diff.c: --ws-check-deleted optionjc/diff-ws-check-deleted

Traditionally, we only cared about whitespace breakages introduced
in new lines.  Some people want to paint whitespace breakages on old
lines, too.  When they see a whitespace breakage on a new line, they
can spot the same kind of whitespace breakage on the corresponding
old line and want to say "Ah, those breakages are there but they
were inherited from the original, so let's not touch them for now."

Enable such use case with the new option, "--ws-check-deleted".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 90 insertions, 1 deletions

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 6cb083aae5..701c087b6f 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -278,6 +278,13 @@ ifndef::git-format-patch[]
 	initial indent of the line are considered whitespace errors.
 	Exits with non-zero status if problems are found. Not compatible
 	with --exit-code.
+
+--ws-check-deleted::
+--no-ws-check-deleted::
+	Highlight whitespace errors in deleted lines in the color
+	specified by `color.diff.whitespace`, as well as in added
+	lines.
+
 endif::git-format-patch[]
 
 --full-index::
diff --git a/diff.c b/diff.c
index 75b134291a..30eeaea06a 100644
--- a/diff.c
+++ b/diff.c
@@ -503,8 +503,22 @@ static void emit_del_line(const char *reset,
 			  const char *line, int len)
 {
 	const char *set = diff_get_color(ecbdata->color_diff, DIFF_FILE_OLD);
+	const char *ws = NULL;
 
-	emit_line_0(ecbdata->opt, set, reset, '-', line, len);
+	if (ecbdata->opt->ws_check_deleted) {
+		ws = diff_get_color(ecbdata->color_diff, DIFF_WHITESPACE);
+		if (!*ws)
+			ws = NULL;
+	}
+
+	if (!ws)
+		emit_line_0(ecbdata->opt, set, reset, '-', line, len);
+	else {
+		/* Emit just the prefix, then the rest. */
+		emit_line_0(ecbdata->opt, set, reset, '-', "", 0);
+		ws_check_emit(line, len, ecbdata->ws_rule,
+			      ecbdata->opt->file, set, reset, ws);
+	}
 }
 
 static void emit_hunk_header(struct emit_callback *ecbdata,
@@ -3823,6 +3837,11 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 	else if (skip_prefix(arg, "--submodule=", &arg))
 		return parse_submodule_opt(options, arg);
 
+	else if (!strcmp(arg, "--ws-check-deleted"))
+		options->ws_check_deleted = 1;
+	else if (!strcmp(arg, "--no-ws-check-deleted"))
+		options->ws_check_deleted = 0;
+
 	/* misc options */
 	else if (!strcmp(arg, "-z"))
 		options->line_termination = 0;
diff --git a/diff.h b/diff.h
index b4a624d235..ba6cf1a482 100644
--- a/diff.h
+++ b/diff.h
@@ -137,6 +137,7 @@ struct diff_options {
 	int dirstat_permille;
 	int setup;
 	int abbrev;
+	int ws_check_deleted;
 	const char *prefix;
 	int prefix_length;
 	const char *stat_sep;
diff --git a/t/t4015-diff-whitespace.sh b/t/t4015-diff-whitespace.sh
index 4da30e5a1e..8f35475b29 100755
--- a/t/t4015-diff-whitespace.sh
+++ b/t/t4015-diff-whitespace.sh
@@ -838,4 +838,66 @@ test_expect_success 'diff that introduces a line with only tabs' '
 	test_cmp expected current
 '
 
+test_expect_success 'diff that introduces and removes ws breakages' '
+	git reset --hard &&
+	{
+		echo "0. blank-at-eol " &&
+		echo "1. blank-at-eol "
+	} >x &&
+	git commit -a --allow-empty -m preimage &&
+	{
+		echo "0. blank-at-eol " &&
+		echo "1. still-blank-at-eol " &&
+		echo "2. and a new line "
+	} >x &&
+
+	git -c color.diff=always diff |
+	test_decode_color >current &&
+
+	cat >expected <<-\EOF &&
+	<BOLD>diff --git a/x b/x<RESET>
+	<BOLD>index d0233a2..700886e 100644<RESET>
+	<BOLD>--- a/x<RESET>
+	<BOLD>+++ b/x<RESET>
+	<CYAN>@@ -1,2 +1,3 @@<RESET>
+	 0. blank-at-eol <RESET>
+	<RED>-1. blank-at-eol <RESET>
+	<GREEN>+<RESET><GREEN>1. still-blank-at-eol<RESET><BLUE> <RESET>
+	<GREEN>+<RESET><GREEN>2. and a new line<RESET><BLUE> <RESET>
+	EOF
+
+	test_cmp expected current
+'
+
+test_expect_success 'the same with --ws-check-deleted' '
+	git reset --hard &&
+	{
+		echo "0. blank-at-eol " &&
+		echo "1. blank-at-eol "
+	} >x &&
+	git commit -a --allow-empty -m preimage &&
+	{
+		echo "0. blank-at-eol " &&
+		echo "1. still-blank-at-eol " &&
+		echo "2. and a new line "
+	} >x &&
+
+	git -c color.diff=always diff --ws-check-deleted |
+	test_decode_color >current &&
+
+	cat >expected <<-\EOF &&
+	<BOLD>diff --git a/x b/x<RESET>
+	<BOLD>index d0233a2..700886e 100644<RESET>
+	<BOLD>--- a/x<RESET>
+	<BOLD>+++ b/x<RESET>
+	<CYAN>@@ -1,2 +1,3 @@<RESET>
+	 0. blank-at-eol <RESET>
+	<RED>-<RESET><RED>1. blank-at-eol<RESET><BLUE> <RESET>
+	<GREEN>+<RESET><GREEN>1. still-blank-at-eol<RESET><BLUE> <RESET>
+	<GREEN>+<RESET><GREEN>2. and a new line<RESET><BLUE> <RESET>
+	EOF
+
+	test_cmp expected current
+'
+
 test_done