tbv2: update std::variant

`std::variant` is the last archetypal container missing in TreeBuilder-v2. The
code for it isn't hugely complicated and relies on pack expansion.

This change introduces a new field to the container specification:
`scoped_extra`. This field allows you to write extra code that will be included
within the TypeHandler in CodeGen. This means it will not have collisions with
other containers, unlike the existing `extra` field. It's used here to write
the recursive `getSizeType` function for `std::variant`.

Tech debt is introduced here by comparing the container name to `std::variant`
in CodeGen to conditionally generate some code. We've worked hard to remove
references to containers in code and move them to `.toml` files. On balance,
this is worth having to include the example of `std::variant`. It should be
moved into a container spec field at some point, the design of which is still
to be determined.

Test plan:
- Activated the OIL `std::variant` tests.
- CI

oi/CodeGen.cpp

@@ -941,6 +941,17 @@ void genContainerTypeHandler(std::unordered_set<const ContainerInfo*>& used,
     containerWithTypes = "OICaptureKeys<" + containerWithTypes + ">";
   }
 
+  // TODO: This is tech debt. This should be moved to a field in the container
+  // spec/`.toml` called something like `codegen.handler_header` or have an
+  // explicit option for variable template parameters. However I'm landing it
+  // anyway to demonstrate how to handle tagged unions in TreeBuilder-v2.
+  if (c.typeName == "std::variant") {
+    code += R"(
+template <typename Ctx, typename... Types>
+struct TypeHandler<Ctx, std::variant<Types...>> {
+  using container_type = std::variant<Types...>;
+)";
+  } else {
     code += "template <typename Ctx";
     types = 0, values = 0;
     for (const auto& p : templateParams) {
@@ -964,6 +975,11 @@ void genContainerTypeHandler(std::unordered_set<const ContainerInfo*>& used,
     code += "struct TypeHandler<Ctx, ";
     code += containerWithTypes;
     code += "> {\n";
+    code += "  using container_type = ";
+    code += containerWithTypes;
+    code += ";\n";
+  }
+
   code += "  using DB = typename Ctx::DataBuffer;\n";
 
   if (c.captureKeys) {
@@ -972,10 +988,6 @@ void genContainerTypeHandler(std::unordered_set<const ContainerInfo*>& used,
     code += "  static constexpr bool captureKeys = false;\n";
   }
 
-  code += "  using container_type = ";
-  code += containerWithTypes;
-  code += ";\n";
-
   code += "  using type = ";
   if (processors.empty()) {
     code += "types::st::Unit<DB>";
@@ -991,11 +1003,11 @@ void genContainerTypeHandler(std::unordered_set<const ContainerInfo*>& used,
   }
   code += ";\n";
 
+  code += c.codegen.scopedExtra;
+
   code += "  static types::st::Unit<DB> getSizeType(\n";
   code += "      Ctx& ctx,\n";
-  code += "      const ";
-  code += containerWithTypes;
-  code += "& container,\n";
+  code += "      const container_type& container,\n";
   code += "      typename TypeHandler<Ctx, ";
   code += containerWithTypes;
   code += ">::type returnArg) {\n";
@@ -1029,7 +1041,6 @@ void genContainerTypeHandler(std::unordered_set<const ContainerInfo*>& used,
     code += "},\n";
   }
   code += "  };\n";
-
   code += "};\n\n";
 }
 

oi/ContainerInfo.cpp

@@ -298,6 +298,10 @@ ContainerInfo::ContainerInfo(const fs::path& path) {
           codegenToml["extra"].value<std::string>()) {
     codegen.extra = std::move(*str);
   }
+  if (std::optional<std::string> str =
+          codegenToml["scoped_extra"].value<std::string>()) {
+    codegen.scopedExtra = std::move(*str);
+  }
 
   if (toml::array* arr = codegenToml["processor"].as_array()) {
     codegen.processors.reserve(arr->size());

oi/ContainerInfo.h

@@ -38,6 +38,7 @@ struct ContainerInfo {
     std::string func;
     std::string traversalFunc = "";
     std::string extra = "";
+    std::string scopedExtra = "";
     std::vector<Processor> processors{};
   };
 

test/integration/std_variant.toml

@@ -9,7 +9,6 @@ definitions = '''
 
 [cases]
   [cases.char_int64_1]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     param_types = ["const std::variant<char, int64_t>&"]
     setup = "return 'a';"
     expect_json = '''[{
@@ -23,7 +22,6 @@ definitions = '''
         {"typeName":"int8_t", "staticSize":1, "exclusiveSize":1, "dynamicSize":0}
       ]}]'''
   [cases.char_int64_2]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     param_types = ["const std::variant<char, int64_t>&"]
     setup = "return 1234;"
     expect_json = '''[{
@@ -38,7 +36,6 @@ definitions = '''
       ]}]'''
 
   [cases.vector_int_1]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     param_types = ["const std::variant<std::vector<int>, int>&"]
     setup = "return std::vector<int>{1,2,3};"
     expect_json = '''[{
@@ -60,7 +57,6 @@ definitions = '''
         }
       ]}]'''
   [cases.vector_int_2]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     param_types = ["const std::variant<std::vector<int>, int>&"]
     setup = "return 123;"
     expect_json = '''[{
@@ -78,7 +74,6 @@ definitions = '''
       ]}]'''
 
   [cases.optional]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     # This test case ensures that the alignment of std::variant is set
     # correctly, as otherwise the size of the std::optional would be wrong
     param_types = ["const std::optional<std::variant<char, int64_t>>&"]
@@ -105,7 +100,6 @@ definitions = '''
       ]}]'''
 
   [cases.empty]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     # https://en.cppreference.com/w/cpp/utility/variant/valueless_by_exception
     param_types = ["const std::variant<int, Thrower>&"]
     setup = '''
@@ -133,7 +127,6 @@ definitions = '''
   # 0xff can be a valid index if there are at least 256 parameters, and that
   # the invalid index value is raised to 0xffff.
   [cases.256_params_256]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     param_types = ["const std::variant<Thrower,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,char>&"]
     setup = "return 'a';"
     expect_json = '''[{
@@ -147,7 +140,6 @@ definitions = '''
         {"typeName":"int8_t", "staticSize":1, "exclusiveSize":1, "dynamicSize":0}
       ]}]'''
   [cases.256_params_empty]
-    oil_skip = "std::variant is not implemented for treebuilder v2" # https://github.com/facebookexperimental/object-introspection/issues/298
     param_types = ["const std::variant<Thrower,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,char>&"]
     setup = '''
       std::variant<Thrower,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,int,char> var{'a'};

types/README.md

@@ -55,6 +55,11 @@ This document describes the format of the container definition files contained i
   `static types::st::Unit<DB> getSizeType(const T& container, ST returnArg)`
   where `ST` defines the combined static type of each supplied processor.
 
+- `scoped_extra`
+
+  Extra C++ code to be included in the generated `TypeHandler` for this
+  container.
+
 - `extra`
 
   Any extra C++ code to be included directly. This code is not automatically

types/std_variant.toml

@@ -36,5 +36,54 @@ void getSizeType(const %1%<Types...> &container, size_t& returnArg)
 }
 """
 
-# TODO: Add tbv2 definitions. The removed intermediate handler is a good
-# template for this, find it in the git logs.
+scoped_extra = """
+template <size_t I = 0>
+static types::st::Unit<DB>
+getSizeTypeRecursive(
+    Ctx& ctx,
+    const typename container_type& container,
+    typename TypeHandler<Ctx, typename container_type>::type returnArg
+) {
+  if constexpr (I < sizeof...(Types)) {
+    if (I == container.index()) {
+      return returnArg.template delegate<I>([&ctx, &container](auto ret) {
+        return OIInternal::getSizeType<Ctx>(ctx, std::get<I>(container), ret);
+      });
+    } else {
+      return getSizeTypeRecursive<I+1>(ctx, container, returnArg);
+    }
+  } else {
+    return returnArg.template delegate<sizeof...(Types)>(std::identity());
+  }
+}
+"""
+
+handler_header = """
+template <typename Ctx, typename... Types>
+struct TypeHandler<Ctx, std::variant<Types...>>
+"""
+
+traversal_func = """
+return getSizeTypeRecursive(ctx, container, returnArg);
+"""
+
+[[codegen.processor]]
+type = "types::st::Sum<Ctx, typename TypeHandler<DB, Types>::type..., types::st::Unit<DB>>"
+func = """
+static constexpr std::array<inst::Field, sizeof...(Types)> children{
+  make_field<Ctx, Types>("*")...,
+};
+
+auto sum = std::get<ParsedData::Sum>(d.val);
+
+el.container_stats = result::Element::ContainerStats {
+  .capacity = 1,
+  .length = sum.index == sizeof...(Types) ? 0 : 1,
+};
+
+if (el.container_stats->length == 0)
+  return;
+
+el.exclusive_size -= children[sum.index].static_size;
+stack_ins(children[sum.index]);
+"""