Add extra support for symbol exporting in modularized projects

Since modules don't see each other's macros, we can use a single, keyword-
like macro for dll-exporting that is managed by the build system (so no
need for an "export" header). For example:

cxx.features.symexport = true

export __symexport void f ();

6 files changed, 74 insertions, 4 deletions

diff --git a/build2/c/init.cxx b/build2/c/init.cxx
index 3461086..159c0b8 100644
--- a/build2/c/init.cxx
+++ b/build2/c/init.cxx
@@ -264,6 +264,7 @@ namespace build2
         cm.tstd,
 
         false, // No C modules yet.
+        false, // No __symexport support since no modules.
 
         cast_null<process_path> (rs["pkgconfig.path"]),
         cast<dir_paths>         (rs[cm.x_sys_lib_dirs]),
diff --git a/build2/cc/common.hxx b/build2/cc/common.hxx
index 24262b7..abeadfc 100644
--- a/build2/cc/common.hxx
+++ b/build2/cc/common.hxx
@@ -115,7 +115,8 @@ namespace build2
 
       const strings& tstd;         // Translated x_std value (options).
 
-      bool modules;                // x.feaures.modules
+      bool modules;                // x.features.modules
+      bool symexport;              // x.features.symexport
 
       const process_path* pkgconfig; // pkgconfig.path (can be NULL).
       const dir_paths& sys_lib_dirs; // x.sys_lib_dirs
@@ -159,6 +160,7 @@ namespace build2
             const target_triplet& tg,
             const strings& std,
             bool fm,
+            bool fs,
             const process_path* pkgc,
             const dir_paths& sld,
             const dir_paths& sid,
@@ -175,6 +177,7 @@ namespace build2
             ctg (tg), tsys (ctg.system), tclass (ctg.class_),
             tstd (std),
             modules (fm),
+            symexport (fs),
             pkgconfig (pkgc), sys_lib_dirs (sld), sys_inc_dirs (sid),
             x_src (src), x_mod (mod), x_hdr (hdr), x_inc (inc) {}
     };
diff --git a/build2/cc/compile.cxx b/build2/cc/compile.cxx
index dc669ce..eddd465 100644
--- a/build2/cc/compile.cxx
+++ b/build2/cc/compile.cxx
@@ -196,6 +196,18 @@ namespace build2
       return nullptr;
     }
 
+    inline void compile::
+    append_symexport_options (cstrings& args, const target& t) const
+    {
+      // With VC if a BMI is compiled with dllexport, then when such BMI is
+      // imported, it is auto-magically treated as dllimport. Let's hope
+      // other compilers follow suit.
+      //
+      args.push_back (t.is_a<bmis> () && tclass == "windows"
+                      ? "-D__symexport=__declspec(dllexport)"
+                      : "-D__symexport=");
+    }
+
     match_result compile::
     match (action act, target& t, const string&) const
     {
@@ -683,10 +695,11 @@ namespace build2
         //
         sha256 cs;
 
-        // This affects how we compile the source as well as the format of
-        // depdb so factor it in.
+        // These flags affect how we compile the source and/or the format of
+        // depdb so factor them in.
         //
         cs.append (&md.pp, sizeof (md.pp));
+        cs.append (&symexport, sizeof (symexport));
 
         if (md.pp != preprocessed::all)
         {
@@ -1392,6 +1405,9 @@ namespace build2
             args.push_back (d.string ().c_str ());
           }
 
+          if (symexport && md.mod)
+            append_symexport_options (args, t);
+
           // Some compile options (e.g., -std, -m) affect the preprocessor.
           //
           // Currently Clang supports importing "header modules" even when in
@@ -2272,6 +2288,9 @@ namespace build2
             args.push_back (d.string ().c_str ());
           }
 
+          if (symexport && md.mod)
+            append_symexport_options (args, t);
+
           append_options (args, t, c_coptions);
           append_options (args, t, x_coptions);
           append_options (args, tstd,
@@ -3108,6 +3127,9 @@ namespace build2
           args.push_back ("-I");
           args.push_back (d.string ().c_str ());
         }
+
+        if (symexport && md.mod)
+          append_symexport_options (args, t);
       }
 
       append_options (args, t, c_coptions);
diff --git a/build2/cc/compile.hxx b/build2/cc/compile.hxx
index be4a2e2..ed9c18d 100644
--- a/build2/cc/compile.hxx
+++ b/build2/cc/compile.hxx
@@ -123,6 +123,9 @@ namespace build2
       const char*
       langopt (const match_data&) const;
 
+      void
+      append_symexport_options (cstrings&, const target&) const;
+
     private:
       const string rule_id;
     };
diff --git a/build2/cxx/init.cxx b/build2/cxx/init.cxx
index 2c59f2c..53ee2d2 100644
--- a/build2/cxx/init.cxx
+++ b/build2/cxx/init.cxx
@@ -55,8 +55,8 @@ namespace build2
         return var_pool.rw (rs).insert<bool> (v, variable_visibility::project);
       };
 
-      bool modules (false); auto& v_m (enter ("cxx.features.modules"));
       //bool concepts (false); auto& v_c (enter ("cxx.features.concepts"));
+      bool modules (false); auto& v_m (enter ("cxx.features.modules"));
 
       // Translate "latest" and "experimental" to the compiler/version-
       // appropriate option(s).
@@ -429,8 +429,16 @@ namespace build2
       if (!cast_false<bool> (rs["cxx.config.loaded"]))
         load_module (rs, rs, "cxx.config", loc, false, hints);
 
+      auto& vp (var_pool.rw (rs));
+
       bool modules (cast<bool> (rs["cxx.features.modules"]));
 
+      bool symexport (
+        modules &&
+        cast_false<bool> (
+          rs[vp.insert<bool> ("cxx.features.symexport",
+                              variable_visibility::project)]));
+
       config_module& cm (*rs.modules.lookup<config_module> ("cxx.config"));
 
       cc::data d {
@@ -451,6 +459,7 @@ namespace build2
         cm.tstd,
 
         modules,
+        symexport,
 
         cast_null<process_path> (rs["pkgconfig.path"]),
         cast<dir_paths> (rs[cm.x_sys_lib_dirs]),
diff --git a/tests/cc/modules/testscript b/tests/cc/modules/testscript
index 23f8946..a65bdac 100644
--- a/tests/cc/modules/testscript
+++ b/tests/cc/modules/testscript
@@ -14,6 +14,8 @@ EOI
 +cat <<EOI >=build/root.build
 cxx.std = experimental
 
+cxx.features.symexport = true
+
 # Force modules.
 #
 cxx.features.modules = true
@@ -313,3 +315,33 @@ $* test clean <<EOI
   exe{test}: cxx{driver} {mxx cxx}{core}
   bmie{foo-core}: mxx{foo-core}
   EOI
+
+: symexport
+:
+: Test the __symexport feature.
+:
+cat <<EOI >=core.mxx;
+  #if __cpp_modules >= 201704
+  export
+  #endif
+  module foo.core;
+
+  export __symexport int f (int);
+
+  __symexport int g_impl (int i) {return i - 1;}
+  export __symexport inline int g (int i) {return g_impl (i);}
+  EOI
+ln -s ../core.cxx core-f.cxx;
+cat <<EOI >=core-g.cxx;
+  module foo.core;
+  int g_impl (int i) {return i - 1;}
+  EOI
+cat <<EOI >=driver.cxx;
+  import foo.core;
+  int main (int argc, char*[]) {return f (argc) + g (argc);}
+  EOI
+$* test clean <<EOI
+  ./: lib{foo} exe{test} # Full build.
+  exe{test}: cxx{driver} lib{foo}
+  lib{foo}: mxx{core} cxx{core-f} # core-g @@ VC
+  EOI