From 09a13244c85de35471ff7a660b17360111b14430 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Wed, 22 Jan 2020 19:08:06 -0800
Subject: [PATCH] Disallow passing non-string-literals to FMT_STRING

---
 include/fmt/format.h | 14 ++++++--------
 test/format-test.cc  |  3 ---
 2 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index cd3bb03f..59dc223b 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -3449,13 +3449,10 @@ template <typename Char> struct udl_arg {
   }
 };
 
-// A constexpr version of strlen.
-template <typename Char> FMT_CONSTEXPR size_t compute_length(const Char* s) {
-  size_t len = 0;
-  while (*s++) ++len;
-  return len;
+template <typename Char, size_t N>
+FMT_CONSTEXPR basic_string_view<Char> literal_to_view(const Char (&s)[N]) {
+  return {s, N - 1};
 }
-
 }  // namespace internal
 
 inline namespace literals {
@@ -3519,7 +3516,8 @@ FMT_END_NAMESPACE
       using char_type = fmt::remove_cvref_t<decltype(*s)>;   \
       __VA_ARGS__ FMT_CONSTEXPR                              \
       operator fmt::basic_string_view<char_type>() const {   \
-        return {s, fmt::internal::compute_length(s)};        \
+        /* FMT_STRING only accepts string literals. */       \
+        return fmt::internal::literal_to_view(s);            \
       }                                                      \
     };                                                       \
     return FMT_STRING();                                     \
@@ -3527,7 +3525,7 @@ FMT_END_NAMESPACE
 
 /**
   \rst
-  Constructs a compile-time format string.
+  Constructs a compile-time format string from a string literal *s*.
 
   **Example**::
 
diff --git a/test/format-test.cc b/test/format-test.cc
index f479ceb6..8bce69be 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1849,10 +1849,7 @@ TEST(FormatTest, UnpackedArgs) {
 struct string_like {};
 fmt::string_view to_string_view(string_like) { return "foo"; }
 
-FMT_CONSTEXPR_DECL const char* format_str_ptr = "0123456789";
-
 TEST(FormatTest, CompileTimeString) {
-  EXPECT_EQ(format_str_ptr, fmt::format(FMT_STRING(format_str_ptr)));
   EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), 42));
   EXPECT_EQ(L"42", fmt::format(FMT_STRING(L"{}"), 42));
   EXPECT_EQ("foo", fmt::format(FMT_STRING("{}"), string_like()));