{fmt} can do the whole format at compile time

std::format checks your format string at compile time, which is the improvement everyone remembers from C++20. What it still does at runtime is parse it: walk the string, find the replacement fields, dispatch on each specifier.

{fmt}, the library std::format came from, has an option to do that work during compilation as well. Wrap the format string in FMT_COMPILE and the parse happens once, at compile time, emitting straight-line formatting code instead of a parse loop.

compile-time formatted: build-0042
fmt version: 120201

The function producing that string is constexpr and returns a fully formatted result:

constexpr auto make_tag() {
    return fmt::format(FMT_COMPILE("{}-{:04d}"), "build", 42);
}

Width, zero-fill and field order are all resolved during compilation. On a hot path that formats with a fixed layout (log lines, wire protocols, filenames) this removes the parse from every call. It is not free in every dimension, since specialised code is emitted per call site, so it is a targeted tool rather than a default.

What 12.2 adds

The current release brings three things worth knowing:

• A type-safe C formatting API (fmt/fmt-c.h), which is an unusual direction for a C++ library and aimed at C code that wants printf ergonomics without printf’s lack of type safety.
• A dedicated C++20 module target, fmt::fmt-module, so import fmt; is a supported configuration rather than something you assemble yourself.
• The full Dragonbox cache enabled by default, which speeds up floating-point formatting at a modest cost in binary size. Dragonbox is the shortest-round-trip float-to-string algorithm underneath both {fmt} and most std::format implementations.

Compiler Explorer currently carries 12.2.1, which is what FMT_VERSION reports as 120201 above.

Why keep using {fmt} when std::format exists

The honest answer is that {fmt} is where the features arrive first, and where the ones that never made it into the standard continue to live. There is no standard equivalent of FMT_COMPILE, nor of the C API or the shipped module target. std::format remains the right default for portable code, while {fmt} is what you reach for when you want compile-time formatting or your toolchain’s <format> is still catching up.

Sources: {fmt} on GitHub and its releases · format-string compilation docs.