Add --require-explicit-ids to require explicit ids (#6277)

* Add --require-explicit-ids to require explicit ids

We just got bit by a well intentioned developer forgetting that field
order by default is the field index.  3 people missed it in review.

I'm looking at ways to make it harder to mess up.  We are requesting
that developers explicitly id all fields in tables.  Automatic (opt in
for others) enforcement of this will help the effort succeed.  This
patch adds a command line flag which lets the user require ids on all
fields in tables.

* Added docs to Compiler.md as well

docs/source/Compiler.md

@@ -218,6 +218,8 @@ Additional options:
 
 -   `--root-type T` : Select or override the default root_type.
 
+-   `--require-explicit-ids` : When parsing schemas, require explicit ids (id: x).
+
 -   `--force-defaults` : Emit default values in binary output from JSON.
 
 -   `--force-empty` : When serializing from object API representation, force

include/flatbuffers/idl.h

@@ -601,6 +601,9 @@ struct IDLOptions {
 
   MiniReflect mini_reflect;
 
+  // If set, require all fields in a table to be explicitly numbered.
+  bool require_explicit_ids;
+
   // The corresponding language bit will be set if a language is included
   // for code generation.
   unsigned long lang_to_generate;
@@ -661,6 +664,7 @@ struct IDLOptions {
         filename_extension(),
         lang(IDLOptions::kJava),
         mini_reflect(IDLOptions::kNone),
+        require_explicit_ids(false),
         lang_to_generate(0),
         set_empty_strings_to_null(true),
         set_empty_vectors_to_null(true) {}

src/flatc.cpp

@@ -163,6 +163,7 @@ std::string FlatCompiler::GetUsageString(const char *program_name) const {
     "  --reflect-types        Add minimal type reflection to code generation.\n"
     "  --reflect-names        Add minimal type/name reflection.\n"
     "  --root-type T          Select or override the default root_type\n"
+    "  --require-explicit-ids When parsing schemas, require explicit ids (id: x).\n"
     "  --force-defaults       Emit default values in binary output from JSON\n"
     "  --force-empty          When serializing from object API representation,\n"
     "                         force strings and vectors to empty rather than null.\n"
@@ -345,6 +346,8 @@ int FlatCompiler::Compile(int argc, const char **argv) {
         opts.mini_reflect = IDLOptions::kTypes;
       } else if (arg == "--reflect-names") {
         opts.mini_reflect = IDLOptions::kTypesAndNames;
+      } else if (arg == "--require-explicit-ids") {
+        opts.require_explicit_ids = true;
       } else if (arg == "--root-type") {
         if (++argi >= argc) Error("missing type following: " + arg, true);
         opts.root_type = argv[argi];

src/idl_parser.cpp

@@ -2379,11 +2379,18 @@ CheckedError Parser::ParseDecl() {
       if ((*it)->attributes.Lookup("id")) num_id_fields++;
     }
     // If any fields have ids..
-    if (num_id_fields) {
+    if (num_id_fields || opts.require_explicit_ids) {
       // Then all fields must have them.
-      if (num_id_fields != fields.size())
-        return Error(
-            "either all fields or no fields must have an 'id' attribute");
+      if (num_id_fields != fields.size()) {
+        if (opts.require_explicit_ids) {
+          return Error(
+              "all fields must have an 'id' attribute when "
+              "--require-explicit-ids is used");
+        } else {
+          return Error(
+              "either all fields or no fields must have an 'id' attribute");
+        }
+      }
       // Simply sort by id, then the fields are the same as if no ids had
       // been specified.
       std::sort(fields.begin(), fields.end(), compareFieldDefs);