Use OnceLock in JobTokenServer::new

Also impls `Send`, `Sync`, `RefUnwindSafe` and `UnwindSafed` when the `T`
meets the criterior.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

src/parallel/job_token.rs

@@ -1,6 +1,6 @@
-use std::{marker::PhantomData, mem::MaybeUninit, sync::Once};
+use std::marker::PhantomData;
 
-use crate::Error;
+use crate::{parallel::once_lock::OnceLock, Error};
 
 pub(crate) struct JobToken(PhantomData<()>);
 
@@ -34,19 +34,13 @@ impl JobTokenServer {
     ///    that has to be static so that it will be shared by all cc
     ///    compilation.
     fn new() -> &'static Self {
-        // TODO: Replace with a OnceLock once MSRV is 1.70
-        static INIT: Once = Once::new();
-        static mut JOBSERVER: MaybeUninit<JobTokenServer> = MaybeUninit::uninit();
-
-        unsafe {
-            INIT.call_once(|| {
-                let server = inherited_jobserver::JobServer::from_env()
-                    .map(Self::Inherited)
-                    .unwrap_or_else(|| Self::InProcess(inprocess_jobserver::JobServer::new()));
-                JOBSERVER.write(server);
-            });
-            JOBSERVER.assume_init_ref()
-        }
+        static JOBSERVER: OnceLock<JobTokenServer> = OnceLock::new();
+
+        JOBSERVER.get_or_init(|| {
+            unsafe { inherited_jobserver::JobServer::from_env() }
+                .map(Self::Inherited)
+                .unwrap_or_else(|| Self::InProcess(inprocess_jobserver::JobServer::new()))
+        })
     }
 }
 
@@ -76,12 +70,9 @@ impl ActiveJobTokenServer {
 }
 
 mod inherited_jobserver {
-    use super::JobToken;
+    use super::{JobToken, OnceLock};
 
-    use crate::{
-        parallel::{async_executor::YieldOnce, once_lock::OnceLock},
-        Error, ErrorKind,
-    };
+    use crate::{parallel::async_executor::YieldOnce, Error, ErrorKind};
 
     use std::{
         io, mem,

src/parallel/once_lock.rs

@@ -3,7 +3,12 @@
 //! is initialised and MSRV is high enoguh to use it.
 
 use std::{
-    cell::UnsafeCell, convert::Infallible, marker::PhantomData, mem::MaybeUninit, sync::Once,
+    cell::UnsafeCell,
+    convert::Infallible,
+    marker::PhantomData,
+    mem::MaybeUninit,
+    panic::{RefUnwindSafe, UnwindSafe},
+    sync::Once,
 };
 
 pub struct OnceLock<T> {
@@ -31,6 +36,12 @@ pub struct OnceLock<T> {
     _marker: PhantomData<T>,
 }
 
+unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
+unsafe impl<T: Send> Send for OnceLock<T> {}
+
+impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceLock<T> {}
+impl<T: UnwindSafe> UnwindSafe for OnceLock<T> {}
+
 impl<T> OnceLock<T> {
     pub const fn new() -> OnceLock<T> {
         OnceLock {
@@ -49,7 +60,6 @@ impl<T> OnceLock<T> {
         }
     }
 
-    #[allow(dead_code)]
     pub fn get_or_init<F>(&self, f: F) -> &T
     where
         F: FnOnce() -> T,