feat(rewrite): introspect container[key] in failure messages

A subscript was opaque: the message showed the value it produced with no
indication of which container or key it came from.  Decompose it the way
attribute access already is.

The container goes through visit_operand() because taking the expression
away from generic_visit() takes away the hoisting that kept it ordered --
without that, `assert box[identity(box := other)] == 1` would start
reading the post-walrus container.  The order-axis guard in the coverage
matrix fails if this is dropped.

Slices keep the generic treatment; decomposing start/stop/step is rarely
what a failure message needs.

Closes the introspect-subscript group in the coverage matrix.

Author: RonnyPfannschmidt

changelog/14815.improvement.rst

@@ -0,0 +1,4 @@
+Assertion failure messages now decompose subscript expressions, showing the container and the key that produced a value::
+
+    assert 1 == 99
+     +  where 1 = {'a': 1, 'b': 2}['a']

src/_pytest/assertion/rewrite.py

@@ -1119,6 +1119,21 @@ def visit_Starred(self, starred: ast.Starred) -> tuple[ast.Starred, str]:
         new_starred = ast.Starred(res, starred.ctx)
         return new_starred, "*" + expl
 
+    def visit_Subscript(self, subscript: ast.Subscript) -> tuple[ast.Name, str]:
+        # For Slice objects (a[1:3]), fall back to generic — decomposing
+        # start/stop/step is rarely useful in assertion messages.
+        if isinstance(subscript.slice, ast.Slice):
+            return self.generic_visit(subscript)
+        value, value_expl = self.visit_operand(subscript.value, [subscript.slice])
+        slice_res, slice_expl = self.visit(subscript.slice)
+        res = self.assign(
+            ast.copy_location(ast.Subscript(value, slice_res, ast.Load()), subscript)
+        )
+        res_expl = self.explanation_param(self.display(res))
+        pat = "%s\n{%s = %s[%s]\n}"
+        expl = pat % (res_expl, res_expl, value_expl, slice_expl)
+        return res, expl
+
     def visit_Attribute(self, attr: ast.Attribute) -> tuple[ast.Name, str]:
         value, value_expl = self.visit(attr.value)
         res = self.assign(

testing/test_assertrewrite_coverage.py

@@ -601,12 +601,8 @@ def check():
 
 
 class TestIntrospectionSubscript:
-    """Subscript / indexing — currently hits generic_visit."""
+    """Subscript / indexing — has a dedicated visitor."""
 
-    @pytest.mark.xfail(
-        strict=True,
-        reason="introspect-subscript: container[key] is not decomposed",
-    )
     def test_dict_subscript_shows_key_and_container(self) -> None:
         assert_introspects(
             """
@@ -617,10 +613,6 @@ def check():
             must_contain=["where 1 = ", "['a']"],
         )
 
-    @pytest.mark.xfail(
-        strict=True,
-        reason="introspect-subscript: container[key] is not decomposed",
-    )
     def test_list_subscript_shows_index_and_container(self) -> None:
         assert_introspects(
             """
@@ -1389,10 +1381,6 @@ def identity(v):
 class TestEdgeCases:
     """Regression and edge-case tests combining multiple expression types."""
 
-    @pytest.mark.xfail(
-        strict=True,
-        reason="introspect-subscript: container[key] is not decomposed",
-    )
     def test_subscript_with_variable_key(self) -> None:
         """Subscript where the key is a variable (not constant)."""
         assert_introspects(