gofmt/main: Added removal of empty declaration groups.

Fixes #7631.

LGTM=gri
R=golang-codereviews, bradfitz, gri
CC=golang-codereviews
https://golang.org/cl/101410046

src/cmd/gofmt/gofmt_test.go

@@ -89,6 +89,7 @@ var tests = []struct {
 	{"testdata/import.input", ""},
 	{"testdata/crlf.input", ""},        // test case for issue 3961; see also TestCRLF
 	{"testdata/typeswitch.input", ""},  // test case for issue 4470
+	{"testdata/emptydecl.input", "-s"}, // test case for issue 7631
 }
 
 func TestRewrite(t *testing.T) {

src/cmd/gofmt/simplify.go

@@ -117,5 +117,34 @@ func simplify(f *ast.File) {
 		}
 	}
 
+	// remove empty declarations such as "const ()", etc
+	removeEmptyDeclGroups(f)
+
 	ast.Walk(&s, f)
 }
+
+func removeEmptyDeclGroups(f *ast.File) {
+	i := 0
+	for _, d := range f.Decls {
+		if g, ok := d.(*ast.GenDecl); !ok || !isEmpty(f, g) {
+			f.Decls[i] = d
+			i++
+		}
+	}
+	f.Decls = f.Decls[:i]
+}
+
+func isEmpty(f *ast.File, g *ast.GenDecl) bool {
+	if g.Doc != nil || g.Specs != nil {
+		return false
+	}
+
+	for _, c := range f.Comments {
+		// if there is a comment in the declaration, it is not considered empty
+		if g.Pos() <= c.Pos() && c.End() <= g.End() {
+			return false
+		}
+	}
+
+	return true
+}

src/cmd/gofmt/testdata/emptydecl.golden

+package main
+
+// Keep this declaration
+var ()
+
+const (
+// Keep this declaration
+)
+
+func main() {}

src/cmd/gofmt/testdata/emptydecl.input

+package main
+
+// Keep this declaration
+var ()
+
+const (
+// Keep this declaration
+)
+
+type ()
+
+func main() {}
\ No newline at end of file