Fix --check -l with stdin. (#3910)

* Fix some possible panics when using `--check` with stdin.

One case which doesn't work is when there are only line ending fixes;
with stdin rustfmt is unable to detect the difference as it stores
the input with Unix line endings.

* Add test for `rustfmt --check -l` with stdin.

1 files changed, 26 insertions, 0 deletions

diff --git a/src/test/mod.rs b/src/test/mod.rs
index e1a7972ec82..db1cf88479c 100644
--- a/src/test/mod.rs
+++ b/src/test/mod.rs
@@ -989,3 +989,29 @@ fn verify_check_works_with_stdin() {
         .expect("Failed to wait on rustfmt child");
     assert!(output.status.success());
 }
+
+#[test]
+fn verify_check_l_works_with_stdin() {
+    init_log();
+
+    let mut child = Command::new(rustfmt().to_str().unwrap())
+        .arg("--check")
+        .arg("-l")
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .spawn()
+        .expect("run with check option failed");
+
+    {
+        let stdin = child.stdin.as_mut().expect("Failed to open stdin");
+        stdin
+            .write_all("fn main()\n{}\n".as_bytes())
+            .expect("Failed to write to rustfmt --check");
+    }
+    let output = child
+        .wait_with_output()
+        .expect("Failed to wait on rustfmt child");
+    assert!(output.status.success());
+    assert_eq!(std::str::from_utf8(&output.stdout).unwrap(), "stdin\n");
+}