go/ast: add Preorder go1.23 iterator

This CL adds a new function Preorder that makes it
easier to iterate over the nodes of a syntax tree.
In particular, break, continue, and return retain
their usual continuations.

Fixes #66339

Change-Id: I438b3c23780c91ed589871ad3b8822d54e8fabc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/570680
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

api/next/66339.txt

@@ -0,0 +1 @@
+pkg go/ast, func Preorder(Node) iter.Seq[Node] #66339

doc/next/6-stdlib/99-minor/go/ast/66339.md

@@ -0,0 +1,2 @@
+The new [Preorder] function returns a convenient iterator over all the
+nodes of a syntax tree.

src/go/ast/example_test.go

@@ -140,6 +140,41 @@ func main() {
 	//     61  }
 }
 
+func ExamplePreorder() {
+	src := `
+package p
+
+func f(x, y int) {
+	print(x + y)
+}
+`
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Print identifiers in order
+	for n := range ast.Preorder(f) {
+		id, ok := n.(*ast.Ident)
+		if !ok {
+			continue
+		}
+		fmt.Println(id.Name)
+	}
+
+	// Output:
+	// p
+	// f
+	// x
+	// y
+	// int
+	// print
+	// x
+	// y
+}
+
 // This example illustrates how to remove a variable declaration
 // in a Go program while maintaining correct comment association
 // using an ast.CommentMap.

src/go/ast/walk.go

@@ -4,7 +4,10 @@
 
 package ast
 
-import "fmt"
+import (
+	"fmt"
+	"iter"
+)
 
 // A Visitor's Visit method is invoked for each node encountered by [Walk].
 // If the result visitor w is not nil, [Walk] visits each of the children
@@ -368,3 +371,20 @@ func (f inspector) Visit(node Node) Visitor {
 func Inspect(node Node, f func(Node) bool) {
 	Walk(inspector(f), node)
 }
+
+// Preorder returns an iterator over all the nodes of the syntax tree
+// beneath (and including) the specified root, in depth-first
+// preorder.
+//
+// For greater control over the traversal of each subtree, use [Inspect].
+func Preorder(root Node) iter.Seq[Node] {
+	return func(yield func(Node) bool) {
+		ok := true
+		Inspect(root, func(n Node) bool {
+			if n != nil && !yield(n) {
+				ok = false
+			}
+			return ok
+		})
+	}
+}