useless-fallthrough: do not report fallthrough before default (#1549)

Co-authored-by: Oleksandr Redko <oleksandr.red+github@gmail.com>

rule/useless_fallthrough.go

@@ -69,8 +69,9 @@ func (w *lintUselessFallthrough) Visit(node ast.Node) ast.Visitor {
 
 		confidence := 1.0
 		if nextCaseClause := switchStmt.Body.List[i+1].(*ast.CaseClause); nextCaseClause.List == nil {
-			// the next case clause is the default clause, report with lower confidence.
-			confidence = 0.8
+			// The next clause is 'default:', and this is a valid pattern.
+			// Skip reporting this fallthrough.
+			continue
 		}
 		if _, ok := w.commentsMap[branchStmt]; ok {
 			// The fallthrough has a comment, report with lower confidence.

testdata/useless_fallthrough.go

@@ -29,14 +29,14 @@ func uselessFallthrough() {
 
 	switch a {
 	case 0:
-		fallthrough // json:{"MATCH": "this \"fallthrough\" can be removed by consolidating this case clause with the next one","Confidence": 0.8}
+		fallthrough
 	default:
 		println()
 	}
 
 	switch a {
 	case 0:
-		fallthrough // json:{"MATCH": "this \"fallthrough\" can be removed by consolidating this case clause with the next one","Confidence": 0.8}
+		fallthrough
 	default:
 		println()
 	case 1:
@@ -47,8 +47,7 @@ func uselessFallthrough() {
 
 	switch a {
 	case 0:
-		// This a comment on the case, the confidence must be 0.5
-		fallthrough // json:{"MATCH": "this \"fallthrough\" can be removed by consolidating this case clause with the next one","Confidence": 0.5}
+		fallthrough
 	default:
 		println()
 	}
@@ -71,7 +70,7 @@ func uselessFallthrough() {
 	switch a {
 	case 0:
 		//foo:bar
-		fallthrough // json:{"MATCH": "this \"fallthrough\" can be removed by consolidating this case clause with the next one","Confidence": 0.5}
+		fallthrough
 	default:
 		println()
 	}