cmd/vet: if a function modifies its args, it's not a print wrapper

Fixes #26486
Updates #26555

Change-Id: I402137b796e574e9b085ab54290d1b4ef73d3fcc
Reviewed-on: https://go-review.googlesource.com/125039Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/vet/print.go

@@ -68,12 +68,12 @@ type printfExport struct {
 var printfImported = make(map[string]map[string]int)
 
 type printfWrapper struct {
-	name       string
-	fn         *ast.FuncDecl
-	format     *ast.Field
-	args       *ast.Field
-	callers    []printfCaller
-	printfLike bool
+	name    string
+	fn      *ast.FuncDecl
+	format  *ast.Field
+	args    *ast.Field
+	callers []printfCaller
+	failed  bool // if true, not a printf wrapper
 }
 
 type printfCaller struct {
@@ -168,6 +168,33 @@ func findPrintfLike(pkg *Package) {
 	for _, w := range wrappers {
 		// Scan function for calls that could be to other printf-like functions.
 		ast.Inspect(w.fn.Body, func(n ast.Node) bool {
+			if w.failed {
+				return false
+			}
+
+			// TODO: Relax these checks; issue 26555.
+			if assign, ok := n.(*ast.AssignStmt); ok {
+				for _, lhs := range assign.Lhs {
+					if match(lhs, w.format) || match(lhs, w.args) {
+						// Modifies the format
+						// string or args in
+						// some way, so not a
+						// simple wrapper.
+						w.failed = true
+						return false
+					}
+				}
+			}
+			if un, ok := n.(*ast.UnaryExpr); ok && un.Op == token.AND {
+				if match(un.X, w.format) || match(un.X, w.args) {
+					// Taking the address of the
+					// format string or args,
+					// so not a simple wrapper.
+					w.failed = true
+					return false
+				}
+			}
+
 			call, ok := n.(*ast.CallExpr)
 			if !ok || len(call.Args) == 0 || !match(call.Args[len(call.Args)-1], w.args) {
 				return true

src/cmd/vet/testdata/print.go

@@ -318,6 +318,9 @@ func PrintfTests() {
 	l.Print("%d", 1)    // ERROR "Print call has possible formatting directive %d"
 	l.Printf("%d", "x") // ERROR "Printf format %d has arg \x22x\x22 of wrong type string"
 	l.Println("%d", 1)  // ERROR "Println call has possible formatting directive %d"
+
+	// Issue 26486
+	dbg("", 1) // no error "call has arguments but no formatting directive"
 }
 
 func someString() string { return "X" }
@@ -650,3 +653,11 @@ func UnexportedStringerOrError() {
 func DisableErrorForFlag0() {
 	fmt.Printf("%0t", true)
 }
+
+// Issue 26486.
+func dbg(format string, args ...interface{}) {
+	if format == "" {
+		format = "%v"
+	}
+	fmt.Printf(format, args...)
+}