consistently use "fallthrough" comments in switches

Gcc 7 adds -Wimplicit-fallthrough, which can warn when a
switch case falls through to the next case. The general idea
is that the compiler can't tell if this was intentional or
not, so you should annotate any intentional fall-throughs as
such, leaving it to complain about any unannotated ones.

There's a GNU __attribute__ which can be used for
annotation, but of course we'd have to #ifdef it away on
non-gcc compilers. Gcc will also recognize
specially-formatted comments, which matches our current
practice. Let's extend that practice to all of the
unannotated sites (which I did look over and verify that
they were behaving as intended).

Ideally in each case we'd actually give some reasons in the
comment about why we're falling through, or what we're
falling through to. And gcc does support that with
-Wimplicit-fallthrough=2, which relaxes the comment pattern
matching to anything that contains "fallthrough" (or a
variety of spelling variants). However, this isn't the
default for -Wimplicit-fallthrough, nor for -Wextra. In the
name of simplicity, it's probably better for us to support
the default level, which requires "fallthrough" to be the
only thing in the comment (modulo some window dressing like
"else" and some punctuation; see the gcc manual for the
complete set of patterns).

This patch suppresses all warnings due to
-Wimplicit-fallthrough. We might eventually want to add that
to the DEVELOPER Makefile knob, but we should probably wait
until gcc 7 is more widely adopted (since earlier versions
will complain about the unknown warning type).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 4 insertions, 1 deletions

diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index 4ccbfaac31..aee280ea2c 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -113,6 +113,7 @@ static int cat_one_file(int opt, const char *exp_type, const char *obj_name,
 
 		if (textconv_object(path, obj_context.mode, &oid, 1, &buf, &size))
 			break;
+		/* else fallthrough */
 
 	case 'p':
 		type = sha1_object_info(oid.hash, NULL);
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 5c202b7af5..d091f06274 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -436,6 +436,7 @@ static int reset_tree(struct tree *tree, const struct checkout_opts *o,
 		 * update paths in the work tree, and we cannot revert
 		 * them.
 		 */
+		/* fallthrough */
 	case 0:
 		return 0;
 	default:
diff --git a/builtin/remote-ext.c b/builtin/remote-ext.c
index bfb21ba7d2..6a9127a33c 100644
--- a/builtin/remote-ext.c
+++ b/builtin/remote-ext.c
@@ -57,7 +57,7 @@ static char *strip_escapes(const char *str, const char *service,
 				special = str[rpos];
 				if (rpos == 1)
 					break;
-				/* Fall-through to error. */
+				/* fallthrough */
 			default:
 				die("Bad remote-ext placeholder '%%%c'.",
 					str[rpos]);
diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 818fe74f0a..f6a5e1af5d 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1189,6 +1189,7 @@ static int push_check(int argc, const char **argv, const char *prefix)
 						break;
 					die("HEAD does not match the named branch in the superproject");
 				}
+				/* fallthrough */
 			default:
 				die("src refspec '%s' must name a ref",
 				    rs->src);