Rollup merge of #23210 - richo:rust-o, r=alexcrichton

rustc will ICE if you specify an outfile path that is bare without a
directory. As a workaround, before this -o ./foo will work

It wasn't clear to me where I could put a test that actually invokes rustc from a shell, although I think I can add doctests to that machinery in librustc_driver that will arrange for this to be called with arguments that would trigger the ICE

src/test/run-make/bare-outfile/Makefile

@@ -0,0 +1,4 @@
+-include ../tools.mk
+
+all:
+	rustc -o foo foo.rs

src/test/run-make/bare-outfile/foo.rs

@@ -0,0 +1,12 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+}