pw_span: Add IWYU pragma

More context behind what these are:

Some projects (including some internal Google infrastructure) use these
pragmas to help surface unused includes. Without this pragma, certain
tooling would believe that all includes to pw_span/span.h were
unnecessary, and/or that pw_span/internal/span_impl.h should instead be

It's likely that other files would also benefit from these pragmas, but
this particular one is currently causing trouble for me.

Change-Id: Ia2de79e6ec8ed356289c809286d73e953fda4506
Commit-Queue: Anqi Dong <>
Reviewed-by: Keir Mierle <>
Pigweed-Auto-Submit: Anqi Dong <>
diff --git a/pw_span/public/pw_span/span.h b/pw_span/public/pw_span/span.h
index af0c63b..69c5521 100644
--- a/pw_span/public/pw_span/span.h
+++ b/pw_span/public/pw_span/span.h
@@ -42,6 +42,6 @@
 // If std::span is not available, use Pigweed's span implementation.
-#include "pw_span/internal/span_impl.h"
+#include "pw_span/internal/span_impl.h"  // IWYU pragma: export
 #endif  // defined(__cpp_lib_span) && __cpp_lib_span >= 202002L