cmd/gofmt: fix error on partial Go code ending with line comment.

Fix issue by always appending newline after user input, before
the closing curly bracket. The adjust func is modified to remove
this new newline.

Add test case (it fails before CL, passes after).

Fixes #8411.

LGTM=gri
R=golang-codereviews, bradfitz, josharian, gri
CC=golang-codereviews
https://golang.org/cl/124700043

src/cmd/gofmt/gofmt.go

@@ -277,14 +277,14 @@ func parse(fset *token.FileSet, filename string, src []byte, stdin bool) (*ast.F
 	// into a function body.  This handles expressions too.
 	// Insert using a ;, not a newline, so that the line numbers
 	// in fsrc match the ones in src.
-	fsrc := append(append([]byte("package p; func _() {"), src...), '}')
+	fsrc := append(append([]byte("package p; func _() {"), src...), '\n', '}')
 	file, err = parser.ParseFile(fset, filename, fsrc, parserMode)
 	if err == nil {
 		adjust := func(orig, src []byte) []byte {
 			// Remove the wrapping.
 			// Gofmt has turned the ; into a \n\n.
 			src = src[len("package p\n\nfunc _() {"):]
-			src = src[:len(src)-len("}\n")]
+			src = src[:len(src)-len("\n}\n")]
 			// Gofmt has also indented the function body one level.
 			// Remove that indent.
 			src = bytes.Replace(src, []byte("\n\t"), []byte("\n"), -1)

src/cmd/gofmt/testdata/stdin5.golden

+//gofmt -stdin
+
+i := 5 // Line comment without newline.
\ No newline at end of file

src/cmd/gofmt/testdata/stdin5.input

+//gofmt -stdin
+
+i  :=5// Line comment without newline.
\ No newline at end of file