fix max_level_hint for empty cases for Option/Vec Subscribe impls

tracing-subscriber/src/subscribe/mod.rs

@@ -1485,7 +1485,10 @@ where
     fn max_level_hint(&self) -> Option<LevelFilter> {
         match self {
             Some(ref inner) => inner.max_level_hint(),
-            None => None,
+            None => {
+                // There is no underlying `Subscribe` so our max level is OFF
+                Some(LevelFilter::OFF)
+            }
         }
     }
 
@@ -1690,7 +1693,8 @@ feature! {
         }
 
         fn max_level_hint(&self) -> Option<LevelFilter> {
-            let mut max_level = LevelFilter::ERROR;
+            // Default to `OFF` if there are no underlying `Subscribe`s
+            let mut max_level = LevelFilter::OFF;
             for s in self {
                 // NOTE(eliza): this is slightly subtle: if *any* subscriber
                 // returns `None`, we have to return `None`, assuming there is

tracing-subscriber/tests/subscriber_filters/main.rs

@@ -3,6 +3,7 @@
 mod support;
 use self::support::*;
 mod filter_scopes;
+mod option;
 mod targets;
 mod trees;
 mod vec;

tracing-subscriber/tests/subscriber_filters/option.rs

@@ -0,0 +1,45 @@
+use super::*;
+use tracing::Collect;
+
+// This test is just used to compare to the tests below
+#[test]
+fn just_layer() {
+    let info = subscriber::named("info")
+        .run()
+        .with_filter(LevelFilter::INFO)
+        .boxed();
+
+    let collector = tracing_subscriber::registry().with(info);
+    assert_eq!(collector.max_level_hint(), Some(LevelFilter::INFO));
+}
+
+#[test]
+fn layer_and_option_layer() {
+    let info = subscriber::named("info")
+        .run()
+        .with_filter(LevelFilter::INFO)
+        .boxed();
+
+    let debug = subscriber::named("debug")
+        .run()
+        .with_filter(LevelFilter::DEBUG)
+        .boxed();
+
+    let mut collector = tracing_subscriber::registry().with(info).with(Some(debug));
+    assert_eq!(collector.max_level_hint(), Some(LevelFilter::DEBUG));
+
+    let error = subscriber::named("error")
+        .run()
+        .with_filter(LevelFilter::ERROR)
+        .boxed();
+    // None means the other layer takes control
+    collector = tracing_subscriber::registry().with(error).with(None);
+    assert_eq!(collector.max_level_hint(), Some(LevelFilter::ERROR));
+}
+
+#[test]
+fn just_option_layer() {
+    // Just a None means everything is off
+    let collector = tracing_subscriber::registry().with(None::<ExpectSubscriber>);
+    assert_eq!(collector.max_level_hint(), Some(LevelFilter::OFF));
+}

tracing-subscriber/tests/subscriber_filters/vec.rs

@@ -115,3 +115,10 @@ fn all_filtered_max_level_hint() {
 
     assert_eq!(collector.max_level_hint(), Some(LevelFilter::DEBUG));
 }
+
+#[test]
+fn empty_vec() {
+    // Just a None means everything is off
+    let collector = tracing_subscriber::registry().with(Vec::<ExpectSubscriber>::new());
+    assert_eq!(collector.max_level_hint(), Some(LevelFilter::OFF));
+}