Include Seastar headers in module purview for automatic export

Changed from using 'export namespace seastar {}' to including Seastar headers
in the module purview (after 'export module'). This automatically exports:
- All symbols in the seastar namespace
- std::hash specializations (marked with SEASTAR_MODULE_EXPORT in headers)

Third-party headers (std, boost, system) remain in global module fragment
and are NOT exported. This matches the intended behavior and avoids the need
for explicit 'using' declarations for every symbol.

Co-authored-by: avikivity <[email protected]>

Author: Copilot

src/seastar.cppm

@@ -21,15 +21,17 @@
 
 // C++20 module implementation for Seastar
 // 
-// This file follows the libstdc++ approach where all headers are included
-// in the global module fragment, and then we use export declarations to
-// re-export the seastar namespace as a module. This avoids putting module
-// declarations inside #ifdef blocks, which is illegal per the C++ standard.
+// This file follows the libstdc++ approach where third-party headers are
+// included in the global module fragment (and not exported), while Seastar
+// headers are included in the module purview (and automatically exported).
+// This avoids putting module declarations inside #ifdef blocks, which
+// is illegal per the C++ standard.
 
 module;
 
-// Put all headers into the global module fragment to prevent attachment
-// to the module. This includes both dependency headers and Seastar headers.
+// Put all third-party headers in the global module fragment to prevent
+// them from being attached to the module. Only Seastar headers will be
+// in the module purview and exported.
 
 #include <any>
 #include <array>
@@ -140,7 +142,11 @@ module;
 #include <ucontext.h>
 #include <unistd.h>
 
-// Include all Seastar public headers in the global module fragment
+export module seastar;
+
+// Include all Seastar public headers in the module purview to export them.
+// This automatically exports everything declared in these headers, including
+// the seastar namespace and std::hash specializations.
 #include <seastar/util/std-compat.hh>
 #include <seastar/core/abortable_fifo.hh>
 #include <seastar/core/abort_on_ebadf.hh>
@@ -293,11 +299,6 @@ module;
 #include <seastar/json/formatter.hh>
 #include <seastar/json/json_elements.hh>
 
-export module seastar;
-
-// Export the seastar namespace and all its symbols
-export namespace seastar {}
-
 module : private;
 
 // Include implementation headers in the private module partition.