use const thread_locals when possible (#2838)

This results in a substantial performance improvement,
and is compatible with our MSRV. 

Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>
Co-authored-by: Eliza Weisman <eliza@buoyant.io>

examples/examples/sloggish/sloggish_collector.rs

@@ -27,7 +27,7 @@ pub struct CurrentSpanPerThread {
 impl CurrentSpanPerThread {
     pub fn new() -> Self {
         thread_local! {
-            static CURRENT: RefCell<Vec<Id>> = RefCell::new(vec![]);
+            static CURRENT: RefCell<Vec<Id>> = const { RefCell::new(Vec::new()) };
         };
         Self { current: &CURRENT }
     }

tracing-core/src/dispatch.rs

@@ -205,9 +205,11 @@ enum Kind<T> {
 
 #[cfg(feature = "std")]
 thread_local! {
-    static CURRENT_STATE: State = State {
-        default: RefCell::new(None),
-        can_enter: Cell::new(true),
+    static CURRENT_STATE: State = const {
+        State {
+            default: RefCell::new(None),
+            can_enter: Cell::new(true),
+        }
     };
 }
 

tracing-subscriber/src/filter/subscriber_filters/mod.rs

@@ -99,11 +99,17 @@ pub struct FilterId(u64);
 ///
 /// [`Registry`]: crate::Registry
 /// [`Filter`]: crate::subscribe::Filter
-#[derive(Default, Copy, Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq)]
 pub(crate) struct FilterMap {
     bits: u64,
 }
 
+impl FilterMap {
+    pub(crate) const fn new() -> Self {
+        Self { bits: 0 }
+    }
+}
+
 /// The current state of `enabled` calls to per-subscriber filters on this
 /// thread.
 ///
@@ -145,7 +151,7 @@ pub(crate) struct FilterState {
 
 /// Extra counters added to `FilterState` used only to make debug assertions.
 #[cfg(debug_assertions)]
-#[derive(Debug, Default)]
+#[derive(Debug)]
 struct DebugCounters {
     /// How many per-subscriber filters have participated in the current `enabled`
     /// call?
@@ -156,8 +162,18 @@ struct DebugCounters {
     in_interest_pass: Cell<usize>,
 }
 
+#[cfg(debug_assertions)]
+impl DebugCounters {
+    const fn new() -> Self {
+        Self {
+            in_filter_pass: Cell::new(0),
+            in_interest_pass: Cell::new(0),
+        }
+    }
+}
+
 thread_local! {
-    pub(crate) static FILTERING: FilterState = FilterState::new();
+    pub(crate) static FILTERING: FilterState = const { FilterState::new() };
 }
 
 /// Extension trait adding [combinators] for combining [`Filter`].
@@ -1072,13 +1088,13 @@ impl fmt::Binary for FilterMap {
 // === impl FilterState ===
 
 impl FilterState {
-    fn new() -> Self {
+    const fn new() -> Self {
         Self {
-            enabled: Cell::new(FilterMap::default()),
+            enabled: Cell::new(FilterMap::new()),
             interest: RefCell::new(None),
 
             #[cfg(debug_assertions)]
-            counters: DebugCounters::default(),
+            counters: DebugCounters::new(),
         }
     }
 
@@ -1087,7 +1103,7 @@ impl FilterState {
         {
             let in_current_pass = self.counters.in_filter_pass.get();
             if in_current_pass == 0 {
-                debug_assert_eq!(self.enabled.get(), FilterMap::default());
+                debug_assert_eq!(self.enabled.get(), FilterMap::new());
             }
             self.counters.in_filter_pass.set(in_current_pass + 1);
             debug_assert_eq!(
@@ -1132,7 +1148,7 @@ impl FilterState {
                 #[cfg(debug_assertions)]
                 {
                     if this.counters.in_filter_pass.get() == 0 {
-                        debug_assert_eq!(this.enabled.get(), FilterMap::default());
+                        debug_assert_eq!(this.enabled.get(), FilterMap::new());
                     }
 
                     // Nothing enabled this event, we won't tick back down the
@@ -1169,7 +1185,7 @@ impl FilterState {
         {
             let in_current_pass = self.counters.in_filter_pass.get();
             if in_current_pass <= 1 {
-                debug_assert_eq!(self.enabled.get(), FilterMap::default());
+                debug_assert_eq!(self.enabled.get(), FilterMap::new());
             }
             self.counters
                 .in_filter_pass
@@ -1199,7 +1215,7 @@ impl FilterState {
         // a panic and the thread-local has been torn down, that's fine, just
         // ignore it ratehr than panicking.
         let _ = FILTERING.try_with(|filtering| {
-            filtering.enabled.set(FilterMap::default());
+            filtering.enabled.set(FilterMap::new());
 
             #[cfg(debug_assertions)]
             filtering.counters.in_filter_pass.set(0);
@@ -1225,7 +1241,7 @@ impl FilterState {
         let map = self.enabled.get();
         #[cfg(debug_assertions)]
         if self.counters.in_filter_pass.get() == 0 {
-            debug_assert_eq!(map, FilterMap::default());
+            debug_assert_eq!(map, FilterMap::new());
         }
 
         map

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -944,7 +944,7 @@ where
 
     fn on_event(&self, event: &Event<'_>, ctx: Context<'_, C>) {
         thread_local! {
-            static BUF: RefCell<String> = RefCell::new(String::new());
+            static BUF: RefCell<String> = const { RefCell::new(String::new()) };
         }
 
         BUF.with(|buf| {

tracing-subscriber/src/registry/sharded.rs

@@ -214,7 +214,7 @@ thread_local! {
     /// track how many subscribers have processed the close.
     /// For additional details, see [`CloseGuard`].
     ///
-    static CLOSE_COUNT: Cell<usize> = Cell::new(0);
+    static CLOSE_COUNT: Cell<usize> = const { Cell::new(0) };
 }
 
 impl Collect for Registry {
@@ -255,7 +255,7 @@ impl Collect for Registry {
                 data.filter_map = crate::filter::FILTERING.with(|filtering| filtering.filter_map());
                 #[cfg(debug_assertions)]
                 {
-                    if data.filter_map != FilterMap::default() {
+                    if data.filter_map != FilterMap::new() {
                         debug_assert!(self.has_per_subscriber_filters());
                     }
                 }
@@ -477,7 +477,7 @@ impl Default for DataInner {
         };
 
         Self {
-            filter_map: FilterMap::default(),
+            filter_map: FilterMap::new(),
             metadata: &NULL_METADATA,
             parent: None,
             ref_count: AtomicUsize::new(0),
@@ -522,7 +522,7 @@ impl Clear for DataInner {
             })
             .clear();
 
-        self.filter_map = FilterMap::default();
+        self.filter_map = FilterMap::new();
     }
 }