/rust-web-backend

name: rust-web-backend
description: Use when building a REST/HTTP backend in Rust — axum routing, extractors, shared state, middleware, error responses, sqlx database access. Not for raw async/concurrency (rust-async-concurrency).
versions:
  axum: "0.8.9"
  tokio: "1.54"
  sqlx: "0.9"
  tracing: "0.1"
  rust-edition: "2024"
user-invocable: false
references: references/architecture.md, references/error-handling.md, references/database.md, references/observability.md, references/templates/rest-service.md
related-skills: rust-async-concurrency, solid-rust

<objective> This skill covers the 2026 standard Rust web-backend stack: axum 0.8.x routing (including the /{id} path syntax that replaced /:id) and extractors, sharing state via State<T>, composing tower-http middleware (trace, cors, timeout, compression), mapping domain errors to IntoResponse instead of unwrapping in handlers, and sqlx 0.9 database access with compile-time-checked query!/query_as! (including the .sqlx/ offline cache for CI).

It also names the honest alternatives — sea-orm for an ActiveRecord-style ORM, diesel for a mature but synchronous alternative needing spawn_blocking — and structured observability via tracing.

Out of scope: raw async/concurrency primitives (channels, locks, task spawning) belong to rust-async-concurrency; cross-domain crate selection belongs to rust-ecosystem-crates. </objective>

Rust Web Backend

Agent Workflow (MANDATORY)

Before building the service, spawn in parallel:

1. **fuse-ai-pilot:explore-codebase** — detect the existing router, state, and DB layer 2. **fuse-ai-pilot:research-expert** — verify current axum/sqlx APIs via Context7/Exa (axum 0.8 changed several APIs) 3. **mcp__context7__query-docs** — pull exact extractor/handler signatures

After building, run **fuse-ai-pilot:sniper**.

---

The 2026 standard stack

| Layer | Crate | Why | |-------|-------|-----| | **Runtime** | tokio | De-facto async runtime; everything targets it | | **HTTP framework** | axum 0.8.x | Tower-based, extractor ergonomics, minimal magic | | **Middleware** | tower / tower-http | Composable layers (trace, cors, timeout, compression) | | **Database** | sqlx 0.9 | Async, `query!` compile-time-checked SQL, no DSL; Postgres/MySQL/SQLite | | **Observability** | tracing + tracing-subscriber | Structured, async-aware spans and logs |

**Honest alternatives:** `sea-orm` (ActiveRecord-style ORM, higher-level than sqlx), `diesel` 2.x (mature, **synchronous** — needs `spawn_blocking` or a sync pool in async apps). Prefer sqlx for the standard stack; reach for these only when their model fits.

---

Critical Rules

1. **axum 0.8 path syntax is `/{id}`, not `/:id`** — `/*rest` became `/{*rest}`. The old `matchit` syntax will not compile. 2. **Errors implement `IntoResponse`** — never `.unwrap()` in a handler. Map domain errors to a status + body via one app error type. 3. **Share state with `State<T>`, wrapped once** — put the pool/config in an `Arc`-friendly struct; extract it with `State`, do not use globals. 4. **`sqlx::query!` needs `DATABASE_URL` at compile time** — or a committed `.sqlx/` offline cache (`cargo sqlx prepare`). Plan this before CI. 5. **No `#[async_trait]` on axum extractors** — 0.8 uses RPITIT; custom `FromRequestParts` impls must drop the macro.

---

Reference Guide

Concepts

| Topic | Reference | When to Consult | |-------|-----------|-----------------| | **Architecture** | [architecture.md](references/architecture.md) | Router, extractors, State, tower middleware layout | | **Error handling** | [error-handling.md](references/error-handling.md) | App error type + `IntoResponse` | | **Database** | [database.md](references/database.md) | sqlx pool, `query!`, migrations, alternatives | | **Observability** | [observability.md](references/observability.md) | tracing spans, subscriber, request logging |

Templates

| Template | When to Use | |----------|-------------| | [rest-service.md](references/templates/rest-service.md) | Complete minimal REST service (router + state + handlers + errors + tracing) |

---

Quick Reference

Router with the 0.8 path syntax

let app = Router::new()
    .route("/users", get(list).post(create))
    .route("/users/{id}", get(show))   // NOT /:id
    .with_state(state);

→ See [architecture.md](references/architecture.md)

Compile-time-checked query

let user = sqlx::query_as!(User, "SELECT id, name FROM users WHERE id = $1", id)
    .fetch_optional(&pool)
    .await?;

→ See [database.md](references/database.md)

---

Best Practices

DO

• Keep one app error type that implements `IntoResponse`.
• Layer cross-cutting concerns (trace, timeout, cors) via `tower-http`.
• Verify queries at compile time with `query!`/`query_as!`.

DON'T

• Use `/:id` route syntax (0.7 and earlier only).
• `.unwrap()` in handlers — return a typed error.
• Reach for diesel in async code without accounting for its sync nature.

---

Sources (verified)

• tokio.rs/blog/2025-01-01-announcing-axum-0-8-0 — path syntax, Option extractor, `#[async_trait]` removal (fetched 2026-07-05)
• crates.io — axum 0.8.9, sqlx 0.9.0, tokio 1.54.1, tracing 0.1.44, tower-http 0.7.1 (current at fetch 2026-11-08)