DOC: add a howto page on debug builds

Closes gh-461
Closes gh-207
Also xref gh-459 and gh-441 which are already closed but contain
discussion relevant to debug builds.

Author: rgommers

docs/how-to-guides/debug-builds.rst

@@ -0,0 +1,49 @@
+.. SPDX-FileCopyrightText: 2023 The meson-python developers
+..
+.. SPDX-License-Identifier: MIT
+
+.. _how-to-guides-debug-builds:
+
+******************
+Using debug builds
+******************
+
+For development work on native code in your Python package, you may want to use
+a debug build. To do so, we need to use the ``buildtype`` option, which is
+equivalent to ``-Ddebug=true -Doptimization=0``, to ``meson setup``. In addition,
+it is likely most useful to set up an editable build with a fixed build
+directory. That way, the shared libraries in the installed debug build will
+contain correct paths, rather than paths pointing to a temporary build
+directory which ``meson-python`` will otherwise use. IDEs and other tools will
+then be able to show correct file locations and line numbers during debugging.
+
+We can do all that with the following ``pip`` invocation:
+
+.. code-block:: console
+
+    $ pip install -e . --no-build-isolation -Csetup-args=-Dbuildtype=debug -Cbuilddir=build-dbg
+
+This debug build of your package will work with either a regular or debug build
+of your Python interpreter. A debug Python interpreter isn't necessary, but may
+be useful. It can be built from source, or you may be able to get it from your
+package manager of choice (typically under a package name like ``python-dbg``).
+Note that using a debug Python interpreter does not yield a debug build of your
+own package - you must use ``-Dbuildtype=debug`` or an equivalent flag for that.
+
+.. warning::
+
+    Inside a Conda environment, environment variables like ``CFLAGS`` and
+    ``CXXFLAGS`` are usually set when the environment is activated. These
+    environment variables contain optimization flags like ``-O2``, which will
+    override the optimization level implied by ``-Dbuildtype=debug``. In order
+    to avoid this, unset these variables:
+
+    .. code-block:: console
+
+        $ unset CFLAGS
+        $ unset CXXFLAGS
+
+Finally, note changing the buildtype from its default value of ``release`` to
+``debug`` will also cause ``meson-python`` to enable (or better, not disable)
+assertions by not defining the ``NDEBUG`` macro (see ``b_ndebug`` under
+:ref:`explanations-default-options`).

docs/index.rst

@@ -77,6 +77,7 @@ for quick questions - one ``meson-python`` maintainer is active there.
    how-to-guides/editable-installs
    how-to-guides/config-settings
    how-to-guides/meson-args
+   how-to-guides/debug-builds
    projects-using-meson-python
 
 .. how-to-guides/build-directory