sync thread_local key conditions exactly with what the macro uses

Author: RalfJung

library/std/src/sys/unix/thread_local_dtor.rs

@@ -17,6 +17,7 @@
     target_os = "redox",
     target_os = "emscripten"
 ))]
+#[cfg_attr(target_family = "wasm", allow(unused))] // might remain unused depending on target details (e.g. wasm32-unknown-emscripten)
 pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern "C" fn(*mut u8)) {
     use crate::mem;
     use crate::sys_common::thread_local_dtor::register_dtor_fallback;

library/std/src/sys/unsupported/thread_local_dtor.rs

@@ -1,5 +1,6 @@
 #![unstable(feature = "thread_local_internals", issue = "none")]
 
+#[cfg_attr(target_family = "wasm", allow(unused))] // unused on wasm32-unknown-unknown
 pub unsafe fn register_dtor(_t: *mut u8, _dtor: unsafe extern "C" fn(*mut u8)) {
     // FIXME: right now there is no concept of "thread exit", but this is likely
     // going to show up at some point in the form of an exported symbol that the

library/std/src/thread/local.rs

@@ -901,7 +901,7 @@ pub mod statik {
 }
 
 #[doc(hidden)]
-#[cfg(target_thread_local)]
+#[cfg(all(target_thread_local, not(all(target_family = "wasm", not(target_feature = "atomics"))),))]
 pub mod fast {
     use super::lazy::LazyKeyInner;
     use crate::cell::Cell;
@@ -1037,7 +1037,10 @@ pub mod fast {
 }
 
 #[doc(hidden)]
-#[cfg(not(target_thread_local))]
+#[cfg(all(
+    not(target_thread_local),
+    not(all(target_family = "wasm", not(target_feature = "atomics"))),
+))]
 pub mod os {
     use super::lazy::LazyKeyInner;
     use crate::cell::Cell;

library/std/src/thread/mod.rs

@@ -192,32 +192,49 @@ pub use scoped::{scope, Scope, ScopedJoinHandle};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::local::{AccessError, LocalKey};
 
-// Select the type used by the thread_local! macro to access TLS keys. There
-// are three types: "static", "fast", "OS". The "OS" thread local key
+// Provide the type used by the thread_local! macro to access TLS keys. This
+// needs to be kept in sync with the macro itself (in `local.rs`).
+// There are three types: "static", "fast", "OS". The "OS" thread local key
 // type is accessed via platform-specific API calls and is slow, while the "fast"
 // key type is accessed via code generated via LLVM, where TLS keys are set up
 // by the elf linker. "static" is for single-threaded platforms where a global
 // static is sufficient.
 
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
-#[cfg(target_thread_local)]
 #[cfg(not(test))]
+#[cfg(all(
+    target_thread_local,
+    not(all(target_family = "wasm", not(target_feature = "atomics"))),
+))]
 #[doc(hidden)]
 pub use self::local::fast::Key as __FastLocalKeyInner;
+
+// when building for tests, use real std's type
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
-#[cfg(target_thread_local)]
-#[cfg(test)] // when building for tests, use real std's key
+#[cfg(test)]
+#[cfg(all(
+    target_thread_local,
+    not(all(target_family = "wasm", not(target_feature = "atomics"))),
+))]
 pub use realstd::thread::__FastLocalKeyInner;
 
+// but import the local one anyway to silence 'unused' warnings
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
-#[cfg(target_thread_local)]
 #[cfg(test)]
-pub use self::local::fast::Key as __FastLocalKeyInnerUnused; // we import this anyway to silence 'unused' warnings
+#[cfg(all(
+    target_thread_local,
+    not(all(target_family = "wasm", not(target_feature = "atomics"))),
+))]
+pub use self::local::fast::Key as __FastLocalKeyInnerUnused;
 
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
+#[cfg(all(
+    not(target_thread_local),
+    not(all(target_family = "wasm", not(target_feature = "atomics"))),
+))]
 #[doc(hidden)]
-#[cfg(not(target_thread_local))]
 pub use self::local::os::Key as __OsLocalKeyInner;
+
 #[unstable(feature = "libstd_thread_internals", issue = "none")]
 #[cfg(all(target_family = "wasm", not(target_feature = "atomics")))]
 #[doc(hidden)]