Reland Support for mustache templating language (#132467)

The last version of this patch had memory leaks due to using the
BumpPtrAllocator for data types that required destructors to run to
release heap memory (e.g. via std::vector and std::string). This version
avoids that by using smart pointers, and dropping support for
BumpPtrAllocator.

We should refactor this code to use the BumpPtrAllocator again, but that
can be addressed in future patches, since those are more invasive
changes that need to refactor many of the core data types to avoid
owning allocations.

Adds Support for the Mustache Templating Language. See specs here:
https://mustache.github.io/mustache.5.html This patch implements
support+tests for majority of the features of the language including:

    - Variables
    - Comments
    - Lambdas
    - Sections

This meant as a library to support places where we have to generate
HTML, such as in clang-doc.

Co-authored-by: Peter Chou <[email protected]>

Author: ilovepi

llvm/include/llvm/Support/Mustache.h

@@ -0,0 +1,128 @@
+//===--- Mustache.h ---------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Implementation of the Mustache templating language supports version 1.4.2
+// currently relies on llvm::json::Value for data input.
+// See the Mustache spec for more information
+// (https://mustache.github.io/mustache.5.html).
+//
+// Current Features Supported:
+// - Variables
+// - Sections
+// - Inverted Sections
+// - Partials
+// - Comments
+// - Lambdas
+// - Unescaped Variables
+//
+// Features Not Supported:
+// - Set Delimiter
+// - Blocks
+// - Parents
+// - Dynamic Names
+//
+// The Template class is a container class that outputs the Mustache template
+// string and is the main class for users. It stores all the lambdas and the
+// ASTNode Tree. When the Template is instantiated it tokenizes the Template
+// String and creates a vector of Tokens. Then it calls a basic recursive
+// descent parser to construct the ASTNode Tree. The ASTNodes are all stored
+// in an arena allocator which is freed once the template class goes out of
+// scope.
+//
+// Usage:
+// \code
+//   // Creating a simple template and rendering it
+//   auto Template = Template("Hello, {{name}}!");
+//   Value Data = {{"name", "World"}};
+//   std::string Out;
+//   raw_string_ostream OS(Out);
+//   T.render(Data, OS);
+//   // Out == "Hello, World!"
+//
+//   // Creating a template with a partial and rendering it
+//   auto Template = Template("{{>partial}}");
+//   Template.registerPartial("partial", "Hello, {{name}}!");
+//   Value Data = {{"name", "World"}};
+//   std::string Out;
+//   raw_string_ostream OS(Out);
+//   T.render(Data, OS);
+//   // Out == "Hello, World!"
+//
+//   // Creating a template with a lambda and rendering it
+//   Value D = Object{};
+//   auto T = Template("Hello, {{lambda}}!");
+//   Lambda L = []() -> llvm::json::Value { return "World"; };
+//   T.registerLambda("lambda", L);
+//   std::string Out;
+//   raw_string_ostream OS(Out);
+//   T.render(D, OS);
+//   // Out == "Hello, World!"
+// \endcode
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_MUSTACHE
+#define LLVM_SUPPORT_MUSTACHE
+
+#include "Error.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/JSON.h"
+#include "llvm/Support/StringSaver.h"
+#include <functional>
+#include <vector>
+
+namespace llvm::mustache {
+
+using Lambda = std::function<llvm::json::Value()>;
+using SectionLambda = std::function<llvm::json::Value(std::string)>;
+
+class ASTNode;
+using AstPtr = std::unique_ptr<ASTNode>;
+
+// A Template represents the container for the AST and the partials
+// and Lambdas that are registered with it.
+class Template {
+public:
+  Template(StringRef TemplateStr);
+
+  Template(const Template &) = delete;
+
+  Template &operator=(const Template &) = delete;
+
+  Template(Template &&Other) noexcept;
+
+  // Define this in the cpp file to  work around ASTNode being an incomplete
+  // type.
+  ~Template();
+
+  Template &operator=(Template &&Other) noexcept;
+
+  void render(const llvm::json::Value &Data, llvm::raw_ostream &OS);
+
+  void registerPartial(std::string Name, std::string Partial);
+
+  void registerLambda(std::string Name, Lambda Lambda);
+
+  void registerLambda(std::string Name, SectionLambda Lambda);
+
+  // By default the Mustache Spec Specifies that HTML special characters
+  // should be escaped. This function allows the user to specify which
+  // characters should be escaped.
+  void overrideEscapeCharacters(DenseMap<char, std::string> Escapes);
+
+private:
+  StringMap<AstPtr> Partials;
+  StringMap<Lambda> Lambdas;
+  StringMap<SectionLambda> SectionLambdas;
+  DenseMap<char, std::string> Escapes;
+  AstPtr Tree;
+};
+} // namespace llvm::mustache
+
+#endif // LLVM_SUPPORT_MUSTACHE

llvm/lib/Support/CMakeLists.txt

@@ -220,6 +220,7 @@ add_llvm_component_library(LLVMSupport
   MD5.cpp
   MSP430Attributes.cpp
   MSP430AttributeParser.cpp
+  Mustache.cpp      
   NativeFormatting.cpp
   OptimizedStructLayout.cpp
   Optional.cpp