---
name: lint-rule-development
description: Step-by-step guide for creating and implementing lint rules in Biome's analyzer. Use when implementing rules like noVar, useConst, or any custom lint/assist rule. Examples:<example>User wants to create a rule that detects unused variables</example><example>User needs to add code actions to fix diagnostic issues</example><example>User is implementing semantic analysis for binding references</example>
---

## Purpose

Use this skill when creating new lint rules or assist actions for Biome. It provides scaffolding commands, implementation patterns, testing workflows, and documentation guidelines.

## Prerequisites

1. Install required tools: `just install-tools`
2. Ensure `cargo`, `just`, and `pnpm` are available
3. Read `crates/biome_analyze/CONTRIBUTING.md` for in-depth concepts

## Common Workflows

### Create a New Lint Rule

Generate scaffolding for a JavaScript lint rule:

```shell
just new-js-lintrule useMyRuleName
```

For other languages:
```shell
just new-css-lintrule myRuleName
just new-json-lintrule myRuleName
just new-graphql-lintrule myRuleName
```

This creates a file in `crates/biome_js_analyze/src/lint/nursery/use_my_rule_name.rs`

### Implement the Rule

Basic rule structure (generated by scaffolding):

```rust
use biome_analyze::{context::RuleContext, declare_lint_rule, Rule, RuleDiagnostic};
use biome_js_syntax::JsIdentifierBinding;
use biome_rowan::AstNode;

declare_lint_rule! {
    /// Disallows the use of prohibited identifiers.
    pub UseMyRuleName {
        version: "next",
        name: "useMyRuleName",
        language: "js",
        recommended: false,
    }
}

impl Rule for UseMyRuleName {
    type Query = Ast<JsIdentifierBinding>;
    type State = ();
    type Signals = Option<Self::State>;
    type Options = ();

    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
        let binding = ctx.query();
        
        // Check if identifier matches your rule logic
        if binding.name_token().ok()?.text() == "prohibited_name" {
            return Some(());
        }
        
        None
    }

    fn diagnostic(ctx: &RuleContext<Self>, _state: &Self::State) -> Option<RuleDiagnostic> {
        let node = ctx.query();
        Some(
            RuleDiagnostic::new(
                rule_category!(),
                node.range(),
                markup! {
                    "Avoid using this identifier."
                },
            )
            .note(markup! {
                "This identifier is prohibited because..."
            }),
        )
    }
}
```

### Using Semantic Model

For rules that need binding analysis:

```rust
use biome_analyze::Semantic;

impl Rule for MySemanticRule {
    type Query = Semantic<JsReferenceIdentifier>;
    
    fn run(ctx: &RuleContext<Self>) -> Self::Signals {
        let node = ctx.query();
        let model = ctx.model();
        
        // Check if binding is declared
        let binding = node.binding(model)?;
        
        // Get all references to this binding
        let all_refs = binding.all_references(model);
        
        // Get only read references
        let read_refs = binding.all_reads(model);
        
        // Get only write references
        let write_refs = binding.all_writes(model);
        
        Some(())
    }
}
```

### Add Code Actions (Fixes)

To provide automatic fixes:

```rust
use biome_analyze::FixKind;

declare_lint_rule! {
    pub UseMyRuleName {
        version: "next",
        name: "useMyRuleName",
        language: "js",
        recommended: false,
        fix_kind: FixKind::Safe, // or FixKind::Unsafe
    }
}

impl Rule for UseMyRuleName {
    fn action(ctx: &RuleContext<Self>, _state: &Self::State) -> Option<JsRuleAction> {
        let node = ctx.query();
        let mut mutation = ctx.root().begin();
        
        // Example: Replace the node
        mutation.replace_node(
            node.clone(),
            make::js_identifier_binding(make::ident("replacement"))
        );
        
        Some(JsRuleAction::new(
            ctx.action_category(ctx.category(), ctx.group()),
            ctx.metadata().applicability(),
            markup! { "Use 'replacement' instead" }.to_owned(),
            mutation,
        ))
    }
}
```

### Quick Testing

Use the quick test for rapid iteration:

```rust
// In crates/biome_js_analyze/tests/quick_test.rs
// Uncomment #[ignore] and modify:

const SOURCE: &str = r#"
const prohibited_name = 1;
"#;

let rule_filter = RuleFilter::Rule("nursery", "useMyRuleName");
```

Run the test:
```shell
cd crates/biome_js_analyze
cargo test quick_test -- --show-output
```

### Create Snapshot Tests

Create test files in `tests/specs/nursery/useMyRuleName/`:

```
tests/specs/nursery/useMyRuleName/
├── invalid.js          # Code that triggers the rule
├── valid.js            # Code that doesn't trigger the rule
└── options.json        # Optional rule configuration
```

Example `invalid.js`:
```javascript
const prohibited_name = 1;
const another_prohibited = 2;
```

Run snapshot tests:
```shell
just test-lintrule useMyRuleName
```

Review snapshots:
```shell
cargo insta review
```

### Generate Analyzer Code

After modifying rules, generate updated boilerplate:

```shell
just gen-analyzer
```

This updates:
- Rule registrations
- Configuration schemas
- Documentation exports
- Type bindings

### Format and Lint

Before committing:
```shell
just f  # Format code
just l  # Lint code
```

## Tips

- **Rule naming**: Use `no*` prefix for rules that forbid something (e.g., `noVar`), `use*` for rules that mandate something (e.g., `useConst`)
- **Nursery group**: All new rules start in the `nursery` group
- **Semantic queries**: Use `Semantic<Node>` query when you need binding/scope analysis
- **Multiple signals**: Return `Vec<Self::State>` or `Box<[Self::State]>` to emit multiple diagnostics
- **Safe vs Unsafe fixes**: Mark fixes as `Unsafe` if they could change program behavior
- **Check for globals**: Always verify if a variable is global before reporting it (use semantic model)
- **Error recovery**: When navigating CST, use `.ok()?` pattern to handle missing nodes gracefully
- **Testing arrays**: Use `.jsonc` files with arrays of code snippets for multiple test cases

## Common Query Types

```rust
// Simple AST query
type Query = Ast<JsVariableDeclaration>;

// Semantic query (needs binding info)
type Query = Semantic<JsReferenceIdentifier>;

// Multiple node types (requires declare_node_union!)
declare_node_union! {
    pub AnyFunctionLike = AnyJsFunction | JsMethodObjectMember | JsMethodClassMember
}
type Query = Semantic<AnyFunctionLike>;
```

## References

- Full guide: `crates/biome_analyze/CONTRIBUTING.md`
- Rule examples: `crates/biome_js_analyze/src/lint/`
- Semantic model: Search for `Semantic<` in existing rules
- Testing guide: Main `CONTRIBUTING.md` testing section