cloudformation-validate-cel-engine 1.12.0

AWS CloudFormation Validate
Documentation 
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# cel-engine

Validates CloudFormation templates using a three-tier evaluation strategy: native Rust rule functions for built-in
rules, a generated rule registry for data-driven rules, and
the [cel-interpreter](https://crates.io/crates/cel-interpreter) for user-provided custom rules. Implements
the [ValidationEngine](../validation-engine/README.md) trait.

## Architecture

```
  CelEngine::evaluate_rules(model, config)
       │
       ├── Tier 1: Native Rust functions (built-in rules, max performance)
       ├── Tier 2: Generated rule descriptors (data-driven rules from schemas)
       ├── Tier 3: Custom CEL expressions (user-provided)
       └── Guard DSL rules (shared Guard evaluator, identical in every engine)
       │
       ▼
  Vec<Diagnostic>
```

- **Tier 1** - Native Rust functions organized by category (Structure, Intrinsic, Reference, BestPractice,
  Resource, Security). Access the `SemanticModel` directly with no serialization overhead.
- **Tier 2** - JSON rule descriptors generated by data-source, evaluated natively against
  the `SemanticModel`. Indexed by resource type for efficient lookup.
- **Tier 3** - User-provided CEL expressions, compiled at engine init and evaluated via the CEL interpreter at
  runtime.
- **Guard DSL rules** are not translated to CEL. They are evaluated by the shared Guard evaluator in
  [validation-engine](../validation-engine/README.md), which every engine calls, so the same Guard rule yields the same
  findings in every engine.

## Custom CEL Rules

Custom rules are provided as JSON:

```json
{
  "rules": [
    {
      "rule_id": "CUSTOM001",
      "severity": "ERROR",
      "category": "custom",
      "resource_type": "AWS::S3::Bucket",
      "expression": "!has(properties.BucketEncryption)",
      "message": "S3 bucket {name} must have encryption enabled",
      "prop_path": "Properties.BucketEncryption",
      "suggested_fix": "Add BucketEncryption configuration"
    }
  ]
}
```

For resource-scoped rules (`resource_type` set), the engine iterates over all resources of that type and evaluates the
CEL expression with resource-specific context. For global rules (no `resource_type`), the expression is evaluated once.

### CEL Context Variables

| Variable              | Type   | Scope        | Content                                                      |
|-----------------------|--------|--------------|--------------------------------------------------------------|
| `resources`           | map    | Global       | All resolved resources keyed by logical ID                   |
| `parameters`          | map    | Global       | All parameters with metadata                                 |
| `conditions`          | map    | Global       | All conditions                                               |
| `outputs`             | map    | Global       | All outputs                                                  |
| `mappings`            | map    | Global       | All mappings                                                 |
| `template`            | map    | Global       | Template metadata (transforms, format version, description)  |
| `name`                | string | Per-resource | Current resource's logical ID                                |
| `resource`            | map    | Per-resource | Current resource's full resolved data                        |
| `properties`          | map    | Per-resource | Current resource's resolved properties                       |
| `resolved_properties` | map    | Per-resource | Properties with intrinsic functions fully resolved to values |