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..c747452983 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 between Git version 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
