Explain deprecated parts of API in README

README.rst

@@ -1,3 +1,6 @@
+API to call PEP 517 hooks
+=========================
+
 `PEP 517 <https://www.python.org/dev/peps/pep-0517/>`_ specifies a standard
 API for systems which build Python packages.
 
@@ -8,28 +11,10 @@ provides:
   the current process.
 - Fallbacks for the optional hooks, so that frontends can call the hooks without
   checking which are defined.
-- Higher-level functions which install the build dependencies into a
-  temporary environment and build a wheel/sdist using them.
 
 Run the tests with ``pytest`` or `tox <https://pypi.org/project/tox>`_.
 
-High level usage, with build requirements handled:
-
-.. code-block:: python
-
-    import os
-    from pep517.envbuild import build_wheel, build_sdist
-
-    src = 'path/to/source'  # Folder containing 'pyproject.toml'
-    destination = 'also/a/folder'
-    whl_filename = build_wheel(src, destination)
-    assert os.path.isfile(os.path.join(destination, whl_filename))
-
-    targz_filename = build_sdist(src, destination)
-    assert os.path.isfile(os.path.join(destination, targz_filename))
-
-Lower level usage—you are responsible for ensuring build requirements are
-available:
+Usage—you are responsible for ensuring build requirements are available:
 
 .. code-block:: python
 
@@ -42,6 +27,8 @@ available:
         build_sys = toml.load(f)['build-system']
 
     print(build_sys['requires'])  # List of static requirements
+    # The caller is responsible for installing these and running the hooks in
+    # an environment where they are available.
 
     hooks = Pep517HookCaller(
         src, 
@@ -52,11 +39,38 @@ available:
     config_options = {}   # Optional parameters for backend
     # List of dynamic requirements:
     print(hooks.get_requires_for_build_wheel(config_options))
+    # Again, the caller is responsible for installing these build requirements
 
     destination = 'also/a/folder'
     whl_filename = hooks.build_wheel(destination, config_options)
     assert os.path.isfile(os.path.join(destination, whl_filename))
 
+Deprecated high-level
+---------------------
+
+For now, ``pep517`` also contains higher-level functions which install the build
+dependencies into a temporary environment and build a wheel/sdist using them.
+This is a rough implementation, e.g. it does not do proper build isolation.
+The `PyPA build project <https://github.com/pypa/build>`_ is recommended as an
+alternative, although it's still quite young in October 2020.
+This layer of functionality in ``pep517`` is now deprecated, but won't be
+removed for some time, as there is code relying on it.
+
+High level usage, with build requirements handled:
+
+.. code-block:: python
+
+    import os
+    from pep517.envbuild import build_wheel, build_sdist
+
+    src = 'path/to/source'  # Folder containing 'pyproject.toml'
+    destination = 'also/a/folder'
+    whl_filename = build_wheel(src, destination)
+    assert os.path.isfile(os.path.join(destination, whl_filename))
+
+    targz_filename = build_sdist(src, destination)
+    assert os.path.isfile(os.path.join(destination, targz_filename))
+
 To test the build backend for a project, run in a system shell:
 
 .. code-block:: shell
@@ -69,6 +83,4 @@ To build a backend into source and/or binary distributions, run in a shell:
 
     python -m pep517.build path/to/source  # source dir containing pyproject.toml
 
-This 'build' module should be considered experimental while the PyPA `decides
-on the best place for this functionality
-<https://github.com/pypa/packaging-problems/issues/219>`_.
+All of this high-level functionality is deprecated.