Use const thread_locals when possible

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 default() -> 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 default() -> 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`].
@@ -1072,7 +1088,7 @@ impl fmt::Binary for FilterMap {
 // === impl FilterState ===
 
 impl FilterState {
-    fn new() -> Self {
+    const fn new() -> Self {
         Self {
             enabled: Cell::new(FilterMap::default()),
             interest: RefCell::new(None),

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 {