PEP 831: clarify build backend flag propagation

peps/pep-0831.rst

@@ -23,8 +23,10 @@ This PEP proposes two things:
    them.**  The default build configuration is changed to compile the
    interpreter with ``-fno-omit-frame-pointer`` and
    ``-mno-omit-leaf-frame-pointer``. The flags are added to ``CFLAGS``, so they
-   apply to the interpreter itself and propagate to C extension modules built
-   against this Python via ``sysconfig``.  An opt-out ``configure`` flag
+   apply to the interpreter itself and are exposed through ``sysconfig`` to
+   build systems that deliberately consume CPython's reported compiler flags,
+   including Setuptools. They do not propagate automatically through every
+   PEP 517 build backend. An opt-out ``configure`` flag
    (``--without-frame-pointers``) is provided for deployments that require
    maximum raw throughput.
 
@@ -397,11 +399,13 @@ breaks the chain for the entire process.
 The need for a continuous chain is precisely why the flags must propagate to
 extension builds. If only the interpreter has frame pointers but extensions do
 not, the chain is still broken at every C extension boundary.  By adding the
-flags to ``CFLAGS`` as reported by ``sysconfig``, extension builds that consume
-CPython's compiler flags (for example via ``pip install``, Setuptools, or
-other build backends) will inherit frame pointers by default.  Extensions
-and libraries with independent build systems still need to enable the same
-flags themselves for the frame-pointer chain to remain continuous.
+flags to ``CFLAGS`` as reported by ``sysconfig``, extension builds that
+deliberately consume CPython's compiler flags (for example, Setuptools or a
+direct ``sysconfig`` consumer) will inherit frame pointers by default. Other
+PEP 517 backends do not necessarily inspect ``sysconfig`` and will not inherit
+the flags automatically. Extensions and libraries with independent build
+systems still need to enable the same flags themselves for the frame-pointer
+chain to remain continuous.
 
 The JIT Compiler Needs Frame Pointers to Be Debuggable
 ------------------------------------------------------
@@ -483,8 +487,11 @@ they propagate to third-party builds via ``sysconfig``.  This ensures:
    the ``python`` binary, ``libpython``, and built-in extension modules under
    ``Modules/``.
 2. The flags **are** written into the ``sysconfig`` data, so that third-party C
-   extensions built against this Python (via ``pip``, Setuptools, or direct
-   ``sysconfig`` queries) inherit frame pointers by default.
+   extensions built with Setuptools or other build systems that deliberately
+   query ``sysconfig`` inherit frame pointers by default. Other PEP 517
+   backends do not necessarily inspect ``sysconfig`` and will not inherit the
+   flags automatically. The future standardized interface for exposing these
+   build details to build backends is ``build-details.json`` (see :pep:`739`).
 
 Several architectures need adjustments to produce a walkable frame-pointer
 chain:
@@ -532,9 +539,11 @@ Ecosystem Impact
 ----------------
 
 Because the flags are in ``CFLAGS``, they propagate automatically to consumers
-that build against CPython's reported compiler flags, such as C extensions
-built via ``pip``, Setuptools, or direct ``sysconfig`` queries.  Those
-consumers need take no additional action to benefit from this change.
+that deliberately build against CPython's reported compiler flags, notably
+Setuptools and projects that query ``sysconfig`` directly. Those consumers
+need take no additional action to benefit from this change. Other PEP 517
+backends do not necessarily inspect ``sysconfig`` and will not inherit the
+flags automatically.
 
 Not all compiled code in the Python ecosystem inherits CPython's ``CFLAGS``.
 Rust extensions built with ``pyo3`` or ``maturin``, C++ libraries with their
@@ -890,11 +899,13 @@ unacceptable may use ``--without-frame-pointers``.
 Extension Build Impact
 ----------------------
 
-C extensions built against Python 3.15+ will inherit
-``-fno-omit-frame-pointer`` and ``-mno-omit-leaf-frame-pointer`` in their
-default ``CFLAGS`` from ``sysconfig``.  This is the same mechanism by which
-extensions already inherit ``-O2``, warning flags, and other compilation
-defaults.
+C extensions built against Python 3.15+ with Setuptools or another build
+backend that deliberately consumes CPython's reported compiler flags will
+inherit ``-fno-omit-frame-pointer`` and ``-mno-omit-leaf-frame-pointer`` in
+their default ``CFLAGS`` from ``sysconfig``.  This is the same mechanism by
+which extensions already inherit ``-O2``, warning flags, and other compilation
+defaults. Other PEP 517 backends do not necessarily inspect ``sysconfig`` and
+will not inherit these flags automatically.
 
 Extensions that set their own ``CFLAGS`` or use ``extra_compile_args`` in
 ``setup.py`` / ``pyproject.toml`` can override this default.  The last flag on