Fix CI: useless cast in the Zmij digit writer and snprintf truncation

- ci_test_gcc (Linux x86-64): static_cast<std::size_t>(d.significand % 100)
  was a useless cast (a std::uint64_t prvalue, the same type as
  std::size_t there); cast a named variable instead.
- ci_test_gcc: -Werror=format-truncation for snprintf("%.*e") in
  unit-to_chars.cpp, whose precision GCC cannot bound; write the
  neighboring decimal with a stream (classic locale, std::scientific),
  which gives the same text.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/conversions/to_chars.hpp

@@ -983,7 +983,8 @@ inline void shortest_digits(char* buf, int& len, int& decimal_exponent, double v
     std::size_t n = digits.size();
     while (d.significand >= 100)
     {
-        const auto i = static_cast<std::size_t>(d.significand % 100) * 2;
+        const std::uint64_t two_digits = d.significand % 100; // a variable: GCC calls a cast of the remainder useless where std::uint64_t is std::size_t
+        const auto i = static_cast<std::size_t>(two_digits) * 2;
         d.significand /= 100;
         n -= 2;
         digits[n] = pairs[i];

single_include/nlohmann/json.hpp

@@ -25427,7 +25427,8 @@ inline void shortest_digits(char* buf, int& len, int& decimal_exponent, double v
     std::size_t n = digits.size();
     while (d.significand >= 100)
     {
-        const auto i = static_cast<std::size_t>(d.significand % 100) * 2;
+        const std::uint64_t two_digits = d.significand % 100; // a variable: GCC calls a cast of the remainder useless where std::uint64_t is std::size_t
+        const auto i = static_cast<std::size_t>(two_digits) * 2;
         d.significand /= 100;
         n -= 2;
         digits[n] = pairs[i];

tests/src/unit-to_chars.cpp

@@ -20,8 +20,11 @@ using nlohmann::detail::dtoa_impl::reinterpret_bits;
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
+#include <iomanip>
 #include <limits>
+#include <locale>
 #include <random>
+#include <sstream>
 #include <string>
 #include <utility>
 #include <vector>
@@ -664,9 +667,11 @@ void check_shortest(double v)
     if (digits.size() > 1)
     {
         // the decimals of one digit fewer next to the value
-        std::array<char, 64> shorter{};
-        const int n = std::snprintf(shorter.data(), shorter.size(), "%.*e", static_cast<int>(digits.size()) - 2, v); // NOLINT(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
-        const auto near = digits_and_exponent(std::string(shorter.data(), static_cast<std::size_t>(n)));
+        // (a stream rather than snprintf("%.*e"), whose output GCC cannot bound)
+        std::ostringstream shorter;
+        shorter.imbue(std::locale::classic());
+        shorter << std::scientific << std::setprecision(static_cast<int>(digits.size()) - 2) << v;
+        const auto near = digits_and_exponent(shorter.str());
         // as an integer with digits.size() - 1 digits
         std::string m = near.first;
         int e = near.second;