Deprecate support for source encodings other than UTF-8

CHANGES.rst

@@ -23,6 +23,9 @@ Deprecated
   Patch by Adam Turner.
 * #13644: Deprecate the :py:attr:`!Parser.config` and :py:attr:`!env` attributes.
   Patch by Adam Turner.
+* #13665: Deprecate support for non-UTF 8 source encodings,
+  scheduled for removal in Sphinx 10.
+  Patch by Adam Turner.
 
 Features added
 --------------

doc/usage/configuration.rst

@@ -1157,6 +1157,9 @@ Options for source files
    The recommended encoding is ``'utf-8-sig'``.
 
    .. versionadded:: 0.5
+   .. deprecated:: 8.3
+      Support for source encodings other than UTF-8 is deprecated.
+      Sphinx 10 will only support UTF-8 files.
 
 .. confval:: source_suffix
    :type: :code-py:`dict[str, str] | Sequence[str] | str`

doc/usage/restructuredtext/basics.rst

@@ -646,10 +646,11 @@ configurations:
 Source encoding
 ---------------
 
-Since the easiest way to include special characters like em dashes or copyright
-signs in reStructuredText is to directly write them as Unicode characters, one has to
-specify an encoding.  Sphinx assumes source files to be encoded in UTF-8 by
-default; you can change this with the :confval:`source_encoding` config value.
+Sphinx supports source files that are encoded in UTF-8.
+This means that the full range of Unicode__ characters may be used
+directly in reStructuredText.
+
+__ https://www.unicode.org/
 
 
 Gotchas

doc/usage/restructuredtext/directives.rst

@@ -971,7 +971,7 @@ __ https://pygments.org/docs/lexers
       :type: text
 
       Explicitly specify the encoding of the file.
-      This overwrites the default encoding (:confval:`source_encoding`).
+      This overwrites the default encoding (UTF-8).
       For example:
 
       .. code-block:: rst

sphinx/config.py

@@ -895,7 +895,21 @@ def check_master_doc(
     return changed
 
 
+def deprecate_source_encoding(_app: Sphinx, config: Config) -> None:
+    """Warn on non-UTF 8 source_encoding."""
+    # RemovedInSphinx10Warning
+    if config.source_encoding.lower() not in {'utf-8', 'utf-8-sig', 'utf8'}:
+        msg = _(
+            'Support for source encodings other than UTF-8 '
+            'is deprecated and will be removed in Sphinx 10. '
+            'Please comment at https://github.com/sphinx-doc/sphinx/issues/13665 '
+            'if this causes a problem.'
+        )
+        logger.warning(msg)
+
+
 def setup(app: Sphinx) -> ExtensionMetadata:
+    app.connect('config-inited', deprecate_source_encoding, priority=790)
     app.connect('config-inited', convert_source_suffix, priority=800)
     app.connect('config-inited', convert_highlight_options, priority=800)
     app.connect('config-inited', init_numfig_format, priority=800)

tests/test_config/test_config.py

@@ -19,14 +19,14 @@
 )
 from sphinx.deprecation import RemovedInSphinx90Warning
 from sphinx.errors import ConfigError, ExtensionError, VersionRequirementError
+from sphinx.testing.util import SphinxTestApp
 from sphinx.util.tags import Tags
 
 if TYPE_CHECKING:
     from collections.abc import Iterable
+    from pathlib import Path
     from typing import TypeAlias
 
-    from sphinx.testing.util import SphinxTestApp
-
     CircularList: TypeAlias = list[int | 'CircularList']
     CircularDict: TypeAlias = dict[str, int | 'CircularDict']
 
@@ -811,3 +811,14 @@ def test_root_doc_and_master_doc_are_synchronized() -> None:
     c.root_doc = '1234'
     assert c.master_doc == '1234'
     assert c.root_doc == c.master_doc
+
+
+def test_source_encoding_deprecation(tmp_path: Path) -> None:
+    (tmp_path / 'conf.py').touch()
+    app = SphinxTestApp(
+        buildername='dummy',
+        srcdir=tmp_path,
+        confoverrides={'source_encoding': 'latin-1'},
+    )
+    expected = 'Support for source encodings other than UTF-8 is deprecated and will be removed'
+    assert expected in app.warning.getvalue()