subscriber: set the max log LevelFilter in init (#1248)

Depends on #1247.

Since `tracing-subscriber`'s `init` and `try_init` functions set the
global default collector, we can use the collector's max-level hint as
the max level for the log crate, as well. This should significantly
improve performance for skipping `log` records that fall below the
collector's max level, as they will not have to call the
`LogTracer::enabled` method.

This will prevent issues like bytecodealliance/wasmtime#2662
from occurring in the future. See also #1249.

In order to implement this, I also changed the `FmtCollector`'s
`try_init` to just use `util::SubscriberInitExt`'s `try_init` function,
so that the same code isn't duplicated in multiple places. I also
added `AsLog` and `AsTrace` conversions for `LevelFilter`s in
the `tracing-log` crate.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-log/src/lib.rs

@@ -358,6 +358,39 @@ impl AsTrace for log::Level {
     }
 }
 
+impl crate::sealed::Sealed for log::LevelFilter {}
+
+impl AsTrace for log::LevelFilter {
+    type Trace = tracing_core::LevelFilter;
+    #[inline]
+    fn as_trace(&self) -> tracing_core::LevelFilter {
+        match self {
+            log::LevelFilter::Off => tracing_core::LevelFilter::OFF,
+            log::LevelFilter::Error => tracing_core::LevelFilter::ERROR,
+            log::LevelFilter::Warn => tracing_core::LevelFilter::WARN,
+            log::LevelFilter::Info => tracing_core::LevelFilter::INFO,
+            log::LevelFilter::Debug => tracing_core::LevelFilter::DEBUG,
+            log::LevelFilter::Trace => tracing_core::LevelFilter::TRACE,
+        }
+    }
+}
+
+impl crate::sealed::Sealed for tracing_core::LevelFilter {}
+
+impl AsLog for tracing_core::LevelFilter {
+    type Log = log::LevelFilter;
+    #[inline]
+    fn as_log(&self) -> Self::Log {
+        match *self {
+            tracing_core::LevelFilter::OFF => log::LevelFilter::Off,
+            tracing_core::LevelFilter::ERROR => log::LevelFilter::Error,
+            tracing_core::LevelFilter::WARN => log::LevelFilter::Warn,
+            tracing_core::LevelFilter::INFO => log::LevelFilter::Info,
+            tracing_core::LevelFilter::DEBUG => log::LevelFilter::Debug,
+            tracing_core::LevelFilter::TRACE => log::LevelFilter::Trace,
+        }
+    }
+}
 /// Extends log `Event`s to provide complete `Metadata`.
 ///
 /// In `tracing-log`, an `Event` produced by a log (through [`AsTrace`]) has an hard coded

tracing-subscriber/src/fmt/mod.rs

@@ -566,12 +566,9 @@ where
     /// because a global collector was already installed by another
     /// call to `try_init`.
     pub fn try_init(self) -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
-        #[cfg(feature = "tracing-log")]
-        tracing_log::LogTracer::init().map_err(Box::new)?;
+        use crate::util::SubscriberInitExt;
+        self.finish().try_init()?;
 
-        tracing_core::dispatch::set_global_default(tracing_core::dispatch::Dispatch::new(
-            self.finish(),
-        ))?;
         Ok(())
     }
 

tracing-subscriber/src/util.rs

@@ -2,6 +2,8 @@
 //! ergonomic.
 use std::{error::Error, fmt};
 use tracing_core::dispatch::{self, Dispatch};
+#[cfg(feature = "tracing-log")]
+use tracing_log::AsLog;
 
 /// Extension trait adding utility methods for subscriber initialization.
 ///
@@ -50,11 +52,20 @@ where
     /// [global default subscriber]: tracing::dispatch#setting-the-default-collector
     /// [`log`]: https://crates.io/log
     fn try_init(self) -> Result<(), TryInitError> {
-        #[cfg(feature = "tracing-log")]
-        tracing_log::LogTracer::init().map_err(TryInitError::new)?;
-
         dispatch::set_global_default(self.into()).map_err(TryInitError::new)?;
 
+        // Since we are setting the global default subscriber, we can
+        // opportunistically go ahead and set its global max level hint as
+        // the max level for the `log` crate as well. This should make
+        // skipping `log` diagnostics much faster.
+        #[cfg(feature = "tracing-log")]
+        tracing_log::LogTracer::builder()
+            // Note that we must call this *after* setting the global default
+            // subscriber, so that we get its max level hint.
+            .with_max_level(tracing_core::LevelFilter::current().as_log())
+            .init()
+            .map_err(TryInitError::new)?;
+
         Ok(())
     }