howto-code-in-rust

name	howto-code-in-rust
description	Use when writing, reviewing, or modifying Rust code - covers error handling with thiserror+miette, type system patterns, async and serde conventions, testing crates, dependency pinning, and module organization
user-invocable	false

Writing Rust

Overview

Rust house style. Applies whenever writing, reviewing, or modifying Rust code.

The two governing values: correctness over convenience, and pragmatic incrementalism. Use the type system aggressively to make invalid states unrepresentable, then evolve the design as patterns repeat rather than building speculative abstractions.

Correctness over convenience

• Model the full error space. No shortcuts or simplified error handling.
• Handle all edge cases: race conditions, signal timing, platform differences.
• Use the type system to encode correctness constraints (newtypes, exhaustive matching, #[must_use]).
• Prefer compile-time guarantees over runtime checks where possible.
• When uncertain, explore and iterate rather than assume.

User-facing error quality

Standard pairing: thiserror for structured error enums, miette for user-facing diagnostics with source spans, help text, and related errors. They are complementary -- thiserror defines the shape, miette adds the diagnostic layer on top.

#[derive(Debug, thiserror::Error, miette::Diagnostic)]
enum ConfigError {
    #[error("failed to parse config at {path}")]
    #[diagnostic(help("check that the file is valid TOML"))]
    Parse { path: String, #[source] source: toml::de::Error },
}

Rules:

• Group errors by category with an ErrorKind enum when a single error type covers many failure modes.
• Two-tier error model: user-facing errors get semantic exit codes and rich diagnostics; internal errors (programming bugs) may panic or use internal error types.
• Error display messages are lowercase sentence fragments suitable for composing as "failed to {message}".
• Cross-platform consistency. Use OS-native logic rather than emulating Unix on Windows or vice versa.
• User-facing messages in clear, present tense.
• Never silently drop unsupported content. When translating or adapting data between formats, return Err for content the target format cannot represent. Silent data loss is a correctness bug. If a caller wants to ignore unsupported content, they can explicitly choose to -- but the library must surface the problem.

Type system patterns

Encode invariants in types:

• Newtypes for domain types. Wrap primitive types to prevent misuse: struct UserId(u64) instead of bare u64.
• Builder patterns for complex construction with many optional parameters.
• Type states encoded in generics when state transitions matter and invalid states should be unrepresentable.
• Lifetimes to avoid unnecessary cloning. Prefer borrows when data has a natural tree structure.
• Restricted visibility. Use pub(crate) and pub(super) liberally. Default to the narrowest visibility that works.
• #[non_exhaustive] on public types in library crates that have stable APIs. Allows adding variants or fields without a breaking change. Internal crates do not need it.

For concurrent code, use message passing or the actor model to avoid data races rather than shared mutable state behind locks.

Pragmatic incrementalism

• Prefer specific, composable logic over abstract frameworks. Do not be overly generic.
• Document non-obvious design decisions and trade-offs in code or commit messages.
• Do not build for hypothetical future requirements. Rule of three: do not abstract until you have seen the pattern three times.

Research before guessing

When you encounter an unfamiliar crate, an unclear API, or a build problem you cannot immediately diagnose, use research agents rather than iterating by trial and error. Speculative iteration wastes build cycles and context.

• morphe:internet-researcher for crate documentation, API behavior, and ecosystem conventions.
• morphe:remote-code-researcher for examining external repositories for patterns and reference implementations.

These run in isolated context and return summaries, so they do not pollute working context.

Testing

• Test comprehensively, including edge cases, race conditions, and platform differences.
• Reuse existing test facilities. Before writing new test helpers, check whether the codebase already has what you need.
• Unit tests belong in the same file as the code they test, inside a #[cfg(test)] mod tests block.
• Integration tests and fixtures go in tests/ at the crate root, not mixed with production sources.

Never skip tests

Tests must never silently skip. If a test requires an environment variable, API key, fixture file, or any other external input, it must fail with a clear error message when that input is unavailable. Never use patterns like let Some(...) = ... else { return } or #[ignore] or early-return guards that turn a missing dependency into a silent pass. A green test suite must mean every test actually ran and verified something. If a test cannot run, it must be red, not invisible.

Preferred testing crates

Crate	Purpose
test-case	Parameterized tests. Annotate a single function with multiple input/output cases.
proptest	Property-based testing. Generates random inputs to find edge cases you would not write by hand.
insta	Snapshot testing. Captures complex output and diffs against stored snapshots.
pretty_assertions	Better assertion output. Colored diffs instead of raw Debug output on failure.

Serde patterns

• Use serde_ignored to detect unused or typo'd fields in configuration deserialization.
• Never use #[serde(flatten)]. The internal buffering breaks serde_ignored warnings, silently swallowing typos in config files.
• Never use #[serde(untagged)] for deserializers. It produces useless error messages like "data did not match any variant." Write custom visitors with an appropriate expecting method instead.

Serialization format changes

When modifying any struct that is serialized to disk or over the wire, trace the full version matrix:

Scenario	Question
Old reader + new data	Can it deserialize? Does it lose information?
New reader + old data	Does #[serde(default)] produce correct values?
Old writer + new data	Can it round-trip without data loss?

The third case is easy to miss. #[serde(default)] allows old readers to deserialize new data, but old writers will still drop unknown fields on write-back, silently corrupting data.

Bump format versions proactively. If adding a field that will be semantically important, bump the version when adding the field, not when first using non-default values. This prevents older versions from silently corrupting data on write-back.

Configuration and environment

Library crates must never read environment variables. All configuration -- API keys, base URLs, auth modes, feature flags -- must be accepted as parameters (in structs, function arguments, or builder methods). Environment variable reading belongs exclusively to application entry points (main.rs, CLI argument parsing, or dedicated configuration loaders). This keeps libraries testable without environment manipulation and prevents hidden coupling to deployment details.

Async patterns

Be selective with async. Use it for I/O and concurrency; keep all other code synchronous. Async infecting non-I/O code makes testing harder and adds complexity for no benefit.

• Runtime: Tokio (multi-threaded). The only production-grade choice.
• Async traits: Use native async fn in traits directly. The async_trait macro is no longer needed on Rust 1.85+.
• Structured concurrency: Use tokio::task::JoinSet for concurrent task groups that need to be awaited together.
• Backpressure: Use bounded mpsc channels. Unbounded channels hide backpressure problems.
• Sync/async boundary: Isolate async to specific modules. Use block_on at application entry points. Do not try to make a single library support both sync and async APIs.

Lints and formatting

• Enable clippy::format_push_string. It catches push_str(&format!(...)) which allocates unnecessarily. Use the write! macro instead.
• Use #[expect(...)] instead of #[allow(...)] for suppressing lints. expect warns when the suppression is no longer needed, preventing stale suppressions from accumulating.

Memory and performance

• Use Arc or borrows for shared immutable data. Avoid cloning when code has a natural tree structure.
• Stream data with iterators where possible rather than buffering into collections.

Module organization

• Use mod.rs files for re-exports only. Put all nontrivial logic in imp.rs or a more specific submodule.
• Platform-specific code goes in separate files: unix.rs, windows.rs. Use #[cfg(unix)] and #[cfg(windows)] for conditional compilation.
• Import all types and functions at the top of the module. The one exception is cfg()-gated imports, which may appear inline.
• Prefer module-level imports over fully qualified paths. Write use std::fmt and then fmt::Display, not std::fmt::Display.
• Importing enum variants for pattern matching is fine.
• Test helpers go in dedicated modules or files, not mixed with production code.

Documentation and style

• Inline comments explain "why," not "what." Do not add narrative comments in function bodies. Only comment when something is non-obvious or needs a deeper "why" explanation.
• Module-level documentation (//!) should explain purpose and responsibilities.
• Periods at the end of code comments.
• Sentence case in headings, never title case.
• Oxford comma. Do not omit articles ("a", "an", "the").

Dependencies

Pinning strategy depends on what you are building.

Applications, binaries, and internal/workspace crates: pin exactly. Cargo's default caret behavior (serde = "1.0" resolves to anything >=1.0.0, <2.0.0) lets dependency resolution shift between cargo update runs. For reproducibility and reviewable bumps, pin exactly:

[dependencies]
serde = "=1.0.219"
tokio = { version = "=1.43.0", features = ["full"] }

Libraries published to crates.io: use the narrowest range that works. Exact pins in a published library break diamond-dependency unification for downstream consumers -- if crate-a pins =1.0.219 and crate-b pins =1.0.220, a consumer depending on both gets two copies of the dependency or a hard resolution failure. Publish caret ranges (or tighter, e.g., >=1.0.219, <1.1) and let downstream Cargo.lock files do the pinning.

Other dependency rules:

• Verify latest versions before adding or bumping. Use morphe:internet-researcher to look up the current version on crates.io. Memorized versions go stale quickly.
• Comment on non-obvious dependency choices: # disable punycode parsing since we only access well-known domains.
• For workspaces, manage shared versions in the root Cargo.toml [workspace.dependencies] table and reference with { workspace = true } in member crates.