runtime: implement newcoro, coroswitch to support package iter

src/runtime/coro.go

@@ -0,0 +1,31 @@
+package runtime
+
+// A naive implementation of coroutines that supports
+// package iter.
+
+type coro struct {
+	f  func(*coro)
+	ch chan struct{}
+}
+
+//go:linkname newcoro
+
+func newcoro(f func(*coro)) *coro {
+	c := &coro{
+		ch: make(chan struct{}),
+		f:  f,
+	}
+	go func() {
+		defer close(c.ch)
+		<-c.ch
+		f(c)
+	}()
+	return c
+}
+
+//go:linkname coroswitch
+
+func coroswitch(c *coro) {
+	c.ch <- struct{}{}
+	<-c.ch
+}