rust-guidelines

name	rust-guidelines
description	Pragmatic Rust conventions to keep code readable, testable, and performant for this project.

Pragmatic Rust Guidelines

This file contains all guidelines concatenated for easy reference.

---

AI Guidelines

Design with AI use in Mind (M-DESIGN-FOR-AI) { #M-DESIGN-FOR-AI }

To maximize the utility you get from letting agents work in your code base. 0.1

As a general rule, making APIs easier to use for humans also makes them easier to use by AI. If you follow the guidelines in this book, you should be in good shape.

Rust's strong type system is a boon for agents, as their lack of genuine understanding can often be counterbalanced by comprehensive compiler checks, which Rust provides in abundance.

With that said, there are a few guidelines which are particularly important to help make AI coding in Rust more effective:

• Create Idiomatic Rust API Patterns. The more your APIs, whether public or internal, look and feel like the majority of Rust code in the world, the better it is for AI. Follow the Rust API Guidelines along with the guidelines from Library / UX.

• Provide Thorough Docs. Agents love good detailed docs. Include docs for all of your modules and public items in your crate. Assume the reader has a solid, but not expert, level of understanding of Rust, and that the reader understands the standard library. Follow C-CRATE-DOC, C-FAILURE, C-LINK, and M-MODULE-DOCS M-CANONICAL-DOCS.

• Provide Thorough Examples. Your documentation should have directly usable examples, the repository should include more elaborate ones. Follow C-EXAMPLE C-QUESTION-MARK.

• Use Strong Types. Avoid primitive obsession by using strong types with strict well-documented semantics. Follow C-NEWTYPE.

• Make Your APIs Testable. Design APIs which allow your customers to test their use of your API in unit tests. This might involve introducing some mocks, fakes, or cargo features. AI agents need to be able to iterate quickly to prove that the code they are writing that calls your API is working correctly.

• Ensure Test Coverage. Your own code should have good test coverage over observable behavior. This enables agents to work in a mostly hands-off mode when refactoring.

---

Application Guidelines

Applications may use Anyhow or Derivatives (M-APP-ERROR) { #M-APP-ERROR }

To simplify application-level error handling. 0.1

Note, this guideline is primarily a relaxation and clarification of M-ERRORS-CANONICAL-STRUCTS.

Applications, and crates in your own repository exclusively used from your application, may use anyhow, eyre or similar application-level error crates instead of implementing their own types.

For example, in your application crates you may just re-export and use eyre's common Result type, which should be able to automatically handle all third party library errors, in particular the ones following M-ERRORS-CANONICAL-STRUCTS.

use eyre::Result;

fn start_application() -> Result<()> {
    start_server()?;
    Ok(())
}

Once you selected your application error crate you should switch all application-level errors to that type, and you should not mix multiple application-level error types.

Libraries (crates used by more than one crate) should always follow M-ERRORS-CANONICAL-STRUCTS instead.

Use Mimalloc for Apps (M-MIMALLOC-APPS) { #M-MIMALLOC-APPS }

To get significant performance for free. 0.1

Applications should set mimalloc as their global allocator. This usually results in notable performance increases along allocating hot paths; we have seen up to 25% benchmark improvements.

Changing the allocator only takes a few lines of code. Add mimalloc to your Cargo.toml like so:

[dependencies]
mimalloc = { version = "0.1" } # Or later version if available

Then use it from your main.rs:

use mimalloc::MiMalloc;

#[global_allocator]
static GLOBAL: MiMalloc = MiMalloc;

---

Documentation

Documentation Has Canonical Sections (M-CANONICAL-DOCS) { #M-CANONICAL-DOCS }

To follow established and expected Rust best practices. 1.0

Public library items must contain the canonical doc sections. The summary sentence must always be present. Extended documentation and examples are strongly encouraged. The other sections must be present when applicable.

/// Summary sentence < 15 words.
///
/// Extended documentation in free form.
///
/// # Examples
/// One or more examples that show API usage like so.
///
/// # Errors
/// If fn returns `Result`, list known error conditions
///
/// # Panics
/// If fn may panic, list when this may happen
///
/// # Safety
/// If fn is `unsafe` or may otherwise cause UB, this section must list
/// all conditions a caller must uphold.
///
/// # Abort
/// If fn may abort the process, list when this may happen.
pub fn foo() {}

In contrast to other languages, you should not create a table of parameters. Instead parameter use is explained in plain text. In other words, do not

/// Copies a file.
///
/// # Parameters
/// - src: The source.
/// - dst: The destination.
fn copy(src: File, dst: File) {}

but instead:

/// Copies a file from `src` to `dst`.
fn copy(src: File, dst: File) {}

Related Reading

• Function docs include error, panic, and safety considerations (C-FAILURE)

Mark pub use Items with #[doc(inline)] (M-DOC-INLINE) { #M-DOC-INLINE }

To make re-exported items 'fit in' with their non re-exported siblings. 1.0

When publicly re-exporting crate items via pub use foo::Foo or pub use foo::*, they show up in an opaque re-export block. In most cases, this is not helpful to the reader:

Instead, you should annotate them with #[doc(inline)] at the use site, for them to be inlined organically:

# pub(crate) mod foo { pub struct Foo; }
#[doc(inline)]
pub use foo::*;

// or

#[doc(inline)]
pub use foo::Foo;

This does not apply to std or 3rd party types; these should always be re-exported without inlining to make it clear they are external.

Still avoid glob exports

The #[doc(inline)] trick above does not change M-NO-GLOB-REEXPORTS; you generally should not re-export items via wildcards.

First Sentence is One Line; Approx. 15 Words (M-FIRST-DOC-SENTENCE) { #M-FIRST-DOC-SENTENCE }

To make API docs easily skimmable. 1.0

When you document your item, the first sentence becomes the "summary sentence" that is extracted and shown in the module summary:

/// This is the summary sentence, shown in the module summary.
///
/// This is other documentation. It is only shown in that item's detail view.
/// Sentences here can be as long as you like and it won't cause any issues.
fn some_item() { }

Since Rust API documentation is rendered with a fixed max width, there is a naturally preferred sentence length you should not exceed to keep things tidy on most screens.

If you keep things in a line, your docs will become easily skimmable. Compare, for example, the standard library:

Otherwise, you might end up with widows and a generally unpleasant reading flow:

As a rule of thumb, the first sentence should not exceed 15 words.

Has Comprehensive Module Documentation (M-MODULE-DOCS) { #M-MODULE-DOCS }

To allow for better API docs navigation. 1.1

Any public library module must have //! module documentation, and the first sentence must follow M-DOC-FIRST-SENTENCE.

pub mod ffi {
    //! Contains FFI abstractions.

    pub struct String {};
}

The rest of the module documentation should be comprehensive, i.e., cover the most relevant technical aspects of the contained items, including

• what the module contains
• when it should be used, possibly when not
• examples
• subsystem specifications (e.g., std::fmt also describes its formatting language)
• observable side effects, including what guarantees are made about these, if any
• relevant implementation details, e.g., the used system APIs

Great examples include:

• std::fmt
• std::pin
• std::option

This does not mean every module should contain all of these items. But if there is something to say about the interaction of the contained types, their module documentation is the right place.

---

FFI Guidelines

Isolate DLL State Between FFI Libraries (M-ISOLATE-DLL-STATE) { #M-ISOLATE-DLL-STATE }

To prevent data corruption and undefined behavior. 0.1

When loading multiple Rust-based dynamic libraries (DLLs) within one application, you may only share 'portable' state between these libraries. Likewise, when authoring such libraries, you must only accept or provide 'portable' data from foreign DLLs.

Portable here means data that is safe and consistent to process regardless of its origin. By definition, this is a subset of FFI-safe types. A type is portable if it is #[repr(C)] (or similarly well-defined), and all of the following:

• It must not have any interaction with any static or thread local.
• It must not have any interaction with any TypeId.
• It must not contain any value, pointer or reference to any non-portable data (it is valid to point into portable data within non-portable data, such as sharing a reference to an ASCII string held in a Box).

Interaction means any computational relationship, and therefore also relates to how the type is used. Sending a u128 between DLLs is OK, using it to exchange a transmuted TypeId isn't.

The underlying issue stems from the Rust compiler treating each DLL as an entirely new compilation artifact, akin to a standalone application. This means each DLL:

• has its own set of static and thread-local variables,
• the type layout of any #[repr(Rust)] type (the default) can differ between compilations,
• has its own set of unique type IDs, differing from any other DLL.

Notably, this affects:

• ⚠️ any allocated instance, e.g., String, Vec<u8>, Box<Foo>, ...
• ⚠️ any library relying on other statics, e.g., tokio, log,
• ⚠️ any struct not #[repr(C)],
• ⚠️ any data structure relying on consistent TypeId.

In practice, transferring any of the above between libraries leads to data loss, state corruption, and usually undefined behavior.

Take particular note that this may also apply to types and methods that are invisible at the FFI boundary:

/// A method in DLL1 that wants to use a common service from DLL2
#[ffi_function]
fn use_common_service(common: &CommonService) {
    // This has at least two issues:
    // - `CommonService`, or ANY type nested deep within might have
    //   a different type layout in DLL2, leading to immediate
    //   undefined behavior (UB) ⚠️
    // - `do_work()` here looks like it will be invoked in DLL2, but
    //   the code executed will actually come from DLL1. This means that
    //   `do_work()` invoked here will see a data structure coming from
    //   DLL2, but will use statics from DLL1 ⚠️
    common.do_work();
}

---

Library Guidelines

---

Performance Guidelines

Identify, Profile, Optimize the Hot Path Early (M-HOTPATH) { #M-HOTPATH }

To end up with high performance code. 0.1

You should, early in the development process, identify if your crate is performance or COGS relevant. If it is:

• identify hot paths and create benchmarks around them,
• regularly run a profiler collecting CPU and allocation insights,
• document or communicate the most performance sensitive areas.

For benchmarks we recommend criterion or divan. If possible, benchmarks should not only measure elapsed wall time, but also used CPU time over all threads (this unfortunately requires manual work and is not supported out of the box by the common benchmark utils).

Profiling Rust on Windows works out of the box with Intel VTune and Superluminal. However, to gain meaningful CPU insights you should enable debug symbols for benchmarks in your Cargo.toml:

[profile.bench]
debug = 1

Documenting the most performance sensitive areas helps other contributors take better decision. This can be as simple as sharing screenshots of your latest profiling hot spots.

Further Reading

• Performance Tips

How much faster?

Some of the most common 'language related' issues we have seen include:

• frequent re-allocations, esp. cloned, growing or format! assembled strings,
• short lived allocations over bump allocations or similar,
• memory copy overhead that comes from cloning Strings and collections,
• repeated re-hashing of equal data structures
• the use of Rust's default hasher where collision resistance wasn't an issue

Anecdotally, we have seen ~15% benchmark gains on hot paths where only some of these String problems were addressed, and it appears that up to 50% could be achieved in highly optimized versions.

Optimize for Throughput, Avoid Empty Cycles (M-THROUGHPUT) { #M-THROUGHPUT }

To ensure COGS savings at scale. 0.1

You should optimize your library for throughput, and one of your key metrics should be items per CPU cycle.

This does not mean to neglect latency—after all you can scale for throughput, but not for latency. However, in most cases you should not pay for latency with empty cycles that come with single-item processing, contended locks and frequent task switching.

Ideally, you should

• partition reasonable chunks of work ahead of time,
• let individual threads and tasks deal with their slice of work independently,
• sleep or yield when no work is present,
• design your own APIs for batched operations,
• perform work via batched APIs where available,
• yield within long individual items, or between chunks of batches (see M-YIELD-POINTS),
• exploit CPU caches, temporal and spatial locality.

You should not:

• hot spin to receive individual items faster,
• perform work on individual items if batching is possible,
• do work stealing or similar to balance individual items.

Shared state should only be used if the cost of sharing is less than the cost of re-computation.

Long-Running Tasks Should Have Yield Points. (M-YIELD-POINTS) { #M-YIELD-POINTS }

To ensure you don't starve other tasks of CPU time. 0.2

If you perform long running computations, they should contain yield_now().await points.

Your future might be executed in a runtime that cannot work around blocking or long-running tasks. Even then, such tasks are considered bad design and cause runtime overhead. If your complex task performs I/O regularly it will simply utilize these await points to preempt itself:

async fn process_items(items: &[items]) {
    // Keep processing items, the runtime will preempt you automatically.
    for i in items {
        read_item(i).await;
    }
}

If your task performs long-running CPU operations without intermixed I/O, it should instead cooperatively yield at regular intervals, to not starve concurrent operations:

async fn process_items(zip_file: File) {
    let items = zip_file.read().async;
    for i in items {
        decompress(i);
        yield_now().await;
    }
}

If the number and duration of your individual operations are unpredictable you should use APIs such as has_budget_remaining() and related APIs to query your hosting runtime.

Yield how often?

In a thread-per-core model the overhead of task switching must be balanced against the systemic effects of starving unrelated tasks.

Under the assumption that runtime task switching takes 100's of ns, in addition to the overhead of lost CPU caches, continuous execution in between should be long enough that the switching cost becomes negligible (<1%).

Thus, performing 10 - 100μs of CPU-bound work between yield points would be a good starting point.

---

Safety Guidelines

Unsafe Implies Undefined Behavior (M-UNSAFE-IMPLIES-UB) { #M-UNSAFE-IMPLIES-UB }

To ensure semantic consistency and prevent warning fatigue. 1.0

The marker unsafe may only be applied to functions and traits if misuse implies the risk of undefined behavior (UB). It must not be used to mark functions that are dangerous to call for other reasons.

// Valid use of unsafe
unsafe fn print_string(x: *const String) { }

// Invalid use of unsafe
unsafe fn delete_database() { }

Unsafe Needs Reason, Should be Avoided (M-UNSAFE) { #M-UNSAFE }

To prevent undefined behavior, attack surface, and similar 'happy little accidents'. 0.2

You must have a valid reason to use unsafe. The only valid reasons are

1. novel abstractions, e.g., a new smart pointer or allocator,
2. performance, e.g., attempting to call .get_unchecked(),
3. FFI and platform calls, e.g., calling into C or the kernel, ...

Unsafe code lowers the guardrails used by the compiler, transferring some of the compiler's responsibilities to the programmer. Correctness of the resulting code relies primarily on catching all mistakes in code review, which is error-prone. Mistakes in unsafe code may introduce high-severity security vulnerabilities.

You must not use ad-hoc unsafe to

• shorten a performant and safe Rust program, e.g., 'simplify' enum casts via transmute,
• bypass Send and similar bounds, e.g., by doing unsafe impl Send ...,
• bypass lifetime requirements via transmute and similar.

Ad-hoc here means unsafe embedded in otherwise unrelated code. It is of course permissible to create properly designed, sound abstractions doing these things.

In any case, unsafe must follow the guidelines outlined below.

Novel Abstractions

• Verify there is no established alternative. If there is, prefer that.
• Your abstraction must be minimal and testable.
• It must be hardened and tested against "adversarial code", esp.
  • If they accept closures they must become invalid (e.g., poisoned) if the closure panics
  • They must assume any safe trait is misbehaving, esp. Deref, Clone and Drop.
• Any use of unsafe must be accompanied by plain-text reasoning outlining its safety
• It must pass Miri, including adversarial test cases
• It must follow all other unsafe code guidelines

Performance

• Using unsafe for performance reasons should only be done after benchmarking
• Any use of unsafe must be accompanied by plain-text reasoning outlining its safety. This applies to both calling unsafe methods, as well as providing _unchecked ones.
• The code in question must pass Miri
• You must follow the unsafe code guidelines

FFI

• We recommend you use an established interop library to avoid unsafe constructs
• You must follow the unsafe code guidelines
• You must document your generated bindings to make it clear which call patterns are permissible

Further Reading

• Nomicon
• Unsafe Code Guidelines
• Miri
• "Adversarial code"

All Code Must be Sound (M-UNSOUND) { #M-UNSOUND }

To prevent unexpected runtime behavior, leading to potential bugs and incompatibilities. 1.0

Unsound code is seemingly safe code that may produce undefined behavior when called from other safe code, or on its own accord.

Meaning of 'Safe'

The terms safe and unsafe are technical terms in Rust.

A function is safe, if its signature does not mark it unsafe. That said, safe functions can still be dangerous (e.g., delete_database()), and unsafe ones are, when properly used, usually quite benign (e.g.,vec.get_unchecked()).

A function is therefore unsound if it appears safe (i.e., it is not marked unsafe), but if any of its calling modes would cause undefined behavior. This is to be interpreted in the strictest sense. Even if causing undefined behavior is only a 'remote, theoretical possibility' requiring 'weird code', the function is unsound.

Also see Unsafe, Unsound, Undefined.

// "Safely" converts types
fn unsound_ref<T>(x: &T) -> &u128 {
    unsafe { std::mem::transmute(x) }
}

// "Clever trick" to work around missing `Send` bounds.
struct AlwaysSend<T>(T);
unsafe impl<T> Send for AlwaysSend<T> {}
unsafe impl<T> Sync for AlwaysSend<T> {}

Unsound abstractions are never permissible. If you cannot safely encapsulate something, you must expose unsafe functions instead, and document proper behavior.

No Exceptions

While you may break most guidelines if you have a good enough reason, there are no exceptions in this case: unsound code is never acceptable.

It's the Module Boundaries

Note that soundness boundaries equal module boundaries! It is perfectly fine, in an otherwise safe abstraction, to have safe functions that rely on behavior guaranteed elsewhere in the same module.

struct MyDevice(*const u8);

impl MyDevice {
    fn new() -> Self {
       // Properly initializes instance ...
       # todo!()
    }

    fn get(&self) -> u8 {
        // It is perfectly fine to rely on `self.0` being valid, despite this
        // function in-and-by itself being unable to validate that.
        unsafe { *self.0 }
    }
}

---

Universal Guidelines

Names are Free of Weasel Words (M-CONCISE-NAMES) { #M-CONCISE-NAMES }

To improve readability. 1.0

Symbol names, especially types and traits names, should be free of weasel words that do not meaningfully add information. Common offenders include Service, Manager, and Factory. For example:

While your library may very well contain or communicate with a booking service—or even hold an HttpClient instance named booking_service—one should rarely encounter a BookingService type in code.

An item handling many bookings can just be called Bookings. If it does anything more specific, then that quality should be appended instead. It submits these items elsewhere? Calling it BookingDispatcher would be more helpful.

The same is true for Managers. Every code manages something, so that moniker is rarely useful. With rare exceptions, life cycle issues should likewise not be made the subject of some manager. Items are created in whatever way they are needed, their disposal is governed by Drop, and only Drop.

Regarding factories, at least the term should be avoided. While the concept FooFactory has its use, its canonical Rust name is Builder (compare M-INIT-BUILDER). A builder that can produce items repeatedly is still a builder.

In addition, accepting factories (builders) as parameters is an unidiomatic import of OO concepts into Rust. If repeatable instantiation is required, functions should ask for an impl Fn() -> Foo over a FooBuilder or similar. In contrast, standalone builders have their use, but primarily to reduce parametric permutation complexity around optional values (again, M-INIT-BUILDER).

Magic Values are Documented (M-DOCUMENTED-MAGIC) { #M-DOCUMENTED-MAGIC }

To ensure maintainability and prevent misunderstandings when refactoring. 1.0

Hardcoded magic values in production code must be accompanied by a comment. The comment should outline:

• why this value was chosen,
• non-obvious side effects if that value is changed,
• external systems that interact with this constant.

You should prefer named constants over inline values.

// Bad: it's relatively obvious that this waits for a day, but not why
wait_timeout(60 * 60 * 24).await // Wait at most a day

// Better
wait_timeout(60 * 60 * 24).await // Large enough value to ensure the server
                                 // can finish. Setting this too low might
                                 // make us abort a valid request. Based on
                                 // `api.foo.com` timeout policies.

// Best

/// How long we wait for the server.
///
/// Large enough value to ensure the server
/// can finish. Setting this too low might
/// make us abort a valid request. Based on
/// `api.foo.com` timeout policies.
const UPSTREAM_SERVER_TIMEOUT: Duration = Duration::from_secs(60 * 60 * 24);

Lint Overrides Should Use #[expect] (M-LINT-OVERRIDE-EXPECT) { #M-LINT-OVERRIDE-EXPECT }

To prevent the accumulation of outdated lints. 1.0

When overriding project-global lints inside a submodule or item, you should do so via #[expect], not #[allow].

Expected lints emit a warning if the marked warning was not encountered, thus preventing the accumulation of stale lints. That said, #[allow] lints are still useful when applied to generated code, and can appear in macros.

Overrides should be accompanied by a reason:

#[expect(clippy::unused_async, reason = "API fixed, will use I/O later")]
pub async fn ping_server() {
  // Stubbed out for now
}

Use Structured Logging with Message Templates (M-LOG-STRUCTURED) { #M-LOG-STRUCTURED }

To minimize the cost of logging and to improve filtering capabilities. 0.1

Logging should use structured events with named properties and message templates following the message templates specification.

Note: Examples use the tracing crate's event! macro, but these principles apply to any logging API that supports structured logging (e.g., log, slog, custom telemetry systems).

Avoid String Formatting

String formatting allocates memory at runtime. Message templates defer formatting until viewing time. We recommend that message template includes all named properties for easier inspection at viewing time.

// Bad: String formatting causes allocations
tracing::info!("file opened: {}", path);
tracing::info!(format!("file opened: {}", path));

// Good: Message templates with named properties
event!(
    name: "file.open.success",
    Level::INFO,
    file.path = path.display(),
    "file opened: {{file.path}}",
);

Note: Use the {{property}} syntax in message templates which preserves the literal text while escaping Rust's format syntax. String formatting is deferred until logs are viewed.

Name Your Events

Use hierarchical dot-notation: <component>.<operation>.<state>

// Bad: Unnamed events
event!(
    Level::INFO,
    file.path = file_path,
    "file {{file.path}} processed succesfully",
);

// Good: Named events
event!(
    name: "file.processing.success", // event identifier
    Level::INFO,
    file.path = file_path,
    "file {{file.path}} processed succesfully",
);

Named events enable grouping and filtering across log entries.

Follow OpenTelemetry Semantic Conventions

Use OTel semantic conventions for common attributes if needed. This enables standardization and interoperability.

event!(
    name: "file.write.success",
    Level::INFO,
    file.path = path.display(),         // Standard OTel name
    file.size = bytes_written,          // Standard OTel name
    file.directory = dir_path,          // Standard OTel name
    file.extension = extension,         // Standard OTel name
    file.operation = "write",           // Custom name
    "{{file.operation}} {{file.size}} bytes to {{file.path}} in {{file.directory}} extension={{file.extension}}",
);

Common conventions:

• HTTP: http.request.method, http.response.status_code, url.scheme, url.path, server.address
• File: file.path, file.directory, file.name, file.extension, file.size
• Database: db.system.name, db.namespace, db.operation.name, db.query.text
• Errors: error.type, error.message, exception.type, exception.stacktrace

Redact Sensitive Data

Do not log plain sensitive data as this might lead to privacy and security incidents.

// Bad: Logs potentially sensitive data
event!(
    name: "file.operation.started",
    Level::INFO,
    user.email = user.email,  // Sensitive data
    file.name = "license.txt",
    "reading file {{file.name}} for user {{user.email}}",
);

// Good: Redact sensitive parts
event!(
    name: "file.operation.started",
    Level::INFO,
    user.email.redacted = redact_email(user.email),
    file.name = "license.txt",
    "reading file {{file.name}} for user {{user.email.redacted}}",
);

Sensitive data includes email addresses, file paths revealing user identity, filenames containing secrets or tokens, file contents with PII, temporary file paths with session IDs and more. Consider using the data_privacy crate for consistent redaction.

Further Reading

• Message Templates Specification
• OpenTelemetry Semantic Conventions
• OWASP Logging Cheat Sheet

Panic Means 'Stop the Program' (M-PANIC-IS-STOP) { #M-PANIC-IS-STOP }

To ensure soundness and predictability. 1.0

Panics are not exceptions. Instead, they suggest immediate program termination.

Although your code must be panic-safe (i.e., a survived panic may not lead to inconsistent state), invoking a panic means this program should stop now. It is not valid to:

• use panics to communicate (errors) upstream,
• use panics to handle self-inflicted error conditions,
• assume panics will be caught, even by your own code.

For example, if the application calling you is compiled with a Cargo.toml containing

[profile.release]
panic = "abort"

then any invocation of panic will cause an otherwise functioning program to needlessly abort. Valid reasons to panic are:

• when encountering a programming error, e.g., x.expect("must never happen"),
• anything invoked from const contexts, e.g., const { foo.unwrap() },
• when user requested, e.g., providing an unwrap() method yourself,
• when encountering a poison, e.g., by calling unwrap() on a lock result (a poisoned lock signals another thread has panicked already).

Any of those are directly or indirectly linked to programming errors.

Detected Programming Bugs are Panics, Not Errors (M-PANIC-ON-BUG) { #M-PANIC-ON-BUG }

To avoid impossible error handling code and ensure runtime consistency. 1.0

As an extension of M-PANIC-IS-STOP above, when an unrecoverable programming error has been detected, libraries and applications must panic, i.e., request program termination.

In these cases, no Error type should be introduced or returned, as any such error could not be acted upon at runtime.

Contract violations, i.e., the breaking of invariants either within a library or by a caller, are programming errors and must therefore panic.

However, what constitutes a violation is situational. APIs are not expected to go out of their way to detect them, as such checks can be impossible or expensive. Encountering must_be_even == 3 during an already existing check clearly warrants a panic, while a function parse(&str) clearly must return a Result. If in doubt, we recommend you take inspiration from the standard library.

// Generally, a function with bad parameters must either
// - Ignore a parameter and/or return the wrong result
// - Signal an issue via Result or similar
// - Panic
// If in this `divide_by` we see that y == 0, panicking is
// the correct approach.
fn divide_by(x: u32, y: u32) -> u32 { ... }

// However, it can also be permissible to omit such checks
// and return an unspecified (but not an undefined) result.
fn divide_by_fast(x: u32, y: u32) -> u32 { ... }

// Here, passing an invalid URI is not a contract violation.
// Since parsing is inherently fallible, a Result must be returned.
fn parse_uri(s: &str) -> Result<Uri, ParseError> { };

Make it 'Correct by Construction'

While panicking on a detected programming error is the 'least bad option', your panic might still ruin someone's day. For any user input or calling sequence that would otherwise panic, you should also explore if you can use the type system to avoid panicking code paths altogether.

Public Types are Debug (M-PUBLIC-DEBUG) { #M-PUBLIC-DEBUG }

To simplify debugging and prevent leaking sensitive data. 1.0

All public types exposed by a crate should implement Debug. Most types can do so via #[derive(Debug)]:

#[derive(Debug)]
struct Endpoint(String);

Types designed to hold sensitive data should also implement Debug, but do so via a custom implementation. This implementation must employ unit tests to ensure sensitive data isn't actually leaked, and will not be in the future.

use std::fmt::{Debug, Formatter};

struct UserSecret(String);

impl Debug for UserSecret {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        write!(f, "UserSecret(...)")
    }
}

#[test]
fn test() {
    let key = "552d3454-d0d5-445d-ab9f-ef2ae3a8896a";
    let secret = UserSecret(key.to_string());
    let rendered = format!("{:?}", secret);

    assert!(rendered.contains("UserSecret"));
    assert!(!rendered.contains(key));
}

Public Types Meant to be Read are Display (M-PUBLIC-DISPLAY) { #M-PUBLIC-DISPLAY }

To improve usability. 1.0

If your type is expected to be read by upstream consumers, be it developers or end users, it should implement Display. This in particular includes:

• Error types, which are mandated by std::error::Error to implement Display
• Wrappers around string-like data

Implementations of Display should follow Rust customs; this includes rendering newlines and escape sequences. The handling of sensitive data outlined in M-PUBLIC-DEBUG applies analogously.

Prefer Regular over Associated Functions (M-REGULAR-FN) { #M-REGULAR-FN }

To improve readability. 1.0

Associated functions should primarily be used for instance creation, not general purpose computation.

In contrast to some OO languages, regular functions are first-class citizens in Rust and need no module or class to host them. Functionality that does not clearly belong to a receiver should therefore not reside in a type's impl block:

struct Database {}

impl Database {
    // Ok, associated function creates an instance
    fn new() -> Self {}

    // Ok, regular method with `&self` as receiver
    fn query(&self) {}

    // Not ok, this function is not directly related to `Database`,
    // it should therefore not live under `Database` as an associated
    // function.
    fn check_parameters(p: &str) {}
}

// As a regular function this is fine
fn check_parameters(p: &str) {}

Regular functions are more idiomatic, and reduce unnecessary noise on the caller side. Associated trait functions are perfectly idiomatic though:

pub trait Default {
    fn default() -> Self;
}

struct Foo;

impl Default for Foo {
    fn default() -> Self { Self }
}

If in Doubt, Split the Crate (M-SMALLER-CRATES) { #M-SMALLER-CRATES }

To improve compile times and modularity. 1.0

You should err on the side of having too many crates rather than too few, as this leads to dramatic compile time improvements—especially during the development of these crates—and prevents cyclic component dependencies.

Essentially, if a submodule can be used independently, its contents should be moved into a separate crate.

Performing this crate split may cause you to lose access to some pub(crate) fields or methods. In many situations, this is a desirable side-effect and should prompt you to design more flexible abstractions that would give your users similar affordances.

In some cases, it is desirable to re-join individual crates back into a single umbrella crate, such as when dealing with proc macros, or runtimes. Functionality split for technical reasons (e.g., a foo_proc proc macro crate) should always be re-exported. Otherwise, re-exports should be used sparingly.

Features vs. Crates

As a rule of thumb, crates are for items that can reasonably be used on their own. Features should unlock extra functionality that can't live on its own. In the case of umbrella crates, see below, features may also be used to enable constituents (but then that functionality was extracted into crates already).

For example, if you defined a web crate with the following modules, users only needing client calls would also have to pay for the compilation of server code:

web::server
web::client
web::protocols

Instead, you should introduce individual crates that give users the ability to pick and choose:

web_server
web_client
web_protocols

Use Static Verification (M-STATIC-VERIFICATION) { #M-STATIC-VERIFICATION }

To ensure consistency and avoid common issues. 1.0

Projects should use the following static verification tools to help maintain the quality of the code. These tools can be configured to run on a developer's machine during normal work, and should be used as part of check-in gates.

• compiler lints offer many lints to avoid bugs and improve code quality.
• clippy lints contain hundreds of lints to avoid bugs and improve code quality.
• rustfmt ensures consistent source formatting.
• cargo-audit verifies crate dependencies for security vulnerabilities.
• cargo-hack validates that all combinations of crate features work correctly.
• cargo-udeps detects unused dependencies in Cargo.toml files.
• miri validates the correctness of unsafe code.

Compiler Lints

The Rust compiler generally produces exceptionally good diagnostics. In addition to the default set of diagnostics, projects should explicitly enable the following set of compiler lints:

[lints.rust]
ambiguous_negative_literals = "warn"
missing_debug_implementations = "warn"
redundant_imports = "warn"
redundant_lifetimes = "warn"
trivial_numeric_casts = "warn"
unsafe_op_in_unsafe_fn = "warn"
unused_lifetimes = "warn"

Clippy Lints

For clippy, projects should enable all major lint categories, and additionally enable some lints from the restriction lint group. Undesired lints (e.g., numeric casts) can be opted back out of on a case-by-case basis:

[lints.clippy]
cargo = { level = "warn", priority = -1 }
complexity = { level = "warn", priority = -1 }
correctness = { level = "warn", priority = -1 }
pedantic = { level = "warn", priority = -1 }
perf = { level = "warn", priority = -1 }
style = { level = "warn", priority = -1 }
suspicious = { level = "warn", priority = -1 }
# nursery = { level = "warn", priority = -1 }  # optional, might cause more false positives

# These lints are from the `restriction` lint group and prevent specific
# constructs being used in source code in order to drive up consistency,
# quality, and brevity
allow_attributes_without_reason = "warn"
as_pointer_underscore = "warn"
assertions_on_result_states = "warn"
clone_on_ref_ptr = "warn"
deref_by_slicing = "warn"
disallowed_script_idents = "warn"
empty_drop = "warn"
empty_enum_variants_with_brackets = "warn"
empty_structs_with_brackets = "warn"
fn_to_numeric_cast_any = "warn"
if_then_some_else_none = "warn"
map_err_ignore = "warn"
redundant_type_annotations = "warn"
renamed_function_params = "warn"
semicolon_outside_block = "warn"
string_to_string = "warn"
undocumented_unsafe_blocks = "warn"
unnecessary_safety_comment = "warn"
unnecessary_safety_doc = "warn"
unneeded_field_pattern = "warn"
unused_result_ok = "warn"

# May cause issues with structured logging otherwise.
literal_string_with_formatting_args = "allow"

# Define custom opt outs here
# ...

Follow the Upstream Guidelines (M-UPSTREAM-GUIDELINES) { #M-UPSTREAM-GUIDELINES }

To avoid repeating mistakes the community has already learned from, and to have a codebase that does not surprise users and contributors. 1.0

The guidelines in this book complement existing Rust guidelines, in particular:

• Rust API Guidelines
• Rust Style Guide
• Rust Design Patterns
• Rust Reference - Undefined Behavior

We recommend you read through these as well, and apply them in addition to this book's items. Pay special attention to the ones below, as they are frequently forgotten:

• C-CONV - Ad-hoc conversions follow as_, to_, into_ conventions
• C-GETTER - Getter names follow Rust convention
• C-COMMON-TRAITS - Types eagerly implement common traits
  • Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Default, Debug
  • Display where type wants to be displayed
• C-CTOR - Constructors are static, inherent methods
  • In particular, have Foo::new(), even if you have Foo::default()
• C-FEATURE - Feature names are free of placeholder words

---

Libraries / Building Guidelines

Features are Additive (M-FEATURES-ADDITIVE) { #M-FEATURES-ADDITIVE }

To prevent compilation breakage in large and complex projects. 1.0

All library features must be additive, and any combination must work, as long as the feature itself would work on the current platform. This implies:

• You must not introduce a no-std feature, use a std feature instead
• Adding any feature foo must not disable or modify any public item
  • Adding enum variants is fine if these enums are #[non_exhaustive]
• Features must not rely on other features to be manually enabled
• Features must not rely on their parent to skip-enable a feature in one of their children

Further Reading

• Feature Unification
• Mutually Exclusive Features

Libraries Work Out of the Box (M-OOBE) { #M-OOBE }

To be easily adoptable by the Rust ecosystem. 1.0

Libraries must just work on all supported platforms, with the exception of libraries that are expressly platform or target specific.

Rust crates often come with dozens of dependencies, applications with 100's. Users expect cargo build and cargo install to just work. Consider this installation of bat that pulls in ~250 dependencies:

Compiling writeable v0.5.5
Compiling strsim v0.11.1
Compiling litemap v0.7.5
Compiling crossbeam-utils v0.8.21
Compiling icu_properties_data v1.5.1
Compiling ident_case v1.0.1
Compiling once_cell v1.21.3
Compiling icu_normalizer_data v1.5.1
Compiling fnv v1.0.7
Compiling regex-syntax v0.8.5
Compiling anstyle v1.0.10
Compiling vcpkg v0.2.15
Compiling utf8parse v0.2.2
Compiling aho-corasick v1.1.3
Compiling utf16_iter v1.0.5
Compiling hashbrown v0.15.2
Building [==>                       ] 29/251: icu_locid_transform_data, serde, winnow, indexma...

This compilation, like practically all other applications and libraries, will just work.

While there are tools targeting specific functionality (e.g., a Wayland compositor) or platform crates like windows; unless a crate is obviously platform specific, the expectation is that it will otherwise just work.

This means crates must build, ultimately

• on all Tier 1 platforms,¹ and
• without any additional prerequisites beyond cargo and rust.²

¹ It is ok to not support Tier 1 platforms "for now", but abstractions must be present so support can easily be extended. This is usually done by introducing an internal HAL (Hardware Abstraction Layer) module with a dummy fallback target.
² A default Rust installation will also have cc and a linker present.

In particular, non-platform crates must not, by default, require the user to install additional tools, or expect environment variables to compile. If tools were somehow needed (like the generation of Rust from .proto files) these tools should be run as part of the publishing workflow or earlier, and the resulting artifacts (e.g., .rs files) be contained inside the published crate.

If a dependency is known to be platform specific, the parent must use conditional (platform) compilation or opt-in feature gates.

Libraries are Responsible for Their Dependencies.

Imagine you author a Copilot crate, which in turn uses an HttpClient, which in turn depends on a perl script to compile.

Then every one of your users, and your user's users, and everyone above, would need to install Perl to compile their crate. In large projects you would have 100's of people who don't know or don't care about your library or Perl, encounter a cryptic compilation error, and now have to figure out how to install it on their system.

In practical terms, such behavior is largely a self-inflicted death sentence in the open source space, since the moment alternatives are available, people will switch to those that just work.

Native -sys Crates Compile Without Dependencies (M-SYS-CRATES) { #M-SYS-CRATES }

To have libraries that 'just work' on all platforms. 0.2

If you author a pair of foo and foo-sys crates wrapping a native foo.lib, you are likely to run into the issues described in M-OOBE.

Follow these steps to produce a crate that just works across platforms:

• fully govern the build of foo.lib from build.rs inside foo-sys. Only use hand-crafted compilation via the cc crate, do not run Makefiles or external build scripts, as that will require the installation of external dependencies,
• make all external tools optional, such as nasm,
• embed the upstream source code in your crate,
• make the embedded sources verifiable (e.g., include Git URL + hash),
• pre-generate bindgen glue if possible,
• support both static linking, and dynamic linking via libloading.

Deviations from these points can work, and can be considered on a case-by-case basis:

If the native build system is available as an OOBE crate, that can be used instead of cc invocations. The same applies to external tools.

Source code might have to be downloaded if it does not fit crates.io size limitations. In any case, only servers with an availability comparable to crates.io should be used. In addition, the specific hashes of acceptable downloads should be stored in the crate and verified.

Downloading sources can fail on hermetic build environments, therefore alternative source roots should also be specifiable (e.g., via environment variables).

---

Libraries / Interoperability Guidelines

Don't Leak External Types (M-DONT-LEAK-TYPES) { #M-DONT-LEAK-TYPES }

To prevent accidental breakage and long-term maintenance cost. 0.1

Where possible, you should prefer std¹ types in public APIs over types coming from external crates. Exceptions should be carefully considered.

Any type in any public API will become part of that API's contract. Since std and constituents are the only crates shipped by default, and since they come with a permanent stability guarantee, their types are the only ones that come without an interoperability risk.

A crate that exposes another crate's type is said to leak that type.

For maximal long term stability your crate should, theoretically, not leak any types. Practically, some leakage is unavoidable, sometimes even beneficial. We recommend you follow this heuristic:

• if you can avoid it, do not leak third-party types
• if you are part of an umbrella crate,² you may freely leak types from sibling crates.
• behind a relevant feature flag, types may be leaked (e.g., serde)
• without a feature only if they give a substantial benefit. Most commonly that is interoperability with significant other parts of the Rust ecosystem based around these types.

¹ In rare instances, e.g., high performance libraries used from embedded, you might even want to limit yourself to core only.

² For example, a runtime crate might be the umbrella of runtime_rt, runtime_app and runtime_clock As users are expected to only interact with the umbrella, siblings may leak each others types.

Native Escape Hatches (M-ESCAPE-HATCHES) { #M-ESCAPE-HATCHES }

To allow users to work around unsupported use cases until alternatives are available. 0.1

Types wrapping native handles should provide unsafe escape hatches. In interop scenarios your users might have gotten a native handle from somewhere else, or they might have to pass your wrapped handle over FFI. To enable these use cases you should provide unsafe conversion methods.

# type HNATIVE = *const u8;
pub struct Handle(HNATIVE);

impl Handle {
    pub fn new() -> Self {
        // Safely creates handle via API calls
        # todo!()
    }

    // Constructs a new Handle from a native handle the user got elsewhere.
    // This method  should then also document all safety requirements that
    // must be fulfilled.
    pub unsafe fn from_native(native: HNATIVE) -> Self {
        Self(native)
    }

    // Various extra methods to permanently or temporarily obtain
    // a native handle.
    pub fn into_native(self) -> HNATIVE { self.0 }
    pub fn to_native(&self) -> HNATIVE { self.0 }
}

Types are Send (M-TYPES-SEND) { #M-TYPES-SEND }

To enable the use of types in Tokio and behind runtime abstractions 1.0

Public types should be Send for compatibility reasons:

• All futures produced (explicitly or implicitly) must be Send
• Most other types should be Send, but there might be exceptions

Futures

When declaring a future explicitly you should ensure it is, and remains, Send.

# use std::future::Future;
# use std::pin::Pin;
# use std::task::{Context, Poll};
#
struct Foo {}

impl Future for Foo {
    // Explicit implementation of `Future` for your type
    # type Output = ();
    #
    # fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<<Self as Future>::Output> { todo!() }
}

// You should assert your type is `Send`
const fn assert_send<T: Send>() {}
const _: () = assert_send::<Foo>();

When returning futures implicitly through async method calls, you should make sure these are Send too. You do not have to test every single method, but you should at least validate your main entry points.

async fn foo() { }

// TODO: We want this as a macro as well
fn assert_send<T: Send>(_: T) {}
_ = assert_send(foo());

Regular Types

Most regular types should be Send, as they otherwise infect futures turning them !Send if held across .await points.

# use std::rc::Rc;
# async fn read_file(x: &str) {}
#
async fn foo() {
    let rc = Rc::new(123);      // <-- Holding this across an .await point prevents
    read_file("foo.txt").await; //     the future from being `Send`.
    dbg!(rc);
}

That said, if the default use of your type is instantaneous, and there is no reason for it to be otherwise held across .await boundaries, it may be !Send.

# use std::rc::Rc;
# struct Telemetry; impl Telemetry { fn ping(&self, _: u32) {} }
# fn telemetry() -> Telemetry  { Telemetry }
# async fn read_file(x: &str) {}
#
async fn foo() {
    // Here a hypothetical instance Telemetry is summoned
    // and used ad-hoc. It may be ok for Telemetry to be !Send.
    telemetry().ping(0);
    read_file("foo.txt").await;
    telemetry().ping(1);
}

The Cost of Send

Ideally, there would be abstractions that are Send in work-stealing runtimes, and !Send in thread-per-core models based on non-atomic types like Rc and RefCell instead.

Practically these abstractions don't exist, preventing Tokio compatibility in the non-atomic case. That in turn means you would have to "reinvent the world" to get anything done in a thread-per-core universe.

The good news is, in most cases atomics and uncontended locks only have a measurable impact if accessed more frequently than every 64 words or so.

Working with a large Vec<AtomicUsize> in a hot loop is a bad idea, but doing the occasional uncontended atomic operation from otherwise thread-per-core async code has no performance impact, but gives you widespread ecosystem compatibility.

---

Libraries / Resilience Guidelines

Avoid Statics (M-AVOID-STATICS) { #M-AVOID-STATICS }

To prevent consistency and correctness issues between crate versions. 1.0

Libraries should avoid static and thread-local items, if a consistent view of the item is relevant for correctness. Essentially, any code that would be incorrect if the static magically had another value must not use them. Statics only used for performance optimizations are ok.

The fundamental issue with statics in Rust is the secret duplication of state.

Consider a crate core with the following function:

# use std::sync::atomic::AtomicUsize;
# use std::sync::atomic::Ordering;
static GLOBAL_COUNTER: AtomicUsize = AtomicUsize::new(0);

pub fn increase_counter() -> usize {
    GLOBAL_COUNTER.fetch_add(1, Ordering::Relaxed)
}

Now assume you have a crate main, calling two libraries library_a and library_b, each invoking that counter:

// Increase global static counter 2 times
library_a::count_up();
library_a::count_up();

// Increase global static counter 3 more times
library_b::count_up();
library_b::count_up();
library_b::count_up();

They eventually report their result:

library_a::print_counter();
library_b::print_counter();
main::print_counter();

At this point, what is the value of said counter; 0, 2, 3 or 5?

The answer is, possibly any (even multiple!) of the above, depending on the crate's version resolution!

Under the hood Rust may link to multiple versions of the same crate, independently instantiated, to satisfy declared dependencies. This is especially observable during a crate's 0.x version timeline, where each x constitutes a separate major version.

If main, library_a and library_b all declared the same version of core, e.g. 0.5, then the reported result will be 5, since all crates actually see the same version of GLOBAL_COUNTER.

However, if library_a declared 0.4 instead, then it would be linked against a separate version of core; thus main and library_b would agree on a value of 3, while library_a reported 2.

Although static items can be useful, they are particularly dangerous before a library's stabilization, and for any state where secret duplication would cause consistency issues when static and non-static variable use interacts. In addition, statics interfere with unit testing, and are a contention point in thread-per-core designs.

I/O and System Calls Are Mockable (M-MOCKABLE-SYSCALLS) { #M-MOCKABLE-SYSCALLS }

To make otherwise hard-to-evoke edge cases testable. 0.2

Any user-facing type doing I/O, or sys calls with side effects, should be mockable to these effects. This includes file and network access, clocks, entropy sources and seeds, and similar. More generally, any operation that is

• non-deterministic,
• reliant on external state,
• depending on the hardware or the environment,
• is otherwise fragile or not universally reproducible

should be mockable.

Mocking Allocations?

Unless you write kernel code or similar, you can consider allocations to be deterministic, hardware independent and practically infallible, thus not covered by this guideline.

However, this does not mean you should expect there to be unlimited memory available. While it is ok to accept caller provided input as-is if your library has a reasonable memory complexity, memory-hungry libraries and code handling external input should provide bounded and / or chunking operations.

This guideline has several implications for libraries, they

• should not perform ad-hoc I/O, i.e., call read("foo.txt")
• should not rely on non-mockable I/O and sys calls
• should not create their own I/O or sys call core themselves
• should not offer MyIoLibrary::default() constructors

Instead, libraries performing I/O and sys calls should either accept some I/O core that is mockable already, or provide mocking functionality themselves:

let lib = Library::new_runtime(runtime_io); // mockable I/O functionality passed in
let (lib, mock) = Library::new_mocked(); // supports inherent mocking

Libraries supporting inherent mocking should implement it as follows:

pub struct Library {
    some_core: LibraryCore // Encapsulates syscalls, I/O, ... compare below.
}

impl Library {
    pub fn new() -> Self { ... }
    pub fn new_mocked() -> (Self, MockCtrl) { ... }
}

Behind the scenes, LibraryCore is a non-public enum, similar to M-RUNTIME-ABSTRACTED, that either dispatches calls to the respective sys call, or to an mocking controller.

// Dispatches calls either to the operating system, or to a
// mocking controller.
enum LibraryCore {
    Native,

    #[cfg(feature = "test-util")]
    Mocked(mock::MockCtrl)
}

impl LibraryCore {
    // Some function you'd forward to the operating system.
    fn random_u32(&self) {
        match self {
            Self::Native => unsafe { os_random_u32() }
            Self::Mocked(m) => m.random_u32()
        }
    }
}


#[cfg(feature = "test-util")]
mod mock {
    // This follows the M-SERVICES-CLONE pattern, so both `LibraryCore` and
    // the user can hold on to the same `MockCtrl` instance.
    pub struct MockCtrl {
        inner: Arc<MockCtrlInner>
    }

    // Implement required logic accordingly, usually forwarding to
    // `MockCtrlInner` below.
    impl MockCtrl {
        pub fn set_next_u32(&self, x: u32) { ... }
        pub fn random_u32(&self) { ... }
    }

    // Contains actual logic, e.g., the next random number we should return.
    struct MockCtrlInner {
        next_call: u32
    }
}

Runtime-aware libraries already build on top of the M-RUNTIME-ABSTRACTED pattern should extend their runtime enum instead:

enum Runtime {
    #[cfg(feature="tokio")]
    Tokio(tokio::Tokio),

    #[cfg(feature="smol")]
    Smol(smol::Smol)

    #[cfg(feature="test-util")]
    Mock(mock::MockCtrl)
}

As indicated above, most libraries supporting mocking should not accept mock controllers, but return them via parameter tuples, with the first parameter being the library instance, the second the mock controller. This is to prevent state ambiguity if multiple instances shared a single controller:

impl Library {
    pub fn new_mocked() -> (Self, MockCtrl) { ... } // good
    pub fn new_mocked_bad(&mut MockCtrl) -> Self { ... } // prone to misuse
}

Don't Glob Re-Export Items (M-NO-GLOB-REEXPORTS) { #M-NO-GLOB-REEXPORTS }

To prevent accidentally leaking unintended types. 1.0

Don't pub use foo::* from other modules, especially not from other crates. You might accidentally export more than you want, and globs are hard to review in PRs. Re-export items individually instead:

pub use foo::{A, B, C};

Glob exports are permissible for technical reasons, like doing platform specific re-exports from a set of HAL (hardware abstraction layer) modules:

#[cfg(target_os = "windows")]
mod windows { /* ... */ }

#[cfg(target_os = "linux")]
mod linux { /* ... */ }

// Acceptable use of glob re-exports, this is a common pattern
// and it is clear everything is just forwarded from a single 
// platform.

#[cfg(target_os = "windows")]
pub use windows::*;

#[cfg(target_os = "linux")]
pub use linux::*;

Use the Proper Type Family (M-STRONG-TYPES) { #M-STRONG-TYPES }

To have and maintain the right data and safety variants, at the right time. 1.0

Use the appropriate std type for your task. In general you should use the strongest type available, as early as possible in your API flow. Common offenders are

Do not use ...	use instead ...	Explanation
String*	PathBuf*	Anything dealing with the OS should be Path-like

That said, you should also follow common Rust std conventions. Purely numeric types at public API boundaries (e.g., window_size()) are expected to be regular numbers, not Saturating<usize>, NonZero<usize>, or similar.

^* Including their siblings, e.g., &str, Path, ...

Test Utilities are Feature Gated (M-TEST-UTIL) { #M-TEST-UTIL }

To prevent production builds from accidentally bypassing safety checks. 0.2

Testing functionality must be guarded behind a feature flag. This includes

• mocking functionality (M-MOCKABLE-SYSCALLS),
• the ability to inspect sensitive data,
• safety check overrides,
• fake data generation.

We recommend you use a single flag only, named test-util. In any case, the feature(s) must clearly communicate they are for testing purposes.

impl HttpClient {
    pub fn get() { ... }

    #[cfg(feature = "test-util")]
    pub fn bypass_certificate_checks() { ... }
}

---

Libraries / UX Guidelines

Avoid Smart Pointers and Wrappers in APIs (M-AVOID-WRAPPERS) { #M-AVOID-WRAPPERS }

To reduce cognitive load and improve API ergonomics. 1.0

As a specialization of M-ABSTRACTIONS-DONT-NEST, generic wrappers and smart pointers like Rc<T>, Arc<T>, Box<T>, or RefCell<T> should be avoided in public APIs.

From a user perspective these are mostly implementation details, and introduce infectious complexity that users have to resolve. In fact, these might even be impossible to resolve once multiple crates disagree about the required type of wrapper.

If wrappers are needed internally, they should be hidden behind a clean API that uses simple types like &T, &mut T, or T directly. Compare:

// Good: simple API
pub fn process_data(data: &Data) -> State { ... }
pub fn store_config(config: Config) -> Result<(), Error> { ... }

// Bad: Exposing implementation details
pub fn process_shared(data: Arc<Mutex<Shared>>) -> Box<Processed> { ... }
pub fn initialize(config: Rc<RefCell<Config>>) -> Arc<Server> { ... }

Smart pointers in APIs are acceptable when:

• The smart pointer is fundamental to the API's purpose (e.g., a new container lib)

• The smart pointer, based on benchmarks, significantly improves performance and the complexity is justified.

Prefer Types over Generics, Generics over Dyn Traits (M-DI-HIERARCHY) { #M-DI-HIERARCHY }

To prevent patterns that don't compose, and design lock-in. 0.1

When asking for async dependencies, prefer concrete types over generics, and generics over dyn Trait.

It is easy to accidentally deviate from this pattern when porting code from languages like C# that heavily rely on interfaces. Consider you are porting a service called Database from C# to Rust and, inspired by the original IDatabase interface, you naively translate it into:

trait Database {
    async fn update_config(&self, file: PathBuf);
    async fn store_object(&self, id: Id, obj: Object);
    async fn load_object(&self, id: Id) -> Object;
}

impl Database for MyDatabase { ... }

// Intended to be used like this:
async fn start_service(b: Rc<dyn Database>) { ... }

Apart from not feeling idiomatic, this approach precludes other Rust constructs that conflict with object safety, can cause issues with asynchronous code, and exposes wrappers (compare M-AVOID-WRAPPERS).

Instead, when more than one implementation is needed, this design escalation ladder should be followed:

If the other implementation is only concerned with providing a sans-io implementation for testing, implement your type as an enum, following M-MOCKABLE-SYSCALLS instead.

If users are expected to provide custom implementations, you should introduce one or more traits, and implement them for your own types on top of your inherent functions. Each trait should be relatively narrow, e.g., StoreObject, LoadObject. If eventually a single trait is needed it should be made a subtrait, e.g., trait DataAccess: StoreObject + LoadObject {}.

Code working with these traits should ideally accept them as generic type parameters as long as their use does not contribute to significant nesting (compare M-ABSTRACTIONS-DONT-NEST).

// Good, generic does not have infectious impact, uses only most specific trait
async fn read_database(x: impl LoadObject) { ... }

// Acceptable, unless further nesting makes this excessive.
struct MyService<T: DataAccess> {
    db: T,
}

Once generics become a nesting problem, dyn Trait can be considered. Even in this case, visible wrapping should be avoided, and custom wrappers should be preferred.

# use std::sync::Arc;
# trait DataAccess {
#     fn foo(&self);
# }
// This allows you to expand or change `DynamicDataAccess` later. You can also
// implement `DataAccess` for `DynamicDataAccess` if needed, and use it with
// regular generic functions.
struct DynamicDataAccess(Arc<dyn DataAccess>);

impl DynamicDataAccess {
    fn new<T: DataAccess + 'static>(db: T) -> Self {
        Self(Arc::new(db))
    }
}

struct MyService {
    db: DynamicDataAccess,
}

The generic wrapper can also be combined with the enum approach from M-MOCKABLE-SYSCALLS:

enum DataAccess {
    MyDatabase(MyDatabase),
    Mock(mock::MockCtrl),
    Dynamic(DynamicDataAccess)
}

async fn read_database(x: &DataAccess) { ... }

Error are Canonical Structs (M-ERRORS-CANONICAL-STRUCTS) { #M-ERRORS-CANONICAL-STRUCTS }

To harmonize the behavior of error types, and provide a consistent error handling. 1.0

Errors should be a situation-specific struct that contain a Backtrace, a possible upstream error cause, and helper methods.

Simple crates usually expose a single error type Error, complex crates may expose multiple types, for example AccessError and ConfigurationError. Error types should provide helper methods for additional information that allows callers to handle the error.

A simple error might look like so:

# use std::backtrace::Backtrace;
# use std::fmt::Display;
# use std::fmt::Formatter;
pub struct ConfigurationError {
    backtrace: Backtrace,
}

impl ConfigurationError {
    pub(crate) fn new() -> Self {
        Self { backtrace: Backtrace::capture() }
    }
}

// Impl Debug + Display

Where appropriate, error types should provide contextual error information, for example:

# use std::backtrace::Backtrace;
# #[derive(Debug)]
# pub struct ConfigurationError {
#    backtrace: Backtrace,
# }
impl ConfigurationError {
    pub fn config_file(&self) -> &Path { }
}

If your API does mixed operations, or depends on various upstream libraries, store an ErrorKind. Error kinds, and more generally enum-based errors, should not be used to avoid creating separate public error types when there is otherwise no error overlap:

// Prefer this
fn download_iso() -> Result<(), DownloadError> {}
fn start_vm() -> Result<(), VmError> {}

// Over that
fn download_iso() -> Result<(), GlobalEverythingErrorEnum> {}
fn start_vm() -> Result<(), GlobalEverythingErrorEnum> {}

// However, not every function warrants a new error type. Errors
// should be general enough to be reused.
fn parse_json() -> Result<(), ParseError> {}
fn parse_toml() -> Result<(), ParseError> {}

If you do use an inner ErrorKind, that enum should not be exposed directly for future-proofing reasons, as otherwise you would expose your callers to all possible failure modes, even the ones you consider internal and unhandleable. Instead, expose various is_xxx() methods as shown below:

# use std::backtrace::Backtrace;
# use std::fmt::Display;
# use std::fmt::Formatter;
#[derive(Debug)]
pub(crate) enum ErrorKind {
    Io(std::io::Error),
    Protocol
}

#[derive(Debug)]
pub struct HttpError {
    kind: ErrorKind,
    backtrace: Backtrace,
}

impl HttpError {
    pub fn is_io(&self) -> bool { matches!(self.kind, ErrorKind::Io(_)) }
    pub fn is_protocol(&self) -> bool { matches!(self.kind, ErrorKind::Protocol) }
}

Most upstream errors don't provide a backtrace. You should capture one when creating an Error instance, either via one of your Error::new() flavors, or when implementing From<UpstreamError> for Error {}.

Error structs must properly implement Display that renders as follows:

impl Display for MyError {
    // Print a summary sentence what happened.
    // Print `self.backtrace`.
    // Print any additional upstream 'cause' information you might have.
#   fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
#       todo!()
#   }
}

Errors must also implement std::error::Error:

impl std::error::Error for MyError { }

Lastly, if you happen to emit lots of errors from your crate, consider creating a private bail!() helper macro to simplify error instantiation.

When You Get Backtraces

Backtraces are an invaluable debug tool in complex or async code, since errors might travel far through a callstack before being surfaced.

That said, they are a development tool, not a runtime diagnostic, and by default Backtrace::capture() will not capture backtraces, as they have a large overhead, e.g., 4μs per capture on the author's PC.

Instead, Rust evaluates a set of environment variables, such as RUST_BACKTRACE, and only walks the call frame when explicitly asked. Otherwise it captures an empty trace, at the cost of only a few CPU instructions.

Essential Functionality Should be Inherent (M-ESSENTIAL-FN-INHERENT) { #M-ESSENTIAL-FN-INHERENT }

To make essential functionality easily discoverable. 1.0

Types should implement core functionality inherently. Trait implementations should forward to inherent functions, and not replace them. Instead of this

# trait Download {
#     fn download_file(&self, url: impl AsRef<str>);
# }
struct HttpClient {}

// Offloading essential functionality into traits means users
// will have to figure out what other traits to `use` to
// actually use this type.
impl Download for HttpClient {
    fn download_file(&self, url: impl AsRef<str>) {
        // ... logic to download a file
    }
}

do this:

# trait Download {
#     fn download_file(&self, url: impl AsRef<str>);
# }
struct HttpClient {}

impl HttpClient {
    fn download_file(&self, url: impl AsRef<str>) {
        // ... logic to download a file
    }
}

// Forward calls to inherent impls. `HttpClient` can be used
impl Download for HttpClient {
    fn download_file(&self, url: impl AsRef<str>) {
        Self::download_file(self, url)
    }
}

Accept impl AsRef<> Where Feasible (M-IMPL-ASREF) { #M-IMPL-ASREF }

To give users flexibility calling in with their own types. 1.0

In function signatures, accept impl AsRef<T> for types that have a clear reference hierarchy, where you do not need to take ownership, or where object creation is relatively cheap.

Instead of ...	accept ...
&str, String	impl AsRef<str>
&Path, PathBuf	impl AsRef<Path>
&[u8], Vec<u8>	impl AsRef<[u8]>

# use std::path::Path;
// Definitely use `AsRef`, the function does not need ownership.
fn print(x: impl AsRef<str>) {}
fn read_file(x: impl AsRef<Path>) {}
fn send_network(x: impl AsRef<[u8]>) {}

// Further analysis needed. In these cases the function wants
// ownership of some `String` or `Vec<u8>`. If those are
// "low freqency, low volume" functions `AsRef` has better ergonomics,
// otherwise accepting a `String` or `Vec<u8>` will have better
// performance.
fn new_instance(x: impl AsRef<str>) -> HoldsString {}
fn send_to_other_thread(x: impl AsRef<[u8]>) {}

In contrast, types should generally not be infected by these bounds:

// Generally not ok. There might be exceptions for performance
// reasons, but those should not be user visible.
struct User<T: AsRef<str>> {
    name: T
}

// Better
struct User {
    name: String
}

Accept impl 'IO' Where Feasible ('Sans IO') (M-IMPL-IO) { #M-IMPL-IO }

To untangle business logic from I/O logic, and have N*M composability. 0.1

Functions and types that only need to perform one-shot I/O during initialization should be written "sans-io", and accept some impl T, where T is the appropriate I/O trait, effectively outsourcing I/O work to another type:

// Bad, caller must provide a File to parse the given data. If this
// data comes from the network, it'd have to be written to disk first.
fn parse_data(file: File) {}

// Much better, accepts
// - Files,
// - TcpStreams,
// - Stdin,
// - &[u8],
// - UnixStreams,
// ... and many more.
fn parse_data(data: impl std::io::Read) {}

Synchronous functions should use std::io::Read and std::io::Write. Asynchronous functions targeting more than one runtime should use futures::io::AsyncRead and similar. Types that need to perform runtime-specific, continuous I/O should follow M-RUNTIME-ABSTRACTED instead.

Accept impl RangeBounds<> Where Feasible (M-IMPL-RANGEBOUNDS) { #M-IMPL-RANGEBOUNDS }

To give users flexibility and clarity when specifying ranges. 1.0

Functions that accept a range of numbers must use a Range type or trait over hand-rolled parameters:

// Bad
fn select_range(low: usize, high: usize) {}
fn select_range(range: (usize, usize)) {}

In addition, functions that can work on arbitrary ranges, should accept impl RangeBounds<T> rather than Range<T>.

# use std::ops::{RangeBounds, Range};
// Callers must call with `select_range(1..3)`
fn select_range(r: Range<usize>) {}

// Callers may call as
//     select_any(1..3)
//     select_any(1..)
//     select_any(..)
fn select_any(r: impl RangeBounds<usize>) {}

Complex Type Construction has Builders (M-INIT-BUILDER) { #M-INIT-BUILDER }

To future-proof type construction in complex scenarios. 0.3

Types that could support 4 or more arbitrary initialization permutations should provide builders. In other words, types with up to