From 8158b8b68606fa101a89d6e761744ea79f969b77 Mon Sep 17 00:00:00 2001
From: Robert Griesemer <gri@golang.org>
Date: Thu, 17 Jul 2014 09:40:27 -0700
Subject: [PATCH] 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          |  1 +
 src/cmd/gofmt/simplify.go            | 14 ++++++++++++--
 src/cmd/gofmt/testdata/ranges.golden | 28 ++++++++++++++++++++++++++++
 src/cmd/gofmt/testdata/ranges.input  | 18 ++++++++++++++++++
 4 files changed, 59 insertions(+), 2 deletions(-)
 create mode 100644 src/cmd/gofmt/testdata/ranges.golden
 create mode 100644 src/cmd/gofmt/testdata/ranges.input

diff --git a/src/cmd/gofmt/gofmt_test.go b/src/cmd/gofmt/gofmt_test.go
index b767a6bf55..ca44f3dcf7 100644
--- a/src/cmd/gofmt/gofmt_test.go
+++ b/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"},
diff --git a/src/cmd/gofmt/simplify.go b/src/cmd/gofmt/simplify.go
index b05aa2480d..69f7bf23c0 100644
--- a/src/cmd/gofmt/simplify.go
+++ b/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
 
diff --git a/src/cmd/gofmt/testdata/ranges.golden b/src/cmd/gofmt/testdata/ranges.golden
new file mode 100644
index 0000000000..42168526d1
--- /dev/null
+++ b/src/cmd/gofmt/testdata/ranges.golden
@@ -0,0 +1,28 @@
+// 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 {
+	}
+}
diff --git a/src/cmd/gofmt/testdata/ranges.input b/src/cmd/gofmt/testdata/ranges.input
new file mode 100644
index 0000000000..4b02d51752
--- /dev/null
+++ b/src/cmd/gofmt/testdata/ranges.input
@@ -0,0 +1,18 @@
+// 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 {}
+}
-- 
2.48.1