This PR adds formatting support for std::stacktrace and std::stacktrace_entry from the C++23 <stacktrace> header.

Motivation

There is currently no way to format std::stacktrace or std::stacktrace_entry with {fmt}. These types are widely useful for diagnostic and logging output, and are a natural fit alongside the existing std::exception and std::source_location formatters already in fmt/std.h.

Example usage this PR enables:

auto trace = std::stacktrace::current();
fmt::print("{}\n", trace);
// Output:
// #0 main at main.cc:10
// #1 __libc_start_main at libc-start.c:308

fmt::print("{:>60}\n", trace[0]);
// Right-aligned: "                    main at main.cc:10"

Changes

include/fmt/std.h

• Added formatter<std::stacktrace_entry> — formats as function at file:line when debug symbols are available, or as a hex address otherwise
• Added formatter<std::stacktrace> — formats one indexed entry per line as #N description
• Both formatters inherit from formatter<string_view> to support fill, align, and width specs
• Include of <stacktrace> is guarded by FMT_CPLUSPLUS >= 202302L && FMT_HAS_INCLUDE(<stacktrace>) && defined(__cpp_lib_stacktrace)
• Both formatters marked FMT_EXPORT for correct C++20 module support

test/std-test.cc

• TEST(std_test, stacktrace_entry) — verifies description, source location formatting, and width/alignment specs
• TEST(std_test, stacktrace) — verifies indexed multi-line output, newline count, and alignment
• TEST(std_test, stacktrace_empty) — verifies an empty stacktrace formats to "" without crashing

doc/api.md

• Documented std::stacktrace and std::stacktrace_entry under the standard library formatters section with a description of the output format

Compatibility

• No impact on builds without C++23 stacktrace support — all new code is guarded by #ifdef __cpp_lib_stacktrace
• Works with: MSVC 2022 (17.x+), GCC 12+, Clang 14+
• No changes to existing formatters or public API