start using go/ast.Preorder

Thanks to being able to use range-over-func, some control flow
in our code gets simplified.

internal/literals/literals.go

@@ -175,7 +175,7 @@ func handleCompositeLiteral(obfRand *obfRand, isPointer bool, node *ast.Composit
 //
 // We don't set any "end" or middle positions, because they seem irrelevant.
 func withPos(node ast.Node, pos token.Pos) ast.Node {
-	ast.Inspect(node, func(node ast.Node) bool {
+	for node := range ast.Preorder(node) {
 		switch node := node.(type) {
 		case *ast.BasicLit:
 			node.ValuePos = pos
@@ -207,8 +207,7 @@ func withPos(node ast.Node, pos token.Pos) ast.Node {
 		case *ast.BranchStmt:
 			node.TokPos = pos
 		}
-		return true
-	})
+	}
 	return node
 }
 

position.go

@@ -71,7 +71,7 @@ func printFile(lpkg *listedPackage, file *ast.File) ([]byte, error) {
 	// we use a list of offsets indexed by identifiers in source order.
 	var origCallOffsets []int
 	nextOffset := -1
-	ast.Inspect(file, func(node ast.Node) bool {
+	for node := range ast.Preorder(file) {
 		switch node := node.(type) {
 		case *ast.CallExpr:
 			nextOffset = fsetFile.Position(node.Pos()).Offset
@@ -79,8 +79,7 @@ func printFile(lpkg *listedPackage, file *ast.File) ([]byte, error) {
 			origCallOffsets = append(origCallOffsets, nextOffset)
 			nextOffset = -1
 		}
-		return true
-	})
+	}
 
 	copied := 0
 	printBuf2.Reset()

reverse.go

@@ -80,7 +80,7 @@ One can reverse a captured panic stack trace as follows:
 		fieldToStruct := computeFieldToStruct(info)
 		for i, file := range files {
 			goFile := lpkg.CompiledGoFiles[i]
-			ast.Inspect(file, func(node ast.Node) bool {
+			for node := range ast.Preorder(file) {
 				switch node := node.(type) {
 
 				// Replace names.
@@ -125,9 +125,7 @@ One can reverse a captured panic stack trace as follows:
 						fmt.Sprintf("%s/%s", lpkg.ImportPath, goFile),
 					)
 				}
-
-				return true
-			})
+			}
 		}
 	}
 	repl := strings.NewReplacer(replaces...)

runtime_patch.go

@@ -201,16 +201,13 @@ func stripRuntime(basename string, file *ast.File) {
 			}
 		case "runtime1.go":
 			usesEnv := func(node ast.Node) bool {
-				seen := false
-				ast.Inspect(node, func(node ast.Node) bool {
+				for node := range ast.Preorder(node) {
 					ident, ok := node.(*ast.Ident)
 					if ok && ident.Name == "gogetenv" {
-						seen = true
-						return false
+						return true
 					}
-					return true
-				})
-				return seen
+				}
+				return false
 			}
 		filenames:
 			switch funcDecl.Name.Name {