staticcheck: improve extractConsts helper

In commit ae60410 we changed
extractConsts to not collect all distinct constants from phi nodes. We
didn't notice that the function no longer returned more than one
constant and didn't need to return a slice anymore.

In this change, we change the signature and name, and replace all
looping with a single if != nil check.

Additionally, we introduce the extractConstExpectKind helper, which
takes care of checking that the constant is of the right kind, and not
untyped nil. This helper removes a lot of boilerplate.

(cherry picked from commit 1735532)

staticcheck/lint.go

@@ -3154,7 +3154,7 @@ func CheckWriterBufferModified(pass *analysis.Pass) (interface{}, error) {
 
 func loopedRegexp(name string) CallCheck {
 	return func(call *Call) {
-		if len(extractConsts(call.Args[0].Value.Value)) == 0 {
+		if extractConst(call.Args[0].Value.Value) == nil {
 			return
 		}
 		if !isInLoop(call.Instr.Block()) {

staticcheck/rules.go

@@ -56,26 +56,28 @@ func (arg *Argument) Invalid(msg string) {
 
 type CallCheck func(call *Call)
 
-func extractConsts(v ir.Value) []*ir.Const {
+func extractConstExpectKind(v ir.Value, kind constant.Kind) *ir.Const {
+	k := extractConst(v)
+	if k == nil || k.Value == nil || k.Value.Kind() != kind {
+		return nil
+	}
+	return k
+}
+
+func extractConst(v ir.Value) *ir.Const {
 	v = irutil.Flatten(v)
 	switch v := v.(type) {
 	case *ir.Const:
-		return []*ir.Const{v}
+		return v
 	case *ir.MakeInterface:
-		return extractConsts(v.X)
+		return extractConst(v.X)
 	default:
 		return nil
 	}
 }
 
 func ValidateRegexp(v Value) error {
-	for _, c := range extractConsts(v.Value) {
-		if c.Value == nil {
-			continue
-		}
-		if c.Value.Kind() != constant.String {
-			continue
-		}
+	if c := extractConstExpectKind(v.Value, constant.String); c != nil {
 		s := constant.StringVal(c.Value)
 		if _, err := regexp.Compile(s); err != nil {
 			return err
@@ -85,13 +87,7 @@ func ValidateRegexp(v Value) error {
 }
 
 func ValidateTimeLayout(v Value) error {
-	for _, c := range extractConsts(v.Value) {
-		if c.Value == nil {
-			continue
-		}
-		if c.Value.Kind() != constant.String {
-			continue
-		}
+	if c := extractConstExpectKind(v.Value, constant.String); c != nil {
 		s := constant.StringVal(c.Value)
 		s = strings.Replace(s, "_", " ", -1)
 		s = strings.Replace(s, "Z", "-", -1)
@@ -104,13 +100,7 @@ func ValidateTimeLayout(v Value) error {
 }
 
 func ValidateURL(v Value) error {
-	for _, c := range extractConsts(v.Value) {
-		if c.Value == nil {
-			continue
-		}
-		if c.Value.Kind() != constant.String {
-			continue
-		}
+	if c := extractConstExpectKind(v.Value, constant.String); c != nil {
 		s := constant.StringVal(c.Value)
 		_, err := url.Parse(s)
 		if err != nil {
@@ -121,13 +111,7 @@ func ValidateURL(v Value) error {
 }
 
 func InvalidUTF8(v Value) bool {
-	for _, c := range extractConsts(v.Value) {
-		if c.Value == nil {
-			continue
-		}
-		if c.Value.Kind() != constant.String {
-			continue
-		}
+	if c := extractConstExpectKind(v.Value, constant.String); c != nil {
 		s := constant.StringVal(c.Value)
 		if !utf8.ValidString(s) {
 			return true
@@ -269,13 +253,7 @@ func validatePort(s string) bool {
 }
 
 func ValidHostPort(v Value) bool {
-	for _, k := range extractConsts(v.Value) {
-		if k.Value == nil {
-			continue
-		}
-		if k.Value.Kind() != constant.String {
-			continue
-		}
+	if k := extractConstExpectKind(v.Value, constant.String); k != nil {
 		s := constant.StringVal(k.Value)
 		_, port, err := net.SplitHostPort(s)
 		if err != nil {
@@ -296,17 +274,11 @@ func ConvertedFrom(v Value, typ string) bool {
 }
 
 func UniqueStringCutset(v Value) bool {
-	for _, c := range extractConsts(v.Value) {
-		if c.Value == nil {
-			continue
-		}
-		if c.Value.Kind() != constant.String {
-			continue
-		}
+	if c := extractConstExpectKind(v.Value, constant.String); c != nil {
 		s := constant.StringVal(c.Value)
 		rs := runeSlice(s)
 		if len(rs) < 2 {
-			continue
+			return true
 		}
 		sort.Sort(rs)
 		for i, r := range rs[1:] {