Fix pathologically slow assertion diffs for large inputs (#8998)

[kirilklein]

Closes #8998.

Problem

Comparing very large strings, lists, or dataclasses inside an assert can hang for a long time (sometimes minutes) while pytest builds the failure diff.

Profiling the reproductions from the issue confirms the root cause is difflib.ndiff:

• its character-level "fancy replace" step is quadratic in the size of the differing region (so two large, mostly-different strings are catastrophic), and
• the underlying SequenceMatcher is quadratic in the number of lines — a large nested structure pretty-prints to a huge number of lines (the dataclass example in the issue pformats to ~418,000 lines).

Approach

Following the maintainer discussion in the issue, this uses a deterministic size heuristic rather than wall-clock timeouts (which are non-deterministic and can't reliably interrupt difflib).

A new helper module _pytest/assertion/_diff.py provides:

• ndiff_too_slow(left_lines, right_lines) — True when the combined input exceeds a character budget or a line-count budget, the two dimensions that make ndiff slow.
• fast_unified_diff(...) — a coarse but fast line-level difflib.unified_diff, capped to a bounded number of lines so it always completes in milliseconds. It notes in the output that a faster diff is being shown (and how many lines were hidden).

Both pathological call sites fall back to it when needed:

• compare_text._diff_text (string comparisons)
• _compare_sequence._compare_eq_iterable (list / dataclass / iterable comparisons)

Comparisons below the cutoffs keep the existing detailed ndiff output unchanged.

Results

On the reproductions from the issue (dataclass with large lists + two large random strings), with -v:

• before: hangs (one repro profiled at ~384s of find_longest_match)
• after: ~0.7s, with a useful fallback diff

Tests

Added regression tests in testing/test_assertion.py: unit tests for the ndiff_too_slow heuristic, and integration tests that large string / many-line / large-iterable comparisons fall back to the fast diff (no ndiff ? guide lines), still show which lines differ, and emit the line-cap notice. Thresholds were chosen from benchmarking.

🤖 Generated with Claude Code

[Pierre-Sassoulas]

We have a flying MR to use generator in assert repr that could help with this when we don't have to show the actual output. (#14523)