unit-tests: do show relative file paths

Visual C interpolates `__FILE__` with the absolute _Windows_ path of
the source file. GCC interpolates it with the relative path, and the
tests even verify that.

So let's make sure that the unit tests only emit such paths.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/test-lib.c

@@ -21,6 +21,46 @@ static struct {
 	.result = RESULT_NONE,
 };
 
+#ifndef _MSC_VER
+#define make_relative(location) location
+#else
+/*
+ * Visual C interpolates the absolute Windows path for `__FILE__`,
+ * but we want to see relative paths, as verified by t0080.
+ */
+#include "dir.h"
+
+static const char *make_relative(const char *location)
+{
+	static char prefix[] = __FILE__, buf[PATH_MAX], *p;
+	static size_t prefix_len;
+
+	if (!prefix_len) {
+		size_t len = strlen(prefix);
+		const char *needle = "\\t\\unit-tests\\test-lib.c";
+		size_t needle_len = strlen(needle);
+
+		if (len < needle_len || strcmp(needle, prefix + len - needle_len))
+			die("unexpected suffix of '%s'", prefix);
+
+		/* let it end in a directory separator */
+		prefix_len = len - needle_len + 1;
+	}
+
+	/* Does it not start with the expected prefix? */
+	if (fspathncmp(location, prefix, prefix_len))
+		return location;
+
+	strlcpy(buf, location + prefix_len, sizeof(buf));
+	/* convert backslashes to forward slashes */
+	for (p = buf; *p; p++)
+		if (*p == '\\')
+			*p = '/';
+
+	return buf;
+}
+#endif
+
 static void msg_with_prefix(const char *prefix, const char *format, va_list ap)
 {
 	fflush(stderr);
@@ -147,7 +187,8 @@ int test__run_end(int was_run UNUSED, const char *location, const char *format,
 			break;
 
 		case RESULT_NONE:
-			test_msg("BUG: test has no checks at %s", location);
+			test_msg("BUG: test has no checks at %s",
+				 make_relative(location));
 			printf("not ok %d", ctx.count);
 			print_description(format, ap);
 			ctx.result = RESULT_FAILURE;
@@ -193,14 +234,16 @@ int test_assert(const char *location, const char *check, int ok)
 	assert(ctx.running);
 
 	if (ctx.result == RESULT_SKIP) {
-		test_msg("skipping check '%s' at %s", check, location);
+		test_msg("skipping check '%s' at %s", check,
+			 make_relative(location));
 		return 1;
 	}
 	if (!ctx.todo) {
 		if (ok) {
 			test_pass();
 		} else {
-			test_msg("check \"%s\" failed at %s", check, location);
+			test_msg("check \"%s\" failed at %s", check,
+				 make_relative(location));
 			test_fail();
 		}
 	}
@@ -225,7 +268,8 @@ int test__todo_end(const char *location, const char *check, int res)
 	if (ctx.result == RESULT_SKIP)
 		return 1;
 	if (res) {
-		test_msg("todo check '%s' succeeded at %s", check, location);
+		test_msg("todo check '%s' succeeded at %s", check,
+			 make_relative(location));
 		test_fail();
 	} else {
 		test_todo();