CMP0132: Don't set compiler environment variables on first run

When running CMake for the first time in a build tree, for some
generators CMake would set compiler environment variables
like CC, CXX, etc. when the corresponding language is enabled.
That behavior was never documented and can result in different
behavior between the first and subsequent runs. Add a policy
to no longer set those environment variables.

Fixes: #21378

Help/manual/cmake-policies.7.rst

@@ -58,6 +58,7 @@ Policies Introduced by CMake 3.24
 .. toctree::
    :maxdepth: 1
 
+   CMP0132: Do not set compiler environment variables on first run. </policy/CMP0132>
    CMP0131: LINK_LIBRARIES supports the LINK_ONLY generator expression. </policy/CMP0131>
    CMP0130: while() diagnoses condition evaluation errors. </policy/CMP0130>
 

Help/policy/CMP0132.rst

@@ -0,0 +1,26 @@
+CMP0132
+-------
+
+.. versionadded:: 3.24
+
+Apart from when using the Xcode generator and some Visual Studio generators,
+CMake 3.23 and below will set environment variables like :envvar:`CC`,
+:envvar:`CXX`, etc. when the corresponding language is enabled.
+This only occurs on the very first time CMake is run in a build directory,
+and the environment variables are only defined at configure time, not build
+time. On subsequent CMake runs, these environment variables are not set,
+opening up the opportunity for different behavior between the first and
+subsequent CMake runs. CMake 3.24 and above prefer to not set these
+environment variables when a language is enabled, even on the first run in
+a build directory.
+
+The ``OLD`` behavior for this policy sets the relevant environment variable
+on the first run when a language is enabled. The ``NEW`` behavior for this
+policy does not set any such environment variables.
+
+This policy was introduced in CMake version 3.24. Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike many policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt

Help/release/dev/set-env-var-first-run.rst

@@ -0,0 +1,6 @@
+set-env-var-first-run
+---------------------
+
+* CMake no longer sets environment variables like :envvar:`CC`, :envvar:`CXX`,
+  etc. when enabling the corresponding language during the first CMake run in
+  a build directory. See policy :policy:`CMP0132`.

Source/cmGlobalGenerator.cxx

@@ -759,7 +759,9 @@ void cmGlobalGenerator::EnableLanguage(
       needTestLanguage[lang] = true;
       // Some generators like visual studio should not use the env variables
       // So the global generator can specify that in this variable
-      if (!mf->GetDefinition("CMAKE_GENERATOR_NO_COMPILER_ENV")) {
+      if ((mf->GetPolicyStatus(cmPolicies::CMP0132) == cmPolicies::OLD ||
+           mf->GetPolicyStatus(cmPolicies::CMP0132) == cmPolicies::WARN) &&
+          !mf->GetDefinition("CMAKE_GENERATOR_NO_COMPILER_ENV")) {
         // put ${CMake_(LANG)_COMPILER_ENV_VAR}=${CMAKE_(LANG)_COMPILER
         // into the environment, in case user scripts want to run
         // configure, or sub cmakes

Source/cmPolicies.h

@@ -393,7 +393,10 @@ class cmMakefile;
          3, 24, 0, cmPolicies::WARN)                                          \
   SELECT(POLICY, CMP0131,                                                     \
          "LINK_LIBRARIES supports the LINK_ONLY generator expression.", 3,    \
-         24, 0, cmPolicies::WARN)
+         24, 0, cmPolicies::WARN)                                             \
+  SELECT(POLICY, CMP0132,                                                     \
+         "Do not set compiler environment variables on first run", 3, 24, 0,  \
+         cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
 #define CM_FOR_EACH_POLICY_ID(POLICY)                                         \

Tests/RunCMake/CMP0132/CMP0132-Common.cmake

@@ -0,0 +1,15 @@
+if(NOT "$ENV{CC}" STREQUAL "")
+  message(STATUS "Test environment already sets CC, test being SKIPPED")
+  return()
+elseif(CMAKE_GENERATOR MATCHES "Xcode|Visual Studio")
+  message(STATUS "This generator never sets CC, test being SKIPPED")
+  return()
+endif()
+
+enable_language(C)
+
+if("$ENV{CC}" STREQUAL "")
+  message(STATUS "CC was left unset")
+else()
+  message(STATUS "CC was set to $ENV{CC}")
+endif()

Tests/RunCMake/CMP0132/CMP0132-NEW-stdout.txt

@@ -0,0 +1 @@
+SKIPPED|CC was left unset

Tests/RunCMake/CMP0132/CMP0132-NEW.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0132 NEW)
+include(CMP0132-Common.cmake)

Tests/RunCMake/CMP0132/CMP0132-OLD-stdout.txt

@@ -0,0 +1 @@
+SKIPPED|CC was set

Tests/RunCMake/CMP0132/CMP0132-OLD.cmake

@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0132 OLD)
+include(CMP0132-Common.cmake)

Tests/RunCMake/CMP0132/CMP0132-WARN-stdout.txt

@@ -0,0 +1 @@
+SKIPPED|CC was set

Tests/RunCMake/CMP0132/CMP0132-WARN.cmake

@@ -0,0 +1,2 @@
+
+include(CMP0132-Common.cmake)

Tests/RunCMake/CMP0132/CMakeLists.txt

@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.23)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)

Tests/RunCMake/CMP0132/RunCMakeTest.cmake

@@ -0,0 +1,7 @@
+include(RunCMake)
+
+if(NOT CMAKE_GENERATOR_NO_COMPILER_ENV)
+  run_cmake(CMP0132-WARN)
+  run_cmake(CMP0132-OLD)
+  run_cmake(CMP0132-NEW)
+endif()

Tests/RunCMake/CMakeLists.txt

@@ -148,6 +148,8 @@ if("${CMAKE_C_COMPILER_ID}" STREQUAL "LCC" OR
   add_RunCMake_test("CMP0129")
 endif()
 
+add_RunCMake_test(CMP0132)
+
 # The test for Policy 65 requires the use of the
 # CMAKE_SHARED_LIBRARY_LINK_CXX_FLAGS variable, which both the VS and Xcode
 # generators ignore.  The policy will have no effect on those generators.