Add GitHub Copilot instructions for Rust (#365)

Add scoped Copilot instruction file that automatically applies Rust
coding conventions and best practices when working with .rs files.

Based on github/awesome-copilot community instructions covering error
handling, API design, ownership patterns, testing, and code style.

Fixes: VECTOR-541.

Author: ewienik

.github/instructions/rust.instructions.md

@@ -0,0 +1,9 @@
+---
+description: 'Rust coding conventions and contribution guidelines for Vector Store'
+applyTo: '**/*.rs'
+---
+
+When working on Rust code, follow the guidelines defined in:
+
+- [docs/rust_instructions.md](../../docs/rust_instructions.md) — Rust coding conventions and best practices
+- [CONTRIBUTING.md](../../CONTRIBUTING.md) — contribution workflow, PR requirements, static checks, and testing

CONTRIBUTING.md

@@ -23,6 +23,10 @@ After submitting a compliant PR, it will be reviewed by one or more maintainers.
 **Current maintainers:**
 - Paweł Pery (@ewienik)
 
+## Rust Coding Conventions
+
+For detailed Rust coding conventions and best practices used in this project, see [docs/rust_instructions.md](docs/rust_instructions.md).
+
 ## Static Checks
 
 To ensure code quality, we use several static analysis tools. All static checks must pass before merging.

docs/rust_instructions.md

@@ -0,0 +1,136 @@
+# Rust Coding Conventions and Best Practices
+
+Follow idiomatic Rust practices and community standards when writing Rust code.
+
+These instructions are based on [The Rust Book](https://doc.rust-lang.org/book/), [Rust API Guidelines](https://rust-lang.github.io/api-guidelines/), [RFC 430 naming conventions](https://github.com/rust-lang/rfcs/blob/master/text/0430-finalizing-naming-conventions.md), and the broader Rust community at [users.rust-lang.org](https://users.rust-lang.org/).
+
+## General Instructions
+
+- Always prioritize readability, safety, and maintainability.
+- Use strong typing and leverage Rust's ownership system for memory safety.
+- Break down complex functions into smaller, more manageable functions.
+- For algorithm-related code, include explanations of the approach used.
+- Write code with good maintainability practices, including comments on why certain design decisions were made.
+- Handle errors gracefully using `Result<T, E>` and provide meaningful error messages.
+- For external dependencies, mention their usage and purpose in documentation.
+- Use consistent naming conventions following [RFC 430](https://github.com/rust-lang/rfcs/blob/master/text/0430-finalizing-naming-conventions.md).
+- Write idiomatic, safe, and efficient Rust code that follows the borrow checker's rules.
+- Ensure code compiles without warnings.
+
+## Patterns to Follow
+
+- Use modules (`mod`) and public interfaces (`pub`) to encapsulate logic.
+- Handle errors properly using `?`, `match`, or `if let`.
+- Use `serde` for serialization and `thiserror` or `anyhow` for custom errors.
+- Implement traits to abstract services or external dependencies.
+- Structure async code using `async/await` and `tokio` or `async-std`.
+- Prefer enums over flags and states for type safety.
+- Use builders for complex object creation.
+- Split binary and library code (`main.rs` vs `lib.rs`) for testability and reuse.
+- Use `rayon` for data parallelism and CPU-bound tasks.
+- Use iterators instead of index-based loops as they're often faster and safer.
+- Use `&str` instead of `String` for function parameters when you don't need ownership.
+- Prefer borrowing and zero-copy operations to avoid unnecessary allocations.
+
+### Ownership, Borrowing, and Lifetimes
+
+- Prefer borrowing (`&T`) over cloning unless ownership transfer is necessary.
+- Use `&mut T` when you need to modify borrowed data.
+- Explicitly annotate lifetimes when the compiler cannot infer them.
+- Use `Rc<T>` for single-threaded reference counting and `Arc<T>` for thread-safe reference counting.
+- Use `RefCell<T>` for interior mutability in single-threaded contexts and `Mutex<T>` or `RwLock<T>` for multi-threaded contexts.
+
+## Patterns to Avoid
+
+- Don't use `unwrap()` or `expect()` unless absolutely necessary—prefer proper error handling.
+- Avoid panics in library code—return `Result` instead.
+- Don't rely on global mutable state—use dependency injection or thread-safe containers.
+- Avoid deeply nested logic—refactor with functions or combinators.
+- Don't ignore warnings—treat them as errors during CI.
+- Avoid `unsafe` unless required and fully documented.
+- Don't overuse `clone()`, use borrowing instead of cloning unless ownership transfer is needed.
+- Avoid premature `collect()`, keep iterators lazy until you actually need the collection.
+- Avoid unnecessary allocations—prefer borrowing and zero-copy operations.
+
+## Code Style and Formatting
+
+- Follow the Rust Style Guide and use `rustfmt` for automatic formatting.
+- Keep lines under 100 characters when possible.
+- Place function and struct documentation immediately before the item using `///`.
+- Use `cargo clippy` to catch common mistakes and enforce best practices.
+
+## Error Handling
+
+- Use `Result<T, E>` for recoverable errors and `panic!` only for unrecoverable errors.
+- Prefer `?` operator over `unwrap()` or `expect()` for error propagation.
+- Create custom error types using `thiserror` or implement `std::error::Error`.
+- Use `Option<T>` for values that may or may not exist.
+- Provide meaningful error messages and context.
+- Error types should be meaningful and well-behaved (implement standard traits).
+- Validate function arguments and return appropriate errors for invalid input.
+
+## API Design Guidelines
+
+### Common Traits Implementation
+
+Eagerly implement common traits where appropriate:
+
+- `Copy`, `Clone`, `Eq`, `PartialEq`, `Ord`, `PartialOrd`, `Hash`, `Debug`, `Display`, `Default`
+- Use standard conversion traits: `From`, `AsRef`, `AsMut`
+- Collections should implement `FromIterator` and `Extend`
+- Note: `Send` and `Sync` are auto-implemented by the compiler when safe; avoid manual implementation unless using `unsafe` code
+
+### Type Safety and Predictability
+
+- Use newtypes to provide static distinctions
+- Arguments should convey meaning through types; prefer specific types over generic `bool` parameters
+- Use `Option<T>` appropriately for truly optional values
+- Functions with a clear receiver should be methods
+- Only smart pointers should implement `Deref` and `DerefMut`
+
+### Future Proofing
+
+- Use sealed traits to protect against downstream implementations
+- Structs should have private fields
+- Functions should validate their arguments
+- All public types must implement `Debug`
+
+## Testing and Documentation
+
+- Write comprehensive unit tests using `#[cfg(test)]` modules and `#[test]` annotations.
+- Use test modules alongside the code they test (`mod tests { ... }`).
+- Write integration tests in `tests/` directory with descriptive filenames.
+- Write clear and concise comments for each function, struct, enum, and complex logic.
+- Ensure functions have descriptive names and include comprehensive documentation.
+- Document all public APIs with rustdoc (`///` comments) following the [API Guidelines](https://rust-lang.github.io/api-guidelines/).
+- Use `#[doc(hidden)]` to hide implementation details from public documentation.
+- Document error conditions, panic scenarios, and safety considerations.
+- Examples should use `?` operator, not `unwrap()` or deprecated `try!` macro.
+
+## Project Organization
+
+- Use semantic versioning in `Cargo.toml`.
+- Include comprehensive metadata: `description`, `license`, `repository`, `keywords`, `categories`.
+- Use feature flags for optional functionality.
+- Organize code into modules using `mod.rs` or named files.
+- Keep `main.rs` or `lib.rs` minimal - move logic to modules.
+
+## Quality Checklist
+
+Before publishing or reviewing Rust code, ensure:
+
+### Core Requirements
+
+- [ ] **Naming**: Follows RFC 430 naming conventions
+- [ ] **Traits**: Implements `Debug`, `Clone`, `PartialEq` where appropriate
+- [ ] **Error Handling**: Uses `Result<T, E>` and provides meaningful error types
+- [ ] **Documentation**: All public items have rustdoc comments with examples
+- [ ] **Testing**: Comprehensive test coverage including edge cases
+
+### Safety and Quality
+
+- [ ] **Safety**: No unnecessary `unsafe` code, proper error handling
+- [ ] **Performance**: Efficient use of iterators, minimal allocations
+- [ ] **API Design**: Functions are predictable, flexible, and type-safe
+- [ ] **Future Proofing**: Private fields in structs, sealed traits where appropriate
+- [ ] **Tooling**: Code passes `cargo fmt`, `cargo clippy`, and `cargo test`