# rusty-sponge — v0.2.0 Feature Layout
**Status**: implementation draft for the v0.2.0 Cargo features convention
backfill (spec 00011, Phase 4 — rusty-sponge).
**Authority**:
- `specs/adrs/0006-cargo-features-convention-for-portfolio-ports.md` (why)
- `project-instructions.md` §Cargo Feature Surface (what)
- This document — the per-port carving + WHY for each leaf, per HINT-003
+ HINT-009 of spec 00011.
**Reference port**: rusty-figlet v0.2.0 (commit b493d57) — see
`rusty-figlet/docs/feature-layout.md` (FROZEN reference port) for the format
anchor. rusty-sponge conforms to the same shape with the minimum-convention
surface dictated by its single-capability scope.
**Iteration model**: v0.2.0 is a **purely additive** SemVer-minor release.
Every v0.1.x feature name and composition is preserved verbatim; new
umbrellas (`full`, `sponge-classic`, `sponge-minimal`) are layered on top
without renaming or narrowing the existing `cli` / `default` / `sponge-alias`
/ `bench` features. Library and binary API surfaces are unchanged.
## Single-capability port — spec 00011 §Scope Edge Cases
rusty-sponge is a **single-capability port**: it has exactly one documented
capability — soak up stdin and write it atomically to a file (a Rust port
of moreutils `sponge`). Spec 00011 §Scope Edge Cases dictates that
single-capability ports apply the **minimum convention**:
> ports with only one capability adopt the minimum convention:
> `full = ["cli"]` and `<port>-classic = ["cli"]` are the required
> umbrellas; ZERO leaves carved beyond those required umbrellas.
This document records the carving exercise and the explicit decision
to NOT split orthogonal sub-capabilities into leaves — every additional
behavior of `rusty-sponge` (Default-mode ergonomics, Strict moreutils
compat, atomic-rename path, signal-driven cleanup, spill-to-tempfile,
`-a` append mode, `completions` subcommand) is part of the single core
capability surface and removing any of them would break either the
documented public CLI / library contract or the atomic-safety guarantee
that is the entire raison d'être of the tool.
## Source-tree walk
`src/` modules (v0.1.0, post-Phase-1 baseline):
| `error.rs` | yes | (thiserror — always-on) | `Error` enum; library + binary need it. |
| `buffer.rs` | yes | (tempfile — always-on) | Hybrid in-memory + tempfile-spill buffer engine. |
| `atomic.rs` | yes | (tempfile — always-on) | Sibling-tempfile + atomic rename path. The headline atomic-safety promise. |
| `writethrough.rs` | yes | (tempfile — always-on) | Non-atomic write-through path for symlink / reparse targets (FR-010). |
| `lib.rs` | yes | none | Public API (`SpongeBuilder`, `Sponge`, `Target`, `CompatibilityMode`). |
| `cli.rs` | no — `cli` | clap | clap-derive `Cli` struct + `Subcommand::Completions`. |
| `mode.rs` | no — `cli` | none (pure helper) | Strict-mode precedence resolver (`--strict` > env > argv[0]). |
| `signal.rs` | no — `cli` | signal-hook (Unix), windows-sys (Windows) | Signal handler install + cleanup-on-exit dispatch. |
| `strict.rs` | no — `cli` | (clap_complete + clap pulled by `cli`) | Hand-rolled Strict-mode argv pre-scanner + byte-equal moreutils dispatcher.|
| `main.rs` | no — `cli` | clap, clap_complete, anyhow, signal-hook | Binary entry; gated by `required-features = ["cli"]`. |
| `bin/sponge.rs` | no — `sponge-alias` | (inherits `cli`) | `sponge` alias binary; gated by `required-features = ["sponge-alias"]`. |
## Leaf-carving criteria (HINT-009)
A capability becomes a leaf when ALL of the following hold:
1. It is **self-containable** — gated cleanly via `#[cfg(feature = "<leaf>")]`
at the module or top-level item boundary (HINT-004).
2. Either (a) it has a **sole optional dependency** that no other leaf needs
(HINT-005), OR (b) it is a pure-cfg-gate of an internal module worth
exposing as a knob.
3. Disabling it does NOT break any always-on library/CLI surface.
A capability does NOT become a leaf when:
- It is foundational (atomic-rename, spill buffer, writethrough fallback)
— disabling it would break the headline atomic-safety guarantee.
- It is part of the single documented capability surface (Default mode,
Strict mode, signal cleanup, `-a` append, completions subcommand).
- It would create more than ~6 leaves (FR-007 + HINT-003 envelope).
## v0.2.0 Carved Leaves
**ZERO new leaves carved at v0.2.0**. Every capability inside rusty-sponge
is either:
1. Foundational always-on library code (atomic-rename procedure, spill
buffer, writethrough fallback) — cannot be stripped without breaking
the public surface or the atomic-safety guarantee.
2. Already gated by the v0.1.x `cli` umbrella (clap-derived argument
parsing, completions subcommand, signal handler install, Strict-mode
pre-scanner).
3. Already gated by the v0.1.x `sponge-alias` feature (the second `sponge`
binary entry).
4. A dev-tooling feature (`bench` → criterion benches) outside the
convention's runtime-capability purview.
### Leaves intentionally NOT carved
The following candidate leaves were considered + rejected:
- **`signal`**: signal handler install + cleanup-on-exit dispatch lives
in `src/signal.rs` behind `dep:signal-hook` (Unix) and `windows-sys`
(Windows, target-conditional always-on). It is part of the FR-011
documented atomic-safety contract — without it, a Ctrl-C mid-write
could leave a partial tempfile in the target's directory. Stripping
this would silently break the headline promise. Rejected per
HINT-009 criterion 3.
- **`completions`**: Could be carved as `["dep:clap_complete"]`, but
per spec 00011 §Scope Edge Cases minimum-convention single-capability
ports declare ZERO new leaves. `clap_complete` is bundled into the
v0.1.x `cli` umbrella verbatim. Carving it would either rename `cli`
(breaking SemVer additivity) or duplicate the surface.
- **`spill-tempfile`**: The hybrid in-memory + tempfile-spill buffer is
the only mechanism by which arbitrarily-large inputs are handled
without exhausting RAM. The `tempfile` dep is always-on library code.
No carving signal.
- **`strict-compat`**: rusty-sponge's Strict mode is dispatched inline
in `lib.rs::run()` (via `mode::resolve` + `strict::run`) — about 40
lines including the argv pre-scanner. The hand-rolled getopt mirror
lives in `src/strict.rs`. Both are gated by the `cli` umbrella in
v0.1.x (since they consume `clap` + `clap_complete`). Carving out a
separate `strict-compat` leaf would require splitting `strict.rs`
away from `cli.rs`, which is more refactoring than the additive
v0.2.0 release allows. The capability survives untouched inside
the existing `cli` composition.
- **`sponge-alias`**: This v0.1.x feature ships a second binary named
`sponge`. It IS retained verbatim per the v0.2.0 SemVer additive
contract — but it is NOT one of the 2 required preset bundles per
FR-007 (those are `sponge-classic` and `sponge-minimal` below).
Documented separately as an installation-time convenience knob.
- **`bench`**: The v0.1.x `bench` feature is a tooling / benchmark
scaffold (criterion benches under `benches/throughput.rs`), not a
runtime capability leaf. It remains a dev-tooling feature outside
the convention's purview (the vendored `tools/feature-lint/lint.sh`
allowlist skips `bench` from leaf-CI-matrix and phantom-leaf checks)
and is retained verbatim from v0.1.0.
## Preset bundles (FR-007 — 2 required for single-capability ports)
Per spec 00011 §Scope Edge Cases + FR-007, even single-capability ports
declare 2 preset bundles to give the keep-list workaround documentation
something concrete to point at.
### `sponge-classic` (REQUIRED — bare port, 1:1 with moreutils `sponge`)
```toml
sponge-classic = ["cli"]
```
- Includes `cli` so the binary exists.
- Single-capability port; the `cli` umbrella IS the bare-port surface.
- Use case: `cargo install rusty-sponge --no-default-features --features sponge-classic`
for a moreutils-`sponge` drop-in replacement (Strict mode is invoked
via the `--strict` flag, `RUSTY_SPONGE_STRICT` env var, or `sponge-alias`
binary name — none of these require additional features).
### `sponge-minimal`
```toml
sponge-minimal = ["cli"]
```
- Same composition as `sponge-classic` (single-capability port has no
smaller subset to carve).
- Use case: explicit "minimal CLI install" alias for users who prefer
the `<port>-minimal` naming convention seen across other Rusty ports
(figlet-minimal, pwgen-minimal, ts-minimal). Documented as an
intentional semantic alias rather than a distinct composition.
### `sponge-alias` (v0.1.x feature retained, NOT a convention preset)
`sponge-alias = ["cli"]` from v0.1.0 ships an additional `sponge` binary
alongside `rusty-sponge`. It is retained verbatim per the v0.2.0 SemVer
additive contract — but it is NOT one of the 2 required preset bundles
per FR-007 (those are `sponge-classic` and `sponge-minimal` above).
`sponge-alias` is documented separately as an installation-time
convenience knob, not a capability subset.
### `bench` (v0.1.x dev-tooling feature retained, NOT a convention preset)
`bench = ["dep:criterion"]` from v0.1.0 enables `benches/throughput.rs`.
It is dev-tooling, not a runtime capability — outside the convention's
purview per the vendored feature-lint allowlist.
## Cross-port glossary candidates
No leaves carved → no cross-port glossary contributions from rusty-sponge
in this iteration. If a future minor release adds an orthogonal capability
(e.g., a `pidfile` leaf for sponge-as-a-service deployments), the leaf
would be a candidate for promotion to `docs/feature-vocabulary.md` per
FR-053.
## CI matrix shape (FR-010..FR-014)
Per plan §Per-Port v0.2.0 CI Matrix, scaled to a zero-leaf port:
- **Tier 1 — `test-default`**: full DDR-003 cross-compile matrix
(5 targets). Post-v0.2.0 `default = ["full"]` and `full = ["cli"]`,
so the kitchen-sink test resolves to the same set as v0.1.0
`default = ["cli"]` — no regression in coverage.
- **Tier 2 — `test-no-default`**: Linux x86_64 only. `cargo test
--no-default-features --lib` + dep-tree audit (SC-001 evidence).
- **Tier 3 — `test-<bundle>`**: one job per preset bundle. Linux only.
- `test-sponge-classic`
- `test-sponge-minimal`
- **Tier 4 — `check-leaf-<leaf>`**: SKIPPED. Zero leaves → no
per-leaf compile-check jobs. A placeholder comment in `ci.yml`
documents why this tier is empty. The `bench` feature is in the
vendored feature-lint allowlist (dev-tooling) and does not require
a Tier-4 entry.
- **Tier 5 — `lint-convention`**: single Linux job invoking the
vendored `tools/feature-lint/run.sh` script.
Per FR-014, bundle/lint jobs are constrained to Linux x86_64.
## Vendored feature-lint
Per spec 00011 §Phase 2 iteration 6 precedent (rusty-figlet vendored
the lint script because the umbrella `jsh562/rustylib` is private and
cross-repo `actions/checkout` cannot reach it), rusty-sponge vendors
`tools/feature-lint/{lint.sh,run.sh,README.md}` from the umbrella into
the port repo. The vendored copy is byte-equal to the umbrella source
of truth as of the freeze commit (post the dev-tooling-allowlist +
benches/tests-search + additive-CHANGELOG-support fixes from rusty-ts
v0.2.0 / E011 Phase 3 iteration 2).
## Why no new leaves — explicit rationale
Spec 00011 §Scope Edge Cases anticipates this case verbatim:
> Some ports have only one orthogonal capability. Those ports adopt the
> minimum convention: `full = ["cli"]` and `<port>-classic = ["cli"]`
> as aliases; the convention SHAPE is consistent across the portfolio
> even when the per-port leaf carving yields zero leaves.
rusty-sponge deliberately chooses the zero-leaf path because:
1. The atomic-safety guarantee is the entire reason this tool exists.
Carving any of its supporting machinery (signal handlers, spill
buffer, atomic rename, writethrough fallback) into an opt-out leaf
would silently change the FR-006 contract for users who turned that
leaf off.
2. The cost of carving a speculative leaf (cfg-gate scaffolding,
per-leaf CI matrix entry, README/CHANGELOG row, glossary candidacy)
outweighs the value when no orthogonal capability exists to gate.
3. The portfolio-wide convention shape (umbrella set, README "Cargo
Features" section, lint compliance) is preserved verbatim — a
downstream library consumer reading the README for rusty-sponge
gets the same one-glance feature matrix UX as one reading
rusty-figlet or rusty-ts.
4. v0.2.0 is **purely additive**. Every v0.1.x feature is preserved
verbatim; no SemVer break. Future minor releases can add leaves
without breaking the v0.2.0 contract: a hypothetical `pidfile`
v0.3.0 feature would slot in as `pidfile = ["dep:atomicwrites"]`
alongside the existing umbrellas with zero migration cost.