298 lines
10 KiB
Markdown
298 lines
10 KiB
Markdown
# ast-grep Rule Reference
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This document provides comprehensive documentation for ast-grep rule syntax, covering all rule types and metavariables.
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## Introduction to ast-grep Rules
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ast-grep rules are declarative specifications for matching and filtering Abstract Syntax Tree (AST) nodes. They enable structural code search and analysis by defining conditions an AST node must meet to be matched.
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### Rule Categories
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ast-grep rules are categorized into three types:
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* **Atomic Rules**: Match individual AST nodes based on intrinsic properties like code patterns (`pattern`), node type (`kind`), or text content (`regex`).
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* **Relational Rules**: Define conditions based on a target node's position or relationship to other nodes (e.g., `inside`, `has`, `precedes`, `follows`).
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* **Composite Rules**: Combine other rules using logical operations (AND, OR, NOT) to form complex matching criteria (e.g., `all`, `any`, `not`, `matches`).
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## Anatomy of an ast-grep Rule Object
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The ast-grep rule object is the core configuration unit defining how ast-grep identifies and filters AST nodes. It's typically written in YAML format.
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### General Structure
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Every field within an ast-grep Rule Object is optional, but at least one "positive" key (e.g., `kind`, `pattern`) must be present.
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A node matches a rule if it satisfies all fields defined within that rule object, implying an implicit logical AND operation.
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For rules using metavariables that depend on prior matching, explicit `all` composite rules are recommended to guarantee execution order.
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### Rule Object Properties
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| Property | Type | Category | Purpose | Example |
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| :--- | :--- | :--- | :--- | :--- |
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| `pattern` | String or Object | Atomic | Matches AST node by code pattern. | `pattern: console.log($ARG)` |
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| `kind` | String | Atomic | Matches AST node by its kind name. | `kind: call_expression` |
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| `regex` | String | Atomic | Matches node's text by Rust regex. | `regex: ^[a-z]+$` |
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| `nthChild` | number, string, Object | Atomic | Matches nodes by their index within parent's children. | `nthChild: 1` |
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| `range` | RangeObject | Atomic | Matches node by character-based start/end positions. | `range: { start: { line: 0, column: 0 }, end: { line: 0, column: 10 } }` |
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| `inside` | Object | Relational | Target node must be inside node matching sub-rule. | `inside: { pattern: class $C { $$$ }, stopBy: end }` |
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| `has` | Object | Relational | Target node must have descendant matching sub-rule. | `has: { pattern: await $EXPR, stopBy: end }` |
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| `precedes` | Object | Relational | Target node must appear before node matching sub-rule. | `precedes: { pattern: return $VAL }` |
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| `follows` | Object | Relational | Target node must appear after node matching sub-rule. | `follows: { pattern: import $M from '$P' }` |
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| `all` | Array<Rule> | Composite | Matches if all sub-rules match. | `all: [ { kind: call_expression }, { pattern: foo($A) } ]` |
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| `any` | Array<Rule> | Composite | Matches if any sub-rules match. | `any: [ { pattern: foo() }, { pattern: bar() } ]` |
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| `not` | Object | Composite | Matches if sub-rule does not match. | `not: { pattern: console.log($ARG) }` |
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| `matches` | String | Composite | Matches if predefined utility rule matches. | `matches: my-utility-rule-id` |
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## Atomic Rules
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Atomic rules match individual AST nodes based on their intrinsic properties.
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### pattern: String and Object Forms
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The `pattern` rule matches a single AST node based on a code pattern.
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**String Pattern**: Directly matches using ast-grep's pattern syntax with metavariables.
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```yaml
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pattern: console.log($ARG)
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```
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**Object Pattern**: Offers granular control for ambiguous patterns or specific contexts.
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* `selector`: Pinpoints a specific part of the parsed pattern to match.
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```yaml
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pattern:
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selector: field_definition
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context: class { $F }
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```
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* `context`: Provides surrounding code context for correct parsing.
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* `strictness`: Modifies the pattern's matching algorithm (`cst`, `smart`, `ast`, `relaxed`, `signature`).
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```yaml
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pattern:
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context: foo($BAR)
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strictness: relaxed
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```
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### kind: Matching by Node Type
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The `kind` rule matches an AST node by its `tree_sitter_node_kind` name, derived from the language's Tree-sitter grammar. Useful for targeting constructs like `call_expression` or `function_declaration`.
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```yaml
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kind: call_expression
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```
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### regex: Text-Based Node Matching
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The `regex` rule matches the entire text content of an AST node using a Rust regular expression. It's not a "positive" rule, meaning it matches any node whose text satisfies the regex, regardless of its structural kind.
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### nthChild: Positional Node Matching
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The `nthChild` rule finds nodes by their 1-based index within their parent's children list, counting only named nodes by default.
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* `number`: Matches the exact nth child. Example: `nthChild: 1`
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* `string`: Matches positions using An+B formula. Example: `2n+1`
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* `Object`: Provides granular control:
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* `position`: `number` or An+B string.
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* `reverse`: `true` to count from the end.
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* `ofRule`: An ast-grep rule to filter the sibling list before counting.
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### range: Position-Based Node Matching
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The `range` rule matches an AST node based on its character-based start and end positions. A `RangeObject` defines `start` and `end` fields, each with 0-based `line` and `column`. `start` is inclusive, `end` is exclusive.
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## Relational Rules
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Relational rules filter targets based on their position relative to other AST nodes. They can include `stopBy` and `field` options.
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### inside: Matching Within a Parent Node
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Requires the target node to be inside another node matching the `inside` sub-rule.
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```yaml
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inside:
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pattern: class $C { $$$ }
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stopBy: end
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```
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### has: Matching with a Descendant Node
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Requires the target node to have a descendant node matching the `has` sub-rule.
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```yaml
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has:
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pattern: await $EXPR
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stopBy: end
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```
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### precedes and follows: Sequential Node Matching
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* `precedes`: Target node must appear before a node matching the `precedes` sub-rule.
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* `follows`: Target node must appear after a node matching the `follows` sub-rule.
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Both include `stopBy` but not `field`.
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### stopBy and field: Refining Relational Searches
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**stopBy**: Controls search termination for relational rules.
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* `"neighbor"` (default): Stops when immediate surrounding node doesn't match.
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* `"end"`: Searches to the end of the direction (root for `inside`, leaf for `has`).
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* `Rule object`: Stops when a surrounding node matches the provided rule (inclusive).
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**field**: Specifies a sub-node within the target node that should match the relational rule. Only for `inside` and `has`.
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**Best Practice**: When unsure, always use `stopBy: end` to ensure the search goes to the end of the direction.
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## Composite Rules
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Composite rules combine atomic and relational rules using logical operations.
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### all: Conjunction (AND) of Rules
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Matches a node only if all sub-rules in the list match. Guarantees order of rule matching, important for metavariables.
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```yaml
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all:
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- kind: call_expression
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- pattern: console.log($ARG)
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```
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### any: Disjunction (OR) of Rules
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Matches a node if any sub-rules in the list match.
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```yaml
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any:
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- pattern: console.log($ARG)
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- pattern: console.warn($ARG)
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- pattern: console.error($ARG)
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```
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### not: Negation (NOT) of a Rule
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Matches a node if the single sub-rule does not match.
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```yaml
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not:
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pattern: console.log($ARG)
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```
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### matches: Rule Reuse and Utility Rules
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Takes a rule-id string, matching if the referenced utility rule matches. Enables rule reuse and recursive rules.
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## Metavariables
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Metavariables are placeholders in patterns to match dynamic content in the AST.
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### $VAR: Single Named Node Capture
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Captures a single named node in the AST.
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* **Valid**: `$META`, `$META_VAR`, `$_`
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* **Invalid**: `$invalid`, `$123`, `$KEBAB-CASE`
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* **Example**: `console.log($GREETING)` matches `console.log('Hello World')`.
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* **Reuse**: `$A == $A` matches `a == a` but not `a == b`.
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### $$VAR: Single Unnamed Node Capture
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Captures a single unnamed node (e.g., operators, punctuation).
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**Example**: To match the operator in `a + b`, use `$$OP`.
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```yaml
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rule:
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kind: binary_expression
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has:
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field: operator
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pattern: $$OP
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```
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### $$$MULTI_META_VARIABLE: Multi-Node Capture
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Matches zero or more AST nodes (non-greedy). Useful for variable numbers of arguments or statements.
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* **Example**: `console.log($$$)` matches `console.log()`, `console.log('hello')`, and `console.log('debug:', key, value)`.
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* **Example**: `function $FUNC($$$ARGS) { $$$ }` matches functions with varying parameters/statements.
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### Non-Capturing Metavariables (_VAR)
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Metavariables starting with an underscore (`_`) are not captured. They can match different content even if named identically, optimizing performance.
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* **Example**: `$_FUNC($_FUNC)` matches `test(a)` and `testFunc(1 + 1)`.
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### Important Considerations for Metavariable Detection
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* **Syntax Matching**: Only exact metavariable syntax (e.g., `$A`, `$$B`, `$$$C`) is recognized.
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* **Exclusive Content**: Metavariable text must be the only text within an AST node.
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* **Non-working**: `obj.on$EVENT`, `"Hello $WORLD"`, `a $OP b`, `$jq`.
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The ast-grep playground is useful for debugging patterns and visualizing metavariables.
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## Common Patterns and Examples
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### Finding Functions with Specific Content
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Find functions that contain await expressions:
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```yaml
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rule:
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kind: function_declaration
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has:
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pattern: await $EXPR
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stopBy: end
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```
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### Finding Code Inside Specific Contexts
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Find console.log calls inside class methods:
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```yaml
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rule:
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pattern: console.log($$$)
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inside:
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kind: method_definition
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stopBy: end
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```
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### Combining Multiple Conditions
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Find async functions that use await but don't have try-catch:
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```yaml
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rule:
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all:
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- kind: function_declaration
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- has:
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pattern: await $EXPR
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stopBy: end
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- not:
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has:
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pattern: try { $$$ } catch ($E) { $$$ }
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stopBy: end
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```
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### Matching Multiple Alternatives
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Find any type of console method call:
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```yaml
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rule:
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any:
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- pattern: console.log($$$)
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- pattern: console.warn($$$)
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- pattern: console.error($$$)
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- pattern: console.debug($$$)
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```
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## Troubleshooting Tips
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1. **Rule doesn't match**: Use `dump_syntax_tree` to see the actual AST structure
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2. **Relational rule issues**: Ensure `stopBy: end` is set for deep searches
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3. **Wrong node kind**: Check the language's Tree-sitter grammar for correct kind names
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4. **Metavariable not working**: Ensure it's the only content in its AST node
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5. **Pattern too complex**: Break it down into simpler sub-rules using `all`
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