gofmt: -s flag simplifies "for _ = range x"

LGTM=adonovan, rsc
R=rsc, adonovan
CC=golang-codereviews
https://golang.org/cl/117800043

src/cmd/gofmt/gofmt_test.go

@@ -75,6 +75,7 @@ var tests = []struct {
 	{"testdata/composites.input", "-s"},
 	{"testdata/slices1.input", "-s"},
 	{"testdata/slices2.input", "-s"},
+	{"testdata/ranges.input", "-s"},
 	{"testdata/old.input", ""},
 	{"testdata/rewrite1.input", "-r=Foo->Bar"},
 	{"testdata/rewrite2.input", "-r=int->bool"},

src/cmd/gofmt/simplify.go

@@ -97,16 +97,26 @@ func (s *simplifier) Visit(node ast.Node) ast.Visitor {
 		//       x, y := b[:n], b[n:]
 
 	case *ast.RangeStmt:
-		// a range of the form: for x, _ = range v {...}
+		// - a range of the form: for x, _ = range v {...}
 		// can be simplified to: for x = range v {...}
-		if ident, _ := n.Value.(*ast.Ident); ident != nil && ident.Name == "_" {
+		// - a range of the form: for _ = range v {...}
+		// can be simplified to: for range v {...}
+		if isBlank(n.Value) {
 			n.Value = nil
 		}
+		if isBlank(n.Key) && n.Value == nil {
+			n.Key = nil
+		}
 	}
 
 	return s
 }
 
+func isBlank(x ast.Expr) bool {
+	ident, ok := x.(*ast.Ident)
+	return ok && ident.Name == "_"
+}
+
 func simplify(f *ast.File) {
 	var s simplifier
 

src/cmd/gofmt/testdata/ranges.golden

+// Test cases for range simplification.
+package p
+
+func _() {
+	for a, b = range x {
+	}
+	for a = range x {
+	}
+	for _, b = range x {
+	}
+	for range x {
+	}
+
+	for a = range x {
+	}
+	for range x {
+	}
+
+	for a, b := range x {
+	}
+	for a := range x {
+	}
+	for _, b := range x {
+	}
+
+	for a := range x {
+	}
+}

src/cmd/gofmt/testdata/ranges.input

+// Test cases for range simplification.
+package p
+
+func _() {
+	for a, b = range x {}
+	for a, _ = range x {}
+	for _, b = range x {}
+	for _, _ = range x {}
+
+	for a = range x {}
+	for _ = range x {}
+
+	for a, b := range x {}
+	for a, _ := range x {}
+	for _, b := range x {}
+
+	for a := range x {}
+}