Makefile: wire up build option for deprecated features

With 57ec9254eb (docs: introduce document to announce breaking changes,
2024-06-14), we have introduced a new document that tracks upcoming
breaking changes in the Git project. In 2454970930 (BreakingChanges:
early adopter option, 2024-10-11) we have amended the document a bit to
mention that any introduced breaking changes must be accompanied by
logic that allows us to enable the breaking change at compile-time.
While we already have two breaking changes lined up, neither of them has
such a switch because they predate those instructions.

Introduce the proposed `WITH_BREAKING_CHANGES` preprocessor macro and
wire it up with both our Makefiles and Meson. This does not yet wire up
the build flag for existing deprecations.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

GIT-BUILD-OPTIONS.in

@@ -45,3 +45,4 @@ GITWEBDIR=@GITWEBDIR@
 USE_GETTEXT_SCHEME=@USE_GETTEXT_SCHEME@
 LOCALEDIR=@LOCALEDIR@
 BROKEN_PATH_FIX=@BROKEN_PATH_FIX@
+WITH_BREAKING_CHANGES=@WITH_BREAKING_CHANGES@

Makefile

@@ -2230,6 +2230,10 @@ ifdef FSMONITOR_OS_SETTINGS
 	COMPAT_OBJS += compat/fsmonitor/fsm-path-utils-$(FSMONITOR_OS_SETTINGS).o
 endif
 
+ifdef WITH_BREAKING_CHANGES
+	BASIC_CFLAGS += -DWITH_BREAKING_CHANGES
+endif
+
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK = NoThanks
 endif
@@ -3187,6 +3191,7 @@ GIT-BUILD-OPTIONS: FORCE
 		-e "s|@USE_GETTEXT_SCHEME@|\'$(USE_GETTEXT_SCHEME)\'|" \
 		-e "s|@LOCALEDIR@|\'$(localedir_SQ)\'|" \
 		-e "s!@BROKEN_PATH_FIX@!\'$(BROKEN_PATH_FIX)\'!" \
+		-e "s|@WITH_BREAKING_CHANGES@|\'$(WITH_BREAKING_CHANGES)\'|" \
 		GIT-BUILD-OPTIONS.in >$@+
 	@if grep -q '^[A-Z][A-Z_]*=@.*@$$' $@+; then echo "Unsubstituted build options in $@" >&2 && exit 1; fi
 	@if cmp $@+ $@ >/dev/null 2>&1; then $(RM) $@+; else mv $@+ $@; fi

contrib/buildsystems/CMakeLists.txt

@@ -1198,6 +1198,7 @@ string(REPLACE "@GITWEBDIR@" "'${GITWEBDIR}'" git_build_options "${git_build_opt
 string(REPLACE "@USE_GETTEXT_SCHEME@" "" git_build_options "${git_build_options}")
 string(REPLACE "@LOCALEDIR@" "'${LOCALEDIR}'" git_build_options "${git_build_options}")
 string(REPLACE "@BROKEN_PATH_FIX@" "" git_build_options "${git_build_options}")
+string(REPLACE "@WITH_BREAKING_CHANGES@" "" git_build_options "${git_build_options}")
 if(USE_VCPKG)
 	string(APPEND git_build_options "PATH=\"$PATH:$TEST_DIRECTORY/../compat/vcbuild/vcpkg/installed/x64-windows/bin\"\n")
 endif()

meson.build

@@ -644,6 +644,12 @@ build_options_config.set('GIT_TEST_UTF8_LOCALE', '')
 build_options_config.set_quoted('LOCALEDIR', fs.as_posix(get_option('prefix') / get_option('localedir')))
 build_options_config.set('GITWEBDIR', fs.as_posix(get_option('prefix') / get_option('datadir') / 'gitweb'))
 
+if get_option('breaking_changes')
+  build_options_config.set('WITH_BREAKING_CHANGES', 'YesPlease')
+else
+  build_options_config.set('WITH_BREAKING_CHANGES', '')
+endif
+
 if get_option('sane_tool_path') != ''
   build_options_config.set_quoted('BROKEN_PATH_FIX', 's|^\# @BROKEN_PATH_FIX@$|git_broken_path_fix "' + get_option('sane_tool_path') + '"|')
 else

meson_options.txt

@@ -43,6 +43,8 @@ option('sha256_backend', type: 'combo', choices: ['openssl', 'nettle', 'gcrypt',
   description: 'The backend used for hashing objects with the SHA256 object format')
 
 # Build tweaks.
+option('breaking_changes', type: 'boolean', value: false,
+  description: 'Enable upcoming breaking changes.')
 option('macos_use_homebrew_gettext', type: 'boolean', value: true,
   description: 'Use gettext from Homebrew instead of the slightly-broken system-provided one.')
 

t/test-lib.sh

@@ -1864,6 +1864,10 @@ test_lazy_prereq CURL '
 	curl --version
 '
 
+test_lazy_prereq WITHOUT_BREAKING_CHANGES '
+	test -z "$WITH_BREAKING_CHANGES"
+'
+
 # SHA1 is a test if the hash algorithm in use is SHA-1.  This is both for tests
 # which will not work with other hash algorithms and tests that work but don't
 # test anything meaningful (e.g. special values which cause short collisions).