Initialize channel Blocks directly on the heap

The channel's `Block::new` was causing a stack overflow because it held
32 item slots, instantiated on the stack before moving to `Box::new`.
The 32x multiplier made modestly-large item sizes untenable.
That block is now initialized directly on the heap.
Fixes#102246

Author: cuviper

library/std/src/sync/mpmc/list.rs

@@ -63,14 +63,14 @@ struct Block<T> {
 
 impl<T>Block<T>{
 /// Creates an empty block.
-fnnew() -> Block<T>{
+fnnew() -> Box<Block<T>>{
 // SAFETY: This is safe because:
 // [1] `Block::next` (AtomicPtr) may be safely zero initialized.
 // [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4].
 // [3] `Slot::msg` (UnsafeCell) may be safely zero initialized because it
 // holds a MaybeUninit.
 // [4] `Slot::state` (AtomicUsize) may be safely zero initialized.
-unsafe{MaybeUninit::zeroed().assume_init()}
+unsafe{Box::new_zeroed().assume_init()}
 }
 
 /// Waits until the next pointer is set.
@@ -199,13 +199,13 @@ impl<T> Channel<T> {
 // If we're going to have to install the next block, allocate it in advance in order to
 // make the wait for other threads as short as possible.
 if offset + 1 == BLOCK_CAP && next_block.is_none(){
- next_block = Some(Box::new(Block::<T>::new()));
+ next_block = Some(Block::<T>::new());
 }
 
 // If this is the first message to be sent into the channel, we need to allocate the
 // first block and install it.
 if block.is_null(){
-let new = Box::into_raw(Box::new(Block::<T>::new()));
+let new = Box::into_raw(Block::<T>::new());
 
 ifself
 .tail

tests/ui/std/channel-stack-overflow-issue-102246.rs

@@ -0,0 +1,28 @@
+//@ run-pass
+//@ compile-flags: -Copt-level=0
+
+// The channel's `Block::new` was causing a stack overflow because it held 32 item slots, which is
+// 1MiB for this test's `BigStruct` -- instantiated on the stack before moving to `Box::new`.
+//
+// That block is now initialized directly on the heap.
+//
+// Ref: https://github.com/rust-lang/rust/issues/102246
+
+use std::sync::mpsc::channel;
+use std::thread;
+
+constN:usize = 32_768;
+structBigStruct{
+_data:[u8;N],
+}
+
+fnmain(){
+let(sender, receiver) = channel::<BigStruct>();
+
+let thread1 = thread::spawn(move || {
+ sender.send(BigStruct{_data:[0u8;N]}).unwrap();
+});
+
+ thread1.join().unwrap();
+for _data in receiver.try_iter(){}
+}