# AGENTS.md

## Project Overview

This project is a small Rust common library and runtime built without Cargo.
CMake invokes `rustc` directly. The library provides a low-footprint subset
of standard-library-like functionality:

- `Result`, `Duration`, and formatting from `core`
- `Vec`, `String`, `vec![]`, and `format!` from `alloc`
- project-local `print!` and `println!` macros
- direct platform stdout, heap calls, and busy-wait sleep

The project contains:

- `src/lib.rs`: the `#![no_std]` common library API
- `src/io.rs`: the `core::fmt::Write` stdout backend
- `src/runtime.rs`: allocator, async polling, monotonic sleep, and platform
  entry helpers
- `macros/src/lib.rs`: host-side `#[rslibc_macros::main]` attribute macro
- `runtime/main.rs`: minimal example entry point
- `cmake/RustLSP.cmake`: generates `rust-project.json` for rust-analyzer
- `CMakeLists.txt`: direct `rustc` library, macro, and executable commands

## No-Std Design

- Keep `#![no_std]` in both the library and runtime.
- Do not add Rust `std` to the target library or runtime, and do not add
  Cargo unless explicitly requested.
- The host-side procedural macro may use `proc_macro` and host libraries;
  it is not linked into the target executable.
- `core` is allowed and is the foundation for `Result`, formatting, slices,
  and other basic language-library functionality.
- `alloc` is allowed through `extern crate alloc`; it supplies `Vec` and
  related heap-backed types.
- The final runtime must provide its own global allocator and panic handler;
  `rslibc::entry!` supplies them to an executable.
- Keep OS integration in small FFI modules. Do not pull in a general-purpose
  runtime or C I/O layer for basic output.
- `rslibc_macros::main` can wrap `async fn main()` with the small `block_on`
  executor. It is only a polling loop and does not provide Tokio-style I/O
  or a reactor; delays use the busy-wait `rslibc::runtime::sleep`.
- Keep unsafe code inside small safe wrappers. Application code should be
  able to use calls such as:

  ```rust
  let values = rslibc::vec![1, 2, 3];
  rslibc::println!("values = {:?}", values);
  ```

## Platform Runtime

- Unix stdout calls libc `write` with file descriptor `1`.
- Windows stdout calls `GetStdHandle` and `WriteFile` through `kernel32`.
- Windows allocation calls `HeapAlloc` and `HeapFree` through the system
  heap.
- Sleep uses a monotonic clock: `clock_gettime` on Unix and
  `QueryPerformanceCounter` on Windows.
- The Windows runtime dynamically links only the platform support needed by
  `alloc` and the compiler runtime. Do not statically link the full C
  runtime.
- Keep `panic=abort` enabled.

## Build System

CMake is configured with `LANGUAGES NONE` and invokes `rustc` through three
custom commands. CMake 4.4's experimental native Rust support
(`CMAKE_EXPERIMENTAL_RUST`) was evaluated and rejected: it compiles each
source file as its own crate, so multi-module crates break; it links Rust
libraries into executables as native `.lib` inputs instead of `--extern`;
and it has no `proc-macro` crate type. Keep the direct `rustc` commands.

The three custom commands are:

1. Compile `macros/src/lib.rs` as a host procedural macro.
2. Compile `src/lib.rs` and its modules to `librslibc.rlib`.
3. Compile the minimal `runtime/main.rs` and link it against that rlib.

Configure and build a release executable with Ninja:

```sh
cmake -S . -B build -GNinja -DCMAKE_BUILD_TYPE=Release
cmake --build build
```

Build only the host macro with:

```sh
cmake --build build --target rslibc_macros
```

Build only the common library with:

```sh
cmake --build build --target rslibc_library
```

Run it on Unix:

```sh
./build/rslibc_example
```

Run it on Windows:

```powershell
.\build\rslibc_example.exe
```

`CMAKE_BUILD_TYPE` is translated explicitly to rustc flags:

- `Release`: `-C opt-level=3 -C debuginfo=0`
- `RelWithDebInfo`: `-C opt-level=3 -C debuginfo=2`
- Other or unset configurations: `-C opt-level=0 -C debuginfo=2`

## LSP

rust-analyzer works without Cargo through a generated manifest:

- `cmake/RustLSP.cmake` runs on every configure and writes
  `rust-project.json` (git-ignored) describing the three crates and the
  toolchain sysroot, so `core`, `alloc`, and proc-macro attributes resolve.
- Requires the rustup components:
  `rustup component add rust-analyzer rust-src`.
- Open the repository root in the editor so rust-analyzer picks up
  `rust-project.json`. Re-run CMake configure after changing the crate
  layout or switching toolchains.

## Verification

After changing the source or build configuration, run a clean release build
and execute the example:

```sh
rm -rf build
cmake -S . -B build -GNinja -DCMAKE_BUILD_TYPE=Release
cmake --build build
./build/rslibc_example
```

On Windows, use `Remove-Item -Recurse -Force build` and run
`build\\rslibc_example.exe` instead. Confirm that stdout contains
`100% done`.

When size matters, inspect the Windows executable after a release build.
Avoid adding formatting, allocation, or OS dependencies unless they are
needed by the library API.

## Commit Messages

- Follow the 50/72 rule: subject lines are at most 50 characters and body
  lines are wrapped at 72 characters.
- Use conventional prefixes such as `feat:`, `fix:`, `docs:`, `chore:`,
  and `ci:`.
- Separate the subject from the body with a blank line.
- Keep commit messages concise.
- Include a `Co-Authored-By:` trailer for every agent or model that
  contributed to the commit, one trailer per co-author:

  ```text
  Co-Authored-By: qwen (lmstudio/qwen3.6-27b-mtp): wrote tests + build
  Co-Authored-By: luna (openrouter/openai/gpt-5.6-luna): reviewed edge cases
  ```

## Editing Guidelines

- Keep changes minimal and directly related to the request.
- Use four-space indentation in Rust and CMake.
- Keep normal text in Markdown within 80 columns where practical.
- Do not add speculative abstractions or dependencies.
- Update this file when the build workflow, library API, supported
  platforms, or source layout changes.