# Task: Verify Generated Code Works in `no_std` / `no_alloc`

**Goal:** Ensure all generated protobuf code compiles and runs in `no_std` + `no_alloc` environments. Features requiring `alloc` must be gated behind feature flags in the proto generator's output.

**Status:** ✅ COMPLETE

---

## Findings

**Already clean (no_std + no_alloc):**
- All reader structs (`Message<'a>`) — zero-copy accessors over `&[u8]`
- All field accessors, enums, builders (`MessageBuilder<'b>`), typed iterators
- Core runtime: `ProtoAccessor`, `ProtoBuilder`, `RevBuilder`, `read_varint`, `write_varint`, all iterators

**Needs `alloc` feature gate:**
- `RotoMessage` trait — uses `bytes::Bytes` (has `Arc` internally)
- `RotoOwned` trait — same issue
- `OwnedMessage` structs generated by codegen
- `RevBuilder::with()` — uses `Vec<_>` to collect raw fields
- `Bytes`, `BytesMut` imports in `DATA_IMPORTS`

**Needs `std` feature gate:**
- All service code (`SERVICE_IMPORTS`) — uses `std::sync::Arc`, `std::pin::Pin`, `String`, etc.
- `std::error::Error for RotoError` (already gated)

---

## Plan

### Phase 1: Runtime (`runtime/src/lib.rs`, `runtime/Cargo.toml`) ✅

1. ~~**Gate `RotoMessage` and `RotoOwned`** behind `#[cfg(feature = "alloc")]`~~
2. ~~**Gate `BufMutBuilder`** behind `#[cfg(feature = "alloc")]`~~
3. ~~**Gate `use bytes::BufMut`** — only import it conditionally~~
4. ~~**Wire up `bytes` features in Cargo.toml:**~~

**Note:** `bytes::Bytes` requires `Arc` internally, which is in `std`. Since `bytes` only has a `std` feature (no separate `alloc`), our `alloc` feature implies `std`:

```toml
[features]
default = ["std", "alloc"]
std = []
alloc = ["std", "bytes/std"]
```

### Phase 2: Codegen — `DATA_IMPORTS` & Generated Data Code (`codegen/src/generator/mod.rs`) ✅

5. ~~**Split `DATA_IMPORTS`** into unconditional + gated sections~~
6. ~~**Gate `OwnedMessage` structs + `RotoMessage` impls** with `#[cfg(feature = "alloc")]`~~
7. ~~**Fix `RevBuilder::with()`** — replaced `Vec::collect()` + `.rev()` with forward iteration~~

### Phase 3: Codegen — Service Code (`codegen/src/generator/mod.rs`) ✅

8. ~~**Gate `SERVICE_IMPORTS`** with `#[cfg(feature = "std")]`~~
9. ~~**Gate all service traits, server impls, and `*Server` structs** with `#[cfg(feature = "std")]`~~

### Phase 4: Generated Consumer Setup ✅

10. ~~**Emit a `Cargo.toml`** (or instruct consumers) with the right feature setup~~

Updated consumer `Cargo.toml` files:
- `examples/hello_world/Cargo.toml`
- `roto-tonic/Cargo.toml`
- `benches/Cargo.toml`

```toml
[features]
default = ["std"]
std = ["roto-runtime/std", "alloc"]
alloc = ["roto-runtime/alloc"]
```

### Phase 5: Validation ✅

11. ~~**Update `examples/no_std_test`** to build with `--no-default-features`~~
12. ~~**Run existing tests** with default features~~
13. ~~**Add a compile-test** that verifies `--no-default-features` builds succeed~~

---

## Dependency Graph

```
no_std + no_alloc  →  readers, enums, builders, iterators
         ↓ alloc   →  + OwnedMessage, RotoMessage trait (implies std via bytes::Bytes)
         ↓ std     →  + service code, std::error::Error
```

## Files Modified

| File | Changes |
|------|---------|
| `runtime/src/lib.rs` | Gated `RotoMessage`, `RotoOwned`, `BufMutBuilder`, `use bytes::BufMut` behind `#[cfg(feature = "alloc")]` |
| `runtime/Cargo.toml` | Made `bytes` optional, wired `alloc` → `bytes/std` |
| `codegen/src/generator/mod.rs` | Split `DATA_IMPORTS`, gated `Owned*` structs, gated service code, fixed `RevBuilder::with()` |
| `examples/hello_world/Cargo.toml` | Added `std`/`alloc` feature wiring |
| `roto-tonic/Cargo.toml` | Added `std`/`alloc` feature wiring |
| `benches/Cargo.toml` | Added `std`/`alloc` feature wiring, `bytes` dependency |
| `examples/no_std_test/Cargo.toml` | Updated for `no_std` target, added feature wiring |
| `examples/no_std_test/src/lib.rs` | Added panic handler for embedded target |
| `examples/no_std_test/src/helloworld.rs` | Rewrote as minimal gated example |

## Validation

- ✅ `cargo build` — all packages build with default features
- ✅ `cargo build --package roto-runtime --no-default-features` — runtime builds without alloc
- ✅ `cargo test --all` — 38 tests pass (runtime + codegen + integration)
- ✅ `examples/no_std_test` builds with `--no-default-features` on `thumbv7em-none-eabihf` target
- ✅ Generated code from integration test (`test_helloworld_build`) compiles successfully