Refactor sphinx.environment: Reimplement filter_messages() as a transform

tests/roots/test-keep_warnings/conf.py

@@ -0,0 +1,4 @@
+# -*- coding: utf-8 -*-
+
+master_doc = 'index'
+keep_warnings = True

tests/roots/test-keep_warnings/index.rst

@@ -0,0 +1,2 @@
+keep_warnings
+=====

tests/test_markup.py

@@ -170,3 +170,21 @@ def test_rst_prolog(app, status, warning):
     # rst_prolog & rst_epilog on exlucding reST parser
     assert not md.rawsource.startswith('*Hello world*.')
     assert not md.rawsource.endswith('*Good-bye world*.\n')
+
+
+@with_app(buildername='dummy', testroot='keep_warnings')
+def test_keep_warnings_is_True(app, status, warning):
+    app.builder.build_all()
+    doctree = pickle.loads((app.doctreedir / 'index.doctree').bytes())
+    assert_node(doctree[0], nodes.section)
+    assert len(doctree[0]) == 2
+    assert_node(doctree[0][1], nodes.system_message)
+
+
+@with_app(buildername='dummy', testroot='keep_warnings',
+          confoverrides={'keep_warnings': False})
+def test_keep_warnings_is_False(app, status, warning):
+    app.builder.build_all()
+    doctree = pickle.loads((app.doctreedir / 'index.doctree').bytes())
+    assert_node(doctree[0], nodes.section)
+    assert len(doctree[0]) == 1