[clang] [CMake] Change GCC_INSTALL_PREFIX from warning to fatal error (PR #85891)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/85891

unless USE_DEPRECATED_GCC_INSTALL_PREFIX (temporary escape hatch) is
set. Setting GCC_INSTALL_PREFIX leads to a warning for Clang 18.1
(#77537) and will be completely removed for Clang 20.

Link: discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link: discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833


>From 14bf5c0a35de1b1eb2515cd5169b3b02034a2db4 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Tue, 19 Mar 2024 20:14:57 -0700
Subject: [PATCH] [CMake] Change GCC_INSTALL_PREFIX from warning to fatal error

unless USE_DEPRECATED_GCC_INSTALL_PREFIX (temporary escape hatch) is
set. Setting GCC_INSTALL_PREFIX leads to a warning for Clang 18.1
(#77537) and will be completely removed for Clang 20.

Link: discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link: discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833
---
 clang/CMakeLists.txt        | 5 +++--
 clang/docs/ReleaseNotes.rst | 3 +++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
index 47fc2e4886cfc2..761dab8c28c134 100644
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -190,11 +190,12 @@ set(CLANG_RESOURCE_DIR "" CACHE STRING
 set(C_INCLUDE_DIRS "" CACHE STRING
   "Colon separated list of directories clang will search for headers.")
 
+set(USE_DEPRECATED_GCC_INSTALL_PREFIX OFF CACHE BOOL "Temporary workaround before GCC_INSTALL_PREFIX is completely removed")
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
 set(DEFAULT_SYSROOT "" CACHE STRING
   "Default <path> to all compiler invocations for --sysroot=<path>." )
-if(GCC_INSTALL_PREFIX)
-  message(WARNING "GCC_INSTALL_PREFIX is deprecated and will be removed. Use "
+if(GCC_INSTALL_PREFIX AND NOT USE_DEPRECATED_GCC_INSTALL_PREFIX)
+  message(FATAL_ERROR "GCC_INSTALL_PREFIX is deprecated and will be removed. Use "
     "configuration files (https://clang.llvm.org/docs/UsersManual.html#configuration-files)"
     "to specify the default --gcc-install-dir= or --gcc-triple=. --gcc-toolchain= is discouraged. "
     "See https://github.com/llvm/llvm-project/pull/77537 for detail.")
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 0ce3cbe3266efd..862aec323899ce 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -37,6 +37,9 @@ These changes are ones which we think may surprise users when upgrading to
 Clang |release| because of the opportunity they pose for disruption to existing
 code bases.
 
+- Setting the deprecated CMake variable ``GCC_INSTALL_PREFIX`` (which sets the
+  default ``--gcc-toolchain=``) now leads to a fatal error.
+
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------