tokio: use const Mutex::new for globals (#5061)

tokio/build.rs

@@ -17,9 +17,17 @@ const ADDR_OF_PROBE: &str = r#"
 }
 "#;
 
+const CONST_MUTEX_NEW_PROBE: &str = r#"
+{
+    static MY_MUTEX: ::std::sync::Mutex<i32> = ::std::sync::Mutex::new(1);
+    *MY_MUTEX.lock().unwrap()
+}
+"#;
+
 fn main() {
     let mut enable_const_thread_local = false;
     let mut enable_addr_of = false;
+    let mut enable_const_mutex_new = false;
 
     match AutoCfg::new() {
         Ok(ac) => {
@@ -57,6 +65,21 @@ fn main() {
                     enable_addr_of = true;
                 }
             }
+
+            // The `Mutex::new` method was made const in 1.63.
+            if ac.probe_rustc_version(1, 64) {
+                enable_const_mutex_new = true;
+            } else if ac.probe_rustc_version(1, 63) {
+                // This compiler claims to be 1.63, but there are some nightly
+                // compilers that claim to be 1.63 without supporting the
+                // feature. Explicitly probe to check if code using them
+                // compiles.
+                //
+                // The oldest nightly that supports the feature is 2022-06-20.
+                if ac.probe_expression(CONST_MUTEX_NEW_PROBE) {
+                    enable_const_mutex_new = true;
+                }
+            }
         }
 
         Err(e) => {
@@ -86,6 +109,14 @@ fn main() {
         autocfg::emit("tokio_no_addr_of")
     }
 
+    if !enable_const_mutex_new {
+        // 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_mutex_new"
+        autocfg::emit("tokio_no_const_mutex_new")
+    }
+
     let target = ::std::env::var("TARGET").unwrap_or_default();
 
     // We emit cfgs instead of using `target_family = "wasm"` that requires Rust 1.54.

tokio/src/loom/std/mutex.rs

@@ -12,6 +12,12 @@ impl<T> Mutex<T> {
         Mutex(sync::Mutex::new(t))
     }
 
+    #[inline]
+    #[cfg(not(tokio_no_const_mutex_new))]
+    pub(crate) const fn const_new(t: T) -> Mutex<T> {
+        Mutex(sync::Mutex::new(t))
+    }
+
     #[inline]
     pub(crate) fn lock(&self) -> MutexGuard<'_, T> {
         match self.0.lock() {

tokio/src/macros/cfg.rs

@@ -481,6 +481,36 @@ macro_rules! cfg_not_has_atomic_u64 {
     }
 }
 
+macro_rules! cfg_has_const_mutex_new {
+    ($($item:item)*) => {
+        $(
+            #[cfg(all(
+                not(all(loom, test)),
+                any(
+                    feature = "parking_lot",
+                    not(tokio_no_const_mutex_new)
+                )
+            ))]
+            $item
+        )*
+    }
+}
+
+macro_rules! cfg_not_has_const_mutex_new {
+    ($($item:item)*) => {
+        $(
+            #[cfg(not(all(
+                not(all(loom, test)),
+                any(
+                    feature = "parking_lot",
+                    not(tokio_no_const_mutex_new)
+                )
+            )))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_not_wasi {
     ($($item:item)*) => {
         $(

tokio/src/process/unix/mod.rs

@@ -34,7 +34,6 @@ use crate::process::kill::Kill;
 use crate::process::SpawnedChild;
 use crate::signal::unix::driver::Handle as SignalHandle;
 use crate::signal::unix::{signal, Signal, SignalKind};
-use crate::util::once_cell::OnceCell;
 
 use mio::event::Source;
 use mio::unix::SourceFd;
@@ -64,10 +63,22 @@ impl Kill for StdChild {
     }
 }
 
-fn get_orphan_queue() -> &'static OrphanQueueImpl<StdChild> {
-    static ORPHAN_QUEUE: OnceCell<OrphanQueueImpl<StdChild>> = OnceCell::new();
+cfg_not_has_const_mutex_new! {
+    fn get_orphan_queue() -> &'static OrphanQueueImpl<StdChild> {
+        use crate::util::once_cell::OnceCell;
 
-    ORPHAN_QUEUE.get(OrphanQueueImpl::new)
+        static ORPHAN_QUEUE: OnceCell<OrphanQueueImpl<StdChild>> = OnceCell::new();
+
+        ORPHAN_QUEUE.get(OrphanQueueImpl::new)
+    }
+}
+
+cfg_has_const_mutex_new! {
+    fn get_orphan_queue() -> &'static OrphanQueueImpl<StdChild> {
+        static ORPHAN_QUEUE: OrphanQueueImpl<StdChild> = OrphanQueueImpl::new();
+
+        &ORPHAN_QUEUE
+    }
 }
 
 pub(crate) struct GlobalOrphanQueue;

tokio/src/process/unix/orphan.rs

@@ -43,10 +43,21 @@ pub(crate) struct OrphanQueueImpl<T> {
 }
 
 impl<T> OrphanQueueImpl<T> {
-    pub(crate) fn new() -> Self {
-        Self {
-            sigchild: Mutex::new(None),
-            queue: Mutex::new(Vec::new()),
+    cfg_not_has_const_mutex_new! {
+        pub(crate) fn new() -> Self {
+            Self {
+                sigchild: Mutex::new(None),
+                queue: Mutex::new(Vec::new()),
+            }
+        }
+    }
+
+    cfg_has_const_mutex_new! {
+        pub(crate) const fn new() -> Self {
+            Self {
+                sigchild: Mutex::const_new(None),
+                queue: Mutex::const_new(Vec::new()),
+            }
         }
     }
 

tokio/src/runtime/task/mod.rs

@@ -507,21 +507,35 @@ impl Id {
     }
 
     cfg_not_has_atomic_u64! {
-        pub(crate) fn next() -> Self {
-            use crate::util::once_cell::OnceCell;
-            use crate::loom::sync::Mutex;
-
-            fn init_next_id() -> Mutex<u64> {
-                Mutex::new(1)
+        cfg_has_const_mutex_new! {
+            pub(crate) fn next() -> Self {
+                use crate::loom::sync::Mutex;
+                static NEXT_ID: Mutex<u64> = Mutex::const_new(1);
+
+                let mut lock = NEXT_ID.lock();
+                let id = *lock;
+                *lock += 1;
+                Self(id)
             }
+        }
 
-            static NEXT_ID: OnceCell<Mutex<u64>> = OnceCell::new();
+        cfg_not_has_const_mutex_new! {
+            pub(crate) fn next() -> Self {
+                use crate::util::once_cell::OnceCell;
+                use crate::loom::sync::Mutex;
 
-            let next_id = NEXT_ID.get(init_next_id);
-            let mut lock = next_id.lock();
-            let id = *lock;
-            *lock += 1;
-            Self(id)
+                fn init_next_id() -> Mutex<u64> {
+                    Mutex::new(1)
+                }
+
+                static NEXT_ID: OnceCell<Mutex<u64>> = OnceCell::new();
+
+                let next_id = NEXT_ID.get(init_next_id);
+                let mut lock = next_id.lock();
+                let id = *lock;
+                *lock += 1;
+                Self(id)
+            }
         }
     }
 

tokio/src/util/mod.rs

@@ -6,14 +6,8 @@ cfg_io_driver! {
 #[cfg(feature = "rt")]
 pub(crate) mod atomic_cell;
 
-cfg_has_atomic_u64! {
-    #[cfg(any(feature = "signal", all(unix, feature = "process")))]
-    pub(crate) mod once_cell;
-}
-cfg_not_has_atomic_u64! {
-    #[cfg(any(feature = "rt", feature = "signal", all(unix, feature = "process")))]
-    pub(crate) mod once_cell;
-}
+#[cfg(any(feature = "rt", feature = "signal", feature = "process"))]
+pub(crate) mod once_cell;
 
 #[cfg(any(
     // io driver uses `WakeList` directly

tokio/src/util/once_cell.rs

@@ -1,4 +1,4 @@
-#![cfg_attr(loom, allow(dead_code))]
+#![allow(dead_code)]
 use std::cell::UnsafeCell;
 use std::mem::MaybeUninit;
 use std::sync::Once;