go/parser: handle last line comments

Fixes #20636

Change-Id: Icea0012fecb73944c95f6037922505c63b57b245
Reviewed-on: https://go-review.googlesource.com/45295Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/parser/parser.go

@@ -327,7 +327,7 @@ func (p *parser) next() {
 			// The comment is on same line as the previous token; it
 			// cannot be a lead comment but may be a line comment.
 			comment, endline = p.consumeCommentGroup(0)
-			if p.file.Line(p.pos) != endline {
+			if p.file.Line(p.pos) != endline || p.tok == token.EOF {
 				// The next token is on a different line, thus
 				// the last comment group is a line comment.
 				p.lineComment = comment

src/go/parser/parser_test.go

@@ -531,3 +531,18 @@ func TestIncompleteSelection(t *testing.T) {
 		}
 	}
 }
+
+func TestLastLineComment(t *testing.T) {
+	const src = `package main
+type x int // comment
+`
+	fset := token.NewFileSet()
+	f, err := ParseFile(fset, "", src, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	comment := f.Decls[0].(*ast.GenDecl).Specs[0].(*ast.TypeSpec).Comment.List[0].Text
+	if comment != "// comment" {
+		t.Errorf("got %q, want %q", comment, "// comment")
+	}
+}