Use a const initializer for GIL_COUNT if possible.

pyo3-build-config/src/lib.rs

@@ -156,6 +156,11 @@ pub fn print_feature_cfgs() {
     if rustc_minor_version >= 53 {
         println!("cargo:rustc-cfg=option_insert");
     }
+
+    // Enable use of const initializer for thread_local! on Rust 1.59 and greater
+    if rustc_minor_version >= 59 {
+        println!("cargo:rustc-cfg=thread_local_const_init");
+    }
 }
 
 /// Private exports used in PyO3's build.rs

src/gil.rs

@@ -10,15 +10,29 @@ use std::{mem, ptr::NonNull, sync::atomic};
 
 static START: Once = Once::new();
 
-thread_local! {
+cfg_if::cfg_if! {
+    if #[cfg(thread_local_const_init)] {
+        use std::thread_local as thread_local_const_init;
+    } else {
+        macro_rules! thread_local_const_init {
+            ($(#[$attr:meta])* static $name:ident: $ty:ty = const { $init:expr };) => (
+                thread_local! { $(#[$attr])* static $name: $ty = $init; }
+            )
+        }
+    }
+}
+
+thread_local_const_init! {
     /// This is an internal counter in pyo3 monitoring whether this thread has the GIL.
     ///
     /// It will be incremented whenever a GILGuard or GILPool is created, and decremented whenever
     /// they are dropped.
     ///
     /// As a result, if this thread has the GIL, GIL_COUNT is greater than zero.
-    static GIL_COUNT: Cell<usize> = Cell::new(0);
+    static GIL_COUNT: Cell<usize> = const { Cell::new(0) };
+}
 
+thread_local! {
     /// Temporarily hold objects that will be released when the GILPool drops.
     static OWNED_OBJECTS: RefCell<Vec<NonNull<ffi::PyObject>>> = RefCell::new(Vec::with_capacity(256));
 }