rust_library sources [chrome]use statement should import items from one module#[derive()] if possibledyn TraitFrom impls with explicit constructor methods#[doc(hidden)] that you don’t ownconst functions responsibly#[non_exhaustive] to publicly-exposed enums#[non_exhaustive] to public structs with public fields#[must_use] on functions#[must_use] on typesmain()expect over unwrapnon_exhaustive enums when matchingpanic::catch_unwindformat! / write! for concatenation and substitutionThis document is a style guide for the usage of the Rust programming language at Google. In this context, “style” refers to more than just formatting; it includes guidance on API and implementation design.
\nRust is used in many places, including google3, Android, ChromeOS, Fuchsia, and Chrome, and we’d like for Rust usage at Google to be consistent both within and across codebases. In addition to bringing consistency to our Rust code, style guidance also helps establish a standard of quality.
\nThe current scope of authority of the rules outlined in this guide:
\nBy and large, we have this document for the same reasons we have a C++ style guide.
\nMost style rules come from one or more of the goals listed below.
\nStyle rules should prioritize readability and maintainability over writeability
\nWe expect our code to be around for long periods of time, and we consider a little bit of extra effort applied during code writing to be worth it in the long run if the style rule improves readability.
\nStyle rules should favor better automation
\nLarge-Scale Changes (LSCs) are common at Google, and consistency helps make LSCs easier to carry out.
\nStyle rules should be consistent with the wider Rust ecosystem where possible
\nThe broader Rust ecosystem already has a pretty good amount of consistency in style, and we should rely on that where possible to reduce surprise and make it less work to learn Google’s Rust style.
\nStyle rules should pull their weight
\nEvery Rust developer and reviewer at Google has to keep these style rules in mind: as such, we should be careful to include rules that have a tangible benefit.
\nStyle rules may help avoid minor style disagreements
\nThere are many cases where reasonable people may disagree about a style, and we expect that disagreement to show up often on CLs. Having a style rule helps avoid having to rehash this each time. Rules in this category that do not have other benefits should be kept to a minimum.
\nFollow the official public Rust style rules to the extent they don’t conflict with this style guide.
\nA “conflict” would be a case where this style guide suggests a pattern that is discouraged, or counter to the primary encouraged pattern in the Rust style guide.
\nWhy does the guidance in this style guide deviate from regular Rust style rules?
\nMost style guides have three goals: consistency, readability, and encouraging good API design. Regardless of the specific choices made, it helps everyone to have these choices made beforehand to limit bikeshedding.
\nThis style guide shares those goals, but it also has an additional goal not seen as often: smoothing the way for Large-Scale Changes (LSCs). Large codebases like many of the ones at Google occasionally need to run codebase-spanning refactorings that are assisted by tooling: for this to work well, the code should be something tooling has an easy time working with. Consistency helps for this, but so does having deliberate refactor-friendly patterns.
\nWhen applying rules that relate to API design, you should distinguish two types of code:
\nLibrary code
\nApplication code
\nFor library code, style guide rules that concern API design are binding.
\nFor application code, the rules on API design are a strong recommendation. As you apply them, keep in mind that since application code is written to solve a specific problem, it should generally favor simplicity and directness over broad reusability. Avoid premature generalization and abstraction; focus on what is required to meet the immediate need.
\nTerminology:
\nA crate is a unit of compilation. A crate always contains a nameless Rust module, the “root module”.
\nA Rust module is a unit of code organization within a crate. Rust modules are used to control visibility of names. Modules can be nested. They can also be used to split code across files and directories.
\nThe following sections convey that in Chromium, it is preferable to not make use of all possibilities of splitting modules across files.
\nPrefer small crates in Chromium; ideally, but not necessarily, a single file (focusing on one abstraction). If your code can be split up into small crates, do so.
\nA crate corresponds to a single rust_static_library (chrome) rule, which may contain one or more .rs files.
Additional considerations:
\nThe root module is allowed to re-export public APIs from leaf submodules, making submodules an implementation detail of the crate (as users don’t need to know about these submodules).
This rule about small crates applies equally to crates with and without submodules. First, evaluate whether the APIs need to be in the same crate, and only then consider whether submodules could help organize the APIs.
\nIf you decide to use submodules, follow the expected file structure.
\nLarger crates are allowed when trait impl coherence rules force multiple types and traits to be included in one library, or if you’re building a large foundational library.
\nRationale
\nFine-grained library targets are recommended in Chromium due to the massive size of our codebase, non-local effects of adding dependencies in the build graph, and to improve performance of parallel compilation and build caching.
\nrust_library sources [chrome]In Chromium .rs source files belonging to one rust_library (one Rust crate) must reside in a single directory.
Don’t use subdirectories to organize files that belong to a single crate. If organizing one crate’s files into subdirectories feels necessary, this is a strong indication that the crate has grown too large and should be split.
\nThis crate layout is intentionally different from the conventional Cargo directory structure typically used in open-source repositories.
\nException: Rust code in //third_party that is also built with Cargo should follow the conventional Cargo directory structure.
Rationale
\nThe approach in Chromium is driven by the following factors:
\nrust_library targets will only contain one source file.rust_library target would result in most directories containing only one source file.rust_library targets should not create dedicated directories for submodules either; this ensures a consistent approach.When a leaf module has a primary user-defined item (e.g., Foo), the module should be named after that item (e.g., mod foo and foo.rs).
Leaf modules that are not primarily about any particular abstraction can be named as desired.
\nRationale
\nThis makes it easy to identify the content/purpose of a source file or leaf module at a glance.
\nThe only items in non-leaf modules should be module declarations (not definitions) and re-exports.
Note that the definition of leaf modules allows the following sub-modules: mod test and cross-language binding modules like #[cxx::bridge].
Exceptions:
\nRationale
\nThis makes it easier to find things, and avoids mixing structure and contents.
\nDon’t use glob imports, except in the following situations where you may optionally do so at your own discretion:
\nuse super::* for inline modules
use enum::* in method bodies of enums, and functions that are primarily dealing with that enum
pub use submodule::* for non-leaf modules
C++ interop: Since the same namespace (in C++) is a submodule in different Rust crates (in Rust), you may merge the namespaces into a private copy using glob imports. For example:
\n\nIn this hypothetical example, you can use absl::Status and absl::StrCat from Rust, the same way you would from C++, instead of spelling them as status::absl::Status and strings::absl::StrCat.
This must only be used for merging C++ namespaces, and the resulting merged module must not be public.
\nIf using third-party crates that encourage a prelude, it is up to the reviewer’s discretion to allow it.
\nThis also means that defining preludes should be avoided.
\nRationale
\nGlob imports increase toil for code janitors running LSCs, and make it harder to know where a type comes from in contexts without xrefs.
\nuse statement should import items from one moduleThis rule corresponds to the rustfmt setting imports_granularity = Module.
Basically, there should not be any ::’s inside the {..} block of a use statement. For example:
use Foo;\nuse Bar;\nuse Baz;\nuse Foo;\nuse ;\nuse ;\nImport traits within function bodies, unless the trait’s methods are used extensively throughout the file.
\n\nTo use top-level functions defined in other modules or crates, import their module and call them with modulename::functionname(). For example:
use mem;\n\nlet x = transmute;\nuse transmute;\n\nlet x = transmute;\nExceptions:
\nWhen importing functions with sufficiently disambiguated names, e.g., functions from C++ (where it is typical to call top-level (free) functions directly).
\nIn Chromium, when importing functions from small, single-purpose first-party crates.
\nWhen the module name itself is prone to collisions. For example, when the module name is a C++ namespace name, and multiple crates define the same submodules.
\nRationale
\nMany Rust codebases consistently keep a style such that unqualified lowercase names are assumed local, so foo() can be assumed to be from the same module. This rule largely exists to avoid reviewer disagreements, the benefit is mostly consistency and a minor edge on reading code without xrefs.
By default, types and traits should be referenced in code without using their module path, e.g., code should use Regex, not regex::Regex, with a use regex::Regex at the top of the file or potentially in the relevant function.
Exceptions:
\nio::Error. It is also acceptable to rename (use std::io::Error as IoError).hir module in rustc which contains a large number of HIR types.use is counterproductive.Rationale
\nThis rule largely exists to avoid reviewer disagreements.
\nchromium::import!().Use the appropriate formatting tool (e.g., git cl format in Chromium).
Configuration (for style guide authors)
\nA codebase may choose to not apply the following rustfmt options if they care about building on stable and the option is unavailable on their stable Rust:
Maximum of 100 columns per line: max_width = 100
Usage of lowercase alphabets in hex literals (0xab123cdef as opposed to 0xAB123CDEF): hex_literal_case = Lower
Macro matcher formatting: format_macro_matchers = true
Usage of escapes for invisible characters in literals. E.g., if you wish to have text containing a Zero-Width joiner, use \\u{200D}.
You may opt out of automatic formatting using #[rustfmt::skip], but only in the following situations:
Important: Formatting should not be skipped for personal style choices.
\nRc and ArcPrefer unique ownership and borrowing over Rc and Arc for managing memory.
Exception:
\nRc and Arc when the alternative ownership and borrowing structure would be unreasonably complex to write, understand, or maintain. This is a judgment call to be discussed during code review.Rationale
\nGenerally, Rust allows representing object graphs without using shared ownership smart pointers. These pointers add runtime overhead and their own unique complexity. Engineers coming from other languages might tend to use Rc and Arc too quickly.
Use .clone() when you genuinely need a deep copy of an object.
Avoid using .clone() as a substitute for borrowing. Design code that works naturally with Rust’s ownership and borrowing rules. However, if the borrowing relationships become excessively complex, .clone() can be a pragmatic choice.
Consider the performance needs of the code while choosing between cloning and borrowing.
\nNote: This advice does not apply to cloning Rc or Arc pointers. Cloning reference-counted pointers does not create a deep copy, and is an expected way to use them.
Rationale
\nDeep copies can be expensive to make due to memory allocation and data copying.
\nWhen functions accept borrowed, immutable data, use a reference type that affords the most flexibility. Pass references to the data itself instead of references to smart pointers, containers, or wrappers.
\n| Use | \nInstead of | \nWhy? | \n
|---|---|---|
&T | \n&Box<T> | \nBorrow the data inside the box | \n
&[T] | \n&Vec<T> | \nAccept any slice instead of specific containers | \n
&str | \n&String | \nAccept a string slice instead of references to owned strings | \n
Option<&T> | \n&Option<T> | \nBorrow the data inside the Option | \n
Exceptions:
\nFor context, see Clippy lints borrowed_box and ptr_arg.
Rationale
\nAccepting reference types that don’t mention specific smart pointers, containers, or wrappers makes functions more versatile and performant: callers can pass data held in a variety of ways without copying it first.
\nPrefer returning iterators (either concrete types or impl Iterator) where possible over returning Vec<T>.
Note: In application code, returning a Vec<T> is fine if it simplifies the code and if flexibility and performance advantages of iterators are not clearly needed.
Exceptions:
\nfor loop and the forced allocation doesn’t matter too much.Rationale
\nHighly reusable library code generally should strive to provide more flexible and performant APIs. Returning an iterator allows callers to choose how to consume the data: collect it into a vector, process it in a loop, or chain further iterator adapters. Some of these options allow users to avoid unnecessary allocations.
\nIn application code, the usage patterns for a function are often easier to predict. If it is clear that performance is not a concern, or that data would be collected into a vector anyway, returning a vector makes for a simpler API.
\nHighly reusable library code should accept iterators or impl IntoIterator if it only needs to iterate over the input.
Note: In application code, accepting a slice &[T] is fine if it simplifies the code and if flexibility and performance advantages of iterators are not clearly needed.
Rationale
\nHighly reusable library code generally should strive to provide more flexible and performant APIs. Functions that accept iterators offer callers:
\nIn application code, the usage patterns for a function are often easier to predict. If it is clear that performance is not a concern, or that callers would always have a vector anyway, accepting a slice makes for a simpler API.
\nIntoWhen designing library APIs that accept a T but users are likely to have various types, consider accepting impl Into<T> instead of T if call-site ergonomics are paramount.
Tip: This should be balanced against code bloat concerns; it is not a requirement to use lots of Into<T> in your code.
Note: Application code should almost never use impl Into<T> parameters.
Rationale
\nUsing impl Into<T> allows a function to directly accept various arguments of various compatible types. For example, a function accepting impl Into<String> could be called with a String or a &str literal directly.
The tradeoff here is balancing the convenience of avoiding a .into() at the callsite against the cost of generics (more complex API signature, increased code size, sometimes even type inference difficulties).
enum over bool in APIsUse enum function parameters instead of bool in cases where the purpose of the parameter is not obvious.
If you do use bool, consider using an inline comment to document the purpose of the bool in an argument list.
Choose the error reporting mechanism that fits your API contract: Option, Result, or panic!.
Use Option for simple failures where the cause is obvious and the caller would not benefit from further details. This is best for standard, anticipated outcomes like a missing value or a parsing mismatch.
// Good: The cause of the failure is obvious (integer overflow). No further\n// details could be reasonably provided.\n\nUse Result for recoverable failures where the caller needs context to handle the error. These are often “unavoidable” errors caused by external factors (like I/O or network) that even correctly implemented code cannot prevent with checks ahead of time. Nevertheless, the caller can reasonably anticipate the kinds of errors (a file-related function can fail with an I/O error). The error details allow the caller to distinguish failure modes and recover appropriately.
// Good: The caller needs to know *why* it failed (NotFound vs PermissionDenied).\n\nUse panic! to report bugs and violated invariants. These errors are avoidable: the program is missing a check, and is calling an operation without fulfilling a precondition. These defects must be fixed in the code, not handled at runtime, although coarse-grained recovery might be possible with catch_unwind.
// Good: Panics if the invariant (index must be in bounds) is violated.\n// This indicates a bug in the caller's logic.\n\nNote: Some codebases may choose to avoid panics entirely, and require even broken invariants to be reported via Result.
Exceptions:
\nUse a newtype instead of a type alias in the following cases:
\nDate type may use newtypes to distinguish Day, Month, and Year numbers in its internal calculations.Prefer types with named fields over tuples, as tuple indexes (.0, .1, …) quickly become confusing.
Exceptions:
\nAs a general rule, low-level library code should anticipate users’ needs proactively, while application code should add trait impls as needed.
\nIf a method matches the purpose and behavior of a standard library trait, implement that trait. For example, instead of writing fn from_str(&str) -> Self on Foo, you should impl FromStr for Foo.
It is acceptable to have both the method and the trait impl for discoverability reasons.
\nFor context, see Clippy lint should_implement_trait (list of traits).
#[derive()] if possibleManual implementations should be avoided if they can be expressed as a #[derive()].
dyn TraitIn libraries, prefer accepting generics (T: Trait or impl Trait) in function parameters over dyn Trait.
Exceptions:
\nRationale
\nEfficiency
\ndyn Trait requires setting up a fat pointer, and a vtable lookup is required for every call of a function on the trait.dyn Trait, the compiler can’t perform certain optimizations that it could if the function was generic, such as inlining calls to functions of the trait.Some traits are not dyn compatible. For example, a trait is not dyn compatible if it has generic functions, associated functions without a self parameter, or functions that return Self.
Define a trait only for the following purposes:
\nTo enable generic programming (to use T: Trait in constraints of generic functions and types).
To accept or return an impl Trait or dyn Trait.
To define an extension trait (but see also the caveats in the section on extension traits).
\nIn all other cases where you want to implement functionality for a type, use an inherent implementation.
\nDon’t use traits merely to organize code.
\nAn extension trait is a trait that adds methods to a type from a different crate.
\nWhat differentiates an extension trait from other traits is that its sole purpose is to add methods to the type; it is not intended for use in generics (T: Trait, impl Trait) or dynamic dispatch (dyn Trait). To indicate this, the name of an extension trait should carry the suffix ~Ext (for example, ForeignTypeExt).
In general, the team that owns the original type should also own the extension trait.
\nExceptions:
\nNote: This rule is specifically about extension traits. It is relatively common to define a trait as an input trait (e.g., DateLike) and implement it on a bunch of foreign types.
Examples
\nextern crate foreign_crate;\nuse ForeignType;\n\n\n\n\nIt is also possible to define an extension trait for every type that implements some foreign trait. Effectively, this is equivalent to adding default methods to the foreign trait:
\nextern crate foreign_crate;\nuse ForeignTrait;\n\n\n\n\nRationale
\nWhen the same team owns the extension trait and the type being extended, this helps avoid method name conflicts between the base type and the extension trait or between two extension traits.
\nTypes with public fields: Consider implementing traits implemented by all subfields.
\nError types: Implement the Copy trait whenever possible. Copy types are simpler to manage and propagate because they can be passed by value without moving ownership. An example of a case where this is not possible is if the error must contain non-Copy fields like a String.
Copy: Implement whenever it is reasonably clear that the type will continue to be Copy in the future.
Clone: Most types should implement Clone unless they represent some external resource.
Default: Implement if the type implements new() or otherwise has a default state. For context, see Clippy lint new_without_default.
PartialEq / Eq: If you want users to use the == operator, prefer implementing PartialEq over Eq. When you do implement Eq, be aware of the additionally required reflexivity (details: https://doc.rust-lang.org/std/cmp/trait.Eq.html).
PartialOrd / Ord: If you want users to use the < or > operators, prefer implementing PartialOrd over Ord. When you do implement Ord, be aware that it requires total ordering (details: https://doc.rust-lang.org/std/cmp/trait.Ord.html).
Map keys
\nHash and Eq.Ord (and any traits it needs).From impls with explicit constructor methodsIf a type has an important From impl that is expected to be one of the primary ways of constructing it, add an inherent ::new(), ::new_from_foo()¸ or ::from_foo() method.
Structs with all-public fields that are primarily used as a “bag of options” for a function should implement Default.
Rationale
\nThis enables convenient use of struct update syntax (Struct { a: 0, ..Default::default()} ).
Never reach inside a poisoned mutex to get at its insides, no matter how tempting the use case may seem.
\nasync / .await [chrome]Don’t use async / .await in Chromium.
In Chromium, prefer to use sequences instead of threads.
\nfuchsia_sync::Mutex [Fuchsia]Use Fuchsia’s mutex type, rather than one from std::sync or parking_lot.
When considering how to express an API, prefer in order:
\nRegular Rust functions, types, traits, objects, and modules. (Avoid macros, code generation, and metaprogramming where possible).
\nmacro_rules!-style macros.
Other code generation strategies (e.g. genrule)
proc_macro macros.
When you decide to use a macro or code generation, please weigh the benefits against the costs:
\nRationale
\nPrefer non-macro facilities: Macros can create dialects of the language that make it harder for engineers to work across the codebase. When a reader sees a usage of a macro, they can make fewer assumptions about that code than if it didn’t use a macro. Macros are harder to write correctly than regular Rust code.
\nUse macro_rules! for simple code generation: Since Rust macros operate on the AST, they can be less fragile than textual code generation.
Follow the Rust standard library style for naming methods. See also the guidance on naming conventions in the Comprehensive Rust course.
\nSpecific considerations:
\nas_...(), to_...(), or into_...() with the appropriate self convention. See also this section of the Rust API Guidelines and the Clippy lint wrong_self_convention.Lifetimes should have descriptive names, especially type lifetimes.
\nExceptions:
\nCow.'_) where they work.for<'a>). Up to two such local lifetimes are fine, if more are needed, we strongly advise you to use descriptive names.Use a short name for your generic parameters (either single or multi character), related to the kind of thing they represent. Single-letter generics are okay, but the letter chosen should be somewhat indicative of the object type, ideally taken from the most important trait bound.
\nT or U are fine when the generics are in a relatively unbound context; i.e. it is an API intended for use with most types, like Vec<T>.
When naming an extension trait, give the name an ~Ext suffix (for example, ForeignTypeExt).
Follow these guidelines when naming initialization functions:
\nConstructors: Use new(), or new_with_foo() if there is more than one. If the constructor returns a Result, use try_new().
Value-to-value conversions: Use from_foo() with an associated From impl. See here for details on when an operation can be considered to be a value-to-value conversion.
Don’t use abbreviations (especially acronyms and initialisms) that someone outside your project is unlikely to know. Don’t abbreviate by deleting letters within a word.
\nSee also the Google C++ Style Guide.
\nExceptions:
\nIpAddr. You and your reviewer should make this determination. In general, the preference should still be not to abbreviate.Add documentation comments to public APIs, potentially using the missing_doc lint to help with this.
Best practices for documentation comments:
\nConsider including examples: It is not necessary for every function to have examples, but enough functionality should be covered in examples somewhere to give an idea of how the crate works.
\nAlways document panic cases for functions which may panic due to malformed input.
\nClearly document invariants for unsafe functions and traits.
\nUse capitalization and punctuation.
\nDon’t explain the basics of the programming language. For example, don’t do the following:
\n\n\n/// A mock implementation of the FileSystem trait, intended to be used in unit tests.\nExceptions:
\nComments should say something that the code does not already say. If there’s nothing insightful to say, it is better to omit the comment than to repeat the function name in a trivial way. For example, don’t do the following (it’s fine to allow the lint in that case):
/// Removes an element.\n\nIt is generally okay to omit documentation on individual enum variants, especially if their name is self-explanatory.
\nHow to format documentation comments
\nFollow these guidelines when writing documentation comments:
\n/// comments to document items following the comment, such as functions, structs, enums, traits, and type aliases.//! comments to document the module or the library itself. These comments should be placed at the top of the file.//) for non-documentation comments, such as implementation comments, TODOs that are not interesting for users, commented out source code, etc./// and //! comments, rather than generating documentation from macros. This makes the documentation directly readable when viewing or modifying the source code.It is acceptable, and even encouraged, to mix doc comments (/// and //!) and ordinary comments (//). If you have one thing you wish to document publicly, and another thing you want to note as an implementation comment, then use both! For example:
/// Finds the first `FooItem` with the given `name`, or `None` if not present.\n// TODO: crbug.com/12345 - `name` should be &str, not String.\n\nRationale
\nMany, though not all, languages have a different commenting convention (such as extra * characters) for doc comments as opposed to implementation comments. Rust and Python are notable for not just having different commenting conventions, but an actual different syntax entirely. This has concrete benefits to the Rust ecosystem:
rustdoc, IDE features, and Kythe.macro_rules! macros and procedural macros.Don’t write unsafe code unless there is no way to implement the same functionality with safe code.
\nIf you do have to write unsafe code:
\nSAFETY: that explains why the code is correct.Don’t use unstable features.
\nExceptions:
\nWriting and naming getters and setters should follow these guidelines:
\nfoo() for a getter that returns a T by value.foo_mut() for a getter that returns a &mut T.take_foo() for a getter that “clears” the value (by setting it to None or some other default state) and returns the old value.set_foo() for a setter that takes a T by value.with_foo() / with_mut_foo() for an accessor that operates via a closure (see details below). Such an accessor is invoked as object.with_foo(|foo| frob(foo)).replace_foo() for a setter that returns the old value.Examples
\n\nAccessors that accept a closure (.with_foo()) are typically invoked as object.with_foo(|foo| do_something_with(foo)). They can be a good choice in the following situations:
When the property is contained in an interior mutability wrapper (e.g. Mutex or RefCell). These mechanisms need to constrain access to the property to a scope, and the closure provides this scope. This is usually preferable to directly returning the RAII object that implements the scope (e.g. MutexGuard<T>) because the latter would leak an implementation detail.
When follow-up work or validation is needed after a property is changed and a set_foo() accessor cannot be used (for example because the property’s type is expensive to clone or cannot be cloned at all).
A with_foo() accessor will often allow the closure to return a value and forward it by returning it from with_foo().
If you need to provide two closure-accepting accessors, one for read-only access and one for mutable access, it is common to name them with_foo() and with_mut_foo(). Otherwise, with_foo() is often also used for mutating accessors.
If you implement len(), also implement is_empty(). For context, see Clippy lint len_without_is_empty.
#[doc(hidden)] that you don’t ownTreat items marked #[doc(hidden)] as private. Don’t access them unless you own the code.
const functions responsiblyMark a public function as const only if you expect it to remain const in the future (or can commit to helping clients migrate when it changes).
Rationale
\nIt’s a breaking change to turn a const function into a regular function.
Use operator overloads for their original mathematical/logical purpose, not just to make the code more concise (as “syntax sugar”).
\n#[non_exhaustive] to publicly-exposed enumsIf an enum is part of a public API used by other teams, mark it #[non_exhaustive].
Exceptions:
\nenum Color { Red, Green, Blue }).enum Visibility { Visible, Hidden })For context, see Clippy lint exhaustive_enums.
Rationale
\nMarking an enum #[non_exhaustive] forces users to include a wildcard arm (_ => ...) in match statements. This ensures code continues to compile when new variants are added to the enum later.
This rule is primarily important for enums used across team boundaries, because the scope of the breakage could be significant. Enums used only a small number of times can’t create a large breakage and thus may ignore this rule.
\n#[non_exhaustive] to public structs with public fieldsStructs with public fields that are part of a public API used by other teams should be marked #[non_exhaustive].
Exceptions:
\nPerson { name: "foo", ..Default::default() }).Rationale
\nAdding a new field (even a private one) to a struct with all public fields is a breaking change, because callers can no longer name all fields in a struct literal.
\nA #[non_exhaustive] struct can’t be constructed with the struct literal syntax, except for the code in the same crate. This allows new private fields to be added without breaking users.
This rule is primarily important for structs used across team boundaries, because the scope of the breakage could be significant. Structs used only a small number of times can’t create a large breakage and thus may ignore this rule.
\n#[must_use] on functionsUse #[must_use] on functions in these cases:
The caller cannot use the function correctly if they ignore the return value. This includes pure functions (functions without side effects), such as HashMap::get().
Any function whose return value represents an error or otherwise critical information about the function’s behavior. If the return type is already annotated #[must_use] (which is true of Result, for example), the function does not need to be annotated #[must_use].
#[must_use] on typesUse #[must_use] on types intended to be returned by functions that would otherwise be annotated #[must_use] according to the rules above.
Examples:
\nThe type is an error type (similar to Result or StatusOr).
The type represents a lazily evaluated computation. In other words, the function call doesn’t do anything by itself, and the return value needs to be accessed to cause some desired side effect.
\nmain()Do not cause code to run before main(), for example by using techniques such as the ctor crate.
Add enough explicit types so that someone familiar with the codebase can figure out what the inferred types are.
\nPrefer to add the types to variable declarations, rather than as turbofishes. If the compiler forces you to add a turbofish, it is generally fine to add a _ inference marker.
Specific situations in which you should probably add types:
\n.into() or .parse().Avoid potentially panicking operations when the input data guarantees they cannot panic. For example, instead of using Option::unwrap on a value known to be valid, use pattern matching to prove correctness to the compiler.
// Good: The match arm proves to the compiler that `x` is `Some`.\n// A panic is obviously not possible.\nif let Some = option \n// Bad: Correctness of `unwrap` relies on a runtime check that `option` is `Some`.\nif option.is_some \nexpect over unwrapIf a panic is necessary because an invariant cannot be expressed in the type system, prefer expect over unwrap to document the reason.
non_exhaustive enums when matchingPrefer to list all enum variants in match statements over non_exhaustive enums.
If you don’t care about specific variants, place them in the wildcard arm. If you only care about specific variants, use an if let or a let … else {}.
Examples:
\n\n\n\n\n\nExceptions:
\nmatch is cleaner than if let.This is covered by the built-in lint non_exhaustive_omitted_patterns which you can enable on your code.
While some assertions are self-explanatory, many benefit from additional context that clarifies why an assertion is expected to hold, or what caused it to be violated. Use the optional format string argument for assert! to convey this context.
You should always consider adding context, but particularly in the following situations:
\nExamples
\nExplaining why an assertion should hold
\n assert!;\nLogging a value to clarify why the assertion failed
\nassert!Flagging misuse of an API
\n\nDon’t use the context string merely to repeat the condition
\nassert;\nAvoid using combinator chains in ways that hurt readability. Specifically:
\nfor loop. Instead, assign the combinator chain to a variable, then consume that variable in the for loop.Iterator::for_each unless you’re convinced you have a good reason. A for loop is usually easier to read.Combinators are preferable for shorter chains and for cases where the combinator itself provides a lot of logic that would otherwise have to be written out in the corresponding for loop.
Examples
\nComplex combinator chains
\nHere’s an example of a combinator chain that is hard to read because it involves conditionals and mutates external state:
\nlet mut error_count = 0;\nlet _processed_items: = items\n .into_iter\n .map\n .map\n .filter_map\n .collect;\nThe corresponding for loop is easier to read:
let mut error_count = 0;\nlet mut processed_items = Vecwith_capacity;\n\nfor item in items \nCombinator chains in for loops
\nAvoid constructing a combinator chain in the header of a for loop:
for bug in bug_descriptions\n .iter\n .filter_map\n .filter\n\nInstead, assign the combinator chain to a variable. This reduces clutter, and the variable name documents what the combinator chain does:
\nlet urgent_bugs = bug_descriptions\n .iter\n .filter_map\n .filter;\n\nfor bug in urgent_bugs \nWhen to prefer a combinator chain
\nHere’s an example where a combinator chain is preferable:
\nlet threshold_exceeded = values.iter.any;\nThe corresponding for loop is harder to read and reinvents the logic contained in any():
let mut threshold_exceeded = false;\nfor v in values \npanic::catch_unwindUse of panic::catch_unwind is prohibited in first-party Rust code.
Exceptions:
\ncatch_unwind recovery mechanism just in case a panic slips through. Code authors must convince the reviewer that the codebase is doing most of what it can to avoid panics. This exception only applies to code that has the possibility of being run with panic=unwind (so this does not apply to Chrome, Android, Fuchsia).Rationale
\nRust code may be used in many environments, and panic::catch_unwind will only function correctly in environments with panic=unwind. While the final panic disposition of prod binaries has not yet been determined, Chrome, Android, and Fuchsia all require panic=abort.
format! / write! for concatenation and substitutionPrefer format!() and write!() to other alternatives in the following situations:
format!("{a}{b}{c}") instead of a + b + c.str::replace).Rationale
\nformat! / write! require fewer allocations than concatenating strings using +.Most codebases use their own specific test framework rather than #[test].
Chrome Use rust_gtest_interop. This makes it possible to integrate Rust tests into C++ test binaries.
\nFuchsia Use #[fuchsia::test]. See here for more details.