diff --git a/include/kunit/assert.h b/include/kunit/assert.h
index db6a0fc..ad889b5 100644
--- a/include/kunit/assert.h
+++ b/include/kunit/assert.h
@@ -9,10 +9,11 @@
 #ifndef _KUNIT_ASSERT_H
 #define _KUNIT_ASSERT_H
 
-#include <kunit/string-stream.h>
 #include <linux/err.h>
+#include <linux/kernel.h>
 
 struct kunit;
+struct string_stream;
 
 /**
  * enum kunit_assert_type - Type of expectation/assertion.
diff --git a/include/kunit/string-stream.h b/include/kunit/string-stream.h
deleted file mode 100644
index fe98a00..0000000
--- a/include/kunit/string-stream.h
+++ /dev/null
@@ -1,51 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0 */
-/*
- * C++ stream style string builder used in KUnit for building messages.
- *
- * Copyright (C) 2019, Google LLC.
- * Author: Brendan Higgins <brendanhiggins@google.com>
- */
-
-#ifndef _KUNIT_STRING_STREAM_H
-#define _KUNIT_STRING_STREAM_H
-
-#include <linux/spinlock.h>
-#include <linux/types.h>
-#include <stdarg.h>
-
-struct string_stream_fragment {
-	struct kunit *test;
-	struct list_head node;
-	char *fragment;
-};
-
-struct string_stream {
-	size_t length;
-	struct list_head fragments;
-	/* length and fragments are protected by this lock */
-	spinlock_t lock;
-	struct kunit *test;
-	gfp_t gfp;
-};
-
-struct kunit;
-
-struct string_stream *alloc_string_stream(struct kunit *test, gfp_t gfp);
-
-int __printf(2, 3) string_stream_add(struct string_stream *stream,
-				     const char *fmt, ...);
-
-int string_stream_vadd(struct string_stream *stream,
-		       const char *fmt,
-		       va_list args);
-
-char *string_stream_get_string(struct string_stream *stream);
-
-int string_stream_append(struct string_stream *stream,
-			 struct string_stream *other);
-
-bool string_stream_is_empty(struct string_stream *stream);
-
-int string_stream_destroy(struct string_stream *stream);
-
-#endif /* _KUNIT_STRING_STREAM_H */
diff --git a/lib/kunit/assert.c b/lib/kunit/assert.c
index 86013d4..d8ae94e 100644
--- a/lib/kunit/assert.c
+++ b/lib/kunit/assert.c
@@ -6,6 +6,7 @@
  * Author: Brendan Higgins <brendanhiggins@google.com>
  */
 #include <kunit/assert.h>
+#include "string-stream-impl.h"
 
 void kunit_base_assert_format(const struct kunit_assert *assert,
 			      struct string_stream *stream)
diff --git a/lib/kunit/string-stream-impl.h b/lib/kunit/string-stream-impl.h
new file mode 100644
index 0000000..0fe4100
--- /dev/null
+++ b/lib/kunit/string-stream-impl.h
@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * C++ stream style string builder used in KUnit for building messages.
+ *
+ * Copyright (C) 2019, Google LLC.
+ * Author: Brendan Higgins <brendanhiggins@google.com>
+ */
+
+#ifndef _KUNIT_STRING_STREAM_IMPL_H
+#define _KUNIT_STRING_STREAM_IMPL_H
+
+#include <linux/spinlock.h>
+#include <linux/types.h>
+#include <stdarg.h>
+
+struct string_stream_fragment {
+	struct kunit *test;
+	struct list_head node;
+	char *fragment;
+};
+
+struct string_stream {
+	size_t length;
+	struct list_head fragments;
+	/* length and fragments are protected by this lock */
+	spinlock_t lock;
+	struct kunit *test;
+	gfp_t gfp;
+};
+
+struct kunit;
+
+struct string_stream *alloc_string_stream(struct kunit *test, gfp_t gfp);
+
+int __printf(2, 3) string_stream_add(struct string_stream *stream,
+				     const char *fmt, ...);
+
+int string_stream_vadd(struct string_stream *stream,
+		       const char *fmt,
+		       va_list args);
+
+char *string_stream_get_string(struct string_stream *stream);
+
+int string_stream_append(struct string_stream *stream,
+			 struct string_stream *other);
+
+bool string_stream_is_empty(struct string_stream *stream);
+
+int string_stream_destroy(struct string_stream *stream);
+
+#endif /* _KUNIT_STRING_STREAM_IMPL_H */
diff --git a/lib/kunit/string-stream-test.c b/lib/kunit/string-stream-test.c
index 76cc05e..25b2cf3 100644
--- a/lib/kunit/string-stream-test.c
+++ b/lib/kunit/string-stream-test.c
@@ -6,9 +6,9 @@
  * Author: Brendan Higgins <brendanhiggins@google.com>
  */
 
-#include <kunit/string-stream.h>
 #include <kunit/test.h>
 #include <linux/slab.h>
+#include "string-stream-impl.h"
 
 static void string_stream_test_empty_on_creation(struct kunit *test)
 {
diff --git a/lib/kunit/string-stream.c b/lib/kunit/string-stream.c
index e6d17aa..a1e005d 100644
--- a/lib/kunit/string-stream.c
+++ b/lib/kunit/string-stream.c
@@ -6,11 +6,12 @@
  * Author: Brendan Higgins <brendanhiggins@google.com>
  */
 
-#include <kunit/string-stream.h>
 #include <kunit/test.h>
 #include <linux/list.h>
 #include <linux/slab.h>
 
+#include "string-stream-impl.h"
+
 struct string_stream_fragment_alloc_context {
 	struct kunit *test;
 	int len;
diff --git a/lib/kunit/test.c b/lib/kunit/test.c
index c83c0fa..017d4fb 100644
--- a/lib/kunit/test.c
+++ b/lib/kunit/test.c
@@ -10,6 +10,7 @@
 #include <kunit/try-catch.h>
 #include <linux/kernel.h>
 #include <linux/sched/debug.h>
+#include "string-stream-impl.h"
 
 static void kunit_set_failure(struct kunit *test)
 {
