Enable the RUF015 lint in Ruff

.ruff.toml

@@ -231,7 +231,7 @@ select = [
     "RUF010",  # Use explicit conversion flag
 #    "RUF012",  # Mutable class attributes should be annotated with `typing.ClassVar`
     "RUF013",  # PEP 484 prohibits implicit `Optional`
-#    "RUF015",  # Prefer `next({iterable})` over single element slice
+    "RUF015",  # Prefer `next({iterable})` over single element slice
     "RUF016",  # Slice in indexed access to type `{value_type}` uses type `{index_type}` instead of an integer
     "RUF017",  # Avoid quadratic list summation
     "RUF018",  # Avoid assignment expressions in `assert` statements

tests/test_domains/test_domain_std.py

@@ -504,19 +504,23 @@ def test_productionlist(app):
     ul = nodes[2]
     cases = []
     for li in list(ul):
-        assert len(list(li)) == 1
-        p = list(li)[0]
+        li_list = list(li)
+        assert len(li_list) == 1
+        p = li_list[0]
         assert p.tag == 'p'
         text = str(p.text).strip(' :')
-        assert len(list(p)) == 1
-        a = list(p)[0]
+        p_list = list(p)
+        assert len(p_list) == 1
+        a = p_list[0]
         assert a.tag == 'a'
         link = a.get('href')
-        assert len(list(a)) == 1
-        code = list(a)[0]
+        a_list = list(a)
+        assert len(a_list) == 1
+        code = a_list[0]
         assert code.tag == 'code'
-        assert len(list(code)) == 1
-        span = list(code)[0]
+        code_list = list(code)
+        assert len(code_list) == 1
+        span = code_list[0]
         assert span.tag == 'span'
         linkText = span.text.strip()
         cases.append((text, link, linkText))

tests/test_extensions/test_ext_coverage.py

@@ -39,7 +39,7 @@ def test_build(app):
     )
     assert len(undoc_c) == 1
     # the key is the full path to the header file, which isn't testable
-    assert list(undoc_c.values())[0] == {('function', 'Py_SphinxTest')}
+    assert next(iter(undoc_c.values())) == {('function', 'Py_SphinxTest')}
 
     assert 'autodoc_target' in undoc_py
     assert 'funcs' in undoc_py['autodoc_target']

tests/test_util/test_util_nodes.py

@@ -180,7 +180,7 @@ def test_extract_messages_without_rawsource():
     document.append(p)
     _transform(document)
     assert_node_count(extract_messages(document), nodes.TextElement, 1)
-    assert [m for n, m in extract_messages(document)][0], 'text sentence'
+    assert next(m for n, m in extract_messages(document)), 'text sentence'
 
 
 def test_clean_astext():