From b94162078d43ee07277a8a746e4176588a7ab93b Mon Sep 17 00:00:00 2001
From: Kornel <kornel@geekhood.net>
Date: Mon, 17 Feb 2025 17:05:21 +0000
Subject: [PATCH 1/3] Highlight thread_local! const init in docs

---
 library/std/src/thread/local.rs | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/library/std/src/thread/local.rs b/library/std/src/thread/local.rs
index ca04aa4ada497..d5a5d10205dd8 100644
--- a/library/std/src/thread/local.rs
+++ b/library/std/src/thread/local.rs
@@ -50,7 +50,8 @@ use crate::fmt;
 /// use std::cell::Cell;
 /// use std::thread;
 ///
-/// thread_local!(static FOO: Cell<u32> = Cell::new(1));
+/// // explicit `const {}` block enables more efficient initialization
+/// thread_local!(static FOO: Cell<u32> = const { Cell::new(1) });
 ///
 /// assert_eq!(FOO.get(), 1);
 /// FOO.set(2);
@@ -138,7 +139,7 @@ impl<T: 'static> fmt::Debug for LocalKey<T> {
 /// use std::cell::{Cell, RefCell};
 ///
 /// thread_local! {
-///     pub static FOO: Cell<u32> = Cell::new(1);
+///     pub static FOO: Cell<u32> = const { Cell::new(1) };
 ///
 ///     static BAR: RefCell<Vec<f32>> = RefCell::new(vec![1.0, 2.0]);
 /// }
@@ -394,7 +395,7 @@ impl<T: 'static> LocalKey<Cell<T>> {
     /// use std::cell::Cell;
     ///
     /// thread_local! {
-    ///     static X: Cell<i32> = Cell::new(1);
+    ///     static X: Cell<i32> = const { Cell::new(1) };
     /// }
     ///
     /// assert_eq!(X.get(), 1);
@@ -423,7 +424,7 @@ impl<T: 'static> LocalKey<Cell<T>> {
     /// use std::cell::Cell;
     ///
     /// thread_local! {
-    ///     static X: Cell<Option<i32>> = Cell::new(Some(1));
+    ///     static X: Cell<Option<i32>> = const { Cell::new(Some(1)) };
     /// }
     ///
     /// assert_eq!(X.take(), Some(1));
@@ -453,7 +454,7 @@ impl<T: 'static> LocalKey<Cell<T>> {
     /// use std::cell::Cell;
     ///
     /// thread_local! {
-    ///     static X: Cell<i32> = Cell::new(1);
+    ///     static X: Cell<i32> = const { Cell::new(1) };
     /// }
     ///
     /// assert_eq!(X.replace(2), 1);

From ad566646cf02163373c5d60033be07c2d903ef90 Mon Sep 17 00:00:00 2001
From: Kornel <kornel@geekhood.net>
Date: Mon, 17 Feb 2025 17:22:01 +0000
Subject: [PATCH 2/3] Use faster thread_local in current_thread_id()

---
 library/std/src/sync/mpmc/mod.rs   |  3 +++
 library/std/src/sync/mpmc/tests.rs | 14 ++++++++++++++
 library/std/src/sync/mpmc/waker.rs |  2 +-
 3 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 library/std/src/sync/mpmc/tests.rs

diff --git a/library/std/src/sync/mpmc/mod.rs b/library/std/src/sync/mpmc/mod.rs
index 8caa2dcfad99c..8712332dd2767 100644
--- a/library/std/src/sync/mpmc/mod.rs
+++ b/library/std/src/sync/mpmc/mod.rs
@@ -1382,3 +1382,6 @@ impl<T> fmt::Debug for Receiver<T> {
         f.pad("Receiver { .. }")
     }
 }
+
+#[cfg(test)]
+mod tests;
diff --git a/library/std/src/sync/mpmc/tests.rs b/library/std/src/sync/mpmc/tests.rs
new file mode 100644
index 0000000000000..6deb4dc2fe0cc
--- /dev/null
+++ b/library/std/src/sync/mpmc/tests.rs
@@ -0,0 +1,14 @@
+// Ensure that thread_local init with `const { 0 }` still has unique address at run-time
+#[test]
+fn waker_current_thread_id() {
+    let first = super::waker::current_thread_id();
+    let t = crate::thread::spawn(move || {
+        let second = super::waker::current_thread_id();
+        assert_ne!(first, second);
+        assert_eq!(second, super::waker::current_thread_id());
+    });
+
+    assert_eq!(first, super::waker::current_thread_id());
+    t.join().unwrap();
+    assert_eq!(first, super::waker::current_thread_id());
+}
diff --git a/library/std/src/sync/mpmc/waker.rs b/library/std/src/sync/mpmc/waker.rs
index 1895466f95d45..f5e764e69bd6e 100644
--- a/library/std/src/sync/mpmc/waker.rs
+++ b/library/std/src/sync/mpmc/waker.rs
@@ -204,6 +204,6 @@ impl Drop for SyncWaker {
 pub fn current_thread_id() -> usize {
     // `u8` is not drop so this variable will be available during thread destruction,
     // whereas `thread::current()` would not be
-    thread_local! { static DUMMY: u8 = 0 }
+    thread_local! { static DUMMY: u8 = const { 0 } }
     DUMMY.with(|x| (x as *const u8).addr())
 }

From 4742dbc765165dc0ece5e350b3527e697ff871dc Mon Sep 17 00:00:00 2001
From: Kornel <kornel@geekhood.net>
Date: Mon, 17 Feb 2025 17:04:36 +0000
Subject: [PATCH 3/3] Use faster thread_local! for stdout

---
 library/std/src/io/stdio.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/library/std/src/io/stdio.rs b/library/std/src/io/stdio.rs
index 661c422811abb..017862c7f3aac 100644
--- a/library/std/src/io/stdio.rs
+++ b/library/std/src/io/stdio.rs
@@ -20,7 +20,7 @@ type LocalStream = Arc<Mutex<Vec<u8>>>;
 
 thread_local! {
     /// Used by the test crate to capture the output of the print macros and panics.
-    static OUTPUT_CAPTURE: Cell<Option<LocalStream>> = {
+    static OUTPUT_CAPTURE: Cell<Option<LocalStream>> = const {
         Cell::new(None)
     }
 }