Ignore intentional empty cases

README.md

@@ -30,7 +30,7 @@ func f() {
 	switch v {
 	case 1:
 	case 2:
-		fmt.Println("case 1 or 2")
+		fmt.Println("execute")
 	}
 }
 ```
@@ -39,3 +39,24 @@ Of course, Println() donesn't be executed.
 However, it takes a while to realize this when you write it.
 
 This Analyzer detects empty cases, making it faster to notice problems.
+
+## Ignore intentional empty cases
+
+Sometimes an empty case is written to indicate that nothing is to be processed on purpose.
+This Analyzer ignores empty cases that have comments.
+
+```go
+func f() {
+	v := 1
+	switch v {
+	case 1: // This is an intentionally empty case.
+	case 2:
+		// This is an intentionally empty case.
+		// If you want to write a comment on a different line from the case statement, you need to put a blank line between it and the next case statement.
+
+	case 3:
+		fmt.Println("execute")
+	}
+}
+
+```

emptycase.go

@@ -1,7 +1,9 @@
 package emptycase
 
 import (
+	"github.com/gostaticanalysis/comment"
 	"go/ast"
+	"strings"
 
 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/go/analysis/passes/inspect"
@@ -20,7 +22,32 @@ var Analyzer = &analysis.Analyzer{
 	},
 }
 
+var analyzerTest = false
+
 func run(pass *analysis.Pass) (interface{}, error) {
+	commentMap := comment.New(pass.Fset, pass.Files)
+
+	// Report a case with no body.
+	// However, if a comment is attached to it, it is considered an intentional empty case and not reported.
+	var emptyCaseChecker = func(caseNode ast.Node, body []ast.Stmt) {
+		if len(body) != 0 {
+			return
+		}
+
+		commentGroup := commentMap.Comments(caseNode)
+		if len(commentGroup) == 0 {
+			pass.Reportf(caseNode.Pos(), "empty case")
+			return
+		}
+
+		if analyzerTest {
+			// Comments that indicate the expected value of the analyzer test are not considered comments.
+			if len(commentGroup) == 1 && strings.HasPrefix(strings.TrimLeft(commentGroup[0].List[0].Text[2:], " "), "want") {
+				pass.Reportf(caseNode.Pos(), "empty case")
+			}
+		}
+	}
+
 	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
 
 	nodeFilter := []ast.Node{
@@ -31,13 +58,9 @@ func run(pass *analysis.Pass) (interface{}, error) {
 	inspect.Preorder(nodeFilter, func(n ast.Node) {
 		switch n := n.(type) {
 		case *ast.CaseClause:
-			if len(n.Body) == 0 {
-				pass.Reportf(n.Pos(), "empty case")
-			}
+			emptyCaseChecker(n, n.Body)
 		case *ast.CommClause:
-			if len(n.Body) == 0 {
-				pass.Reportf(n.Pos(), "empty case")
-			}
+			emptyCaseChecker(n, n.Body)
 		}
 	})
 

emptycase_test.go

@@ -1,15 +1,15 @@
-package emptycase_test
+package emptycase
 
 import (
-	"github.com/gostaticanalysis/emptycase"
 	"testing"
 
 	"github.com/gostaticanalysis/testutil"
 	"golang.org/x/tools/go/analysis/analysistest"
 )
 
-// TestAnalyzer is a test for Analyzer.
+// TestAnalyzer is a analyzerTest for Analyzer.
 func TestAnalyzer(t *testing.T) {
 	testdata := testutil.WithModules(t, analysistest.TestData(), nil)
-	analysistest.Run(t, testdata, emptycase.Analyzer, "a")
+	analyzerTest = true
+	analysistest.Run(t, testdata, Analyzer, "a")
 }

go.mod

@@ -3,6 +3,7 @@ module github.com/gostaticanalysis/emptycase
 go 1.16
 
 require (
+	github.com/gostaticanalysis/comment v1.4.2
 	github.com/gostaticanalysis/testutil v0.4.0
 	golang.org/x/tools v0.1.1-0.20210205202024-ef80cdb6ec6d
 )

go.sum

@@ -1,5 +1,9 @@
-github.com/google/go-cmp v0.5.2 h1:X2ev0eStA3AbceY54o37/0PQ/UWqKEiiO2dKL5OPaFM=
 github.com/google/go-cmp v0.5.2/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+github.com/google/go-cmp v0.5.4 h1:L8R9j+yAqZuZjsqh/z+F1NCffTKKLShY6zXTItVIZ8M=
+github.com/google/go-cmp v0.5.4/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+github.com/gostaticanalysis/comment v1.4.2 h1:hlnx5+S2fY9Zo9ePo4AhgYsYHbM2+eAv8m/s1JiCd6Q=
+github.com/gostaticanalysis/comment v1.4.2/go.mod h1:KLUTGDv6HOCotCH8h2erHKmpci2ZoR8VPu34YA2uzdM=
+github.com/gostaticanalysis/testutil v0.3.1-0.20210208050101-bfb5c8eec0e4/go.mod h1:D+FIZ+7OahH3ePw/izIEeH5I06eKs1IKI4Xr64/Am3M=
 github.com/gostaticanalysis/testutil v0.4.0 h1:nhdCmubdmDF6VEatUNjgUZBJKWRqugoISdUv3PPQgHY=
 github.com/gostaticanalysis/testutil v0.4.0/go.mod h1:bLIoPefWXrRi/ssLFWX1dx7Repi5x3CuviD3dgAZaBU=
 github.com/hashicorp/go-version v1.2.1 h1:zEfKbn2+PDgroKdiOzqiE8rsmLqU2uwi5PB5pBJ3TkI=

testdata/src/a/a.go

@@ -7,7 +7,10 @@ func f() {
 	switch v {
 	case 1: // want "empty case"
 	case 2:
-		fmt.Println("case 2")
+		// Intentional empty case. Not treated as an error.
+
+	case 3:
+		fmt.Println("execute")
 	}
 }
 
@@ -17,6 +20,6 @@ func g() {
 	select {
 	case <-ch: // want "empty case"
 	default:
-		fmt.Println("no value")
+		fmt.Println("execute")
 	}
 }