Initialize CMAKE_EXPORT_COMPILE_COMMANDS from envvar of the same name

Fixes: #18386

Help/envvar/CMAKE_EXPORT_COMPILE_COMMANDS.rst

@@ -0,0 +1,9 @@
+CMAKE_EXPORT_COMPILE_COMMANDS
+-----------------------------
+
+.. include:: ENV_VAR.txt
+
+The default value for :variable:`CMAKE_EXPORT_COMPILE_COMMANDS` when there
+is no explicit configuration given on the first run while creating a new
+build tree.  On later runs in an existing build tree the value persists in
+the cache as :variable:`CMAKE_EXPORT_COMPILE_COMMANDS`.

Help/manual/cmake-env-variables.7.rst

@@ -23,6 +23,7 @@ Environment Variables that Control the Build
 
    /envvar/CMAKE_BUILD_PARALLEL_LEVEL
    /envvar/CMAKE_CONFIG_TYPE
+   /envvar/CMAKE_EXPORT_COMPILE_COMMANDS
    /envvar/CMAKE_GENERATOR
    /envvar/CMAKE_GENERATOR_INSTANCE
    /envvar/CMAKE_GENERATOR_PLATFORM

Help/release/dev/export-compile-commands-environment-variable.rst

@@ -0,0 +1,6 @@
+export-compile-commands-environment-variable
+--------------------------------------------
+
+* The :variable:`CMAKE_EXPORT_COMPILE_COMMANDS` variable now takes its
+  initial value from the :envvar:`CMAKE_EXPORT_COMPILE_COMMANDS` environment
+  variable if no explicit configuration is given.

Help/variable/CMAKE_EXPORT_COMPILE_COMMANDS.rst

@@ -25,6 +25,9 @@ form.  The format of the JSON file looks like:
     }
   ]
 
+This is initialized by the :envvar:`CMAKE_EXPORT_COMPILE_COMMANDS` environment
+variable.
+
 .. note::
   This option is implemented only by :ref:`Makefile Generators`
   and the :generator:`Ninja`.  It is ignored on other generators.

Modules/CMakeGenericSystem.cmake

@@ -51,16 +51,16 @@ if(CMAKE_GENERATOR MATCHES "Make")
     set_property(GLOBAL PROPERTY TARGET_MESSAGES ${CMAKE_TARGET_MESSAGES})
   endif()
   if(CMAKE_GENERATOR MATCHES "Unix Makefiles")
-    set(CMAKE_EXPORT_COMPILE_COMMANDS OFF CACHE BOOL
-      "Enable/Disable output of compile commands during generation."
+    set(CMAKE_EXPORT_COMPILE_COMMANDS "$ENV{CMAKE_EXPORT_COMPILE_COMMANDS}"
+      CACHE BOOL "Enable/Disable output of compile commands during generation."
       )
     mark_as_advanced(CMAKE_EXPORT_COMPILE_COMMANDS)
   endif()
 endif()
 
 if(CMAKE_GENERATOR MATCHES "Ninja")
-  set(CMAKE_EXPORT_COMPILE_COMMANDS OFF CACHE BOOL
-    "Enable/Disable output of compile commands during generation."
+  set(CMAKE_EXPORT_COMPILE_COMMANDS "$ENV{CMAKE_EXPORT_COMPILE_COMMANDS}"
+    CACHE BOOL "Enable/Disable output of compile commands during generation."
     )
   mark_as_advanced(CMAKE_EXPORT_COMPILE_COMMANDS)
 endif()

Tests/EnforceConfig.cmake.in

@@ -33,5 +33,6 @@ unset(ENV{CMAKE_GENERATOR})
 unset(ENV{CMAKE_GENERATOR_INSTANCE})
 unset(ENV{CMAKE_GENERATOR_PLATFORM})
 unset(ENV{CMAKE_GENERATOR_TOOLSET})
+unset(ENV{CMAKE_EXPORT_COMPILE_COMMANDS})
 
 @TEST_HOME_ENV_CODE@

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

@@ -254,6 +254,24 @@ function(run_EnvironmentGenerator)
 endfunction()
 run_EnvironmentGenerator()
 
+function(run_EnvironmentExportCompileCommands)
+  set(RunCMake_TEST_SOURCE_DIR ${RunCMake_SOURCE_DIR}/env-export-compile-commands)
+
+  run_cmake(env-export-compile-commands-unset)
+
+  set(ENV{CMAKE_EXPORT_COMPILE_COMMANDS} ON)
+  run_cmake(env-export-compile-commands-set)
+
+  set(RunCMake_TEST_OPTIONS -DCMAKE_EXPORT_COMPILE_COMMANDS=OFF)
+  run_cmake(env-export-compile-commands-override)
+
+  unset(ENV{CMAKE_EXPORT_COMPILE_COMMANDS})
+endfunction(run_EnvironmentExportCompileCommands)
+
+if(RunCMake_GENERATOR MATCHES "Unix Makefiles" OR RunCMake_GENERATOR MATCHES "Ninja")
+  run_EnvironmentExportCompileCommands()
+endif()
+
 if(RunCMake_GENERATOR STREQUAL "Ninja")
   # Use a single build tree for a few tests without cleaning.
   set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/Build-build)

Tests/RunCMake/CommandLine/env-export-compile-commands-override-check.cmake

@@ -0,0 +1,3 @@
+if(EXISTS "${RunCMake_TEST_BINARY_DIR}/compile_commands.json")
+  set(RunCMake_TEST_FAILED "compile_commands.json generated with CMAKE_EXPORT_COMPILE_COMMANDS overridden")
+endif()

Tests/RunCMake/CommandLine/env-export-compile-commands-set-check.cmake

@@ -0,0 +1,3 @@
+if(NOT EXISTS "${RunCMake_TEST_BINARY_DIR}/compile_commands.json")
+  set(RunCMake_TEST_FAILED "compile_commands.json not generated with CMAKE_EXPORT_COMPILE_COMMANDS set")
+endif()

Tests/RunCMake/CommandLine/env-export-compile-commands-unset-check.cmake

@@ -0,0 +1,3 @@
+if(EXISTS "${RunCMake_TEST_BINARY_DIR}/compile_commands.json")
+  set(RunCMake_TEST_FAILED "compile_commands.json generated with CMAKE_EXPORT_COMPILE_COMMANDS unset")
+endif()

Tests/RunCMake/CommandLine/env-export-compile-commands/CMakeLists.txt

@@ -0,0 +1,7 @@
+cmake_minimum_required(VERSION 3.14)
+project(env-export-compile-commands C)
+
+# Add target with a source file to make sure compile_commands.json gets
+# generated.
+file(TOUCH ${CMAKE_CURRENT_BINARY_DIR}/main.c)
+add_executable(env-export-compile-commands ${CMAKE_CURRENT_BINARY_DIR}/main.c)