add df.global:_field method. add test case to check for overlapping globals.

library/LuaTypes.cpp

@@ -633,6 +633,20 @@ static int meta_struct_field_reference(lua_State *state)
     return 1;
 }
 
+/**
+ * Method: _field for df.global.
+ */
+static int meta_global_field_reference(lua_State *state)
+{
+    if (lua_gettop(state) != 2)
+        luaL_error(state, "Usage: object._field(name)");
+    auto field = (struct_field_info*)find_field(state, 2, "reference");
+    if (!field)
+        field_error(state, 2, "builtin property or method", "reference");
+    field_reference(state, field, *(void**)field->offset);
+    return 1;
+}
+
 /**
  * Metamethod: __newindex for structures.
  */
@@ -1410,7 +1424,10 @@ void struct_identity::build_metatable(lua_State *state)
 
 void global_identity::build_metatable(lua_State *state)
 {
+    int base = lua_gettop(state);
     MakeFieldMetatable(state, this, meta_global_index, meta_global_newindex, true);
+    SetStructMethod(state, base+1, base+2, meta_global_field_reference, "_field");
+    SetPtrMethods(state, base+1, base+2);
 }
 
 /**

test/library/structures.lua

@@ -0,0 +1,23 @@
+function test.overlappingGlobals()
+    local globals = {}
+    for name, _ in pairs(df.global) do
+        local gvar = df.global:_field(name)
+        local size, addr = gvar:sizeof()
+        table.insert(globals, {
+            name = name,
+            first = addr,
+            last = addr + size - 1
+        })
+    end
+
+    table.sort(globals, function(a, b)
+        return a.first < b.first
+    end)
+
+    for i = 2, #globals do
+        local prev = globals[i - 1]
+        local cur = globals[i]
+
+        expect.true_(prev.last < cur.first, "global variable " .. prev.name .. " overlaps global variable " .. cur.name)
+    end
+end