cmd/vet: fix copylocks false positive on len(array) and cap(array).

This is a follow-up for https://golang.org/cl/24340.

Updates #14664.
Fixes #18374.

Change-Id: I2831556a9014d30ec70d5f91943d18c33db5b390
Reviewed-on: https://go-review.googlesource.com/34630Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/copylock.go

@@ -93,9 +93,11 @@ func checkCopyLocksReturnStmt(f *File, rs *ast.ReturnStmt) {
 
 // checkCopyLocksCallExpr detects lock copy in the arguments to a function call
 func checkCopyLocksCallExpr(f *File, ce *ast.CallExpr) {
-	if id, ok := ce.Fun.(*ast.Ident); ok && id.Name == "new" && f.pkg.types[id].IsBuiltin() {
-		// Skip 'new(Type)' for built-in 'new'
-		return
+	if id, ok := ce.Fun.(*ast.Ident); ok && f.pkg.types[id].IsBuiltin() {
+		switch id.Name {
+		case "new", "len", "cap":
+			return
+		}
 	}
 	for _, x := range ce.Args {
 		if path := lockPathRhs(f, x); path != nil {

src/cmd/vet/testdata/copylock.go

@@ -88,6 +88,20 @@ func BadFunc() {
 	fmuSlice := fmuA[:] // OK
 }
 
+func LenAndCapOnLockArrays() {
+	var a [5]sync.Mutex
+	aLen := len(a) // OK
+	aCap := cap(a) // OK
+
+	// override 'len' and 'cap' keywords
+
+	len := func(interface{}) {}
+	len(a) // ERROR "function call copies lock value: sync.Mutex"
+
+	cap := func(interface{}) {}
+	cap(a) // ERROR "function call copies lock value: sync.Mutex"
+}
+
 // SyncTypesCheck checks copying of sync.* types except sync.Mutex
 func SyncTypesCheck() {
 	// sync.RWMutex copying