From b1341db700dc280c0caa3cb5a652c110bdde08ee Mon Sep 17 00:00:00 2001
From: Corentin Wallez <cwallez@chromium.org>
Date: Wed, 23 May 2018 16:56:55 -0400
Subject: [PATCH] Add a NXT_NO_DISCARD that is C++17 [[nodiscard]] where
 available

---
 CMakeLists.txt        |  7 +++++++
 src/common/Compiler.h | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 83bea7193e..af9b75eb95 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -49,6 +49,7 @@ option(NXT_ENABLE_NULL "Enable compilation of the Null backend" ON)
 option(NXT_ENABLE_OPENGL "Enable compilation of the OpenGL backend" ON)
 option(NXT_ENABLE_VULKAN "Enable compilation of the Vulkan backend" OFF)
 option(NXT_ALWAYS_ASSERT "Enable assertions on all build types" OFF)
+option(NXT_USE_CPP17 "Use some optional C++17 features for compile-time checks" OFF)
 
 ################################################################################
 # Precompute compile flags and defines, functions to set them
@@ -64,6 +65,12 @@ set(NXT_ENABLE_ASSERTS $<OR:$<CONFIG:Debug>,$<BOOL:${NXT_ALWAYS_ASSERT}>>)
 
 list(APPEND NXT_DEFS $<${NXT_ENABLE_ASSERTS}:NXT_ENABLE_ASSERTS>)
 
+if (NXT_USE_CPP17)
+    list(APPEND NXT_INTERNAL_DEFS "NXT_CPP_VERSION=17")
+else()
+    list(APPEND NXT_INTERNAL_DEFS "NXT_CPP_VERSION=14")
+endif()
+
 if (NXT_ENABLE_D3D12)
     list(APPEND NXT_INTERNAL_DEFS "NXT_ENABLE_BACKEND_D3D12")
 endif()
diff --git a/src/common/Compiler.h b/src/common/Compiler.h
index ba7aa48d9e..8b1cb333fe 100644
--- a/src/common/Compiler.h
+++ b/src/common/Compiler.h
@@ -19,6 +19,7 @@
 //  - NXT_COMPILER_[CLANG|GCC|MSVC]: Compiler detection
 //  - NXT_BREAKPOINT(): Raises an exception and breaks in the debugger
 //  - NXT_BUILTIN_UNREACHABLE(): Hints the compiler that a code path is unreachable
+//  - NXT_NO_DISCARD: An attribute that is C++17 [[nodiscard]] where available
 
 // Clang and GCC
 #if defined(__GNUC__)
@@ -36,6 +37,19 @@
 
 #    define NXT_BUILTIN_UNREACHABLE() __builtin_unreachable()
 
+#    if !defined(__has_cpp_attribute)
+#        define __has_cpp_attribute(name) 0
+#    endif
+
+// Use warn_unused_result on clang otherwise we can a c++1z extension warning in C++14 mode
+// Also avoid warn_unused_result with GCC because it is only a function attribute and not a type
+// attribute.
+#    if __has_cpp_attribute(warn_unused_result) && defined(__clang__)
+#        define NXT_NO_DISCARD __attribute__((warn_unused_result))
+#    elif NXT_CPP_VERSION >= 17 && __has_cpp_attribute(nodiscard)
+#        define NXT_NO_DISCARD [[nodiscard]]
+#    endif
+
 // MSVC
 #elif defined(_MSC_VER)
 #    define NXT_COMPILER_MSVC
@@ -45,8 +59,18 @@ extern void __cdecl __debugbreak(void);
 
 #    define NXT_BUILTIN_UNREACHABLE() __assume(false)
 
+// Visual Studio 2017 15.3 adds support for [[nodiscard]]
+#    if _MSC_VER >= 1911 && NXT_CPP_VERSION >= 17
+#        define NXT_NO_DISCARD [[nodiscard]]
+#    endif
+
 #else
 #    error "Unsupported compiler"
 #endif
 
+// Add noop replacements for macros for features that aren't supported by the compiler.
+#if !defined(NXT_NO_DISCARD)
+#    define NXT_NO_DISCARD
+#endif
+
 #endif  // COMMON_COMPILER_H_