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
+}

testdata/go1.23/main.go

@@ -1,14 +1,29 @@
 package main
 
+import "iter"
+
 func main() {
 	testFuncRange(counter)
+	testIterPull(counter)
+	println("go1.23 has lift-off!")
 }
 
-func testFuncRange(f func(yield func(int) bool)) {
+func testFuncRange(it iter.Seq[int]) {
-	for i := range f {
+	for i := range it {
+		println(i)
+	}
+}
+
+func testIterPull(it iter.Seq[int]) {
+	next, stop := iter.Pull(it)
+	defer stop()
+	for {
+		i, ok := next()
+		if !ok {
+			break
+		}
 		println(i)
 	}
-	println("go1.23 has lift-off!")
 }
 
 func counter(yield func(int) bool) {

testdata/go1.23/out.txt

@@ -8,4 +8,14 @@
 3
 2
 1
+10
+9
+8
+7
+6
+5
+4
+3
+2
+1
 go1.23 has lift-off!