chore: import upstream snapshot with attribution

rules/rust/patterns.md

@@ -0,0 +1,168 @@
+---
+paths:
+  - "**/*.rs"
+---
+# Rust Patterns
+
+> This file extends [common/patterns.md](../common/patterns.md) with Rust-specific content.
+
+## Repository Pattern with Traits
+
+Encapsulate data access behind a trait:
+
+```rust
+pub trait OrderRepository: Send + Sync {
+    fn find_by_id(&self, id: u64) -> Result<Option<Order>, StorageError>;
+    fn find_all(&self) -> Result<Vec<Order>, StorageError>;
+    fn save(&self, order: &Order) -> Result<Order, StorageError>;
+    fn delete(&self, id: u64) -> Result<(), StorageError>;
+}
+```
+
+Concrete implementations handle storage details (Postgres, SQLite, in-memory for tests).
+
+## Service Layer
+
+Business logic in service structs; inject dependencies via constructor:
+
+```rust
+pub struct OrderService {
+    repo: Box<dyn OrderRepository>,
+    payment: Box<dyn PaymentGateway>,
+}
+
+impl OrderService {
+    pub fn new(repo: Box<dyn OrderRepository>, payment: Box<dyn PaymentGateway>) -> Self {
+        Self { repo, payment }
+    }
+
+    pub fn place_order(&self, request: CreateOrderRequest) -> anyhow::Result<OrderSummary> {
+        let order = Order::from(request);
+        self.payment.charge(order.total())?;
+        let saved = self.repo.save(&order)?;
+        Ok(OrderSummary::from(saved))
+    }
+}
+```
+
+## Newtype Pattern for Type Safety
+
+Prevent argument mix-ups with distinct wrapper types:
+
+```rust
+struct UserId(u64);
+struct OrderId(u64);
+
+fn get_order(user: UserId, order: OrderId) -> anyhow::Result<Order> {
+    // Can't accidentally swap user and order IDs at call sites
+    todo!()
+}
+```
+
+## Enum State Machines
+
+Model states as enums — make illegal states unrepresentable:
+
+```rust
+enum ConnectionState {
+    Disconnected,
+    Connecting { attempt: u32 },
+    Connected { session_id: String },
+    Failed { reason: String, retries: u32 },
+}
+
+fn handle(state: &ConnectionState) {
+    match state {
+        ConnectionState::Disconnected => connect(),
+        ConnectionState::Connecting { attempt } if *attempt > 3 => abort(),
+        ConnectionState::Connecting { .. } => wait(),
+        ConnectionState::Connected { session_id } => use_session(session_id),
+        ConnectionState::Failed { retries, .. } if *retries < 5 => retry(),
+        ConnectionState::Failed { reason, .. } => log_failure(reason),
+    }
+}
+```
+
+Always match exhaustively — no wildcard `_` for business-critical enums.
+
+## Builder Pattern
+
+Use for structs with many optional parameters:
+
+```rust
+pub struct ServerConfig {
+    host: String,
+    port: u16,
+    max_connections: usize,
+}
+
+impl ServerConfig {
+    pub fn builder(host: impl Into<String>, port: u16) -> ServerConfigBuilder {
+        ServerConfigBuilder {
+            host: host.into(),
+            port,
+            max_connections: 100,
+        }
+    }
+}
+
+pub struct ServerConfigBuilder {
+    host: String,
+    port: u16,
+    max_connections: usize,
+}
+
+impl ServerConfigBuilder {
+    pub fn max_connections(mut self, n: usize) -> Self {
+        self.max_connections = n;
+        self
+    }
+
+    pub fn build(self) -> ServerConfig {
+        ServerConfig {
+            host: self.host,
+            port: self.port,
+            max_connections: self.max_connections,
+        }
+    }
+}
+```
+
+## Sealed Traits for Extensibility Control
+
+Use a private module to seal a trait, preventing external implementations:
+
+```rust
+mod private {
+    pub trait Sealed {}
+}
+
+pub trait Format: private::Sealed {
+    fn encode(&self, data: &[u8]) -> Vec<u8>;
+}
+
+pub struct Json;
+impl private::Sealed for Json {}
+impl Format for Json {
+    fn encode(&self, data: &[u8]) -> Vec<u8> { todo!() }
+}
+```
+
+## API Response Envelope
+
+Consistent API responses using a generic enum:
+
+```rust
+#[derive(Debug, serde::Serialize)]
+#[serde(tag = "status")]
+pub enum ApiResponse<T: serde::Serialize> {
+    #[serde(rename = "ok")]
+    Ok { data: T },
+    #[serde(rename = "error")]
+    Error { message: String },
+}
+```
+
+## References
+
+See skill: `rust-patterns` for comprehensive patterns including ownership, traits, generics, concurrency, and async.