# 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.

---

## 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")]` — only useful with `BytesMut`, which requires alloc
3. **Gate `use bytes::BufMut`** — only import it conditionally
4. **Wire up `bytes` features in Cargo.toml:**
   ```toml
   bytes = { version = "1.7", default-features = false }
   # ...
   [features]
   default = ["std", "alloc"]
   std = ["bytes/std"]
   alloc = ["bytes"]
   ```

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

5. **Split `DATA_IMPORTS`** into unconditional + gated sections:
   ```rust
   // Always emitted
   use roto_runtime::{ProtoAccessor, ProtoBuilder, RevBuilder, Result, RotoError, read_varint, RepeatedFieldIterator};
   use core::str;
   use bytes::{Buf, BufMut};

   // Only emitted when alloc feature is enabled
   #[cfg(feature = "alloc")]
   use bytes::{Bytes, BytesMut};
   #[cfg(feature = "alloc")]
   use roto_runtime::RotoMessage;
   ```

6. **Gate `OwnedMessage` structs + `RotoMessage` impls** with `#[cfg(feature = "alloc")]`

7. **Fix `RevBuilder::with()`** — replace `let fields: Vec<_> = ... .collect()` with a forward iteration that doesn't allocate. The reverse order doesn't matter for protobuf encoding (fields are independent):
   ```rust
   // Before: let fields: Vec<_> = msg.accessor.raw_fields().collect();
   //         for (raw, _) in fields.iter().rev() { ... }
   // After:  for (raw, _) in msg.accessor.raw_fields() { ... }
   ```
   Alternatively, gate the `with()` method itself behind `#[cfg(feature = "alloc")]`.

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

### Phase 5: Validation

11. **Update `examples/no_std_test`** to build with `--no-default-features` and verify compilation
12. **Run existing tests** with default features to ensure nothing breaks
13. **Add a compile-test** that verifies `--no-default-features` builds succeed for generated code

---

## Dependency Graph

```
no_std + no_alloc  →  readers, enums, builders, iterators
         ↓ alloc   →  + OwnedMessage, RotoMessage trait
         ↓ std     →  + service code, std::error::Error
```

## Risk Assessment

| Risk | Mitigation |
|------|-----------|
| Breaking existing consumers who use `OwnedMessage` | `OwnedMessage` was already behind a manual `#[cfg]` in the `no_std_test` example — consumers aren't relying on it unconditionally |
| `BufMutBuilder` removal in no-alloc mode | It's only useful with `BytesMut` anyway, which requires alloc |
| `RevBuilder::with()` change | Forward iteration is semantically equivalent for protobuf encoding |

## Files to Modify

| File | Changes |
|------|---------|
| `runtime/src/lib.rs` | Gate `RotoMessage`, `RotoOwned`, `BufMutBuilder`, `use bytes::BufMut` |
| `runtime/Cargo.toml` | Wire `alloc` → `bytes` dependency, add `bytes/std` to `std` feature |
| `codegen/src/generator/mod.rs` | Split `DATA_IMPORTS`, gate `OwnedMessage`, gate service code, fix `RevBuilder::with()` |
| `examples/no_std_test/` | Update to use new feature structure |

## Execution Order

1. Phase 1 (runtime) — foundation
2. Phase 2 (codegen data imports) — depends on runtime
3. Phase 3 (codegen services) — independent
4. Phase 4–5 (consumer setup + validation) — depends on all above