prefer-mypy-2-performance-milestone

Detects mypy versions below known performance milestones in the 2.x series and suggests incremental upgrades within the major version.

What it detects

• pyproject.toml, requirements.txt, setup.cfg, setup.py, poetry.lock, or similar files pinning mypy below a known 2.x speed milestone:
  • 2.0.x, 2.1.x, 2.2.x, or 2.3.x → suggest 2.4

The 1.x ladder is handled by prefer-mypy-performance-milestone, and the jump from 1.x to 2.x is handled by consider-mypy-2-upgrade.

Why it matters

mypy 2.4 enables the native Rust parser by default (significantly faster parsing), makes parallel type checking non-experimental with automatic worker selection (up to 5x with 8 workers), and speeds up generators and coroutines. It also includes the internal performance improvements from mypy 2.2. The 2.x migration is light, so this rule points straight at the latest 2.x milestone.

These are non-breaking upgrades within the 2.x series, so they are treated as warning rather than an advisory.

Suggested action

Move mypy to the next 2.x milestone and validate the CI type-check job afterward.

Measurement

Compare type-check times before and after upgrading, and with and without --num-workers.