Coding Conventions

Official site: Link.

General Philosophy ​

• rust-analyzer's approach to clean code:

  • Velocity over perfection: Do not block functional PRs on purely stylistic changes.

  • "Show, don't just tell": For complex style issues, reviewers are encouraged to merge the PR and then send a follow-up cleanup PR themselves. This resolves the issue faster and teaches the author "by example" rather than through endless comment threads.

• If a review comment applies generally, update the Style Guide instead of leaving a one-off comment. This way, temporary feedback is turned into permanent documentation.

• Small, atomic cleanup PRs (even just renaming a variable) are explicitly encouraged to keep the codebase healthy.

Scale of Changes ​

• Generally, small & focused PRs are preferred; but sometimes, that isn't possible.

• rust-analyzer categorizes PRs into 3 groups.

Internal Changes (Low Risk) ​

• Definition: Changes confined to the internals of a single component. No pub items are changed or added (no changes to the interfaces and no new dependencies).

• Review standard: Easy Merge.

  • Does the happy path work?

  • Are there tests?

  • Does it avoid panicking on the unhappy path?

API Expansion (Medium Risk) ​

• Definition: Adding new pub functions or types that expose internal capabilities to other crates.

• Review standard: High scrutiny.

  • The interface matters more than the implementation. It must be correct and future-proof.

• rust-analyzer's guideline: If you start a "Type 1" change and realize you need to change the API, stop. Split the API change into its own separate, focused PR first.

Dependency Changes (High Risk) ​

• Definition: Introducing new connections between components via pub use re-exports or Cargo.toml dependencies.

• Review standard: Rare & dangerous.

  • These break encapsulation.

  • Even an innocent-looking pub use can accidentally degrade the architecture by leaking abstractions across boundaries.

Crates.io Dependencies ​

• Restrict external dependencies: Be extremely conservative with crates.io usage to minimize compile times and breakage risks.

• Do not use small "helper" libraries (allowed exceptions: itertools, either).

• Internalize utilities: Place general, reusable logic into the internal stdx crate rather than adding a dependency.

• Audit dependency tree: Periodically review Cargo.lock to prune irrational transitive dependencies.

Rationale ​

• Compilation speed:

  • Rust compiles dependencies from source.

  • Avoiding bloat is the best way to keep build times and feedback loops fast.

• Transitive bloat:

  • Small "helper" crates often pull in deep chains of hidden dependencies (the "iceberg" effect).

• Stability & Security:

  • Reduce the risk of upstream abandonment, breaking changes, or supply chain attacks.

• Self-reliance: If logic is simple enough for a micro-crate, it belongs in the internal stdx library, not as an external liability.

Commit Style ​

• Document for changelogs to avoid release burden on the maintainers.

• Changelogs > Clean history.

Git History ​

• Clean git history is strongly encouraged but not mandated.

• Use a rebase workflow. It is explicitly acceptable to rewrite history (force push) during the PR review process.

• Before the final merge, use interactive rebase to squash small "fixup" commits into logical units.

Commit Message & PR Description ​

• Do not @mention users in commit messages or PR descriptions.

  • Reason: Rebasing re-commits the message, spamming the mentioned user with duplicate notifications.

• User-centric titles: Write PR titles/descriptions describing the user benefit, not the implementation details.

  • Good: "Make goto definition work inside macros".

  • Bad: "Use original span for FileId".

• Changelog automation: You must categorize PRs so release notes can be auto-generated. Use one of two methods:

  • Title prefix: feat:, fix:, internal:, or minor: (e.g., feat: Add hover support).

  • Magic comment: changelog [fix] Description here in the PR body.

• Visuals: For UI changes, include a GIF in the description to demonstrate the feature.

Linting ​

• Clippy is used.

Code ​

Minimal Tests ​

• Tests must use the absolute minimum code necessary to reproduce the case. Aggressively strip "noise" from copy-pasted real-world code.

• Format declarative code densely (e.g., enum E { A, B } on a single line) to keep the test concise, provided it remains readable.

• Unindented raw strings: Use r#...# literals for multiline fixtures. Ensure the content is unindented (starts at column 0) so that character offsets in the test match the actual file positions exactly.

• Rationale:

  • Reduce visual noise and scrolling, making the actual test case immediately obvious.

  • Lower execution time and keeps debug logs clean.

  • Unindented formatting allows you to use your editor's "selection character count" to verify byte offsets directly, without needing to manually subtract indentation whitespace.

Marked Tests ​

• Marked test: A technique used to verify that a specific, often hard-to-reach line of code was actually executed during a test.

• Use cov_mark::hit! (in code) and cov_mark::check! (in tests) to create a strictly unique link between a specific edge case in the implementation and its corresponding test.

• Principle: Only maintain one mark per test and one mark per code branch.

• Never place multiple marks in a single test, and never reuse the same mark across different tests.

• Rationale: This ensures that searching for a mark immediately reveals the single canonical test responsible for verifying that specific code branch, eliminating ambiguity.

#[should_panic] ​

• #[should_panic] is prohibited - None and Err should be explicitly checked.

• Rationale:

  • #[should_panic] is a tool for library authors to make sure that the API does not fail silently when misused.

  • rust-analyzer is a long-running server, not a library. It must handle all input gracefully, even invalid input (returning Err or None). It should never intentionally crash.

  • Expected panics still dump stack traces into the test logs. This "noise" creates confusion, making it difficult to distinguish between a test verifying a panic and an actual bug causing a crash.

#[ignore] ​

• Never ignore tests. Explicitly assert the wrong behavor and add a FIXME comment.

• Rationale:

  • Visibility: It ensures the test fails immediately if the bug is accidentally fixed (alerting you to update the test).

  • Safety: It proves the bug causes incorrect output rather than a server crash (panic), which is a critical distinction for a long-running service.

Function Preconditions ​

Type Encoding ​

• Function's assumptions should be expressed in types.

• The caller must be enforced to provide them.

// GOOD
fn is_zero(n: i32) -> bool {
  ...
}

// BAD
fn is_zero(n: Option<i32>) -> bool {
   let n = match n {
       Some(it) => ...,
       None => ...,
   };
}

• Rationale:

  • The caller has more context as to why to the callee's assumptions do not hold.

  • The control flow is therefore more explicit at the call site.

Parse, Don't Validate ​

• Bad practice:

  • One function validates that the data is valid (validate the assumption).

  • Another function uses that data based on the assumptions.

• Good practice: Validate and immediately use the data in the same place (like match instead of bare if).

• Reasons:

  • The bad practice is prone to decay over time. The maintainer has to memorize the assumptions and make sure refactoring efforts of checks actually verify the assumptions.

  • The good practice always ensure that the assumptions hold when manipulating the data.

• Example from rust-analyzer

  rust

    // GOOD
    fn main() {
        let s: &str = ...;
        if let Some(contents) = string_literal_contents(s) {
  
        }
    }
  
    fn string_literal_contents(s: &str) -> Option<&str> {
        if s.starts_with('"') && s.ends_with('"') {
            Some(&s[1..s.len() - 1])
        } else {
            None
        }
    }
  
    // BAD
    fn main() {
        let s: &str = ...;
        if is_string_literal(s) {
            let contents = &s[1..s.len() - 1];
        }
    }
  
    fn is_string_literal(s: &str) -> bool {
        s.starts_with('"') && s.ends_with('"')
    }

• Remarks:

  • This pattern perfectly illustrates Robert Harper's concept of "Boolean Blindness". By reducing a complex check to a simple bool (true/false), we discard the proof of validity. The compiler sees a "true" flag, but it doesn't see the "valid data," forcing us to rely on faith later in the code.

  • I learned this the hard way while building a DBML parser. I designed a system where the "validation phase" was separate from the "execution phase". Because the validation step didn't return a new, safe type (it just returned true), the execution phase had to blindly trust that the validation had run correctly.

  • While systems like TypeScript uses Flow Typing to mitigate this (by inferring types inside if blocks), that safety is often local only. As soon as you pass that variable into a different function, the "flow context" is lost unless explicitly redefined.

Control Flow ​

• Push "if"s up and "for"s down.

Assertions ​

• Use stdx::never! liberally instead of assert!.

• never! checks a condition and logs a backtrace if it fails, but returns a bool instead of crashing. This allows you to write: if stdx::never!(condition) { return; }.

• Rationale: rust-analyzer is a long-running server. A bug in a minor feature (like a specific completion) should log an error and bail out of that specific request, not crash the entire IDE session.

Getters & Setters ​

• Two cases to consider:

  • No invariants: If a field can hold any value safely, just make it pub. Don't write boilerplate code.

  • Invariants exist: If the data has rules (e.g., "cannot be empty"), make the field private, enforce the rule in the Constructor, and provide a Getter.

• Never provide setters. If data needs to change, it should likely be done via a specific behavior method or by creating a new instance, ensuring invariants are never bypassed.

• Getters should return borrowed data. rust-analyzer's example:

  rust

    struct Person {
      // Invariant: never empty
      first_name: String,
      middle_name: Option<String>
    }
  
    // GOOD
    impl Person {
        fn first_name(&self) -> &str { self.first_name.as_str() }
        fn middle_name(&self) -> Option<&str> { self.middle_name.as_ref() }
    }
  
    // BAD
    impl Person {
        fn first_name(&self) -> String { self.first_name.clone() }
        fn middle_name(&self) -> &Option<String> { &self.middle_name }
    }

• Rationale:

  • The APIs are internal so (internal) breaking changes can be allowed to move fast:

    • Using a pub field (with no invariants) introduces less boilerplate but may be breaking if the pub field is suddenly imposed an invariant and has to be changed to private.

    • Using an accessor can prevent breaking changes, but it means implicitly promising a contract and imposing some maintenance boilerplate.

  • Privacy helps make invariants local to prevent code rot.

  • A type that is too specific (borrow owned types like &String) leaks irrelevant details (neither right nor wrong), which creates noise and the client may accidentally rely on those irrelevent details.

Useless Types ​

• Prefer general types.

• If generality is not important, consistency is important.

// GOOD      BAD
&[T]         &Vec<T>
&str         &String
Option<&T>   &Option<T>
&Path        &PathBuf

• Rationale:

  • General types are more flexible.

  • General types leak fewer irrelevant details (which the client may accidentally rely on).

Constructors ​

• If a new function accepts zero arguments, then use the Default trait (either derive or manually implemented).

  • Rationale:

    • Less boilerplate.

    • Consistent: Less cognitive load for the caller - "Should I call new() or default()?"

• Use Vec::new instead of vec![].

  • Rationale:

    • Strength reduction.

    • Uniformity.

• Do not provide Default if the type doesn't have sensible default value (many possible defaults or defaults that has invalid states).

  • Preserve invariants.

  • The user does not need to wonder if the provided default is their desired initial values.

// GOOD
#[derive(Default)] // 1. Best case: Derive it automatically
struct Options {
    check_on_save: bool,
}

// GOOD (Manual Implementation)
struct Buffer {
    data: Vec<u8>,
}

impl Default for Buffer {
    fn default() -> Self {
        Self {
            // 2. Use Vec::new() instead of vec![] (Strength Reduction)
            // It is semantically lighter (function vs macro) and more uniform.
            data: Vec::new(),
        }
    }
}

// BAD
struct OptionsBad {
    check_on_save: bool,
}

impl OptionsBad {
    // 3. Avoid zero-arg new().
    // It forces users to remember "Do I call new() or default() for this type?"
    fn new() -> Self {
        Self { check_on_save: false }
    }
}

Functions Over Objects ​

• Public API: Prefer simple functions (do_thing()) over transient objects that exist only to execute one method (ThingDoer::new().do()).

• Internal logic: It is acceptable (and encouraged) to use "Context" structs inside the function to manage complex state or arguments during execution.

• Rationale:

  • The "Iceberg" pattern: The user sees a simple function interface; the developer uses a structured object implementation behind the scenes.

  • Implementor API is not mixed with user API.

• Middle ground: If a struct is preferred for namespacing, provide a static do() helper method that handles the instantiation and execution in one step.

• Rationale:

  • Reduce boilerplate for the caller.

  • Prevent implementation details (like temporary state management) from leaking into the public API.

// BAD (Caller has to build and run)
ThingDoer::new(arg1, arg2).do();

// GOOD (Caller just acts)
do_thing(arg1, arg2);

// ACCEPTABLE INTERNAL IMPLEMENTATION (Using a struct to organize code)
pub fn do_thing(arg1: Arg1, arg2: Arg2) -> Res {
    // The struct is an implementation detail, hidden from the user
    let mut ctx = Ctx { arg1, arg2 };
    ctx.run()
}

Functions With Many Parameters ​

• Use Config struct:

  rust

    // BAD (Call site is confusing)
    fn annotations(db, file_id, true, false, true);
  
    // GOOD (Call site is explicit and flexible)
    fn annotations(db, file_id, AnnotationConfig {
        binary_target: true,
        annotate_runnables: false,
        annotate_impls: true,
    });

• Rationale:

  • Call site is clearer.

  • Encapsulating volatile parameters in a struct shields intermediate functions from breaking changes, allowing you to add new options without updating the signature of every function in the call chain.

• No Default for Config: Do not implement Default. Force the caller to provide explicit context.

  • Rationale: They know better than the struct what the initial state should be.

• Pass configuration as an argument to the function, do not store it in the object's state.

  • Rationale: This allows the same object to handle multiple requests with different configurations dynamically.

• Command pattern: If a set of parameters can yield different return types (e.g., Vec<T> vs Option<T>), wrap parameters in a "Command" struct.

  • Rationale: This avoids creating multiple top-level functions (query_all, query_first) with identical argument lists.

// GOOD (Command Pattern)
// Captures arguments once, offers multiple execution paths
pub struct Query {
    pub name: String,
    pub case_sensitive: bool,
}

impl Query {
    // Return type A
    pub fn all(self) -> Vec<Item> { ... }
    // Return type B
    pub fn first(self) -> Option<Item> { ... }
}

// BAD (Parameter Duplication)
// Requires repeating arguments for every variation of the result
fn query_all(name: String, case_sensitive: bool) -> Vec<Item> { ... }
fn query_first(name: String, case_sensitive: bool) -> Option<Item> { ... }

• Remarks: Does the Command rule conflict with the rule "Do not store Config in state"?

  • The Config rule applies to long-lived services (e.g., Database). These should remain stateless so they can handle diverse requests without needing to be reset.

  • The Command rule applies to short-lived tasks (e.g., Query). These objects exist solely to bundle parameters for a single operation and are discarded immediately after use.

  • Relationship: The "Command" is effectively a temporary container that you pass to (or use with) the "Service".

Prefer Separate Functions Over Parameters ​

• Split "flag" arguments: If a function is solely invoked with hardcoded literals (true/None), refactor it into distinct named functions (e.g., process_fast() vs process_full()) to eliminate internal branching and "false sharing" of unrelated logic.

• Rationale:

  • Functions with flag arguments often display false sharing. There's often if branching to distinguish between different cases.

    rust

        // Caller
        process_data(true);
        process_data(false);
    
        // Callee
        fn process_data(is_fast: bool) {
            if is_fast {
                // Fast algorithm
            } else {
                // Accurate algorithm
            }
        }

  • These functions seem to share logic and are related and it seems that it makes sense to merge them into 1 function. However, over time, they diverge. Therefore, splitting the control flows into separate functions simplify the logic & eliminate irrelevant details (such as the maintainer would fail to see the cross-dependencies between the if branches).

  • Split the common code of the if branches into a common helper instead.

• Remark: This binder-refactoring PR is a hard lesson for me illustrating this point, both the problem and the solution (splitting into separate classes & extract common helpers). Although there is still lots of room for improvement, this is already significantly better.

Appropriate String Types ​

• When calling OS APIs (file systems, env vars, arguments), use OsString and &OsStr, never String or &str.

• Rust String guarantees valid UTF-8. Operating Systems do not.

  • Linux/Unix: Paths are arbitrary byte sequences (except null).

  • Windows: Paths are potentially ill-formed UTF-16 sequences.

• Rationale: This creates a strict type-level boundary.

  • If you hold a String, you know it is safe, clean text.

  • If you hold an OsString, you know it is "dirty" data from the outside world.

  • Using OsString prevents accidental panics or data corruption when encountering a file named with invalid encoding.

• Avoid the standard std::Path, use the custom AbsPathBuf and AbsPath wrapper that guarantees the path inside is absolute.

• Rationale:

  • CWD is global mutable state.

  • If you use a relative path like Path::new("src/main.rs"), the OS resolves it relative to where the server binary started, not where the project is.

Premature Pessimization ​

Avoid Allocations ​

• Zero-allocation default: Prefer stack-based structures (iterators) over heap-based collections (Vec, String) unless you specifically need ownership or long-term storage.

• Lazy vs eager: collect::<Vec>() eagerly processes the entire sequence and allocates memory immediately. Iterators are lazy and compute items only on demand.

use itertools::Itertools;

// BAD (Heavy)
// 1. Allocates heap memory. 2. Processes entire string. 3. Frees memory.
let parts: Vec<&str> = text.split(',').collect();
if parts.len() == 3 {
    process(parts[0], parts[1], parts[2]);
}

// GOOD (Light)
// 1. No allocation. 2. Stops after 3 items. 3. Stores ptrs on Stack.
if let Some((a, b, c)) = text.split(',').collect_tuple() {
    process(a, b, c);
}

Push Allocations to the Call Site ​

• Rationale: The cost of calling a function becomes more explicit.

Collection Types ​

• rustc_hash's map and set are preferred over those in std::collections.

• Rationale:

  • Faster hasher.

  • Slightly reduce code size if applied consistently.

Avoid Intermediate Collections ​

• Use an accumulator parameter (list, set, map) as the first parameter to collect values.

Avoid Monomorphization ​

• Minimize generics: Avoid heavy generic logic at crate boundaries to prevent compile-time bloat caused by monomorphization (code duplication).

• "Inner dyn" pattern: Use thin generic wrappers that immediately delegate to private, non-generic functions using dynamic dispatch (dyn Trait).

• Concrete types: Avoid AsRef polymorphism; prefer concrete types like &Path unless writing a widely-used library.

• Rationale: Optimize for compile speed by default; runtime performance only matters for the "hot" 20% of code, but compilation costs affect 100%.

use std::fmt::Display;

// --- 1. The Wrapper (Generic) ---
// This function is "monomorphized" (duplicated) for every type T.
// But since it's only 1 line, the duplication cost is negligible.
pub fn log_message<T: Display>(msg: T) {
    // Coerce the specific type T into a trait object (&dyn Display)
    log_message_impl(&msg);
}

// --- 2. The Implementation (Dynamic) ---
// This function is compiled ONLY ONCE.
// It handles the heavy lifting via dynamic dispatch (vtable).
fn log_message_impl(msg: &dyn Display) {
    // Imagine 500 lines of complex logging logic here...
    println!("Timestamp: [INFO] {}", msg);
    // ... extensive I/O, formatting, or network calls ...
}

fn main() {
    log_message("Hello");       // T is &str -> Wrapper created for &str
    log_message(42);            // T is i32  -> Wrapper created for i32
    log_message(3.14);          // T is f64  -> Wrapper created for f64

    // Result: 3 tiny wrappers, but the heavy `log_message_impl` exists only once.
}

Style ​

Order of Imports ​

• Modules first: Declare modules (mod x;) at the top, before any imports, ordered by "suggested reading."

• Import structure:

  • Grouping: Use one use statement per crate (group items inside { ... }).

  • Spacing: Separate different groups with blank lines.

  • Import order:

    1. std: Standard library (use std::...).

    2. External: Third-party and workspace crates.

    3. Local: Current crate (use crate::...).

    4. Relative: Parent/child modules (use super::...), though crate:: is preferred.

• Re-exports: Place pub use after all imports, as they are treated as item definitions.

• Rationale: Ensures consistency, improves readability for new contributors, and highlights dependencies clearly.

Import Style ​

• Qualify layer types: Always qualify items from hir and ast to prevent ambiguity and clarify the architectural layer.

  • Good: use syntax::ast; ... func: hir::Function

  • Bad: use hir::Function; ... func: Function

• Trait implementations: Import the module (e.g., std::fmt), not the trait itself, when implementing standard traits.

  • Good: impl fmt::Display for ...

• Rationale: Reduces typing and clearly distinguishes implementation from usage.

• Avoid local globs: Do not use use MyEnum::*; inside functions.

• Absolute paths: Prefer use crate::foo over relative paths like super:: or self:: for consistency.

• No re-exports: Avoid re-exports in non-library code to prevent multiple access paths and maintain consistency.

Order of Items ​

• Public API first: Always place public items (pub or pub(crate)) at the very top, before any private helpers or implementation details.

• Types before logic: Define data structures (struct, enum) before functions and impl blocks.

• Top-down: Order type definitions by dependency, place the "parent" container before the "child" component it contains.

• Rationale:

  • Optimize for a new reader scanning top-to-bottom.

  • When code is folded, the file structure should read like API documentation.

Context Parameters ​

• Context-first: Always pass "context" parameters (invariant data threaded through many calls) as the first arguments.

  • Rationale:

    • This creates a visual hierarchy: "setting" "actors".

    • It mimics the self convention in OOP (where the context/object always comes first).

    • When scanning a function call, you immediately see where the operation is happening (the context) before seeing what is being processed (the variable data).

// BAD: Context (db) is hidden at the end.
// In a long list of arguments, you might miss which 'db' is being used.
fn transform_data(data: String, id: usize, force: bool, db: &Database) { ... }

// Usage
transform_data(raw_input, 42, true, &primary_db);

// GOOD: Context (db) is front and center.
// It establishes the environment immediately.
fn transform_data(db: &Database, data: String, id: usize, force: bool) { ... }

// Usage
transform_data(&primary_db, raw_input, 42, true);

• If there are multiple context parameters, bundle them into a struct and pass it as &self.

// BAD: Parameter explosion.
// Every helper function needs to accept all three context items.
fn parse(db: &Db, cfg: &Config, cache: &Cache, text: &str) {
    validate(db, cfg, cache, text); // Tedious repetition
}

// GOOD: Packed Context.
struct ParseCtx<'a> {
    db: &'a Db,
    cfg: &'a Config,
    cache: &'a Cache,
}

impl<'a> ParseCtx<'a> {
    // The signature is clean. Context is implicit in `&self`.
    fn parse(&self, text: &str) {
        self.validate(text);
    }

    fn validate(&self, text: &str) { ... }
}

• The "dangling argument" problem:

  • Context-first works better when non-context parameters are lambdas (closures).

  • Rationale:

    • Rust closures often span multiple lines.

    • If the context parameter is placed after the closure, it ends up "dangling" after the closing brace }. This is visually confusing and looks like a syntax error or a forgotten fragment.

    • Placing context first ensures the closure body is the last thing the eye sees, resulting in a clean termination of the statement.

// BAD: Context Last
// The `&context` argument is easy to miss or looks like it belongs
// to a different statement because it follows a large code block.
apply_changes(|item| {
    // ... complex multi-line logic ...
    // ... complex multi-line logic ...
    item.finalize()
}, &context); // <--- The "Dangler"


// GOOD: Context First
// The statement starts with the context, and the closure flows naturally
// until the end of the function call.
apply_changes(&context, |item| {
    // ... complex multi-line logic ...
    // ... complex multi-line logic ...
    item.finalize()
}); // <--- Clean, standard closure syntax

Variable Naming ​

• Prioritize verbose & boring but clear names: prefer long names that mirror the type (e.g., global_state: GlobalState). Rely on code completion, not brevity.

• Standard variables:

  • res: Function result.

  • it: Generic item (when identity is irrelevant).

  • n_foos: Count (preferred over foo_count).

  • foo_idx: Index.

• Keyword collisions: Avoid r#ident syntax. Use these consistent replacements: crate -> krate, enum -> enum_, fn -> func, impl -> imp, macro -> mac, mod -> module, struct -> strukt, trait -> trait_, type -> ty.

• Spelling & acronyms:

  • Use American spelling (color).

  • Avoid ad-hoc acronyms; stick to common ones (db, ctx).

Error Handling Trivia ​

• Use anyhow::Result instead of the bare Result.

  • Rationale: Makes the return type immediately clear without checking imports.

• Macro choice: Prefer anyhow::format_err! over anyhow::anyhow.

  • Rationale: More "boring" (standard), consistent, and avoids the stuttering of anyhow::anyhow.

• Message formatting:

  • There are no strict rules on message structure. Rust standard library uses lowecase, while anyhow uses uppercase.

  • Do not end error or context messages with a period (.).

Early Returns ​

• Handle negative cases immediately with an early return rather than wrapping the "happy path" inside a large if/else block.

• Rationale: Flattens code nesting and reduces "cognitive stack usage" (mental load).

• Explicit error returns: Use return Err(e) to exit with an error. Avoid using Err(e)? to simulate a throw.

• Rationale: return evaluates to the "never type" (!), which allows the compiler to strictly identify dead code, whereas ? resolves to a generic type that can mask unreachable code.

Comparisons ​

• Prefer less-than:

  • Always use < or <= comparisons.

  • Avoid > or >=.

• Rationale:

  • Spatial intuition: Corresponds to the real number line where values increase from left to right (0->infinity).

  • Visual ordering: lo <= x && x <= hi visually places x in the middle, whereas x >= lo forces a mental "flip."

If-let ​

• Prefer match over if let ... else: When you need to handle both the "success" and "failure" cases, use a full match statement.

• Rationale:

  • Compactness: match is usually cleaner and requires less syntax for simple alternatives.

  • Precision: The else block in if let is implicit (it covers everything else). match forces you, or allows you, to be explicit about what the negative case is (e.g., None vs Err(_)), making the code more robust to type changes.

Match Ergonomics ​

• Avoid ref: Do not use the ref keyword in patterns.

Rationale: - Obsolescence: ref is largely redundant due to "match ergonomics" (introduced in recent Rust editions). - Simplicity: Relying on match ergonomics is cleaner and avoids mixing legacy syntax with modern style.

Empty Match Arms ​

• Use the unit value (), for empty match arms, rather than an empty block {}.

// GOOD
Err(err) => error!("{}", err),
Ok(_) => (), // <--- Clean, single-line comma style

// BAD
Err(err) => error!("{}", err),
Ok(_) => {}  // <--- Block style breaks the visual rhythm

• Rationale:

  • In Rust, () is the value "nothing," while {} is a block of code that evaluates to nothing.

  • While they are functionally identical here, using => (), keeps the match arm visibly distinct as a "value" rather than a "scope," maintaining a consistent visual rhythm in long match statements.

Functional Combinators ​

• Use functional combinators (map, and_then) only when they fit naturally.

• Prefer imperative control flow (if, for, match) over "forced" combinators like bool::then or Option::filter.

• The philosophy: Code should be dense in computation (doing work) but sparse in structure (fewer indirections per line).

• Rationale:

  • Rust has strong support for imperative flow (loops, early returns).

  • Rust functions are "less first-class" because of effects like ? (try-operator) and .await. These do not compose well inside long chains of closures (e.g., trying to ? inside a filter closure is painful).

Turbofish ​

• Prefer explicit type ascription (let x: Vec<i32> = ...) over the "turbofish" syntax (...collect::<Vec<i32>>()).

• Rationale: We want to able to read everything from left-to-right without maintaining to much context:

  • Good: let names: Vec<String> = users.iter().map(...).collect();

    • Why: When you read the line left-to-right, you immediately know what is being built (Vec<String>). This context helps you understand the complex iterator chain that follows.

  • Bad: let names = users.iter().map(...).collect::<Vec<String>>();

    • Why: You have to read the entire chain until the very end to figure out what type is actually being produced.

  • No placeholders (_): Avoid let x: Vec<_> = ....

• Rationale: If the compiler struggles to infer the type (forcing you to add a hint), a human reader will likely struggle too. Be kind to the reader and write the full type Vec<i32>.

Helper Functions ​

• Avoid single-Use helpers: Do not create a separate function if it is only called once.

• Alternative: Use a block { ... }.

• Rationale: This isolates the scope but keeps access to the context variables.

• Exception: Create a function if you need early returns (return) or error propagation (?).

• Local helpers: Place nested helper functions at the end of the enclosing function.

  • Structure: Main logic -> return result; -> fn helper() { ... }.

  • Limit: Do not nest more than 1 level deep.

Helper Variables ​

• Create boolean helper variables for complex conditions (e.g., inside match guards).

• Rationale:

  • Act as a "cognitively cheap" abstraction (names the logic without hiding the context).

  • Make debugging easier (you can inspect/print the variable).

Syntax Macros ​

• Tokens: Use the T![token] macro instead of SyntaxKind::TOKEN_KW.

• Example: T![true] instead of SyntaxKind::TRUE_KW.

• Rationale: Familiar syntax, avoids ambiguity (e.g., curly brace vs bracket).

Documentation ​

• Comments: Write proper sentences. Start with a capital, end with a dot ..

• Markdown: Use sentence-per-line. Do not wrap lines hard; press Enter after every sentence.

• Rationale:

  • Makes diffs cleaner and editing easier.

  • Formatting a comment as a sentence (capital + period) forces the brain to switch from "scribbling notes" to "explaining thoughts."

  • Context dump: It tricks you into emptying your "mental RAM" (assumptions, constraints) into the code, rather than leaving vague fragments like // fix this.