cmd/gofmt: fix gofmt -s for 3-index slices

3-index slices of the form s[:len(s):len(s)]
cannot be simplified to s[::len(s)].

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/108330043

src/cmd/gofmt/simplify.go

@@ -68,9 +68,10 @@ func (s *simplifier) Visit(node ast.Node) ast.Visitor {
 		// a slice expression of the form: s[a:len(s)]
 		// can be simplified to: s[a:]
 		// if s is "simple enough" (for now we only accept identifiers)
-		if s.hasDotImport {
-			// if dot imports are present, we cannot be certain that an
-			// unresolved "len" identifier refers to the predefined len()
+		if n.Max != nil || s.hasDotImport {
+			// - 3-index slices always require the 2nd and 3rd index
+			// - if dot imports are present, we cannot be certain that an
+			//   unresolved "len" identifier refers to the predefined len()
 			break
 		}
 		if s, _ := n.X.(*ast.Ident); s != nil && s.Obj != nil {

src/cmd/gofmt/testdata/slices1.golden

@@ -15,6 +15,7 @@ var (
 	_ = a[3:(len(a))]
 	_ = a[len(a) : len(a)-1]
 	_ = a[0:len(b)]
+	_ = a[2:len(a):len(a)]
 
 	_ = a[:]
 	_ = a[:10]
@@ -22,6 +23,7 @@ var (
 	_ = a[:(len(a))]
 	_ = a[:len(a)-1]
 	_ = a[:len(b)]
+	_ = a[:len(a):len(a)]
 
 	_ = s[0:]
 	_ = s[1:10]
@@ -29,6 +31,7 @@ var (
 	_ = s[3:(len(s))]
 	_ = s[len(a) : len(s)-1]
 	_ = s[0:len(b)]
+	_ = s[2:len(s):len(s)]
 
 	_ = s[:]
 	_ = s[:10]
@@ -36,6 +39,7 @@ var (
 	_ = s[:(len(s))]
 	_ = s[:len(s)-1]
 	_ = s[:len(b)]
+	_ = s[:len(s):len(s)]
 
 	_ = t.s[0:]
 	_ = t.s[1:10]
@@ -43,6 +47,7 @@ var (
 	_ = t.s[3:(len(t.s))]
 	_ = t.s[len(a) : len(t.s)-1]
 	_ = t.s[0:len(b)]
+	_ = t.s[2:len(t.s):len(t.s)]
 
 	_ = t.s[:]
 	_ = t.s[:10]
@@ -50,6 +55,7 @@ var (
 	_ = t.s[:(len(t.s))]
 	_ = t.s[:len(t.s)-1]
 	_ = t.s[:len(b)]
+	_ = t.s[:len(t.s):len(t.s)]
 )
 
 func _() {

src/cmd/gofmt/testdata/slices1.input

@@ -15,6 +15,7 @@ var (
 	_ = a[3:(len(a))]
 	_ = a[len(a) : len(a)-1]
 	_ = a[0:len(b)]
+	_ = a[2:len(a):len(a)]
 
 	_ = a[:]
 	_ = a[:10]
@@ -22,6 +23,7 @@ var (
 	_ = a[:(len(a))]
 	_ = a[:len(a)-1]
 	_ = a[:len(b)]
+	_ = a[:len(a):len(a)]
 
 	_ = s[0:]
 	_ = s[1:10]
@@ -29,6 +31,7 @@ var (
 	_ = s[3:(len(s))]
 	_ = s[len(a) : len(s)-1]
 	_ = s[0:len(b)]
+	_ = s[2:len(s):len(s)]
 
 	_ = s[:]
 	_ = s[:10]
@@ -36,6 +39,7 @@ var (
 	_ = s[:(len(s))]
 	_ = s[:len(s)-1]
 	_ = s[:len(b)]
+	_ = s[:len(s):len(s)]
 
 	_ = t.s[0:]
 	_ = t.s[1:10]
@@ -43,6 +47,7 @@ var (
 	_ = t.s[3:(len(t.s))]
 	_ = t.s[len(a) : len(t.s)-1]
 	_ = t.s[0:len(b)]
+	_ = t.s[2:len(t.s):len(t.s)]
 
 	_ = t.s[:]
 	_ = t.s[:10]
@@ -50,6 +55,7 @@ var (
 	_ = t.s[:(len(t.s))]
 	_ = t.s[:len(t.s)-1]
 	_ = t.s[:len(b)]
+	_ = t.s[:len(t.s):len(t.s)]
 )
 
 func _() {