coverage.md

Coverage: every guide rule and what enforces it

Each section of the Google Rust Style Guide is assigned to the cheapest layer that can hold it:

Layer	Meaning
Tool	rustfmt, clippy, rustc or cargo-deny fails the build. Verified by the fixtures in this repo.
Tool, partial	A lint catches the common or mechanical case; AGENTS.md and review cover the rest.
Agent + review	No tool can check it. AGENTS.md tells coding agents, human-review.md tells reviewers.
Out of scope	Specific to Chromium, Fuchsia or Android build systems. See why-not.md.

Lint names link to the clippy docs at https://rust-lang.github.io/rust-clippy/stable/index.html.

Summary

Layer	Rows
Tool	19
Tool, partial	13
Agent + review	30
Out of scope	7

The tables below have one row per guide section, except documentation, which is split into three rows. Every in-scope row has a home: tooling covers 32 of the 62 fully or partially, and the rest are design judgment no linter can make.

Background

Guide section	Layer	Enforced by
Follow public Rust style rules	Tool, partial	rustfmt defaults, clippy's default groups (clippy::all); API Guidelines judgment in review
Apply API design rules in context	Agent + review	Decide library or application first; see AGENTS.md

Code organization

Guide section	Layer	Enforced by
Prefer small crates [chrome]	Out of scope	Chromium build rule
Use flat file layout for rust_library sources [chrome]	Out of scope	Chromium build rule
Name source files and leaf modules after the primary user-defined item	Agent + review
Don't implement APIs in non-leaf modules	Agent + review
Don't use glob imports	Tool	clippy::wildcard_imports. It already permits use Enum::* inside functions, pub use submodule::*, and use super::* in test modules. use super::* in other inline modules needs an #[expect]. Defining preludes is review-only.
A use statement should import items from one module	Tool	rustfmt imports_granularity = "Module" (nightly rustfmt)
Prefer locally-scoped trait imports	Agent + review
Qualify calls to top-level functions with a module name	Tool, partial	clippy::absolute_paths (max 2 segments) rejects crate::a::f() and std::fs::read() inline. A bare f() imported with use a::f is review-only.
Type names should be imported and used unqualified	Tool, partial	clippy::absolute_paths rejects paths of 3+ segments such as std::io::ErrorKind. Two-segment paths like regex::Regex are review-only.
Use the appropriate import mechanism [chrome]	Out of scope	chromium::import!()

Code formatting and aesthetics

Guide section	Layer	Enforced by
Use the appropriate formatting tool	Tool	rustfmt max_width = 100, hex_literal_case = "Lower", format_macro_matchers = true (the last two need nightly rustfmt); clippy::invisible_characters (deny) for unescaped invisible characters
Opting out of automatic formatting	Agent + review	#[rustfmt::skip] only for manual alignment or generated code

Ownership and borrowing

Guide section	Layer	Enforced by
Avoid Rc and Arc	Tool	clippy::disallowed_types on std::rc::Rc and std::sync::Arc. Every use needs #[expect(clippy::disallowed_types, reason = "...")], so the judgment call the guide asks for has a written reason for review.
Prefer borrows over deep copies	Tool, partial	clippy::redundant_clone, clippy::implicit_clone, clippy::needless_pass_by_value. Clones used to get past the borrow checker are review-only.

Function inputs and outputs

Guide section	Layer	Enforced by
Accept flexible references for immutable data	Tool	clippy::ptr_arg (&Vec<T>, &String), clippy::borrowed_box (&Box<T>), clippy::ref_option (&Option<T>)
Iterators vs. owned collections in return types	Agent + review
Iterators vs. slices in function parameters	Agent + review
Accepting Into	Agent + review
Prefer enum over bool in APIs	Tool, partial	clippy::fn_params_excessive_bools and clippy::struct_excessive_bools with a threshold of one, so two or more bools fail. A single unclear bool is review-only.
Reporting errors	Agent + review	missing_panics_doc and missing_errors_doc make the chosen channel visible in docs

Types

Guide section	Layer	Enforced by
Newtypes versus type aliases	Agent + review
Prefer named fields over tuples and non-newtype tuple structs	Agent + review

Traits

Guide section	Layer	Enforced by
Add trait implementations when they become necessary	Agent + review
Implement known standard library traits instead of custom methods	Tool	clippy::should_implement_trait, clippy::inherent_to_string, clippy::from_over_into
Use #[derive()] if possible	Tool, partial	clippy::derivable_impls, clippy::expl_impl_clone_on_copy, clippy::non_canonical_clone_impl (default). Other derivable impls are review-only.
Generics versus dyn Trait	Agent + review
When to define a trait versus an inherent implementation	Agent + review
Extension traits	Agent + review
Specific guidance for important traits	Tool, partial	clippy::new_without_default for Default. Copy, Clone, PartialEq, PartialOrd and Hash choices are review-only.
Pair important From impls with explicit constructor methods	Agent + review
Options structs should implement Default	Agent + review	exhaustive_structs forces the decision on all-public structs, which surfaces options structs for review

Concurrency

Guide section	Layer	Enforced by
Handling poison	Tool	clippy::disallowed_methods on PoisonError::into_inner, get_ref and get_mut
Don't use async / .await [chrome]	Out of scope
Don't start new threads through the Rust standard library [chrome]	Out of scope	Add std::thread::spawn to disallowed-methods if your codebase has its own scheduler
Use fuchsia_sync::Mutex [Fuchsia]	Out of scope	Add std::sync::Mutex to disallowed-types in Fuchsia code

Macros

Guide section	Layer	Enforced by
Macros	Agent + review

Naming

Guide section	Layer	Enforced by
Naming methods	Tool	clippy::wrong_self_convention (as_, to_, into_, is_, from_)
Naming lifetimes	Agent + review
Naming generic parameters	Agent + review
Naming extension traits	Agent + review
Naming initialization functions	Tool, partial	clippy::new_ret_no_self, clippy::new_without_default. new_with_x and try_new naming is review-only.
Avoid abbreviations	Agent + review

When and how to write documentation

Guide section	Layer	Enforced by
Documentation on public APIs	Tool	missing_docs (rustc), rustdoc::broken_intra_doc_links
Document panics, errors and unsafe invariants	Tool	clippy::missing_panics_doc, clippy::missing_errors_doc, clippy::missing_safety_doc
Markdown, capitalization, examples, saying something the code does not	Tool, partial	clippy::doc_markdown catches unformatted code identifiers; the prose quality is review-only

Other Rust features

Guide section	Layer	Enforced by
Unsafe code	Tool, partial	clippy::undocumented_unsafe_blocks, clippy::unnecessary_safety_comment, unsafe_op_in_unsafe_fn (rustc), clippy::missing_safety_doc. Whether unsafe was necessary, and the technical justification, are review-only.
No unstable features	Tool	rust-toolchain.toml pins stable; unstable_features = "deny"
Getters and setters	Agent + review
More on accessors that accept closures	Agent + review
Implement expected API sets together	Tool	clippy::len_without_is_empty
Do not access items marked #[doc(hidden)] that you don't own	Agent + review
Use const functions responsibly	Agent + review
Avoid overloading for non-mathematical purposes	Agent + review
Apply #[non_exhaustive] to publicly-exposed enums	Tool	clippy::exhaustive_enums; the guide's exceptions are #[expect] with a reason
Apply #[non_exhaustive] to public structs with public fields	Tool	clippy::exhaustive_structs (fires only when every field is public, matching the guide)
#[must_use] on functions	Tool, partial	clippy::must_use_candidate for pure public functions. Functions returning critical status are review-only.
#[must_use] on types	Agent + review
Do not run code before main()	Tool	cargo-deny bans ctor, inventory and static_init

Implementation concerns

Guide section	Layer	Enforced by
Don't overuse type inference	Agent + review
Prefer pattern matching over check-then-unwrap	Tool	clippy::unnecessary_unwrap
Prefer expect over unwrap	Tool	clippy::unwrap_used (tests exempt via allow-unwrap-in-tests)
Don't omit patterns on non_exhaustive enums when matching	Agent + review	The guide's suggested non_exhaustive_omitted_patterns lint is still nightly-only. See why-not.md.
Use context strings to clarify assertions	Tool	clippy::missing_assert_message (skips tests); whether the message adds context is review-only
Don't use iterator combinator chains when direct iteration is clearer	Tool, partial	clippy::needless_for_each. Long chains and chains in for headers are review-only.
Don't call panic::catch_unwind	Tool	clippy::disallowed_methods; the defense-in-depth exception is #[expect] with a reason
Prefer format! / write! for concatenation and substitution	Tool, partial	clippy::format_push_string. Three-way + chains and str::replace templating are review-only.

Testing

Guide section	Layer	Enforced by
Use the right per-codebase test framework	Out of scope	Chromium and Fuchsia test harnesses

Repository-wide policy

Policy	Enforced by
Every exception records why	clippy::allow_attributes_without_reason; AGENTS.md asks for #[expect]
The configuration does what this table says	scripts/check-fixtures.sh, scripts/check-rustfmt.sh and scripts/check-deny.sh in CI