[PATCH/RFC 2/4] skip_prefix: return a non-const pointer

[Jeff King <peff@peff.net>]

The const rules in C are such that one cannot write a
function that takes a const or non-const pointer and returns
a pointer that matches the input in const-ness. Instead, you
must take a const pointer (because you are promising not to
modify it), and then either return a const pointer (which is
safer, but annoying to callers who originally had a
non-const pointer) or a non-const pointer (less safe, as you
may accidentally drop constness, but less annoying).

This is a well-known problem, and the standard string
functions like strchr take the "less annoying" approach.
Let's mimic them. Even though this is technically less safe,
skip_prefix tends to be used alongside standard string
manipulation functions already, so it is not really
introducing a new problem.

Signed-off-by: Jeff King <peff@peff.net>
---
I have mixed feelings on this. It _is_ less safe, and this is a known
bug in the C standard. Still, it seems like the more idiomatic C thing
to do.

My main motivation is to avoid a bunch of casts in the next patch.

 builtin/commit.c  | 2 +-
 git-compat-util.h | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index 3348aa1..bb6890b 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -895,7 +895,7 @@ static int template_untouched(struct strbuf *sb)
 		return 0;
 
 	stripspace(&tmpl, cleanup_mode == CLEANUP_ALL);
-	start = (char *)skip_prefix(sb->buf, tmpl.buf);
+	start = skip_prefix(sb->buf, tmpl.buf);
 	if (!start)
 		start = sb->buf;
 	strbuf_release(&tmpl);
diff --git a/git-compat-util.h b/git-compat-util.h
index b7eaaa9..56c066b 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -320,10 +320,10 @@ static inline const char *skip_prefix(const char *str, const char *prefix)
 extern int prefixcmp(const char *str, const char *prefix);
 extern int suffixcmp(const char *str, const char *suffix);
 
-static inline const char *skip_prefix(const char *str, const char *prefix)
+static inline char *skip_prefix(const char *str, const char *prefix)
 {
 	size_t len = strlen(prefix);
-	return strncmp(str, prefix, len) ? NULL : str + len;
+	return strncmp(str, prefix, len) ? NULL : (char *)(str + len);
 }
 
 #if defined(NO_MMAP) || defined(USE_WIN32_MMAP)
-- 
1.8.1.4.4.g265d2fa