Specify a concrete stack size in channel tests

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

@@ -10,20 +10,30 @@
 // Ref: https://github.com/rust-lang/rust/issues/102246
 
 use std::sync::mpsc::channel;
-use std::thread;
+use std::thread::Builder;
 
 const N: usize = 32_768;
+const SLOTS: usize = 32;
+// Use a stack size that's smaller than N * SLOTS, proving the allocation is on the heap.
+//
+// The test explicitly specifies the stack size, because not all platforms have the same default
+// size.
+const STACK_SIZE: usize = (N*SLOTS) - 1;
+
 struct BigStruct {
     _data: [u8; N],
 }
 
 fn main() {
     let (sender, receiver) = channel::<BigStruct>();
 
-    let thread1 = thread::spawn(move || {
+    let thread1 = Builder::new().stack_size(STACK_SIZE).spawn(move || {
         sender.send(BigStruct { _data: [0u8; N] }).unwrap();
-    });
-
+    }).expect("thread1 should spawn successfully");
     thread1.join().unwrap();
-    for _data in receiver.try_iter() {}
+
+    let thread2 = Builder::new().stack_size(STACK_SIZE).spawn(move || {
+        for _data in receiver.try_iter() {}
+    }).expect("thread2 should spawn successfully");
+    thread2.join().unwrap();
 }