Implement rendering for partials

Summary:
This diff implements rendering for partial blocks and statements as defined in the spec.

To achieve this, we have an *implementation-defined* `partial_definition` class that is exposed to Whisker using a `native_handle<>`. I especially enjoy the demonstrate of the capabilities of partials in the [collatz conjecture](https://en.wikipedia.org/wiki/Collatz_conjecture) example.

Differential Revision: D68687668

fbshipit-source-id: c85e044109ec70ad8260056a23c7fb65332ffc5f

Author: praihan

thrift/compiler/whisker/eval_context.cc

@@ -104,11 +104,14 @@ object::ptr eval_context::lexical_scope::lookup_property(
   return find_property(*this_ref_, identifier);
 }
 
-eval_context::eval_context(map globals)
-    : global_scope_(manage_owned<object>(
-          w::make_native_object<global_scope_object>(std::move(globals)))),
+eval_context::eval_context(object::ptr globals)
+    : global_scope_(std::move(globals)),
       stack_({lexical_scope(global_scope_)}) {}
 
+eval_context::eval_context(map globals)
+    : eval_context(manage_owned<object>(
+          w::make_native_object<global_scope_object>(std::move(globals)))) {}
+
 /* static */ eval_context eval_context::with_root_scope(
     object::ptr root_scope, map globals) {
   eval_context result{std::move(globals)};

thrift/compiler/whisker/eval_context.h

@@ -23,6 +23,7 @@
 #include <deque>
 #include <functional>
 #include <string>
+#include <type_traits>
 #include <variant>
 #include <vector>
 
@@ -204,8 +205,8 @@ class eval_context {
    *   - eval_name_already_bound_error if the name already exists as a local in
    *     the current scope.
    */
-  expected<std::monostate, eval_name_already_bound_error> bind_local(
-      std::string name, object::ptr value);
+  [[nodiscard]] expected<std::monostate, eval_name_already_bound_error>
+  bind_local(std::string name, object::ptr value);
 
   /**
    * Performs a lexical search for an object by name (chain of properties)
@@ -228,7 +229,17 @@ class eval_context {
       std::variant<eval_scope_lookup_error, eval_property_lookup_error>>
   lookup_object(const std::vector<std::string>& path);
 
+  /**
+   * Creates a new "derived" eval_context from the current one.
+   *
+   * A derived eval_context has a fresh stack (empty) but retains the same
+   * global scope from the current one.
+   */
+  eval_context make_derived() const { return eval_context(global_scope_); }
+
  private:
+  explicit eval_context(object::ptr globals);
+
   /**
    * A lexical scope which determines how name lookups are performed within the
    * Whisker templating language.
@@ -285,5 +296,7 @@ class eval_context {
   // references passed around to those scope objects.
   std::deque<lexical_scope> stack_;
 };
+static_assert(std::is_copy_constructible_v<eval_context>);
+static_assert(std::is_move_constructible_v<eval_context>);
 
 } // namespace whisker