25 files changed, 97 insertions, 101 deletions
diff --git a/libbuild2/algorithm.hxx b/libbuild2/algorithm.hxx
index f105fbe..5000c47 100644
--- a/libbuild2/algorithm.hxx
+++ b/libbuild2/algorithm.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_ALGORITHM_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/action.hxx>
@@ -15,11 +16,6 @@
 
 namespace build2
 {
-  class scope;
-  class context;
-  class prerequisite;
-  class prerequisite_key;
-
   // The default prerequisite search implementation. It first calls the
   // prerequisite-type-specific search function. If that doesn't yeld
   // anything, it creates a new target.
diff --git a/libbuild2/cc/utility.hxx b/libbuild2/cc/utility.hxx
index 3ee07bd..2dd06a8 100644
--- a/libbuild2/cc/utility.hxx
+++ b/libbuild2/cc/utility.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_CC_UTILITY_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/target.hxx>
@@ -15,8 +16,6 @@
 
 namespace build2
 {
-  struct variable;
-
   namespace cc
   {
     // To form the complete path do:
diff --git a/libbuild2/config/utility.hxx b/libbuild2/config/utility.hxx
index cfb7177..2a0909f 100644
--- a/libbuild2/config/utility.hxx
+++ b/libbuild2/config/utility.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_CONFIG_UTILITY_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/scope.hxx>
@@ -16,8 +17,6 @@
 
 namespace build2
 {
-  class scope;
-
   namespace config
   {
     // Set, if necessary, a required config.* variable.
diff --git a/libbuild2/context.hxx b/libbuild2/context.hxx
index 75147ef..08b023e 100644
--- a/libbuild2/context.hxx
+++ b/libbuild2/context.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_CONTEXT_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 // NOTE: this file is included by pretty much every other build state header
@@ -20,26 +21,6 @@
 
 namespace build2
 {
-  class context;
-
-  class scope;
-  class scope_map;
-  class target_set;
-  class target_type_map;
-
-  class value;
-  using values = small_vector<value, 1>;
-
-  struct variable;
-  class variable_pool;
-  struct variable_override;
-  using variable_overrides = vector<variable_override>;
-  class variable_override_cache;
-
-  class function_map;
-
-  struct opspec;
-
   class loaded_modules_lock;
 
   class LIBBUILD2_SYMEXPORT run_phase_mutex
diff --git a/libbuild2/diagnostics.hxx b/libbuild2/diagnostics.hxx
index 26f602a..6d06dec 100644
--- a/libbuild2/diagnostics.hxx
+++ b/libbuild2/diagnostics.hxx
@@ -8,6 +8,7 @@
 #include <libbutl/diagnostics.mxx>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/export.hxx>
@@ -441,11 +442,6 @@ namespace build2
   //
   // info << "while " << diag_doing (a, t);
   //
-  class scope;
-  class target;
-  class context;
-  struct action;
-
   struct diag_phrase
   {
     const action& a;
diff --git a/libbuild2/dump.hxx b/libbuild2/dump.hxx
index aead805..d3e3239 100644
--- a/libbuild2/dump.hxx
+++ b/libbuild2/dump.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_DUMP_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/action.hxx>
@@ -14,10 +15,6 @@
 
 namespace build2
 {
-  class scope;
-  class target;
-  class context;
-
   // Dump the build state to diag_stream. If action is specified, then assume
   // rules have been matched for this action and dump action-specific
   // information (like rule-specific variables).
diff --git a/libbuild2/file.hxx b/libbuild2/file.hxx
index 009ba14..3d3c38b 100644
--- a/libbuild2/file.hxx
+++ b/libbuild2/file.hxx
@@ -8,6 +8,7 @@
 #include <map>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/scope.hxx>
@@ -17,10 +18,6 @@
 
 namespace build2
 {
-  class target;
-  class location;
-  class prerequisite_key;
-
   using subprojects = std::map<project_name, dir_path>;
 
   LIBBUILD2_SYMEXPORT ostream&
diff --git a/libbuild2/forward.hxx b/libbuild2/forward.hxx
new file mode 100644
index 0000000..1c1eb77
--- /dev/null
+++ b/libbuild2/forward.hxx
@@ -0,0 +1,73 @@
+// file      : libbuild2/forward.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_FORWARD_HXX
+#define LIBBUILD2_FORWARD_HXX
+
+#include <libbuild2/types.hxx>
+
+namespace build2
+{
+  // Forward declarations for the build state.
+
+  // <libbuild2/action.hxx>
+  //
+  struct action;
+
+  // <libbuild2/spec.hxx>
+  //
+  struct opspec;
+
+  // <libbuild2/variable.hxx>
+  //
+  class value;
+  using values = small_vector<value, 1>;
+  struct lookup;
+
+  struct variable;
+  class variable_pool;
+  class variable_map;
+  struct variable_override;
+  using variable_overrides = vector<variable_override>;
+  class variable_override_cache;
+
+  // <libbuild2/function.hxx>
+  //
+  class function_map;
+
+  // <libbuild2/scope.hxx>
+  //
+  class scope;
+  class scope_map;
+
+  // <libbuild2/target-type.hxx>
+  //
+  class target_type_map;
+
+  // <libbuild2/target-key.hxx>
+  //
+  class target_key;
+
+  // <libbuild2/target.hxx>
+  //
+  class target;
+  class target_set;
+  class include_type;
+  struct prerequisite_member;
+
+  // <libbuild2/prerequisite.hxx>
+  //
+  class prerequisite;
+  class prerequisite_key;
+
+  // <libbuild2/rule.hxx>
+  //
+  class rule;
+
+  // <libbuild2/context.hxx>
+  //
+  class context;
+}
+
+#endif // LIBBUILD2_FORWARD_HXX
diff --git a/libbuild2/function.hxx b/libbuild2/function.hxx
index d1de4ac..dd31024 100644
--- a/libbuild2/function.hxx
+++ b/libbuild2/function.hxx
@@ -10,6 +10,7 @@
 #include <type_traits>   // aligned_storage
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/variable.hxx>
diff --git a/libbuild2/module.hxx b/libbuild2/module.hxx
index 50ca7da..34c7245 100644
--- a/libbuild2/module.hxx
+++ b/libbuild2/module.hxx
@@ -8,6 +8,7 @@
 #include <map>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/context.hxx>
@@ -24,9 +25,6 @@ namespace build2
   // module is "bootstrapped" (or "booted" for short) and then "initialized"
   // (or "inited").
 
-  class scope;
-  class location;
-
   class module_base
   {
   public:
diff --git a/libbuild2/operation.hxx b/libbuild2/operation.hxx
index 125c692..7b6310f 100644
--- a/libbuild2/operation.hxx
+++ b/libbuild2/operation.hxx
@@ -8,6 +8,7 @@
 #include <libbutl/string-table.mxx>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/action.hxx>
@@ -17,19 +18,6 @@
 
 namespace build2
 {
-  class location;
-  class scope;
-  class target;
-  class target_key;
-  class context;
-  class include_type;
-  struct prerequisite_member;
-
-  class value;
-  using values = small_vector<value, 1>;
-
-  struct opspec;
-
   // Meta-operation info.
   //
 
diff --git a/libbuild2/parser.hxx b/libbuild2/parser.hxx
index d0e5b57..8b6179c 100644
--- a/libbuild2/parser.hxx
+++ b/libbuild2/parser.hxx
@@ -8,6 +8,7 @@
 #include <stack>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/spec.hxx>
@@ -20,11 +21,6 @@
 
 namespace build2
 {
-  class scope;
-  class target;
-  class context;
-  class prerequisite;
-
   class LIBBUILD2_SYMEXPORT parser
   {
   public:
diff --git a/libbuild2/prerequisite.hxx b/libbuild2/prerequisite.hxx
index fe6d10a..c826e3b 100644
--- a/libbuild2/prerequisite.hxx
+++ b/libbuild2/prerequisite.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_PREREQUISITE_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/scope.hxx>
@@ -18,8 +19,6 @@
 
 namespace build2
 {
-  class target;
-
   // Light-weight (by being shallow-pointing) prerequisite key, similar
   // to (and based on) target key.
   //
diff --git a/libbuild2/rule-map.hxx b/libbuild2/rule-map.hxx
index 26f80a8..7994583 100644
--- a/libbuild2/rule-map.hxx
+++ b/libbuild2/rule-map.hxx
@@ -10,14 +10,13 @@
 #include <libbutl/prefix-map.mxx>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/action.hxx>
 
 namespace build2
 {
-  class rule;
-
   using hint_rule_map =
     butl::prefix_map<string, reference_wrapper<const rule>, '.'>;
 
diff --git a/libbuild2/rule.hxx b/libbuild2/rule.hxx
index abd754e..88c6c7d 100644
--- a/libbuild2/rule.hxx
+++ b/libbuild2/rule.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_RULE_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/action.hxx>
diff --git a/libbuild2/scope.hxx b/libbuild2/scope.hxx
index b29a981..2737958 100644
--- a/libbuild2/scope.hxx
+++ b/libbuild2/scope.hxx
@@ -9,6 +9,7 @@
 #include <unordered_set>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/module.hxx>
diff --git a/libbuild2/search.hxx b/libbuild2/search.hxx
index 63ea6b1..1b018c4 100644
--- a/libbuild2/search.hxx
+++ b/libbuild2/search.hxx
@@ -6,16 +6,13 @@
 #define LIBBUILD2_SEARCH_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/export.hxx>
 
 namespace build2
 {
-  class target;
-  class context;
-  class prerequisite_key;
-
   // Search for an existing target in this prerequisite's scope. Scope can be
   // NULL if directories are absolute.
   //
diff --git a/libbuild2/spec.hxx b/libbuild2/spec.hxx
index b18f665..b8e65fe 100644
--- a/libbuild2/spec.hxx
+++ b/libbuild2/spec.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_SPEC_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/variable.hxx>
@@ -14,8 +15,6 @@
 
 namespace build2
 {
-  class scope;
-
   struct targetspec
   {
     typedef build2::name name_type;
diff --git a/libbuild2/target-key.hxx b/libbuild2/target-key.hxx
index 4df2b1f..dd937c9 100644
--- a/libbuild2/target-key.hxx
+++ b/libbuild2/target-key.hxx
@@ -8,9 +8,8 @@
 #include <map>
 #include <cstring> // strcmp()
 
-#include <libbutl/utility.mxx> // compare_c_string
-
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/target-type.hxx>
diff --git a/libbuild2/target-type.hxx b/libbuild2/target-type.hxx
index 3325df1..120e6e0 100644
--- a/libbuild2/target-type.hxx
+++ b/libbuild2/target-type.hxx
@@ -8,18 +8,13 @@
 #include <map>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/export.hxx>
 
 namespace build2
 {
-  class scope;
-  class target;
-  class context;
-  class target_key;
-  class prerequisite_key;
-
   // Target type.
   //
   // Note that we assume there is always a single instance of this class for
diff --git a/libbuild2/target.hxx b/libbuild2/target.hxx
index e91c41a..258daef 100644
--- a/libbuild2/target.hxx
+++ b/libbuild2/target.hxx
@@ -12,6 +12,7 @@
 #include <libbutl/multi-index.mxx> // map_iterator_adapter
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/scope.hxx>
@@ -27,9 +28,6 @@
 
 namespace build2
 {
-  class rule;
-  class target;
-
   // From <libbuild2/algorithm.hxx>.
   //
   LIBBUILD2_SYMEXPORT const target&
diff --git a/libbuild2/test/script/parser.hxx b/libbuild2/test/script/parser.hxx
index 4d3fa49..c8a1408 100644
--- a/libbuild2/test/script/parser.hxx
+++ b/libbuild2/test/script/parser.hxx
@@ -6,6 +6,7 @@
 #define LIBBUILD2_TEST_SCRIPT_PARSER_HXX
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/parser.hxx>
@@ -16,8 +17,6 @@
 
 namespace build2
 {
-  class context;
-
   namespace test
   {
     namespace script
diff --git a/libbuild2/test/script/script.hxx b/libbuild2/test/script/script.hxx
index 8b34be8..30bc6e8 100644
--- a/libbuild2/test/script/script.hxx
+++ b/libbuild2/test/script/script.hxx
@@ -8,6 +8,7 @@
 #include <set>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/variable.hxx>
@@ -18,8 +19,6 @@
 
 namespace build2
 {
-  class target;
-
   namespace test
   {
     namespace script
diff --git a/libbuild2/utility.hxx b/libbuild2/utility.hxx
index 1d5c7bb..43b556e 100644
--- a/libbuild2/utility.hxx
+++ b/libbuild2/utility.hxx
@@ -21,6 +21,7 @@
 #include <unordered_set>
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 
 // "Fake" version values used during bootstrap.
 //
@@ -157,8 +158,6 @@ namespace build2
   LIBBUILD2_SYMEXPORT extern optional<path> config_sub;   // --config-sub
   LIBBUILD2_SYMEXPORT extern optional<path> config_guess; // --config-guess
 
-  class location;
-
   LIBBUILD2_SYMEXPORT void
   check_build_version (const standard_version_constraint&, const location&);
 
@@ -502,8 +501,6 @@ namespace build2
   //
   // If excl is not NULL, then filter this option out (note: case sensitive).
   //
-  struct variable;
-
   template <typename T>
   void
   append_options (cstrings&, T&, const variable&, const char* excl = nullptr);
@@ -530,9 +527,6 @@ namespace build2
 
   // As above but from the strings value directly.
   //
-  class value;
-  struct lookup;
-
   LIBBUILD2_SYMEXPORT void
   append_options (cstrings&, const lookup&, const char* excl = nullptr);
 
diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index 51cdf49..6a2c681 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -14,6 +14,7 @@
 #include <libbutl/multi-index.mxx> // map_key
 
 #include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
 #include <libbuild2/utility.hxx>
 
 #include <libbuild2/context.hxx>
@@ -34,10 +35,6 @@ namespace build2
   //    during a phase other than load must make sure the variable has already
   //    been typified.
 
-  class value;
-  struct variable;
-  struct lookup;
-
   struct value_type
   {
     const char* name;  // Type name for diagnostics.
@@ -481,8 +478,6 @@ namespace build2
   // A variable can be undefined, NULL, or contain a (potentially empty)
   // value.
   //
-  class variable_map;
-
   struct lookup
   {
     using value_type = build2::value;