Documentation: Improve guidance on compile-time options for binary size reduction

[teruyamato0731]

Hello, and first of all, thank you for the excellent {fmt} library. It's a powerful and essential tool for modern C++ development.

I am opening this issue to suggest an improvement to the documentation, specifically regarding the compile-time options available to reduce binary size.

Motivation

I am currently using {fmt} in an embedded environment as a replacement for std::format, with the primary goal of minimizing binary size. The library's documentation rightly highlights its small binary footprint, which is a major reason for its adoption in resource-constrained environments like mine.

While I was ultimately successful in significantly reducing the binary size by passing specific compile-time flags, I found that these options are not documented in a centralized or easily discoverable location.

The Challenge

My experience finding the right options involved:

• Sifting through changelogs where some flags were mentioned.
• Difficulty searching, as the term "macro" is generic and results are often mixed with deprecated flags from older versions or notes about them being disabled.
• A process of trial and error to find the combination that worked.

This makes it challenging for users, especially those who are specifically choosing {fmt} for its size benefits, to achieve those benefits without significant self-directed research.

Concrete Results

To demonstrate the impact of these flags, here are the binary size measurements from my environment. The difference is substantial.

Method	Binary Size
std::format	317 KiB
{fmt} (default configuration)	210 KiB
{fmt} with size-optimization options	29 KiB
<cstdio> printf only (for baseline)	18 KiB

The options I used to achieve the 29 KiB size were:

• FMT_BUILTIN_TYPES=0
• FMT_OPTIMIZE_SIZE=1
• FMT_USE_LOCALE=0

Proposal

I believe the documentation would be greatly improved by adding a dedicated section that covers compile-time configuration. This section could include:

1. A comprehensive list of all available compile-time options (e.g., FMT_HEADER_ONLY, FMT_USE_EXCEPTIONS, etc.).
2. A clear explanation of what each option enables, disables, or changes.
3. A specific guide or "recipe" for users who want to optimize for minimal binary size, explaining which flags are most effective.
4. A complete, buildable example, including a sample CMakeLists.txt, that demonstrates how to apply these settings to achieve a minimal footprint.

For reference, these are some of the options I came across during my research:

• FMT_HEADER_ONLY
• FMT_USE_EXCEPTIONS
• FMT_OS
• FMT_BUILTIN_TYPES
• FMT_OPTIMIZE_SIZE
• FMT_USE_LOCALE

Making this information more accessible would be incredibly valuable for the embedded community and anyone else concerned with code size. It would solidify {fmt}'s position as the go-to solution for high-performance, low-overhead formatting.

[vitaut]

I think it's a good idea. Would you be willing to submit an initial version of the doc with the options that you found useful as a new section in https://github.com/fmtlib/fmt/blob/master/doc/api.md?

[teruyamato0731]

Thank you for the positive response! Yes, I'd be happy to contribute. I'll start working on a draft for the new section in api.md and will submit a pull request in the coming days.