Reduced the library size. For example, on macOS a stripped test binary statically linked with {fmt} shrank from ~368k to less than 100k.

Added a simpler and more efficient format string compilation API:

#include <fmt/compile.h>

// Converts 42 into std::string using the most efficient method and no
// runtime format string processing.
std::string s = fmt::format(FMT_COMPILE("{}"), 42);

The old fmt::compile API is now deprecated.

Thanks @ldionne (Louis Dionne) for the suggestion and @hanickadot (Hana Dusíková) for the initial prototype.

Optimized integer formatting: format_to with format string compilation and a stack-allocated buffer is now faster than to_chars on both libc++ and libstdc++.

Optimized handling of small format strings. For example,

fmt::format("Result: {}: ({},{},{},{})", str1, str2, str3, str4, str5)

is now ~40% faster (#1685).

Applied extern templates to improve compile times when using the core API and fmt/format.h (#1452). For example, on macOS with clang the compile time of a test translation unit dropped from 2.3s to 0.3s with -O2 and from 0.6s to 0.3s with the default settings (-O0).

Before (-O2):

% time c++ -c test.cc -I include -std=c++17 -O2
c++ -c test.cc -I include -std=c++17 -O2  2.22s user 0.08s system 99% cpu 2.311 total

After (-O2):

% time c++ -c test.cc -I include -std=c++17 -O2
c++ -c test.cc -I include -std=c++17 -O2  0.26s user 0.04s system 98% cpu 0.303 total

Before (default):

% time c++ -c test.cc -I include -std=c++17
c++ -c test.cc -I include -std=c++17  0.53s user 0.06s system 98% cpu 0.601 total

After (default):

% time c++ -c test.cc -I include -std=c++17
c++ -c test.cc -I include -std=c++17  0.24s user 0.06s system 98% cpu 0.301 total

It is still recommended to use fmt/core.h instead of fmt/format.h but the compile time difference is now smaller. Thanks @alex3d for the suggestion.

Named arguments are now stored on stack (no dynamic memory allocations) and the compiled code is more compact and efficient. For example

#include <fmt/core.h>

int main() {
  fmt::print("The answer is {answer}\n", fmt::arg("answer", 42));
}

compiles to just (godbolt)

.LC0:
        .string "answer"
.LC1:
        .string "The answer is {answer}\n"
main:
        sub     rsp, 56
        mov     edi, OFFSET FLAT:.LC1
        mov     esi, 23
        movabs  rdx, 4611686018427387905
        lea     rax, [rsp+32]
        lea     rcx, [rsp+16]
        mov     QWORD PTR [rsp+8], 1
        mov     QWORD PTR [rsp], rax
        mov     DWORD PTR [rsp+16], 42
        mov     QWORD PTR [rsp+32], OFFSET FLAT:.LC0
        mov     DWORD PTR [rsp+40], 0
        call    fmt::v6::vprint(fmt::v6::basic_string_view<char>,
                                fmt::v6::format_args)
        xor     eax, eax
        add     rsp, 56
        ret

    .L.str.1:
            .asciz  "answer"

Implemented compile-time checks for dynamic width and precision (#1614):

#include <fmt/format.h>

int main() {
  fmt::print(FMT_STRING("{0:{1}}"), 42);
}

now gives a compilation error because argument 1 doesn't exist:

In file included from test.cc:1:
include/fmt/format.h:2726:27: error: constexpr variable 'invalid_format' must be
initialized by a constant expression
  FMT_CONSTEXPR_DECL bool invalid_format =
                          ^
...
include/fmt/core.h:569:26: note: in call to
'&checker(s, {}).context_->on_error(&"argument not found"[0])'
    if (id >= num_args_) on_error("argument not found");
                        ^

Added sentinel support to fmt::join (#1689)

struct zstring_sentinel {};
bool operator==(const char* p, zstring_sentinel) { return *p == '\0'; }
bool operator!=(const char* p, zstring_sentinel) { return *p != '\0'; }

struct zstring {
  const char* p;
  const char* begin() const { return p; }
  zstring_sentinel end() const { return {}; }
};

auto s = fmt::format("{}", fmt::join(zstring{"hello"}, "_"));
// s == "h_e_l_l_o"

Thanks @BRevzin (Barry Revzin).

Added support for named args, clear and reserve to dynamic_format_arg_store (#1655, #1663, #1674, #1677). Thanks @vsolontsov-ll (Vladimir Solontsov).

Added support for the 'c' format specifier to integral types for compatibility with std::format (#1652).

Replaced the 'n' format specifier with 'L' for compatibility with std::format (#1624). The 'n' specifier can be enabled via the FMT_DEPRECATED_N_SPECIFIER macro.

The '=' format specifier is now disabled by default for compatibility with std::format. It can be enabled via the FMT_DEPRECATED_NUMERIC_ALIGN macro.

Removed the following deprecated APIs:

• FMT_STRING_ALIAS and fmt macros - replaced by FMT_STRING

• fmt::basic_string_view::char_type - replaced by fmt::basic_string_view::value_type

• convert_to_int

• format_arg_store::types

• *parse_context - replaced by *format_parse_context

• FMT_DEPRECATED_INCLUDE_OS

• FMT_DEPRECATED_PERCENT - incompatible with std::format

• *writer - replaced by compiled format API

Renamed the internal namespace to detail (#1538). The former is still provided as an alias if the FMT_USE_INTERNAL macro is defined.

Improved compatibility between fmt::printf with the standard specs (#1595, #1682, #1683, #1687, #1699). Thanks @rimathia.

Fixed handling of operator<< overloads that use copyfmt (#1666).

Added the FMT_OS CMake option to control inclusion of OS-specific APIs in the fmt target. This can be useful for embedded platforms (#1654, #1656). Thanks @kwesolowski (Krzysztof Wesolowski).

Replaced FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION with the FMT_FUZZ macro to prevent interferring with fuzzing of projects using {fmt} (#1650). Thanks @asraa (Asra Ali).

Fixed compatibility with emscripten (#1636, #1637). Thanks @ArthurSonzogni (Arthur Sonzogni).

Improved documentation (#704, #1643, #1660, #1681, #1691, #1706, #1714, #1721, #1739, #1740, #1741, #1751). Thanks @senior7515 (Alexander Gallego), @lsr0 (Lindsay Roberts), @puetzk (Kevin Puetz), @fpelliccioni (Fernando Pelliccioni), Alexey Kuzmenko, @jelly (jelle van der Waa), @claremacrae (Clare Macrae), @jiapengwen (文佳鹏), @gsjaardema (Greg Sjaardema), @alexey-milovidov.

Implemented various build configuration fixes and improvements (#1603, #1657, #1702, #1728). Thanks @scramsby (Scott Ramsby), @jtojnar (Jan Tojnar), @orivej (Orivej Desh), @flagarde.

Fixed various warnings and compilation issues (#1616, #1620, #1622, #1625, #1627, #1628, #1629, #1631, #1633, #1649, #1658, #1661, #1667, #1668, #1669, #1692, #1696, #1697, #1707, #1712, #1716, #1722, #1724, #1729, #1738, #1742, #1743, #1744, #1747, #1750). Thanks @gsjaardema (Greg Sjaardema), @gabime (Gabi Melman), @johnor (Johan), @gabime (Dmitry Kurkin), @invexed (James Beach), @peterbell10, @daixtrose (Markus Werle), @petrutlucian94 (Lucian Petrut), @Neargye (Daniil Goncharov), @ambitslix (Attila M. Szilagyi), @gabime (Gabi Melman), @erthink (Leonid Yuriev), @tohammer (Tobias Hammer), @0x8000-0000 (Florin Iucha).