chore: fix version detection in build script

tokio/build.rs

@@ -1,11 +1,38 @@
 use autocfg::AutoCfg;
 
+const TASK_LOCAL_PROBE: &str = r#"
+{
+    thread_local! {
+        static MY_PROBE: usize = const { 10 };
+    }
+
+    MY_PROBE.with(|val| *val)
+}
+"#;
+
 fn main() {
+    let mut enable_const_thread_local = false;
+
     match AutoCfg::new() {
         Ok(ac) => {
-            // Const-initialized thread locals were stabilized in 1.59
-            if ac.probe_rustc_version(1, 59) {
-                autocfg::emit("tokio_const_thread_local")
+            // These checks prefer to call only `probe_rustc_version` if that is
+            // enough to determine whether the feature is supported. This is
+            // because the `probe_expression` call involves a call to rustc,
+            // which the `probe_rustc_version` call avoids.
+
+            // Const-initialized thread locals were stabilized in 1.59.
+            if ac.probe_rustc_version(1, 60) {
+                enable_const_thread_local = true;
+            } else if ac.probe_rustc_version(1, 59) {
+                // This compiler claims to be 1.59, but there are some nightly
+                // compilers that claim to be 1.59 without supporting the
+                // feature. Explicitly probe to check if code using them
+                // compiles.
+                //
+                // The oldest nightly that supports the feature is 2021-12-06.
+                if ac.probe_expression(TASK_LOCAL_PROBE) {
+                    enable_const_thread_local = true;
+                }
             }
         }
 
@@ -19,4 +46,12 @@ fn main() {
             );
         }
     }
+
+    if !enable_const_thread_local {
+        // To disable this feature on compilers that support it, you can
+        // explicitly pass this flag with the following environment variable:
+        //
+        // RUSTFLAGS="--cfg tokio_no_const_thread_local"
+        autocfg::emit("tokio_no_const_thread_local")
+    }
 }

tokio/src/macros/thread_local.rs

@@ -10,12 +10,14 @@ macro_rules! thread_local {
     ($($tts:tt)+) => { loom::thread_local!{ $($tts)+ } }
 }
 
-#[cfg(all(tokio_const_thread_local, not(all(loom, test))))]
+#[cfg(not(tokio_no_const_thread_local))]
+#[cfg(not(all(loom, test)))]
 macro_rules! thread_local {
     ($($tts:tt)+) => { ::std::thread_local!{ $($tts)+ } }
 }
 
-#[cfg(all(not(tokio_const_thread_local), not(all(loom, test))))]
+#[cfg(tokio_no_const_thread_local)]
+#[cfg(not(all(loom, test)))]
 macro_rules! thread_local {
     ($(#[$attrs:meta])* $vis:vis static $name:ident: $ty:ty = const { $expr:expr } $(;)?) => {
         ::std::thread_local! {

tokio/src/task/task_local.rs

@@ -47,7 +47,7 @@ macro_rules! task_local {
 }
 
 #[doc(hidden)]
-#[cfg(tokio_const_thread_local)]
+#[cfg(not(tokio_no_const_thread_local))]
 #[macro_export]
 macro_rules! __task_local_inner {
     ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty) => {
@@ -62,7 +62,7 @@ macro_rules! __task_local_inner {
 }
 
 #[doc(hidden)]
-#[cfg(not(tokio_const_thread_local))]
+#[cfg(tokio_no_const_thread_local)]
 #[macro_export]
 macro_rules! __task_local_inner {
     ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty) => {