)
-        .with_writer(non_blocking_writer)
+        .with_writer(writer)
         .init();
 
     tracing::debug!(?app_args, "using");