scripts: add an assert-rewrite diff tool

[RonnyPfannschmidt]

Split out of #14447 at @Pierre-Sassoulas' request — the tooling there was
review aid, not part of the fix, and it is useful on its own.

What it does

scripts/diff-assert-rewrite.py dumps what the rewriter generates for a
snippet and diffs two sides. A side is plain (the source as written),
worktree (this checkout's src/), or a released pytest version, fetched on
demand with uv run --with pytest==VERSION — nothing under comparison has to
be installed. Output is either rewritten source (ast.unparse) or an AST.

The default — plain against worktree, as source — answers @bluetech's
question on #14447 directly, on main as it stands today:

$ python scripts/diff-assert-rewrite.py -c 'def test_walrus_boolop():
    assert (x := side_effect()) and (x := False)
'
--- plain
+++ worktree
@@ -1,2 +1,18 @@
+import builtins as @py_builtins
+import _pytest.assertion.rewrite as @pytest_ar
+
 def test_walrus_boolop():
-    assert (x := side_effect()) and (x := False)
+    @py_assert1 = []
+    @py_assert0 = (x := side_effect())
+    if (x := side_effect()):
+        @py_assert0 = (x := False)
+    if not @py_assert0:
+        @py_format3 = '%(py2)s' % {'py2': @pytest_ar._saferepr((x := side_effect())) if 'x' in @py_builtins.locals() or @pytest_ar._should_repr_global_name((x := side_effect())) else 'x'}
+        @py_assert1.append(@py_format3)
+        if (x := side_effect()):
+            @py_format5 = '%(py4)s' % {'py4': @pytest_ar._saferepr((x := False)) if 'x' in @py_builtins.locals() or @pytest_ar._should_repr_global_name((x := False)) else 'x'}
+            @py_assert1.append(@py_format5)
+        @py_format6 = @pytest_ar._format_boolop(@py_assert1, 0) % {}
+        @py_format8 = ('' + 'assert %(py7)s') % {'py7': @py_format6}
+        raise AssertionError(@pytest_ar._format_explanation(@py_format8))
+    @py_assert0 = @py_assert1 = None

side_effect() appears four times where the user wrote it once — that is
#14445, visible without reading the rewriter. Other modes:

# a behaviour change against a release, over a whole file
$ python scripts/diff-assert-rewrite.py --left 8.3.4 testing/example.py

# as AST, when the source form hides the difference
$ python scripts/diff-assert-rewrite.py --left 8.3.4 --format ast -c 'assert a == b'

# two releases against each other; also reads stdin
$ echo 'assert a == b' | python scripts/diff-assert-rewrite.py --left 6.2.5 --right 8.3.4

It exits 1 when the sides differ, so it works as a check as well as a viewer.

Notes

Down from the three files (410 lines) that were in #14447 to one file
(130 lines): the separate dump script is gone (it existed only to be
re-invoked by the diff script, which now spawns the two workers itself), the
example_asserts.py fixture is replaced by -c/stdin, and the compact AST
format is folded into ast — position attributes were never wanted.

@Pierre-Sassoulas: on the primer idea — running this over a selection of open
source repos to show output changes per PR is a bigger step (corpus choice,
a stored baseline, CI plumbing) and I would rather do it separately. This is
the piece it would be built on: a stable, scriptable "what does the rewriter
emit" that already exits non-zero on a difference.

Made with Claude Code

[Pierre-Sassoulas]

Just a nit, LGTM

[Pierre-Sassoulas]

Do we want to set the expected python version as 3.14 or more ?

[RonnyPfannschmidt]

Should probably be a cli arg

[Pierre-Sassoulas]

We could set the python interpreter version through uv