rebase: remove the rebase.useBuiltin setting

Remove the rebase.useBuiltin setting, which was added as an escape
hatch to disable the builtin version of rebase first released with Git
2.20.

See [1] for the initial implementation of rebase.useBuiltin, and [2]
and [3] for the documentation and corresponding
GIT_TEST_REBASE_USE_BUILTIN option.

Carrying the legacy version is a maintenance burden as seen in
7e097e27d3 ("legacy-rebase: backport -C<n> and --whitespace=<option>
checks", 2018-11-20) and 9aea5e9286 ("rebase: fix regression in
rebase.useBuiltin=false test mode", 2019-02-13). Since the built-in
version has been shown to be stable enough let's remove the legacy
version.

As noted in [3] having use_builtin_rebase() shell out to get its
config doesn't make any sense anymore, that was done for the purposes
of spawning the legacy rebase without having modified any global
state. Let's instead handle this case in rebase_config().

There's still a bunch of references to git-legacy-rebase in po/*.po,
but those will be dealt with in time by the i18n effort.

Even though this configuration variable only existed two releases
let's not entirely delete the entry from the docs, but note its
absence. Individual versions of git tend to be around for a while due
to distro packaging timelines, so e.g. if we're "lucky" a given
version like 2.21 might be installed on say OSX for half a decade.

That'll mean some people probably setting this in config, and then
when they later wonder if it's needed they can Google search the
config option name or check it in git-config. It also allows us to
refer to the docs from the warning for details.

1. 55071ea248 ("rebase: start implementing it as a builtin",
   2018-08-07)
2. d8d0a546f0 ("rebase doc: document rebase.useBuiltin", 2018-11-14)
3. 62c23938fa ("tests: add a special setup where rebase.useBuiltin is
   off", 2018-11-14)
3. https://public-inbox.org/git/nycvar.QRO.7.76.6.1903141544110.41@tvgsbejvaqbjf.bet/

Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

8 files changed, 35 insertions, 833 deletions

diff --git a/.gitignore b/.gitignore
index 7374587f9d..5cb84f1d1a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -82,7 +82,6 @@
 /git-init-db
 /git-interpret-trailers
 /git-instaweb
-/git-legacy-rebase
 /git-log
 /git-ls-files
 /git-ls-remote
diff --git a/Documentation/config/rebase.txt b/Documentation/config/rebase.txt
index 331d250e04..d98e32d812 100644
--- a/Documentation/config/rebase.txt
+++ b/Documentation/config/rebase.txt
@@ -1,16 +1,9 @@
 rebase.useBuiltin::
-	Set to `false` to use the legacy shellscript implementation of
-	linkgit:git-rebase[1]. Is `true` by default, which means use
-	the built-in rewrite of it in C.
-+
-The C rewrite is first included with Git version 2.20. This option
-serves an an escape hatch to re-enable the legacy version in case any
-bugs are found in the rewrite. This option and the shellscript version
-of linkgit:git-rebase[1] will be removed in some future release.
-+
-If you find some reason to set this option to `false` other than
-one-off testing you should report the behavior difference as a bug in
-git.
+	Unused configuration variable. Used in Git versions 2.20 and
+	2.21 as an escape hatch to enable the legacy shellscript
+	implementation of rebase. Now the built-in rewrite of it in C
+	is always used. Setting this will emit a warning, to alert any
+	remaining users that setting this now does nothing.
 
 rebase.stat::
 	Whether to show a diffstat of what changed upstream since the last
diff --git a/Makefile b/Makefile
index 537493822b..9f1159cffc 100644
--- a/Makefile
+++ b/Makefile
@@ -632,7 +632,6 @@ SCRIPT_SH += git-merge-one-file.sh
 SCRIPT_SH += git-merge-resolve.sh
 SCRIPT_SH += git-mergetool.sh
 SCRIPT_SH += git-quiltimport.sh
-SCRIPT_SH += git-legacy-rebase.sh
 SCRIPT_SH += git-remote-testgit.sh
 SCRIPT_SH += git-request-pull.sh
 SCRIPT_SH += git-stash.sh
diff --git a/builtin/rebase.c b/builtin/rebase.c
index 52114cbf0d..bc3dc629c1 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -46,29 +46,6 @@ enum rebase_type {
 	REBASE_PRESERVE_MERGES
 };
 
-static int use_builtin_rebase(void)
-{
-	struct child_process cp = CHILD_PROCESS_INIT;
-	struct strbuf out = STRBUF_INIT;
-	int ret, env = git_env_bool("GIT_TEST_REBASE_USE_BUILTIN", -1);
-
-	if (env != -1)
-		return env;
-
-	argv_array_pushl(&cp.args,
-			 "config", "--bool", "rebase.usebuiltin", NULL);
-	cp.git_cmd = 1;
-	if (capture_command(&cp, &out, 6)) {
-		strbuf_release(&out);
-		return 1;
-	}
-
-	strbuf_trim(&out);
-	ret = !strcmp("true", out.buf);
-	strbuf_release(&out);
-	return ret;
-}
-
 struct rebase_options {
 	enum rebase_type type;
 	const char *state_dir;
@@ -106,6 +83,7 @@ struct rebase_options {
 	char *strategy, *strategy_opts;
 	struct strbuf git_format_patch_opt;
 	int reschedule_failed_exec;
+	int use_legacy_rebase;
 };
 
 static int is_interactive(struct rebase_options *opts)
@@ -869,6 +847,11 @@ static int rebase_config(const char *var, const char *value, void *data)
 		return 0;
 	}
 
+	if (!strcmp(var, "rebase.usebuiltin")) {
+		opts->use_legacy_rebase = !git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, data);
 }
 
@@ -1143,22 +1126,6 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	};
 	int i;
 
-	/*
-	 * NEEDSWORK: Once the builtin rebase has been tested enough
-	 * and git-legacy-rebase.sh is retired to contrib/, this preamble
-	 * can be removed.
-	 */
-
-	if (!use_builtin_rebase()) {
-		const char *path = mkpath("%s/git-legacy-rebase",
-					  git_exec_path());
-
-		if (sane_execvp(path, (char **)argv) < 0)
-			die_errno(_("could not exec %s"), path);
-		else
-			BUG("sane_execvp() returned???");
-	}
-
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage_with_options(builtin_rebase_usage,
 				   builtin_rebase_options);
@@ -1169,6 +1136,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 
 	git_config(rebase_config, &options);
 
+	if (options.use_legacy_rebase ||
+	    !git_env_bool("GIT_TEST_REBASE_USE_BUILTIN", -1))
+		warning(_("the rebase.useBuiltin support has been removed!\n"
+			  "See its entry in 'git help config' for details."));
+
 	strbuf_reset(&buf);
 	strbuf_addf(&buf, "%s/applying", apply_dir());
 	if(file_exists(buf.buf))
diff --git a/git-legacy-rebase.sh b/git-legacy-rebase.sh
deleted file mode 100755
index 5c2c4e5276..0000000000
--- a/git-legacy-rebase.sh
+++ /dev/null
@@ -1,770 +0,0 @@
-#!/bin/sh
-#
-# Copyright (c) 2005 Junio C Hamano.
-#
-
-SUBDIRECTORY_OK=Yes
-OPTIONS_KEEPDASHDASH=
-OPTIONS_STUCKLONG=t
-OPTIONS_SPEC="\
-git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] [<upstream>] [<branch>]
-git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] --root [<branch>]
-git rebase --continue | --abort | --skip | --edit-todo
---
- Available options are
-v,verbose!         display a diffstat of what changed upstream
-q,quiet!           be quiet. implies --no-stat
-autostash          automatically stash/stash pop before and after
-fork-point         use 'merge-base --fork-point' to refine upstream
-onto=!             rebase onto given branch instead of upstream
-r,rebase-merges?   try to rebase merges instead of skipping them
-p,preserve-merges! try to recreate merges instead of ignoring them
-s,strategy=!       use the given merge strategy
-X,strategy-option=! pass the argument through to the merge strategy
-no-ff!             cherry-pick all commits, even if unchanged
-f,force-rebase!    cherry-pick all commits, even if unchanged
-m,merge!           use merging strategies to rebase
-i,interactive!     let the user edit the list of commits to rebase
-x,exec=!           add exec lines after each commit of the editable list
-k,keep-empty	   preserve empty commits during rebase
-allow-empty-message allow rebasing commits with empty messages
-stat!              display a diffstat of what changed upstream
-n,no-stat!         do not show diffstat of what changed upstream
-verify             allow pre-rebase hook to run
-rerere-autoupdate  allow rerere to update index with resolved conflicts
-root!              rebase all reachable commits up to the root(s)
-autosquash         move commits that begin with squash!/fixup! under -i
-signoff            add a Signed-off-by: line to each commit
-committer-date-is-author-date! passed to 'git am'
-ignore-date!       passed to 'git am'
-whitespace=!       passed to 'git apply'
-ignore-whitespace! passed to 'git apply'
-C=!                passed to 'git apply'
-S,gpg-sign?        GPG-sign commits
- Actions:
-continue!          continue
-abort!             abort and check out the original branch
-skip!              skip current patch and continue
-edit-todo!         edit the todo list during an interactive rebase
-quit!              abort but keep HEAD where it is
-show-current-patch! show the patch file being applied or merged
-reschedule-failed-exec automatically reschedule failed exec commands
-"
-. git-sh-setup
-set_reflog_action rebase
-require_work_tree_exists
-cd_to_toplevel
-
-LF='
-'
-ok_to_skip_pre_rebase=
-
-squash_onto=
-unset onto
-unset restrict_revision
-cmd=
-strategy=
-strategy_opts=
-do_merge=
-merge_dir="$GIT_DIR"/rebase-merge
-apply_dir="$GIT_DIR"/rebase-apply
-verbose=
-diffstat=
-test "$(git config --bool rebase.stat)" = true && diffstat=t
-autostash="$(git config --bool rebase.autostash || echo false)"
-fork_point=auto
-git_am_opt=
-git_format_patch_opt=
-rebase_root=
-force_rebase=
-allow_rerere_autoupdate=
-# Non-empty if a rebase was in progress when 'git rebase' was invoked
-in_progress=
-# One of {am, merge, interactive}
-type=
-# One of {"$GIT_DIR"/rebase-apply, "$GIT_DIR"/rebase-merge}
-state_dir=
-# One of {'', continue, skip, abort}, as parsed from command line
-action=
-rebase_merges=
-rebase_cousins=
-preserve_merges=
-autosquash=
-keep_empty=
-allow_empty_message=--allow-empty-message
-signoff=
-reschedule_failed_exec=
-test "$(git config --bool rebase.autosquash)" = "true" && autosquash=t
-case "$(git config --bool commit.gpgsign)" in
-true)	gpg_sign_opt=-S ;;
-*)	gpg_sign_opt= ;;
-esac
-test "$(git config --bool rebase.reschedulefailedexec)" = "true" &&
-reschedule_failed_exec=--reschedule-failed-exec
-. git-rebase--common
-
-read_basic_state () {
-	test -f "$state_dir/head-name" &&
-	test -f "$state_dir/onto" &&
-	head_name=$(cat "$state_dir"/head-name) &&
-	onto=$(cat "$state_dir"/onto) &&
-	# We always write to orig-head, but interactive rebase used to write to
-	# head. Fall back to reading from head to cover for the case that the
-	# user upgraded git with an ongoing interactive rebase.
-	if test -f "$state_dir"/orig-head
-	then
-		orig_head=$(cat "$state_dir"/orig-head)
-	else
-		orig_head=$(cat "$state_dir"/head)
-	fi &&
-	test -f "$state_dir"/quiet && GIT_QUIET=t
-	test -f "$state_dir"/verbose && verbose=t
-	test -f "$state_dir"/strategy && strategy="$(cat "$state_dir"/strategy)"
-	test -f "$state_dir"/strategy_opts &&
-		strategy_opts="$(cat "$state_dir"/strategy_opts)"
-	test -f "$state_dir"/allow_rerere_autoupdate &&
-		allow_rerere_autoupdate="$(cat "$state_dir"/allow_rerere_autoupdate)"
-	test -f "$state_dir"/gpg_sign_opt &&
-		gpg_sign_opt="$(cat "$state_dir"/gpg_sign_opt)"
-	test -f "$state_dir"/signoff && {
-		signoff="$(cat "$state_dir"/signoff)"
-		force_rebase=t
-	}
-	test -f "$state_dir"/reschedule-failed-exec &&
-		reschedule_failed_exec=t
-}
-
-finish_rebase () {
-	rm -f "$(git rev-parse --git-path REBASE_HEAD)"
-	apply_autostash &&
-	{ git gc --auto || true; } &&
-	rm -rf "$state_dir"
-}
-
-run_interactive () {
-	GIT_CHERRY_PICK_HELP="$resolvemsg"
-	export GIT_CHERRY_PICK_HELP
-
-	test -n "$keep_empty" && keep_empty="--keep-empty"
-	test -n "$rebase_merges" && rebase_merges="--rebase-merges"
-	test -n "$rebase_cousins" && rebase_cousins="--rebase-cousins"
-	test -n "$autosquash" && autosquash="--autosquash"
-	test -n "$verbose" && verbose="--verbose"
-	test -n "$force_rebase" && force_rebase="--no-ff"
-	test -n "$restrict_revision" && \
-		restrict_revision="--restrict-revision=^$restrict_revision"
-	test -n "$upstream" && upstream="--upstream=$upstream"
-	test -n "$onto" && onto="--onto=$onto"
-	test -n "$squash_onto" && squash_onto="--squash-onto=$squash_onto"
-	test -n "$onto_name" && onto_name="--onto-name=$onto_name"
-	test -n "$head_name" && head_name="--head-name=$head_name"
-	test -n "$strategy" && strategy="--strategy=$strategy"
-	test -n "$strategy_opts" && strategy_opts="--strategy-opts=$strategy_opts"
-	test -n "$switch_to" && switch_to="--switch-to=$switch_to"
-	test -n "$cmd" && cmd="--cmd=$cmd"
-	test -n "$action" && action="--$action"
-
-	exec git rebase--interactive "$action" "$keep_empty" "$rebase_merges" "$rebase_cousins" \
-		"$upstream" "$onto" "$squash_onto" "$restrict_revision" \
-		"$allow_empty_message" "$autosquash" "$verbose" \
-		"$force_rebase" "$onto_name" "$head_name" "$strategy" \
-		"$strategy_opts" "$cmd" "$switch_to" \
-		"$allow_rerere_autoupdate" "$gpg_sign_opt" "$signoff" \
-		"$reschedule_failed_exec"
-}
-
-run_specific_rebase () {
-	if [ "$interactive_rebase" = implied ]; then
-		GIT_SEQUENCE_EDITOR=:
-		export GIT_SEQUENCE_EDITOR
-		autosquash=
-	fi
-
-	if test -n "$interactive_rebase" -a -z "$preserve_merges"
-	then
-		run_interactive
-	else
-		. git-rebase--$type
-
-		if test -z "$preserve_merges"
-		then
-			git_rebase__$type
-		else
-			git_rebase__preserve_merges
-		fi
-	fi
-
-	ret=$?
-	if test $ret -eq 0
-	then
-		finish_rebase
-	elif test $ret -eq 2 # special exit status for rebase -p
-	then
-		apply_autostash &&
-		rm -rf "$state_dir" &&
-		die "Nothing to do"
-	fi
-	exit $ret
-}
-
-run_pre_rebase_hook () {
-	if test -z "$ok_to_skip_pre_rebase" &&
-	   test -x "$(git rev-parse --git-path hooks/pre-rebase)"
-	then
-		"$(git rev-parse --git-path hooks/pre-rebase)" ${1+"$@"} ||
-		die "$(gettext "The pre-rebase hook refused to rebase.")"
-	fi
-}
-
-test -f "$apply_dir"/applying &&
-	die "$(gettext "It looks like 'git am' is in progress. Cannot rebase.")"
-
-if test -d "$apply_dir"
-then
-	type=am
-	state_dir="$apply_dir"
-elif test -d "$merge_dir"
-then
-	type=interactive
-	if test -d "$merge_dir"/rewritten
-	then
-		type=preserve-merges
-		interactive_rebase=explicit
-		preserve_merges=t
-	elif test -f "$merge_dir"/interactive
-	then
-		interactive_rebase=explicit
-	fi
-	state_dir="$merge_dir"
-fi
-test -n "$type" && in_progress=t
-
-total_argc=$#
-while test $# != 0
-do
-	case "$1" in
-	--no-verify)
-		ok_to_skip_pre_rebase=yes
-		;;
-	--verify)
-		ok_to_skip_pre_rebase=
-		;;
-	--continue|--skip|--abort|--quit|--edit-todo|--show-current-patch)
-		test $total_argc -eq 2 || usage
-		action=${1##--}
-		;;
-	--onto=*)
-		onto="${1#--onto=}"
-		;;
-	--exec=*)
-		cmd="${cmd}exec ${1#--exec=}${LF}"
-		test -z "$interactive_rebase" && interactive_rebase=implied
-		;;
-	--interactive)
-		interactive_rebase=explicit
-		;;
-	--keep-empty)
-		keep_empty=yes
-		;;
-	--allow-empty-message)
-		allow_empty_message=--allow-empty-message
-		;;
-	--no-keep-empty)
-		keep_empty=
-		;;
-	--rebase-merges)
-		rebase_merges=t
-		test -z "$interactive_rebase" && interactive_rebase=implied
-		;;
-	--rebase-merges=*)
-		rebase_merges=t
-		case "${1#*=}" in
-		rebase-cousins) rebase_cousins=t;;
-		no-rebase-cousins) rebase_cousins=;;
-		*) die "Unknown mode: $1";;
-		esac
-		test -z "$interactive_rebase" && interactive_rebase=implied
-		;;
-	--preserve-merges)
-		preserve_merges=t
-		test -z "$interactive_rebase" && interactive_rebase=implied
-		;;
-	--autosquash)
-		autosquash=t
-		;;
-	--no-autosquash)
-		autosquash=
-		;;
-	--fork-point)
-		fork_point=t
-		;;
-	--no-fork-point)
-		fork_point=
-		;;
-	--merge)
-		do_merge=t
-		;;
-	--strategy-option=*)
-		strategy_opts="$strategy_opts $(git rev-parse --sq-quote "--${1#--strategy-option=}" | sed -e s/^.//)"
-		do_merge=t
-		test -z "$strategy" && strategy=recursive
-		;;
-	--strategy=*)
-		strategy="${1#--strategy=}"
-		do_merge=t
-		;;
-	--no-stat)
-		diffstat=
-		;;
-	--stat)
-		diffstat=t
-		;;
-	--autostash)
-		autostash=true
-		;;
-	--no-autostash)
-		autostash=false
-		;;
-	--verbose)
-		verbose=t
-		diffstat=t
-		GIT_QUIET=
-		;;
-	--quiet)
-		GIT_QUIET=t
-		git_am_opt="$git_am_opt -q"
-		verbose=
-		diffstat=
-		;;
-	--whitespace=*)
-		git_am_opt="$git_am_opt --whitespace=${1#--whitespace=}"
-		case "${1#--whitespace=}" in
-		fix|strip)
-			force_rebase=t
-			;;
-		warn|nowarn|error|error-all)
-			;; # okay, known whitespace option
-		*)
-			die "fatal: Invalid whitespace option: '${1#*=}'"
-			;;
-		esac
-		;;
-	--ignore-whitespace)
-		git_am_opt="$git_am_opt $1"
-		;;
-	--signoff)
-		signoff=--signoff
-		;;
-	--no-signoff)
-		signoff=
-		;;
-	--committer-date-is-author-date|--ignore-date)
-		git_am_opt="$git_am_opt $1"
-		force_rebase=t
-		;;
-	-C*[!0-9]*)
-		die "fatal: switch \`C' expects a numerical value"
-		;;
-	-C*)
-		git_am_opt="$git_am_opt $1"
-		;;
-	--root)
-		rebase_root=t
-		;;
-	--force-rebase|--no-ff)
-		force_rebase=t
-		;;
-	--rerere-autoupdate|--no-rerere-autoupdate)
-		allow_rerere_autoupdate="$1"
-		;;
-	--gpg-sign)
-		gpg_sign_opt=-S
-		;;
-	--gpg-sign=*)
-		gpg_sign_opt="-S${1#--gpg-sign=}"
-		;;
-	--reschedule-failed-exec)
-		reschedule_failed_exec=--reschedule-failed-exec
-		;;
-	--no-reschedule-failed-exec)
-		reschedule_failed_exec=
-		;;
-	--)
-		shift
-		break
-		;;
-	*)
-		usage
-		;;
-	esac
-	shift
-done
-test $# -gt 2 && usage
-
-if test -n "$action"
-then
-	test -z "$in_progress" && die "$(gettext "No rebase in progress?")"
-	# Only interactive rebase uses detailed reflog messages
-	if test -n "$interactive_rebase" && test "$GIT_REFLOG_ACTION" = rebase
-	then
-		GIT_REFLOG_ACTION="rebase -i ($action)"
-		export GIT_REFLOG_ACTION
-	fi
-fi
-
-if test "$action" = "edit-todo" && test -z "$interactive_rebase"
-then
-	die "$(gettext "The --edit-todo action can only be used during interactive rebase.")"
-fi
-
-case "$action" in
-continue)
-	# Sanity check
-	git rev-parse --verify HEAD >/dev/null ||
-		die "$(gettext "Cannot read HEAD")"
-	git update-index --ignore-submodules --refresh &&
-	git diff-files --quiet --ignore-submodules || {
-		echo "$(gettext "You must edit all merge conflicts and then
-mark them as resolved using git add")"
-		exit 1
-	}
-	read_basic_state
-	run_specific_rebase
-	;;
-skip)
-	output git reset --hard HEAD || exit $?
-	read_basic_state
-	run_specific_rebase
-	;;
-abort)
-	git rerere clear
-	read_basic_state
-	case "$head_name" in
-	refs/*)
-		git symbolic-ref -m "rebase: aborting" HEAD $head_name ||
-		die "$(eval_gettext "Could not move back to \$head_name")"
-		;;
-	esac
-	output git reset --hard $orig_head
-	finish_rebase
-	exit
-	;;
-quit)
-	exec rm -rf "$state_dir"
-	;;
-edit-todo)
-	run_specific_rebase
-	;;
-show-current-patch)
-	run_specific_rebase
-	die "BUG: run_specific_rebase is not supposed to return here"
-	;;
-esac
-
-# Make sure no rebase is in progress
-if test -n "$in_progress"
-then
-	state_dir_base=${state_dir##*/}
-	cmd_live_rebase="git rebase (--continue | --abort | --skip)"
-	cmd_clear_stale_rebase="rm -fr \"$state_dir\""
-	die "
-$(eval_gettext 'It seems that there is already a $state_dir_base directory, and
-I wonder if you are in the middle of another rebase.  If that is the
-case, please try
-	$cmd_live_rebase
-If that is not the case, please
-	$cmd_clear_stale_rebase
-and run me again.  I am stopping in case you still have something
-valuable there.')"
-fi
-
-if test -n "$rebase_root" && test -z "$onto"
-then
-	test -z "$interactive_rebase" && interactive_rebase=implied
-fi
-
-if test -n "$keep_empty"
-then
-	test -z "$interactive_rebase" && interactive_rebase=implied
-fi
-
-actually_interactive=
-if test -n "$interactive_rebase"
-then
-	if test -z "$preserve_merges"
-	then
-		type=interactive
-	else
-		type=preserve-merges
-	fi
-	actually_interactive=t
-	state_dir="$merge_dir"
-elif test -n "$do_merge"
-then
-	interactive_rebase=implied
-	type=interactive
-	state_dir="$merge_dir"
-else
-	type=am
-	state_dir="$apply_dir"
-fi
-
-if test -t 2 && test -z "$GIT_QUIET"
-then
-	git_format_patch_opt="$git_format_patch_opt --progress"
-fi
-
-incompatible_opts=$(echo " $git_am_opt " | \
-		    sed -e 's/ -q / /g' -e 's/^ \(.*\) $/\1/')
-if test -n "$incompatible_opts"
-then
-	if test -n "$actually_interactive" || test "$do_merge"
-	then
-		die "$(gettext "fatal: cannot combine am options with either interactive or merge options")"
-	fi
-fi
-
-if test -n "$signoff"
-then
-	test -n "$preserve_merges" &&
-		die "$(gettext "fatal: cannot combine '--signoff' with '--preserve-merges'")"
-	git_am_opt="$git_am_opt $signoff"
-	force_rebase=t
-fi
-
-if test -n "$preserve_merges"
-then
-	# Note: incompatibility with --signoff handled in signoff block above
-	# Note: incompatibility with --interactive is just a strong warning;
-	#       git-rebase.txt caveats with "unless you know what you are doing"
-	test -n "$rebase_merges" &&
-		die "$(gettext "fatal: cannot combine '--preserve-merges' with '--rebase-merges'")"
-
-	test -n "$reschedule_failed_exec" &&
-		die "$(gettext "error: cannot combine '--preserve-merges' with '--reschedule-failed-exec'")"
-fi
-
-if test -n "$rebase_merges"
-then
-	test -n "$strategy_opts" &&
-		die "$(gettext "fatal: cannot combine '--rebase-merges' with '--strategy-option'")"
-	test -n "$strategy" &&
-		die "$(gettext "fatal: cannot combine '--rebase-merges' with '--strategy'")"
-fi
-
-if test -z "$rebase_root"
-then
-	case "$#" in
-	0)
-		if ! upstream_name=$(git rev-parse --symbolic-full-name \
-			--verify -q @{upstream} 2>/dev/null)
-		then
-			. git-parse-remote
-			error_on_missing_default_upstream "rebase" "rebase" \
-				"against" "git rebase $(gettext '<branch>')"
-		fi
-
-		test "$fork_point" = auto && fork_point=t
-		;;
-	*)	upstream_name="$1"
-		if test "$upstream_name" = "-"
-		then
-			upstream_name="@{-1}"
-		fi
-		shift
-		;;
-	esac
-	upstream=$(peel_committish "${upstream_name}") ||
-	die "$(eval_gettext "invalid upstream '\$upstream_name'")"
-	upstream_arg="$upstream_name"
-else
-	if test -z "$onto"
-	then
-		empty_tree=$(git hash-object -t tree /dev/null)
-		onto=$(git commit-tree $empty_tree </dev/null)
-		squash_onto="$onto"
-	fi
-	unset upstream_name
-	unset upstream
-	test $# -gt 1 && usage
-	upstream_arg=--root
-fi
-
-# Make sure the branch to rebase onto is valid.
-onto_name=${onto-"$upstream_name"}
-case "$onto_name" in
-*...*)
-	if	left=${onto_name%...*} right=${onto_name#*...} &&
-		onto=$(git merge-base --all ${left:-HEAD} ${right:-HEAD})
-	then
-		case "$onto" in
-		?*"$LF"?*)
-			die "$(eval_gettext "\$onto_name: there are more than one merge bases")"
-			;;
-		'')
-			die "$(eval_gettext "\$onto_name: there is no merge base")"
-			;;
-		esac
-	else
-		die "$(eval_gettext "\$onto_name: there is no merge base")"
-	fi
-	;;
-*)
-	onto=$(peel_committish "$onto_name") ||
-	die "$(eval_gettext "Does not point to a valid commit: \$onto_name")"
-	;;
-esac
-
-# If the branch to rebase is given, that is the branch we will rebase
-# $branch_name -- branch/commit being rebased, or HEAD (already detached)
-# $orig_head -- commit object name of tip of the branch before rebasing
-# $head_name -- refs/heads/<that-branch> or "detached HEAD"
-switch_to=
-case "$#" in
-1)
-	# Is it "rebase other $branchname" or "rebase other $commit"?
-	branch_name="$1"
-	switch_to="$1"
-
-	# Is it a local branch?
-	if git show-ref --verify --quiet -- "refs/heads/$branch_name" &&
-	   orig_head=$(git rev-parse -q --verify "refs/heads/$branch_name")
-	then
-		head_name="refs/heads/$branch_name"
-	# If not is it a valid ref (branch or commit)?
-	elif orig_head=$(git rev-parse -q --verify "$branch_name")
-	then
-		head_name="detached HEAD"
-
-	else
-		die "$(eval_gettext "fatal: no such branch/commit '\$branch_name'")"
-	fi
-	;;
-0)
-	# Do not need to switch branches, we are already on it.
-	if branch_name=$(git symbolic-ref -q HEAD)
-	then
-		head_name=$branch_name
-		branch_name=$(expr "z$branch_name" : 'zrefs/heads/\(.*\)')
-	else
-		head_name="detached HEAD"
-		branch_name=HEAD
-	fi
-	orig_head=$(git rev-parse --verify HEAD) || exit
-	;;
-*)
-	die "BUG: unexpected number of arguments left to parse"
-	;;
-esac
-
-if test "$fork_point" = t
-then
-	new_upstream=$(git merge-base --fork-point "$upstream_name" \
-			"${switch_to:-HEAD}")
-	if test -n "$new_upstream"
-	then
-		restrict_revision=$new_upstream
-	fi
-fi
-
-if test "$autostash" = true && ! (require_clean_work_tree) 2>/dev/null
-then
-	stash_sha1=$(git stash create "autostash") ||
-	die "$(gettext 'Cannot autostash')"
-
-	mkdir -p "$state_dir" &&
-	echo $stash_sha1 >"$state_dir/autostash" &&
-	stash_abbrev=$(git rev-parse --short $stash_sha1) &&
-	echo "$(eval_gettext 'Created autostash: $stash_abbrev')" &&
-	git reset --hard
-fi
-
-require_clean_work_tree "rebase" "$(gettext "Please commit or stash them.")"
-
-# Now we are rebasing commits $upstream..$orig_head (or with --root,
-# everything leading up to $orig_head) on top of $onto
-
-# Check if we are already based on $onto with linear history,
-# but this should be done only when upstream and onto are the same
-# and if this is not an interactive rebase.
-mb=$(git merge-base "$onto" "$orig_head")
-if test -z "$actually_interactive" && test "$upstream" = "$onto" &&
-	test "$mb" = "$onto" && test -z "$restrict_revision" &&
-	# linear history?
-	! (git rev-list --parents "$onto".."$orig_head" | sane_grep " .* ") > /dev/null
-then
-	if test -z "$force_rebase"
-	then
-		# Lazily switch to the target branch if needed...
-		test -z "$switch_to" ||
-		GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $switch_to" \
-			git checkout -q "$switch_to" --
-		if test "$branch_name" = "HEAD" &&
-			 ! git symbolic-ref -q HEAD
-		then
-			say "$(eval_gettext "HEAD is up to date.")"
-		else
-			say "$(eval_gettext "Current branch \$branch_name is up to date.")"
-		fi
-		finish_rebase
-		exit 0
-	else
-		if test "$branch_name" = "HEAD" &&
-			 ! git symbolic-ref -q HEAD
-		then
-			say "$(eval_gettext "HEAD is up to date, rebase forced.")"
-		else
-			say "$(eval_gettext "Current branch \$branch_name is up to date, rebase forced.")"
-		fi
-	fi
-fi
-
-# If a hook exists, give it a chance to interrupt
-run_pre_rebase_hook "$upstream_arg" "$@"
-
-if test -n "$diffstat"
-then
-	if test -n "$verbose"
-	then
-		if test -z "$mb"
-		then
-			echo "$(eval_gettext "Changes to \$onto:")"
-		else
-			echo "$(eval_gettext "Changes from \$mb to \$onto:")"
-		fi
-	fi
-	mb_tree="${mb:-$(git hash-object -t tree /dev/null)}"
-	# We want color (if set), but no pager
-	GIT_PAGER='' git diff --stat --summary "$mb_tree" "$onto"
-fi
-
-if test -z "$actually_interactive" && test "$mb" = "$orig_head"
-then
-	say "$(eval_gettext "Fast-forwarded \$branch_name to \$onto_name.")"
-	GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $onto_name" \
-		git checkout -q "$onto^0" || die "could not detach HEAD"
-	# If the $onto is a proper descendant of the tip of the branch, then
-	# we just fast-forwarded.
-	git update-ref ORIG_HEAD $orig_head
-	move_to_original_branch
-	finish_rebase
-	exit 0
-fi
-
-test -n "$interactive_rebase" && run_specific_rebase
-
-# Detach HEAD and reset the tree
-say "$(gettext "First, rewinding head to replay your work on top of it...")"
-
-GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $onto_name" \
-	git checkout -q "$onto^0" || die "could not detach HEAD"
-git update-ref ORIG_HEAD $orig_head
-
-if test -n "$rebase_root"
-then
-	revisions="$onto..$orig_head"
-else
-	revisions="${restrict_revision-$upstream}..$orig_head"
-fi
-
-run_specific_rebase
diff --git a/t/README b/t/README
index 7a3d582267..385262357b 100644
--- a/t/README
+++ b/t/README
@@ -379,10 +379,6 @@ the --no-sparse command-line argument.
 GIT_TEST_PRELOAD_INDEX=<boolean> exercises the preload-index code path
 by overriding the minimum number of cache entries required per thread.
 
-GIT_TEST_REBASE_USE_BUILTIN=<boolean>, when false, disables the
-builtin version of git-rebase. See 'rebase.useBuiltin' in
-git-config(1).
-
 GIT_TEST_INDEX_THREADS=<n> enables exercising the multi-threaded loading
 of the index for the whole test suite by bypassing the default number of
 cache entries and thread minimums. Setting this to 1 will make the
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 3e73f7584c..a809de9304 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -311,4 +311,20 @@ test_expect_success 'rebase--merge.sh and --show-current-patch' '
 	)
 '
 
+test_expect_success 'rebase -c rebase.useBuiltin=false warning' '
+	expected="rebase.useBuiltin support has been removed" &&
+
+	# Only warn when the legacy rebase is requested...
+	test_must_fail git -c rebase.useBuiltin=false rebase 2>err &&
+	test_i18ngrep "$expected" err &&
+	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=false git rebase 2>err &&
+	test_i18ngrep "$expected" err &&
+
+	# ...not when we would have used the built-in anyway
+	test_must_fail git -c rebase.useBuiltin=true rebase 2>err &&
+	test_must_be_empty err &&
+	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=true git rebase 2>err &&
+	test_must_be_empty err
+'
+
 test_done
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index b60b11f9f2..1723e1a858 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -149,12 +149,10 @@ test_expect_success 'rebase -i with the exec command checks tree cleanness' '
 
 test_expect_success 'rebase -x with empty command fails' '
 	test_when_finished "git rebase --abort ||:" &&
-	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=true \
-		git rebase -x "" @ 2>actual &&
+	test_must_fail env git rebase -x "" @ 2>actual &&
 	test_write_lines "error: empty exec command" >expected &&
 	test_i18ncmp expected actual &&
-	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=true \
-		git rebase -x " " @ 2>actual &&
+	test_must_fail env git rebase -x " " @ 2>actual &&
 	test_i18ncmp expected actual
 '
 
@@ -162,8 +160,7 @@ LF='
 '
 test_expect_success 'rebase -x with newline in command fails' '
 	test_when_finished "git rebase --abort ||:" &&
-	test_must_fail env GIT_TEST_REBASE_USE_BUILTIN=true \
-		git rebase -x "a${LF}b" @ 2>actual &&
+	test_must_fail env git rebase -x "a${LF}b" @ 2>actual &&
 	test_write_lines "error: exec commands cannot contain newlines" \
 			 >expected &&
 	test_i18ncmp expected actual