Rust frontend — integer overflow mode + format-string parser

• Type: feature — Track A
• Status: backlog
• Owner: —
• Opened: 2026-07-03
• Umbrella: [[feature-rust-frontend]] — sub-ticket 9/12. Depends on [[feature-rust-frontend-skeleton]]. Bundled together as two small, self-contained, independently-testable pieces — not because they're related in concept.

What it does

1. Integer overflow mode. Rust panics on integer overflow in debug builds, wraps in release. PXX currently just wraps everywhere (like C). Add a mode flag, same pattern as the existing --no-div-check opt-out (see [[project_div_zero_re200_v135]] for the precedent this mirrors): a pre-op overflow check that raises/panics when the mode is on, no-ops (plain wrap, current behavior) when off. Cheap — one flag, one check site pattern already proven for div-by-zero.

2. Format-string mini-parser. println!("{} {:?}", a, b) needs a real format-spec parser: {} (Display), {:?} (Debug), positional/named args, width/precision/fill specifiers. Self-contained, bounded scope — nobody's designed this yet in this codebase. Feeds:

• [[feature-rust-rtl-macros-io]] (the println!/format! intrinsics)
• [[feature-rust-derive-macros]]'s Debug derive (needs to produce {:?}-compatible output)

Scope

• Format parser: cover {}, {:?}, positional ({0}), named ({name}), and the common width/precision/fill subset. Skip exotic specifiers ({:#x}-style alternate hex, {:+} sign-forcing) unless real usage demands them later — don't build for hypothetical format strings.
• Overflow mode: default matches Rust's own convention (panic in debug-equivalent build, wrap in release-equivalent) — tie to whatever PXX's existing debug/release distinction is (or add one if none exists yet at the flag level).

Acceptance

• {}/{:?}/positional/named/width/precision all format correctly against a small corpus of derived-Debug and Display-able types.
• Overflow mode: an Int32 add past MAX_INT panics when mode is on, wraps identically to current behavior when off; zero behavior change to existing Pascal/C code (opt-in flag, not a default-on change to shared codegen).

Log

• 2026-07-09 — the format-string half landed on master (ports-back pass, commit bbd15a52): {}/{:...} placeholder splitting behind println!/print!, lowered onto AN_WRITE/AN_WRITELN (the Zig frontend's splitter shape). {{ }} escapes and the integer-overflow mode remain open.

• 2026-07-03 — split from [[feature-rust-frontend]] umbrella at ticket-craft time. No code written yet.