struct-tag warns on private fields with tags (#131)

rule/struct-tag.go

@@ -2,11 +2,12 @@ package rule
 
 import (
 	"fmt"
-	"github.com/fatih/structtag"
-	"github.com/mgechev/revive/lint"
 	"go/ast"
 	"strconv"
 	"strings"
+
+	"github.com/fatih/structtag"
+	"github.com/mgechev/revive/lint"
 )
 
 // StructTagRule lints struct tags.
@@ -58,6 +59,10 @@ func (w lintStructTagRule) Visit(node ast.Node) ast.Visitor {
 // checkTaggedField checks the tag of the given field.
 // precondition: the field has a tag
 func (w lintStructTagRule) checkTaggedField(f *ast.Field) {
+	if len(f.Names) > 0 && !f.Names[0].IsExported() {
+		w.addFailure(f, "tag on not-exported field "+f.Names[0].Name)
+	}
+
 	tags, err := structtag.Parse(strings.Trim(f.Tag.Value, "`"))
 	if err != nil || tags == nil {
 		w.addFailure(f.Tag, "malformed tag")