Subject: [PATCH 11/12] diff: highlight and error out on incomplete lines

From: Junio C Hamano <[email-masked]>
Date: Mon, 3 Nov 2025 18:09:27 -0800

Teach "git diff" to highlight "\ No newline at end of file" message
as a whitespace error when incomplete-line whitespace error class is
in effect.  Thanks to the previous refactoring of complete rewrite
code path, we can do this at a single place.

Unlike whitespace errors in the payload where we need to annotate in
line, possibly using colors, the line that has whitespace problems,
we have a dedicated line already that can serve as the error
message, so paint it as a whitespace error message.

Also teach "git diff --check" to notice incomplete lines as
whitespace errors and report when incomplete-line whitespace error
class is in effect.

Signed-off-by: Junio C Hamano <[email-masked]>
---
 diff.c                     | 29 ++++++++++++++++--
 t/t4015-diff-whitespace.sh | 63 +++++++++++++++++++++++++++++++++++---
 2 files changed, 86 insertions(+), 6 deletions(-)

diff --git a/diff.c b/diff.c
index 8d03146aaa..965b97f7f0 100644
--- a/diff.c
+++ b/diff.c
@@ -1370,7 +1370,11 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 		emit_line(o, "", "", line, len);
 		break;
 	case DIFF_SYMBOL_CONTEXT_INCOMPLETE:
-		set = diff_get_color_opt(o, DIFF_CONTEXT);
+		if ((flags & WS_INCOMPLETE_LINE) &&
+		    (flags & o->ws_error_highlight))
+			set = diff_get_color_opt(o, DIFF_WHITESPACE);
+		else
+			set = diff_get_color_opt(o, DIFF_CONTEXT);
 		reset = diff_get_color_opt(o, DIFF_RESET);
 		emit_line(o, set, reset, line, len);
 		break;
@@ -1666,8 +1670,14 @@ static void emit_context_line(struct emit_callback *ecbdata,
 static void emit_incomplete_line(struct emit_callback *ecbdata,
 				 const char *line, int len)
 {
+	int last_line_kind = ecbdata->last_line_kind;
+	unsigned flags = (last_line_kind == '+'
+			  ? WSEH_NEW
+			  : last_line_kind == '-'
+			  ? WSEH_OLD
+			  : WSEH_CONTEXT) | ecbdata->ws_rule;
 	emit_diff_symbol(ecbdata->opt, DIFF_SYMBOL_CONTEXT_INCOMPLETE,
-			 line, len, 0);
+			 line, len, flags);
 }
 
 static void emit_hunk_header(struct emit_callback *ecbdata,
@@ -3257,6 +3267,7 @@ struct checkdiff_t {
 	struct diff_options *o;
 	unsigned ws_rule;
 	unsigned status;
+	int last_line_kind;
 };
 
 static int is_conflict_marker(const char *line, int marker_size, unsigned long len)
@@ -3295,6 +3306,7 @@ static void checkdiff_consume_hunk(void *priv,
 static int checkdiff_consume(void *priv, char *line, unsigned long len)
 {
 	struct checkdiff_t *data = priv;
+	int last_line_kind;
 	int marker_size = data->conflict_marker_size;
 	const char *ws = diff_get_color(data->o->use_color, DIFF_WHITESPACE);
 	const char *reset = diff_get_color(data->o->use_color, DIFF_RESET);
@@ -3305,6 +3317,8 @@ static int checkdiff_consume(void *priv, char *line, unsigned long len)
 	assert(data->o);
 	line_prefix = diff_line_prefix(data->o);
 
+	last_line_kind = data->last_line_kind;
+	data->last_line_kind = line[0];
 	if (line[0] == '+') {
 		unsigned bad;
 		data->lineno++;
@@ -3327,6 +3341,17 @@ static int checkdiff_consume(void *priv, char *line, unsigned long len)
 			      data->o->file, set, reset, ws);
 	} else if (line[0] == ' ') {
 		data->lineno++;
+	} else if (line[0] == '\\') {
+		/* no newline at the end of the line */
+		if ((data->ws_rule & WS_INCOMPLETE_LINE) &&
+		    (last_line_kind == '+')) {
+			unsigned bad = WS_INCOMPLETE_LINE;
+			data->status |= bad;
+			err = whitespace_error_string(bad);
+			fprintf(data->o->file, "%s%s:%d: %s.\n",
+				line_prefix, data->filename, data->lineno, err);
+			free(err);
+		}
 	}
 	return 0;
 }
diff --git a/t/t4015-diff-whitespace.sh b/t/t4015-diff-whitespace.sh
index 9de7f73f42..138730cbce 100755
--- a/t/t4015-diff-whitespace.sh
+++ b/t/t4015-diff-whitespace.sh
@@ -43,6 +43,49 @@ do
 	'
 done
 
+test_expect_success "incomplete line in both pre- and post-image context" '
+	(echo foo && echo baz | tr -d "\012") >x &&
+	git add x &&
+	(echo bar && echo baz | tr -d "\012") >x &&
+	git diff x &&
+	git -c core.whitespace=incomplete diff --check x &&
+	git diff -R x &&
+	git -c core.whitespace=incomplete diff -R --check x
+'
+
+test_expect_success "incomplete lines on both pre- and post-image" '
+	# The interpretation taken here is "since you are toucing
+	# the line anyway, you would better fix the incomplete line
+	# while you are at it."  but this is debatable.
+	echo foo | tr -d "\012" >x &&
+	git add x &&
+	echo bar | tr -d "\012" >x &&
+	git diff x &&
+	test_must_fail git -c core.whitespace=incomplete diff --check x &&
+	git diff -R x &&
+	test_must_fail git -c core.whitespace=incomplete diff -R --check x
+'
+
+test_expect_success "fix incomplete line in pre-image" '
+	echo foo | tr -d "\012" >x &&
+	git add x &&
+	echo bar >x &&
+	git diff x &&
+	git -c core.whitespace=incomplete diff --check x &&
+	git diff -R x &&
+	test_must_fail git -c core.whitespace=incomplete diff -R --check x
+'
+
+test_expect_success "new incomplete line in post-image" '
+	echo foo >x &&
+	git add x &&
+	echo bar | tr -d "\012" >x &&
+	git diff x &&
+	test_must_fail git -c core.whitespace=incomplete diff --check x &&
+	git diff -R x &&
+	git -c core.whitespace=incomplete diff -R --check x
+'
+
 test_expect_success "Ray Lehtiniemi's example" '
 	cat <<-\EOF >x &&
 	do {
@@ -1040,7 +1083,8 @@ test_expect_success 'ws-error-highlight test setup' '
 	{
 		echo "0. blank-at-eol " &&
 		echo "1. still-blank-at-eol " &&
-		echo "2. and a new line "
+		echo "2. and a new line " &&
+		printf "3. and more"
 	} >x &&
 	new_hash_x=$(git hash-object x) &&
 	after=$(git rev-parse --short "$new_hash_x") &&
@@ -1050,11 +1094,13 @@ test_expect_success 'ws-error-highlight test setup' '
 	<BOLD>index $before..$after 100644<RESET>
 	<BOLD>--- a/x<RESET>
 	<BOLD>+++ b/x<RESET>
-	<CYAN>@@ -1,2 +1,3 @@<RESET>
+	<CYAN>@@ -1,2 +1,4 @@<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>
+	<GREEN>+<RESET><GREEN>3. and more<RESET>
+	<BLUE>\ No newline at end of file<RESET>
 	EOF
 
 	cat >expect.all <<-EOF &&
@@ -1062,11 +1108,13 @@ test_expect_success 'ws-error-highlight test setup' '
 	<BOLD>index $before..$after 100644<RESET>
 	<BOLD>--- a/x<RESET>
 	<BOLD>+++ b/x<RESET>
-	<CYAN>@@ -1,2 +1,3 @@<RESET>
+	<CYAN>@@ -1,2 +1,4 @@<RESET>
 	 <RESET>0. blank-at-eol<RESET><BLUE> <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>
+	<GREEN>+<RESET><GREEN>3. and more<RESET>
+	<BLUE>\ No newline at end of file<RESET>
 	EOF
 
 	cat >expect.none <<-EOF
@@ -1074,16 +1122,19 @@ test_expect_success 'ws-error-highlight test setup' '
 	<BOLD>index $before..$after 100644<RESET>
 	<BOLD>--- a/x<RESET>
 	<BOLD>+++ b/x<RESET>
-	<CYAN>@@ -1,2 +1,3 @@<RESET>
+	<CYAN>@@ -1,2 +1,4 @@<RESET>
 	 0. blank-at-eol <RESET>
 	<RED>-1. blank-at-eol <RESET>
 	<GREEN>+1. still-blank-at-eol <RESET>
 	<GREEN>+2. and a new line <RESET>
+	<GREEN>+3. and more<RESET>
+	\ No newline at end of file<RESET>
 	EOF
 
 '
 
 test_expect_success 'test --ws-error-highlight option' '
+	git config core.whitespace blank-at-eol,incomplete-line &&
 
 	git diff --color --ws-error-highlight=default,old >current.raw &&
 	test_decode_color <current.raw >current &&
@@ -1100,6 +1151,7 @@ test_expect_success 'test --ws-error-highlight option' '
 '
 
 test_expect_success 'test diff.wsErrorHighlight config' '
+	git config core.whitespace blank-at-eol,incomplete-line &&
 
 	git -c diff.wsErrorHighlight=default,old diff --color >current.raw &&
 	test_decode_color <current.raw >current &&
@@ -1116,6 +1168,7 @@ test_expect_success 'test diff.wsErrorHighlight config' '
 '
 
 test_expect_success 'option overrides diff.wsErrorHighlight' '
+	git config core.whitespace blank-at-eol,incomplete-line &&
 
 	git -c diff.wsErrorHighlight=none \
 		diff --color --ws-error-highlight=default,old >current.raw &&
@@ -1135,6 +1188,8 @@ test_expect_success 'option overrides diff.wsErrorHighlight' '
 '
 
 test_expect_success 'detect moved code, complete file' '
+	git config core.whitespace blank-at-eol &&
+
 	git reset --hard &&
 	cat <<-\EOF >test.c &&
 	#include<stdio.h>
-- 
2.52.0-rc0