Dynamically generate field info in __index

Constructing the complete tables when the types were initialized made it
impossible to populate the "type" field, because not all types had been added to
the global type tables yet.

library/LuaTypes.cpp

@@ -1367,6 +1367,11 @@ static void IndexFields(lua_State *state, int base, struct_identity *pstruct, in
 
 static void PushFieldInfoSubTable(lua_State *state, const struct_field_info *field)
 {
+    if (!field) {
+        lua_pushnil(state);
+        return;
+    }
+
     lua_newtable(state);    // new field info
     Lua::TableInsert(state, "mode", field->mode);
     Lua::TableInsert(state, "name", field->name);
@@ -1401,6 +1406,51 @@ static void PushFieldInfoSubTable(lua_State *state, const struct_field_info *fie
     // freeze_table(state);  // TODO: make pairs() work
 }
 
+/**
+ * Metamethod: __index for struct._fields
+ *
+ * upvalue 1: name -> struct_field_info* table
+ */
+static int meta_fieldinfo_index(lua_State *state)
+{
+    luaL_checktype(state, -1, LUA_TSTRING);
+
+    lua_gettable(state, lua_upvalueindex(1));
+    auto field = static_cast<const struct_field_info*>(lua_touserdata(state, -1));
+    lua_pop(state, 1);
+    PushFieldInfoSubTable(state, field);
+
+    return 1;
+}
+
+/**
+ * Metamethod: iterator for struct._fields
+ *
+ * upvalue 1: name -> struct_field_info* table
+ * upvalue 3: field table (int <-> name)
+ */
+static int meta_fieldinfo_next(lua_State *state)
+{
+    if (lua_gettop(state) < 2) lua_pushnil(state);
+
+    int len = lua_rawlen(state, UPVAL_FIELDTABLE);
+    int idx = cur_iter_index(state, len+1, 2, 0);
+    if (idx == len)
+        return 0;
+
+    lua_rawgeti(state, UPVAL_FIELDTABLE, idx+1);
+
+    // modified from meta_struct_next:
+    // retrieve the struct_field_info* from the table and convert it
+    lua_dup(state);
+    lua_gettable(state, lua_upvalueindex(1));
+    auto field = static_cast<const struct_field_info*>(lua_touserdata(state, -1));
+    lua_pop(state, 1);
+    PushFieldInfoSubTable(state, field);
+
+    return 2;
+}
+
 static void AddFieldInfoTable(lua_State *state, int ftable_idx, struct_identity *pstruct)
 {
     Lua::StackUnwinder base{state};
@@ -1418,7 +1468,10 @@ static void AddFieldInfoTable(lua_State *state, int ftable_idx, struct_identity
     int ix_fielditer = lua_gettop(state);
     IndexFields(state, base, pstruct, IndexFieldsFlags::RAW);
 
-    PushStructMethod(state, ix_meta, ix_fielditer, meta_struct_next);
+    PushStructMethod(state, ix_meta, ix_fielditer, meta_fieldinfo_next);
+    // change upvalue 1 to the field info table since we don't need the original
+    lua_pushvalue(state, ix_fieldinfo);
+    lua_setupvalue(state, -2, 1);
     SetPairsMethod(state, ix_meta, "__pairs");
 
     // field table (name -> table representation of struct_field_info)
@@ -1432,7 +1485,8 @@ static void AddFieldInfoTable(lua_State *state, int ftable_idx, struct_identity
     // set up metatable for the wrapper
     // use field table for __index
     lua_pushstring(state, "__index");
-    lua_pushvalue(state, ix_fields);
+    lua_pushvalue(state, ix_fieldinfo);
+    lua_pushcclosure(state, meta_fieldinfo_index, 1);
     lua_settable(state, ix_meta);
 
     // use change_error() for __newindex

test/structures/struct_fields.lua

@@ -60,3 +60,15 @@ function test.readonly()
     expect.eq(df.coord._fields.x.name, 'x')
     expect.nil_(df.coord._fields.nonexistent)
 end
+
+function test.circular_refs_init()
+    expect.eq(df.job._fields.list_link.type, df.job_list_link)
+    expect.eq(df.job_list_link._fields.item.type, df.job)
+end
+
+function test.subclass_match()
+    for f, parent in pairs(df.viewscreen._fields) do
+        local child = df.viewscreen_titlest._fields[f]
+        expect.table_eq(parent, child, f)
+    end
+end