Add thread Builder::no_hooks().

library/std/src/thread/mod.rs

@@ -269,6 +269,8 @@ pub struct Builder {
     name: Option<String>,
     // The size of the stack for the spawned thread in bytes
     stack_size: Option<usize>,
+    // Skip running and inheriting the thread spawn hooks
+    no_hooks: bool,
 }
 
 impl Builder {
@@ -292,7 +294,7 @@ impl Builder {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new() -> Builder {
-        Builder { name: None, stack_size: None }
+        Builder { name: None, stack_size: None, no_hooks: false }
     }
 
     /// Names the thread-to-be. Currently the name is used for identification
@@ -348,6 +350,16 @@ impl Builder {
         self
     }
 
+    /// Disables running and inheriting [spawn hooks](add_spawn_hook).
+    ///
+    /// Use this if the parent thread is in no way relevant for the child thread.
+    /// For example, when lazily spawning threads for a thread pool.
+    #[unstable(feature = "thread_spawn_hook", issue = "none")]
+    pub fn no_hooks(mut self) -> Builder {
+        self.no_hooks = true;
+        self
+    }
+
     /// Spawns a new thread by taking ownership of the `Builder`, and returns an
     /// [`io::Result`] to its [`JoinHandle`].
     ///
@@ -472,7 +484,7 @@ impl Builder {
         T: Send + 'a,
         'scope: 'a,
     {
-        let Builder { name, stack_size } = self;
+        let Builder { name, stack_size, no_hooks } = self;
 
         let stack_size = stack_size.unwrap_or_else(|| {
             static MIN: AtomicUsize = AtomicUsize::new(0);
@@ -498,7 +510,11 @@ impl Builder {
             )
         });
 
-        let hooks = spawnhook::run_spawn_hooks(&my_thread);
+        let hooks = if no_hooks {
+            spawnhook::ChildSpawnHooks::default()
+        } else {
+            spawnhook::run_spawn_hooks(&my_thread)
+        };
 
         let their_thread = my_thread.clone();
 

library/std/src/thread/spawnhook.rs

@@ -104,7 +104,7 @@ where
 /// Called on the parent thread.
 ///
 /// Returns the functions to be called on the newly spawned thread.
-pub(super) fn run_spawn_hooks(thread: &Thread) -> SpawnHookResults {
+pub(super) fn run_spawn_hooks(thread: &Thread) -> ChildSpawnHooks {
     // Get a snapshot of the spawn hooks.
     // (Increments the refcount to the first node.)
     let hooks = SPAWN_HOOKS.with(|hooks| {
@@ -121,19 +121,20 @@ pub(super) fn run_spawn_hooks(thread: &Thread) -> SpawnHookResults {
     }
     // Pass on the snapshot of the hooks and the results to the new thread,
     // which will then run SpawnHookResults::run().
-    SpawnHookResults { hooks, to_run }
+    ChildSpawnHooks { hooks, to_run }
 }
 
 /// The results of running the spawn hooks.
 ///
 /// This struct is sent to the new thread.
 /// It contains the inherited hooks and the closures to be run.
-pub(super) struct SpawnHookResults {
+#[derive(Default)]
+pub(super) struct ChildSpawnHooks {
     hooks: SpawnHooks,
     to_run: Vec<Box<dyn FnOnce() + Send>>,
 }
 
-impl SpawnHookResults {
+impl ChildSpawnHooks {
     // This is run on the newly spawned thread, directly at the start.
     pub(super) fn run(self) {
         SPAWN_HOOKS.set(self.hooks);