avoid quadratic tree traversal in pre_order/post_order/leaves

[sahvx655-wq]

Description

Profiling blackd on a deeply nested expression such as a long a ** b ** c ** ... power chain showed pre_order and leaves in blib2to3/pytree.py dominating the trace and growing quadratically. get_features_used runs one pre_order walk over the whole tree on every format pass, so it is an easy place to measure: that walk alone took 8.8ms at 500 terms, 33.9ms at 1000, 132ms at 2000 and 520ms at 4000, quadrupling every time the depth doubled. All three traversal helpers (pre_order, post_order, leaves) descend with yield from child.<method>(), and because yield from resumes every delegating generator in the chain on each step, one full walk costs O(depth) per node and is O(n^2) once the tree is n levels deep. This pass runs before any formatting, so a deeply nested submission is a cheap pre-auth amplification vector through unauthenticated blackd.

Replacing the recursive delegation with an explicit stack visits each node once regardless of depth. The traversal order is unchanged: I checked it leaf-for-leaf and in pre/post-order against the recursive form on assorted trees, and formatting is byte-identical across the whole tests/data corpus in stable, preview and unstable modes plus the full test suite (434 passed). Keeping the fix in the traversal helpers themselves linearises every caller at once (get_features_used, normalize_fmt_off, the --line-ranges walk and the rest) instead of each having to special-case deep input; the same 4000-term walk drops from ~520ms to ~5.9ms and the curve goes from quadratic to linear.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[github-actions]

diff-shades results comparing this PR (faaeb51) to main (51abf53):

--preview style: no changes

--stable style: no changes

---

What is this? | Workflow run | diff-shades documentation