Add JSON configuration file support

This change adds support for JSON-formatted TFLint configuration files
(.tflint.json) in addition to the existing HCL format (.tflint.hcl).

Changes:
- Update config loading to detect file extension and parse JSON/HCL accordingly
- Add support for .tflint.json in default file discovery
- Maintain HCL preference over JSON files
- Add comprehensive unit tests for JSON config loading scenarios
- Add integration tests for JSON config functionality and precedence
- Update user guide documentation with JSON format examples

The implementation uses the existing hclparse.ParseJSON() from the HCL
library, following the same JSON structure conventions as Terraform's
.tf.json files. All existing functionality remains unchanged.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: rorychatterton

docs/user-guide/config.md

@@ -4,10 +4,12 @@ You can change the behavior not only in CLI flags but also in config files. TFLi
 
 1. File passed by the `--config` option
 2. File set by the `TFLINT_CONFIG_FILE` environment variable
-3. Current directory (`./.tflint.hcl`)
-4. Home directory (`~/.tflint.hcl`)
+3. Current directory `./.tflint.hcl`
+4. Current directory `./.tflint.json`
+5. Home directory `~/.tflint.hcl`
+6. Home directory `~/.tflint.json`
 
-The config file is written in [HCL](https://github.com/hashicorp/hcl). An example is shown below:
+The config file can be written in either [HCL](https://github.com/hashicorp/hcl) or JSON format, determined by the file extension. JSON files use the [HCL-compatible JSON syntax](https://developer.hashicorp.com/terraform/language/syntax/json), following the same structure as Terraform's `.tf.json` files. An HCL example is shown below:
 
 ```hcl
 tflint {
@@ -42,10 +44,47 @@ rule "aws_instance_invalid_type" {
 }
 ```
 
+The same configuration can be written in JSON format as `.tflint.json`:
+
+```json
+{
+  "tflint": {
+    "required_version": ">= 0.50"
+  },
+  "config": {
+    "format": "compact",
+    "plugin_dir": "~/.tflint.d/plugins",
+    "call_module_type": "local",
+    "force": false,
+    "disabled_by_default": false,
+    "ignore_module": {
+      "terraform-aws-modules/vpc/aws": true,
+      "terraform-aws-modules/security-group/aws": true
+    },
+    "varfile": ["example1.tfvars", "example2.tfvars"],
+    "variables": ["foo=bar", "bar=[\"baz\"]"]
+  },
+  "plugin": {
+    "aws": {
+      "enabled": true,
+      "version": "0.4.0",
+      "source": "github.com/terraform-linters/tflint-ruleset-aws"
+    }
+  },
+  "rule": {
+    "aws_instance_invalid_type": {
+      "enabled": false
+    }
+  }
+}
+```
+
 The file path is resolved relative to the module directory when `--chdir` or `--recursive` is used. To use a config file from the working directory when recursing, pass an absolute path:
 
 ```sh
 tflint --recursive --config "$(pwd)/.tflint.hcl"
+# or
+tflint --recursive --config "$(pwd)/.tflint.json"
 ```
 
 ### `required_version`

integrationtest/cli/cli_test.go

@@ -71,6 +71,21 @@ func TestIntegration(t *testing.T) {
 			status:  cmd.ExitCodeOK,
 			stdout:  "[]",
 		},
+		{
+			name:    "JSON format config",
+			command: "./tflint",
+			dir:     "json_config",
+			status:  cmd.ExitCodeOK,
+			stdout: `<?xml version="1.0" encoding="UTF-8"?>
+<checkstyle></checkstyle>`,
+		},
+		{
+			name:    "HCL precedence over JSON config",
+			command: "./tflint",
+			dir:     "hcl_json_precedence",
+			status:  cmd.ExitCodeOK,
+			stdout:  "",
+		},
 		{
 			name:    "`--force` option with no issues",
 			command: "./tflint --force",

integrationtest/cli/hcl_json_precedence/.tflint.hcl

@@ -0,0 +1,3 @@
+config {
+  format = "compact"
+}

integrationtest/cli/hcl_json_precedence/.tflint.json

@@ -0,0 +1,5 @@
+{
+  "config": {
+    "format": "checkstyle"
+  }
+}

integrationtest/cli/json_config/.tflint.json

@@ -0,0 +1,5 @@
+{
+  "config": {
+    "format": "checkstyle"
+  }
+}

tflint/config.go

@@ -20,7 +20,9 @@ import (
 )
 
 var defaultConfigFile = ".tflint.hcl"
+var defaultConfigFileJSON = ".tflint.json"
 var fallbackConfigFile = "~/.tflint.hcl"
+var fallbackConfigFileJSON = "~/.tflint.json"
 
 var configSchema = &hcl.BodySchema{
 	Blocks: []hcl.BlockHeaderSchema{
@@ -136,11 +138,15 @@ func EmptyConfig() *Config {
 //
 // 1. file passed by the --config option
 // 2. file set by the TFLINT_CONFIG_FILE environment variable
-// 3. current directory (./.tflint.hcl)
-// 4. home directory (~/.tflint.hcl)
+// 3. current directory ./.tflint.hcl
+// 4. current directory ./.tflint.json
+// 5. home directory ~/.tflint.hcl
+// 6. home directory ~/.tflint.json
 //
-// For 1 and 2, if the file does not exist, an error will be returned immediately.
-// If 3 fails, fallback to 4, and If it fails, an empty configuration is returned.
+// Files are parsed as HCL or JSON based on their file extension.
+// JSON files use HCL-compatible JSON syntax, following Terraform's .tf.json conventions.
+// For steps 1-2, if the file does not exist, an error will be returned immediately.
+// For steps 3-6, each step is tried in order until a file is found or all fail.
 //
 // It also automatically enables bundled plugin if the "terraform"
 // plugin block is not explicitly declared.
@@ -174,7 +180,7 @@ func LoadConfig(fs afero.Afero, file string) (*Config, error) {
 		return cfg.enableBundledPlugin(), nil
 	}
 
-	// Load the default config file
+	// Load the default config file (prefer .hcl over .json)
 	log.Printf("[INFO] Load config: %s", defaultConfigFile)
 	if f, err := fs.Open(defaultConfigFile); err == nil {
 		cfg, err := loadConfig(f)
@@ -185,7 +191,18 @@ func LoadConfig(fs afero.Afero, file string) (*Config, error) {
 	}
 	log.Printf("[INFO] file not found")
 
-	// Load the fallback config file
+	// Try JSON config file if HCL not found
+	log.Printf("[INFO] Load config: %s", defaultConfigFileJSON)
+	if f, err := fs.Open(defaultConfigFileJSON); err == nil {
+		cfg, err := loadConfig(f)
+		if err != nil {
+			return nil, err
+		}
+		return cfg.enableBundledPlugin(), nil
+	}
+	log.Printf("[INFO] file not found")
+
+	// Load the fallback config file (prefer .hcl over .json)
 	fallback, err := homedir.Expand(fallbackConfigFile)
 	if err != nil {
 		return nil, err
@@ -200,6 +217,21 @@ func LoadConfig(fs afero.Afero, file string) (*Config, error) {
 	}
 	log.Printf("[INFO] file not found")
 
+	// Try JSON fallback config file if HCL not found
+	fallbackJSON, err := homedir.Expand(fallbackConfigFileJSON)
+	if err != nil {
+		return nil, err
+	}
+	log.Printf("[INFO] Load config: %s", fallbackJSON)
+	if f, err := fs.Open(fallbackJSON); err == nil {
+		cfg, err := loadConfig(f)
+		if err != nil {
+			return nil, err
+		}
+		return cfg.enableBundledPlugin(), nil
+	}
+	log.Printf("[INFO] file not found")
+
 	// Use the default config
 	log.Print("[INFO] Use default config")
 	return EmptyConfig().enableBundledPlugin(), nil
@@ -212,7 +244,17 @@ func loadConfig(file afero.File) (*Config, error) {
 	}
 
 	parser := hclparse.NewParser()
-	f, diags := parser.ParseHCL(src, file.Name())
+	var f *hcl.File
+	var diags hcl.Diagnostics
+
+	// Parse based on file extension
+	switch {
+	case strings.HasSuffix(strings.ToLower(file.Name()), ".json"):
+		f, diags = parser.ParseJSON(src, file.Name())
+	default:
+		f, diags = parser.ParseHCL(src, file.Name())
+	}
+
 	if diags.HasErrors() {
 		return nil, diags
 	}