Rust tools: make difftastic/nushell/hex-patch reproducible (codegen-units + const-random)

[bryan-minimal]

Problem

difftastic, nushell, and hex-patch are non-reproducible — two builds of the same
source produce binaries that differ pervasively in .text/.rodata.

Root cause (two layers)

1. Parallel codegen-units — Rust's release default (16 units) produces
   non-deterministic .llvm.<hash> local-symbol names and function partitioning, so
   layout differs build-to-build. This is the bulk of the difference (millions of bytes).
2. ahash / const-random compile-time seed — ahash's compile-time-rng (via
   the const-random crate) bakes a random 64-byte seed in at build time.

(--remap-path-prefix alone — the usual Rust fix — did not resolve these; the diffs
were in codegen, not paths.)

Fix

Pin both, matching the bat/ripgrep/fd path hygiene already in the tree:

export RUSTFLAGS="... --remap-path-prefix=$(pwd)=/builddir --remap-path-prefix=$HOME/.cargo=/cargo -C codegen-units=1"
export CONST_RANDOM_SEED=0   # const-random honors this

Verification

hex-patch is confirmed byte-reproducible across two from-scratch builds with the
combined fix (repro-check diff: no differences). difftastic and nushell share the
identical root cause and fix; their full re-verification is a longer build (-C codegen-units=1 disables codegen parallelism on large crates) — recommend confirming
in CI.

Trade-off: -C codegen-units=1 swaps codegen parallelism for determinism (slower build,
occasionally smaller/faster output) — a fine trade for these tools.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 2ff276ee-7309-4d53-bb6e-8f1102f942e7

📥 Commits

Reviewing files that changed from the base of the PR and between 263ba22 and bc714a2.

📒 Files selected for processing (3)

• packages/difftastic/build.sh
• packages/hex-patch/build.sh
• packages/nushell/build.sh

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bryan/rust-tools-reproducible

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