chore: collect all #define PYBIND11_HAS_... macros in pybind11/detail/common.h

[rwgk]

Description

The trigger for working on this PR was seeing complicated code like this.

With the help of my best friend Chat Geepeetee:

This PR consolidates all PYBIND11_HAS_... feature macros into pybind11/detail/common.h.

Benefits:

• Simplifies user-side compatibility checks:

  • Users can reliably use #ifdef guards on PYBIND11_HAS_... macros without needing to worry about which pybind11 headers have been included.

• Enables safer header inclusion patterns:

  • Users can conditionally include newer pybind11 headers based on feature availability, regardless of current include state.

• Improves internal maintainability:

  • Simplifies refactoring within pybind11 itself by eliminating the need to manage include order just to ensure a given PYBIND11_HAS_... macro is visible.

This change promotes cleaner and more robust code, both for users and within pybind11's internals.

---

Minor details:

• All PYBIND11_HAS_... macros are now consistently defined as 1.
• The awkward PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL macro is no longer needed (see changes in include/pybind11/stl/filesystem.h and tests/test_stl.cpp).

---

Before this PR:

$ git switch master
Switched to branch 'master'
Your branch is up to date with 'upstream/master'.

$ git grep '^# *define  *PYBIND11_HAS_' | grep -v '^include/pybind11/detail/common.h:' | cutniq
include/pybind11/detail/cpp_conduit.h
include/pybind11/detail/internals.h
include/pybind11/detail/native_enum_data.h
include/pybind11/functional.h
include/pybind11/stl/filesystem.h
tests/test_stl.cpp

With this PR:

$ git switch collect_define_pybind11_has
Switched to branch 'collect_define_pybind11_has'

$ git grep '^# *define  *PYBIND11_HAS_' | grep -v '^include/pybind11/detail/common.h:' | cutniq

(no output)

Suggested changelog entry:

Consolidated all ``PYBIND11_HAS_...`` feature macros into ``pybind11/detail/common.h`` to streamline backward compatibility checks and simplify internal refactoring. This change ensures consistent macro availability regardless of header inclusion order.

[rwgk]

@XuehaiPan for a second set of eyes

[henryiii]

With the help of my best friend Chat Geepeete

I think you can just click "request Copilot review" and Copilot basically generates this, only as a followup comment.

[Copilot]

Pull Request Overview

This PR consolidates all feature macros starting with PYBIND11_HAS_ into a single header (pybind11/detail/common.h) to simplify user compatibility checks and improve internal header inclusion order. Key changes include:

• Removing separate definitions of PYBIND11_HAS_* macros from various files and centralizing them in common.h.
• Updating filesystem support in both tests and header code to rely on the consolidated filesystem macros.
• Eliminating legacy or redundant macros such as PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL and PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_stl.cpp	Removed legacy filesystem optional macro and updated include guard logic.
include/pybind11/stl/filesystem.h	Refactored conditional filesystem includes to use centralized macros.
include/pybind11/functional.h	Removed standalone PYBIND11_HAS_TYPE_CASTER_STD_FUNCTION_SPECIALIZATIONS macro.
include/pybind11/detail/native_enum_data.h	Removed legacy native enum macro definition.
include/pybind11/detail/internals.h	Removed backward compatibility macro for smart holder support.
include/pybind11/detail/cpp_conduit.h	Removed standalone cpp conduit macro definition.
include/pybind11/detail/common.h	Centralized all feature macros and updated conditions for newer standards.

[Copilot]

The removal of the PYBIND11_HAS_FILESYSTEM_IS_OPTIONAL branch changes the behavior for environments where neither nor <experimental/filesystem> is available. Consider updating the documentation or migration notes to inform users of this breaking change.

[rwgk]

I reasoned this out with the help of Chat:

https://chatgpt.com/share/681da436-a5fc-8008-8d13-402e8bcbaa8e

✅ Conclusion
You're preserving — and even clarifying — expected behavior by:

Removing a test-only macro,

Consolidating detection macros in a single header,

Ensuring that #include <pybind11/stl/filesystem.h> fails clearly if the compiler lacks support.

So: you’re not overlooking anything, and the PR is correct as is.

[rwgk]

Thanks for the reviews @henryiii and @XuehaiPan!