Implement optional module loading

The syntax is:

using? cli

Now each module use results in two bool variables: <module>.loaded and
<module>.configured.

Also implement variable visibility (the above two variables are limited
to project).

1 files changed, 28 insertions, 9 deletions

diff --git a/build/module.cxx b/build/module.cxx
index 0f2b1b2..5f1aeff 100644
--- a/build/module.cxx
+++ b/build/module.cxx
@@ -7,6 +7,7 @@
 #include <utility> // make_pair()
 
 #include <build/scope>
+#include <build/variable>
 #include <build/diagnostics>
 
 using namespace std;
@@ -15,13 +16,16 @@ namespace build
 {
   available_module_map builtin_modules;
 
-  void
-  load_module (const string& name, scope& root, scope& base, const location& l)
+  bool
+  load_module (bool opt,
+               const string& name,
+               scope& rs,
+               scope& bs,
+               const location& loc)
   {
-    // First see if this modules has already been loaded for this
-    // project.
+    // First see if this modules has already been loaded for this project.
     //
-    loaded_module_map& lm (root.modules);
+    loaded_module_map& lm (rs.modules);
     auto i (lm.find (name));
     bool f (i == lm.end ());
 
@@ -32,11 +36,26 @@ namespace build
       auto j (builtin_modules.find (name));
 
       if (j == builtin_modules.end ())
-        fail (l) << "unknown module " << name;
-
-      i = lm.emplace (name, make_pair (j->second, nullptr)).first;
+      {
+        if (!opt)
+          fail (loc) << "unknown module " << name;
+      }
+      else
+        i = lm.emplace (name, make_pair (j->second, nullptr)).first;
     }
 
-    i->second.first (root, base, l, i->second.second, f);
+    bool l (i != lm.end ());
+    bool c (l && i->second.first (rs, bs, loc, i->second.second, f, opt));
+
+    const variable& lv (var_pool.find (name + ".loaded",
+                                       variable_visibility::project,
+                                       bool_type));
+    const variable& cv (var_pool.find (name + ".configured",
+                                       variable_visibility::project,
+                                       bool_type));
+    bs.assign (lv) = l;
+    bs.assign (cv) = c;
+
+    return l && c;
   }
 }