Only enable tracing when MIRI_TRACING is set

Also remove --features from ci.sh as now tracing won't be enabled anymore.

Author: Stypox

CONTRIBUTING.md

@@ -158,6 +158,15 @@ compiler that has `debug=true` set in `bootstrap.toml`.
 You can set `MIRI_BACKTRACE=1` to get a backtrace of where an
 evaluation error was originally raised.
 
+#### Tracing
+
+You can generate a Chrome trace file from a Miri execution by passing `--features=tracing` during
+build and then setting `MIRI_TRACING=1` when running Miri. This will generate a `.json` file that
+you can visualize in [Perfetto](https://ui.perfetto.dev/). For example:
+
+```sh
+MIRI_TRACING=1 ./miri run --features=tracing tests/pass/hello.rs
+```
 
 ### UI testing
 

Cargo.toml

@@ -65,8 +65,6 @@ harness = false
 
 [features]
 default = ["stack-cache"]
-# When adding a new feature that is relevant for some tests,
-# make sure to add it to the FEATURES variable in `ci/ci.sh`.
 genmc = []
 stack-cache = []
 stack-cache-consistency-check = ["stack-cache"]

ci/ci.sh

@@ -28,17 +28,16 @@ begingroup "Building Miri"
 export RUSTFLAGS="-D warnings"
 export CARGO_INCREMENTAL=0
 export CARGO_EXTRA_FLAGS="--locked"
-# We enable all features (except tracing) to make sure the Stacked Borrows consistency check runs.
-export FEATURES="--features=genmc,stack-cache,stack-cache-consistency-check"
 
 # Determine configuration for installed build (used by test-cargo-miri and `./miri bench`).
-# $FEATURES is not passed here as "stack-cache-consistency-check" makes things quite slow.
 echo "Installing release version of Miri"
 time ./miri install
 
 # Prepare debug build for direct `./miri` invocations.
+# We enable all features to make sure the Stacked Borrows consistency check runs.
 echo "Building debug version of Miri"
-time ./miri build "$FEATURES" # the build that all the `./miri test` below will use
+export CARGO_EXTRA_FLAGS="$CARGO_EXTRA_FLAGS --all-features"
+time ./miri build # the build that all the `./miri test` below will use
 
 endgroup
 
@@ -62,9 +61,9 @@ function run_tests {
 
   ## ui test suite
   if [ -n "${GC_STRESS-}" ]; then
-    time MIRIFLAGS="${MIRIFLAGS-} -Zmiri-provenance-gc=1" ./miri test "$FEATURES" $TARGET_FLAG
+    time MIRIFLAGS="${MIRIFLAGS-} -Zmiri-provenance-gc=1" ./miri test $TARGET_FLAG
   else
-    time ./miri test "$FEATURES" $TARGET_FLAG
+    time ./miri test $TARGET_FLAG
   fi
 
   ## advanced tests
@@ -75,20 +74,20 @@ function run_tests {
     # them. Also error locations change so we don't run the failing tests.
     # We explicitly enable debug-assertions here, they are disabled by -O but we have tests
     # which exist to check that we panic on debug assertion failures.
-    time MIRIFLAGS="${MIRIFLAGS-} -O -Zmir-opt-level=4 -Cdebug-assertions=yes" MIRI_SKIP_UI_CHECKS=1 ./miri test "$FEATURES" $TARGET_FLAG tests/{pass,panic}
+    time MIRIFLAGS="${MIRIFLAGS-} -O -Zmir-opt-level=4 -Cdebug-assertions=yes" MIRI_SKIP_UI_CHECKS=1 ./miri test $TARGET_FLAG tests/{pass,panic}
   fi
   if [ -n "${MANY_SEEDS-}" ]; then
     # Run many-seeds tests. (Also tests `./miri run`.)
     time for FILE in tests/many-seeds/*.rs; do
-      ./miri run "$FEATURES" $TARGET_FLAG "-Zmiri-many-seeds=0..$MANY_SEEDS" "$FILE"
+      ./miri run "-Zmiri-many-seeds=0..$MANY_SEEDS" $TARGET_FLAG "$FILE"
     done
   fi
   if [ -n "${TEST_BENCH-}" ]; then
     # Check that the benchmarks build and run, but only once.
     time HYPERFINE="hyperfine -w0 -r1 --show-output" ./miri bench $TARGET_FLAG --no-install
   fi
   # Smoke-test `./miri run --dep`.
-  ./miri run "$FEATURES" $TARGET_FLAG --dep tests/pass-dep/getrandom.rs
+  ./miri run $TARGET_FLAG --dep tests/pass-dep/getrandom.rs
 
   ## test-cargo-miri
   # On Windows, there is always "python", not "python3" or "python2".
@@ -126,7 +125,7 @@ function run_tests_minimal {
     exit 1
   fi
 
-  time ./miri test "$FEATURES" $TARGET_FLAG "$@"
+  time ./miri test $TARGET_FLAG "$@"
 
   # Ensure that a small smoke test of cargo-miri works.
   time cargo miri run --manifest-path test-cargo-miri/no-std-smoke/Cargo.toml $TARGET_FLAG

src/bin/log/setup.rs

@@ -4,7 +4,6 @@ use std::sync::{Mutex, OnceLock};
 
 use rustc_middle::ty::TyCtxt;
 use rustc_session::{CtfeBacktrace, EarlyDiagCtxt};
-use tracing_subscriber::Registry;
 
 /// The tracing layer from `tracing-chrome` starts a thread in the background that saves data to
 /// file and closes the file when stopped. If the thread is not stopped properly, the file will be
@@ -57,21 +56,38 @@ static LOGGER_INITED: OnceLock<Mutex<Option<TracingGuard>>> = OnceLock::new();
 fn init_logger_once(early_dcx: &EarlyDiagCtxt) {
     // If the logger is not yet initialized, initialize it.
     LOGGER_INITED.get_or_init(|| {
-        #[cfg(feature = "tracing")]
-        let (chrome_layer, chrome_guard) =
-            super::tracing_chrome::ChromeLayerBuilder::new().include_args(true).build();
-        rustc_driver::init_logger_with_additional_layer(early_dcx, rustc_logger_config(), || {
-            let registry = Registry::default();
-            #[cfg(feature = "tracing")]
-            let registry = tracing_subscriber::layer::SubscriberExt::with(registry, chrome_layer);
-            registry
-        });
+        let guard = if env::var_os("MIRI_TRACING").is_some() {
+            #[cfg(not(feature = "tracing"))]
+            {
+                eprintln!(
+                    "fatal error: cannot enable MIRI_TRACING since Miri was not built with the \"tracing\" feature"
+                );
+                std::process::exit(1)
+            }
 
-        Mutex::new(Some(TracingGuard {
             #[cfg(feature = "tracing")]
-            _chrome: chrome_guard,
-            _no_construct: (),
-        }))
+            {
+                let (chrome_layer, chrome_guard) =
+                    super::tracing_chrome::ChromeLayerBuilder::new().include_args(true).build();
+                rustc_driver::init_logger_with_additional_layer(
+                    early_dcx,
+                    rustc_logger_config(),
+                    || {
+                        tracing_subscriber::layer::SubscriberExt::with(
+                            tracing_subscriber::Registry::default(),
+                            chrome_layer,
+                        )
+                    },
+                );
+
+                Some(TracingGuard { _chrome: chrome_guard, _no_construct: () })
+            }
+        } else {
+            // initialize the logger without any tracing enabled
+            rustc_driver::init_logger(early_dcx, rustc_logger_config());
+            None
+        };
+        Mutex::new(guard)
     });
 }
 

src/bin/miri.rs

@@ -10,6 +10,7 @@
 
 // Some "regular" crates we want to share with rustc
 extern crate tracing;
+#[cfg(feature = "tracing")]
 extern crate tracing_subscriber;
 
 // The rustc crates we need