Rollup merge of #147613 - dingxiangfei2009:investigate-log, r=jdonszelmann

Make logging filters work again by moving EnvFilter into its own layer

`tracing` at the time of writing has a feature (?) in its Filter implementation, so that filters like EnvFilter are consulted for status of a span or event and whether it is marked as interesting for logging. Combining a Filter with another layer through the `with_filter` combinator produces a filtered layer that enables an event unless it is statically determined that the event is uninteresting. However, if the filter is dynamic, because of filtering on span names or field values as an example, events are **always** enabled by design. There is an `event_enabled` predicate on `EnvFilter` implementation but it falls back to default and, thus, the dynamic filters are **unused**.

Previously, `RUSTC_LOG=[]` or `RUSTC_LOG=[garbage]` enables all events, even when spans do not match.

This patch re-enables span- and field-based filters. With `RUSTC_LOG=[garbage]` one should expect no events are enabled again. This will help with development greatly because we can meaningfully filter internal logs again.

compiler/rustc_log/src/lib.rs

@@ -39,11 +39,11 @@ use std::io::{self, IsTerminal};
 
 use tracing::dispatcher::SetGlobalDefaultError;
 use tracing::{Event, Subscriber};
+use tracing_subscriber::Registry;
 use tracing_subscriber::filter::{Directive, EnvFilter, LevelFilter};
 use tracing_subscriber::fmt::FmtContext;
 use tracing_subscriber::fmt::format::{self, FormatEvent, FormatFields};
 use tracing_subscriber::layer::SubscriberExt;
-use tracing_subscriber::{Layer, Registry};
 
 /// The values of all the environment variables that matter for configuring a logger.
 /// Errors are explicitly preserved so that we can share error handling.
@@ -152,18 +152,19 @@ where
         }
     }
 
-    let subscriber = build_subscriber().with(layer.with_filter(filter));
+    let subscriber = build_subscriber();
+    // NOTE: It is important to make sure that the filter is applied on the last layer
     match cfg.backtrace {
         Ok(backtrace_target) => {
             let fmt_layer = tracing_subscriber::fmt::layer()
                 .with_writer(io::stderr)
                 .without_time()
                 .event_format(BacktraceFormatter { backtrace_target });
-            let subscriber = subscriber.with(fmt_layer);
+            let subscriber = subscriber.with(layer).with(fmt_layer).with(filter);
             tracing::subscriber::set_global_default(subscriber)?;
         }
         Err(_) => {
-            tracing::subscriber::set_global_default(subscriber)?;
+            tracing::subscriber::set_global_default(subscriber.with(layer).with(filter))?;
         }
     };