feat: add JSON configuration file support with centralized plugin version checking

- Add support for JSON configuration files (.tflint.json)
- Detect JSON configs by file extension, not filename pattern
- Centralize all plugin version checking logic into plugin/plugin_version.go
- Enforce SDK version 0.23+ for JSON configuration support
- Split version checking: TFLint constraints checked before ApplyGlobalConfig,
  SDK versions checked after to avoid plugin initialization failures
- Add comprehensive unit tests for SDK version validation
- Update documentation to mention JSON configuration support

This allows users to use JSON configuration files with any name when
using the --config flag, while ensuring proper plugin compatibility.

Author: rorychatterton

cmd/inspect.go

@@ -8,15 +8,12 @@ import (
 	"os"
 	"path/filepath"
 
-	"github.com/hashicorp/go-version"
 	"github.com/hashicorp/hcl/v2"
 	"github.com/spf13/afero"
 	"github.com/terraform-linters/tflint-plugin-sdk/hclext"
 	"github.com/terraform-linters/tflint/plugin"
 	"github.com/terraform-linters/tflint/terraform"
 	"github.com/terraform-linters/tflint/tflint"
-	"google.golang.org/grpc/codes"
-	"google.golang.org/grpc/status"
 )
 
 func (cli *CLI) inspect(opts Options) int {
@@ -125,22 +122,10 @@ func (cli *CLI) inspectModule(opts Options, dir string, filterFiles []string) (t
 		return issues, changes, err
 	}
 
-	// Check preconditions
-	sdkVersions := map[string]*version.Version{}
-	for name, ruleset := range rulesetPlugin.RuleSets {
-		sdkVersion, err := ruleset.SDKVersion()
-		if err != nil {
-			if st, ok := status.FromError(err); ok && st.Code() == codes.Unimplemented {
-				// SDKVersion endpoint is available in tflint-plugin-sdk v0.14+.
-				return issues, changes, fmt.Errorf(`Plugin "%s" SDK version is incompatible. Compatible versions: %s`, name, plugin.SDKVersionConstraints)
-			} else {
-				return issues, changes, fmt.Errorf(`Failed to get plugin "%s" SDK version; %w`, name, err)
-			}
-		}
-		if !plugin.SDKVersionConstraints.Check(sdkVersion) {
-			return issues, changes, fmt.Errorf(`Plugin "%s" SDK version (%s) is incompatible. Compatible versions: %s`, name, sdkVersion, plugin.SDKVersionConstraints)
-		}
-		sdkVersions[name] = sdkVersion
+	// Validate and collect plugin versions
+	sdkVersions, err := plugin.ValidatePluginVersions(rulesetPlugin, cli.config.IsJSONConfig())
+	if err != nil {
+		return issues, changes, err
 	}
 
 	// Run inspection
@@ -265,19 +250,20 @@ func launchPlugins(config *tflint.Config, fix bool) (*plugin.Plugin, error) {
 	pluginConf := config.ToPluginConfig()
 	pluginConf.Fix = fix
 
-	// Check version constraints and apply a config to plugins
+	// Apply config to plugins
 	for name, ruleset := range rulesetPlugin.RuleSets {
+		// Check TFLint version constraints before applying config
 		constraints, err := ruleset.VersionConstraints()
 		if err != nil {
-			if st, ok := status.FromError(err); ok && st.Code() == codes.Unimplemented {
+			if plugin.IsVersionConstraintsUnimplemented(err) {
 				// VersionConstraints endpoint is available in tflint-plugin-sdk v0.14+.
-				return rulesetPlugin, fmt.Errorf(`Plugin "%s" SDK version is incompatible. Compatible versions: %s`, name, plugin.SDKVersionConstraints)
-			} else {
-				return rulesetPlugin, fmt.Errorf(`Failed to get TFLint version constraints to "%s" plugin; %w`, name, err)
+				// Plugin is too old
+				return rulesetPlugin, fmt.Errorf(`Plugin "%s" SDK version is incompatible. Compatible versions: %s`, name, plugin.DefaultSDKVersionConstraints)
 			}
+			return rulesetPlugin, fmt.Errorf(`Failed to get TFLint version constraints to "%s" plugin; %w`, name, err)
 		}
-		if !constraints.Check(tflint.Version) {
-			return rulesetPlugin, fmt.Errorf("Failed to satisfy version constraints; tflint-ruleset-%s requires %s, but TFLint version is %s", name, constraints, tflint.Version)
+		if err := plugin.CheckTFLintVersionConstraints(name, constraints); err != nil {
+			return rulesetPlugin, err
 		}
 
 		if err := ruleset.ApplyGlobalConfig(pluginConf); err != nil {

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"
+  }
+}

integrationtest/inspection/inspection_test.go

@@ -1,4 +1,4 @@
-package main
+package inspection
 
 import (
 	"bytes"

langserver/handler.go

@@ -21,8 +21,6 @@ import (
 	"github.com/terraform-linters/tflint/plugin"
 	"github.com/terraform-linters/tflint/terraform"
 	"github.com/terraform-linters/tflint/tflint"
-	"google.golang.org/grpc/codes"
-	"google.golang.org/grpc/status"
 )
 
 // NewHandler returns a new JSON-RPC handler
@@ -48,30 +46,35 @@ func NewHandler(configPath string, cliConfig *tflint.Config) (jsonrpc2.Handler,
 	for name, ruleset := range rulsetPlugin.RuleSets {
 		constraints, err := ruleset.VersionConstraints()
 		if err != nil {
-			if st, ok := status.FromError(err); ok && st.Code() == codes.Unimplemented {
+			if plugin.IsVersionConstraintsUnimplemented(err) {
 				// VersionConstraints endpoint is available in tflint-plugin-sdk v0.14+.
-				return nil, nil, fmt.Errorf(`Plugin "%s" SDK version is incompatible. Compatible versions: %s`, name, plugin.SDKVersionConstraints)
+				return nil, nil, fmt.Errorf(`Plugin "%s" SDK version is incompatible. Compatible versions: %s`, name, plugin.DefaultSDKVersionConstraints)
 			} else {
 				return nil, nil, fmt.Errorf(`Failed to get TFLint version constraints to "%s" plugin; %w`, name, err)
 			}
 		}
-		if !constraints.Check(tflint.Version) {
-			return nil, nil, fmt.Errorf("Failed to satisfy version constraints; tflint-ruleset-%s requires %s, but TFLint version is %s", name, constraints, tflint.Version)
+		if err := plugin.CheckTFLintVersionConstraints(name, constraints); err != nil {
+			return nil, nil, err
 		}
 
-		clientSDKVersions[name], err = ruleset.SDKVersion()
+		sdkVersion, err := ruleset.SDKVersion()
 		if err != nil {
-			if st, ok := status.FromError(err); ok && st.Code() == codes.Unimplemented {
+			if plugin.IsSDKVersionUnimplemented(err) {
 				// SDKVersion endpoint is available in tflint-plugin-sdk v0.14+.
-				return nil, nil, fmt.Errorf(`Plugin "%s" SDK version is incompatible. Compatible versions: %s`, name, plugin.SDKVersionConstraints)
+				// Plugin is too old, treat as nil
+				sdkVersion = nil
 			} else {
 				return nil, nil, fmt.Errorf(`Failed to get plugin "%s" SDK version; %w`, name, err)
 			}
 		}
-		if !plugin.SDKVersionConstraints.Check(clientSDKVersions[name]) {
-			return nil, nil, fmt.Errorf(`Plugin "%s" SDK version (%s) is incompatible. Compatible versions: %s`, name, clientSDKVersions[name], plugin.SDKVersionConstraints)
+
+		// Check if plugin SDK version meets minimum requirements for the config type
+		if err := plugin.CheckSDKVersionSatisfiesConstraints(name, sdkVersion, cfg.IsJSONConfig()); err != nil {
+			return nil, nil, err
 		}
 
+		clientSDKVersions[name] = sdkVersion
+
 		rulesets = append(rulesets, ruleset)
 	}
 	if err := cliConfig.ValidateRules(rulesets...); err != nil {

plugin/plugin.go

@@ -2,7 +2,6 @@ package plugin
 
 import (
 	plugin "github.com/hashicorp/go-plugin"
-	"github.com/hashicorp/go-version"
 	"github.com/terraform-linters/tflint-plugin-sdk/plugin/host2plugin"
 )
 
@@ -13,9 +12,6 @@ var (
 	localPluginRoot = "./.tflint.d/plugins"
 )
 
-// SDKVersionConstraints is the version constraint of the supported SDK version.
-var SDKVersionConstraints = version.MustConstraints(version.NewConstraint(">= 0.16.0"))
-
 // Plugin is an object handling plugins
 // Basically, it is a wrapper for go-plugin and provides an API to handle them collectively.
 type Plugin struct {

plugin/plugin_version.go

@@ -0,0 +1,105 @@
+package plugin
+
+import (
+	"fmt"
+
+	"github.com/hashicorp/go-version"
+	"github.com/terraform-linters/tflint/tflint"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+// Version constraints for plugin compatibility
+var (
+	// DefaultSDKVersionConstraints is the minimum SDK version for basic functionality
+	DefaultSDKVersionConstraints = version.MustConstraints(version.NewConstraint(">= 0.16.0"))
+
+	// JSONConfigSDKVersionConstraints is the minimum SDK version for JSON configuration support
+	JSONConfigSDKVersionConstraints = version.MustConstraints(version.NewConstraint(">= 0.23.0"))
+
+	// EphemeralMarksMinVersion is when ephemeral marks support was added
+	EphemeralMarksMinVersion = version.Must(version.NewVersion("0.22.0"))
+)
+
+// CheckTFLintVersionConstraints validates if TFLint version meets the plugin's requirements
+func CheckTFLintVersionConstraints(pluginName string, constraints version.Constraints) error {
+	if !constraints.Check(tflint.Version) {
+		return fmt.Errorf("Failed to satisfy version constraints; tflint-ruleset-%s requires %s, but TFLint version is %s", pluginName, constraints, tflint.Version)
+	}
+	return nil
+}
+
+// CheckSDKVersionSatisfiesConstraints validates if a plugin's SDK version meets the minimum requirements.
+// For HCL configs, requires SDK >= 0.16.0. For JSON configs, requires SDK >= 0.23.0.
+func CheckSDKVersionSatisfiesConstraints(pluginName string, sdkVersion *version.Version, isJSONConfig bool) error {
+	// If sdkVersion is nil, the plugin doesn't have SDKVersion endpoint (SDK < 0.14)
+	if sdkVersion == nil {
+		return fmt.Errorf(`Plugin "%s" SDK version is incompatible. Compatible versions: %s`, pluginName, DefaultSDKVersionConstraints)
+	}
+
+	constraints := DefaultSDKVersionConstraints
+	if isJSONConfig {
+		constraints = JSONConfigSDKVersionConstraints
+	}
+
+	if !constraints.Check(sdkVersion) {
+		if isJSONConfig {
+			return fmt.Errorf(`Plugin "%s" SDK version (%s) is incompatible with JSON configuration. Minimum required: %s`, pluginName, sdkVersion, JSONConfigSDKVersionConstraints)
+		}
+		return fmt.Errorf(`Plugin "%s" SDK version (%s) is incompatible. Compatible versions: %s`, pluginName, sdkVersion, DefaultSDKVersionConstraints)
+	}
+	return nil
+}
+
+// SupportsEphemeralMarks checks if the plugin SDK version supports ephemeral marks
+func SupportsEphemeralMarks(sdkVersion *version.Version) bool {
+	if sdkVersion == nil {
+		return false
+	}
+	return sdkVersion.GreaterThanOrEqual(EphemeralMarksMinVersion)
+}
+
+// IsSDKVersionUnimplemented checks if an error indicates the SDK version endpoint is not implemented
+func IsSDKVersionUnimplemented(err error) bool {
+	if st, ok := status.FromError(err); ok {
+		return st.Code() == codes.Unimplemented
+	}
+	return false
+}
+
+// IsVersionConstraintsUnimplemented checks if an error indicates the version constraints endpoint is not implemented
+func IsVersionConstraintsUnimplemented(err error) bool {
+	if st, ok := status.FromError(err); ok {
+		return st.Code() == codes.Unimplemented
+	}
+	return false
+}
+
+// ValidatePluginVersions checks plugin SDK version requirements and returns SDK versions for later use
+// Note: TFLint version constraints are checked separately in launchPlugins before ApplyGlobalConfig
+func ValidatePluginVersions(rulesetPlugin *Plugin, isJSONConfig bool) (map[string]*version.Version, error) {
+	sdkVersions := map[string]*version.Version{}
+
+	for name, ruleset := range rulesetPlugin.RuleSets {
+		// Get SDK version
+		sdkVersion, err := ruleset.SDKVersion()
+		if err != nil {
+			if IsSDKVersionUnimplemented(err) {
+				// SDKVersion endpoint is available in tflint-plugin-sdk v0.14+.
+				// Plugin is too old, treat as nil
+				sdkVersion = nil
+			} else {
+				return nil, fmt.Errorf(`Failed to get plugin "%s" SDK version; %w`, name, err)
+			}
+		}
+
+		// Check if SDK version meets minimum requirements
+		if err := CheckSDKVersionSatisfiesConstraints(name, sdkVersion, isJSONConfig); err != nil {
+			return nil, err
+		}
+
+		sdkVersions[name] = sdkVersion
+	}
+
+	return sdkVersions, nil
+}