docs(r12): Rust macros for systems — example 43 + chapter 43 (Deep Dives)

[patterncatalyst]

The book's first single-language deep dive (Rust only): a cargo workspace with a proc-macro crate sysmacros and the app binary that uses it.

Example 43-rust-macros-for-systems (mode: local, langs: [rust])

• #[derive(SysError)] — a miniature thiserror: Display + std::error::Error generated from #[error("...")] on named/tuple enum variants via syn 2.0 + quote; malformed input becomes a compile_error!, not a panic.
• #[instrument] — an attribute macro logging a fn's entry (name+args), exit (return value), and Instant-measured duration.
• A sound unsafe unaligned read of a #[repr(C)] record from a byte slice, validated clean by miri; a defect-named --oob-read path reads 4 bytes past the 8-byte buffer, which miri catches as out-of-bounds UB.
• verify.lua asserts behaviour on the pinned stable toolchain (PASS 8 / FAIL 0); miri (nightly) is exercised by the chapter, not the CI gate.

First single-language example: demo.sh dispatches rust only, verify.lua skips non-rust, the runner renders cpp/go as -.

Chapter 43

To the spine (Part "Deep Dives"): Tools-used box, verbatim rust excerpts (no tri-language codetabs), Figures 43.1 (proc-macro pipeline) + 43.2 (miri bounds check), verified footer with the real miri diagnostic.

Gate

• --only 43 → 1 passed / 0 failed
• full --mode local → 97 passed / 0 failed / 3 skipped (skips are 38-otel, lgtm down)
• validate.py: OK

miri (first-hand): sound path exit 0 (no UB); --oob-read → Undefined Behavior: memory access failed: attempting to access 8 bytes, but got alloc25+0x4 which is only 4 bytes from the end at main.rs:57. Stable vs miri timing: 4µs vs 491785µs. Crates: syn 2.0, quote 1.0, proc-macro2 1.0; stable 1.97.1 + nightly miri.