Bun adds Rust porting guide

This is less a rewrite announcement than a formalization of the rewrite process, which is the more important signal. Once a project documents a mechanical translation path, the port stops being an experiment and becomes an engineering program.

• –The guide spells out a two-phase workflow: draft `.rs` files first, make them compile later
• –It prioritizes faithful structure over idiomatic Rust, which should make review and diffing far easier
• –The batch script suggests the team expects to port code at scale, not file-by-file ad hoc
• –The move is a maintainability bet, but it also raises the usual risks around parity, performance regressions, and borrow-checker churn
• –For Bun users, the near-term impact is mostly on contributor workflow rather than runtime behavior