Add new terraform_deprecated_interpolation rule

docs/rules/README.md

@@ -69,6 +69,7 @@ These rules suggest to better ways.
 | --- | --- |
 |[terraform_dash_in_resource_name](terraform_dash_in_resource_name.md)||
 |[terraform_dash_in_output_name](terraform_dash_in_output_name.md)||
+|[terraform_deprecated_interpolation](terraform_deprecated_interpolation.md)|✔|
 |[terraform_documented_outputs](terraform_documented_outputs.md)||
 |[terraform_documented_variables](terraform_documented_variables.md)||
 |[terraform_module_pinned_source](terraform_module_pinned_source.md)|✔|

docs/rules/terraform_deprecated_interpolation.md

@@ -0,0 +1,38 @@
+# terraform_deprecated_interpolation
+
+Disallow deprecated (0.11-style) interpolation
+
+## Example
+
+```hcl
+resource "aws_instance" "deprecated" {
+    instance_type = "${var.type}"
+}
+
+resource "aws_instance" "new" {
+    instance_type = var.type
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Warning: Interpolation-only expressions are deprecated in Terraform v0.12.14 (terraform_deprecated_interpolation)
+
+  on example.tf line 2:
+   2:     instance_type = "${var.type}"
+
+Reference: https://github.com/terraform-linters/tflint/blob/v0.14.0/docs/rules/terraform_deprecated_interpolation.md
+
+```
+
+## Why
+
+Terraform v0.12 introduces a new interpolation syntax, but continues to support the old 0.11-style interpolation syntax for compatibility.
+
+Terraform will now warn at planning/applying due to its policy to make old syntax errors in the next major release (v0.13). TFLint emits an issue instead of a warning with the same logic.
+
+## How To Fix
+
+Switch to the new interpolation syntax. See the release notes for Terraform 0.12.14 for details: https://github.com/hashicorp/terraform/releases/tag/v0.12.14

integration/variables/result.json

@@ -15,7 +15,7 @@
         },
         "end": {
           "line": 28,
-          "column": 35
+          "column": 30
         }
       },
       "callers": []
@@ -35,7 +35,7 @@
         },
         "end": {
           "line": 32,
-          "column": 47
+          "column": 42
         }
       },
       "callers": []
@@ -55,7 +55,7 @@
         },
         "end": {
           "line": 36,
-          "column": 44
+          "column": 39
         }
       },
       "callers": []
@@ -75,7 +75,7 @@
         },
         "end": {
           "line": 40,
-          "column": 39
+          "column": 34
         }
       },
       "callers": []
@@ -95,7 +95,7 @@
         },
         "end": {
           "line": 44,
-          "column": 31
+          "column": 26
         }
       },
       "callers": []

integration/variables/template.tf

@@ -21,25 +21,25 @@ variable "var" {
 }
 
 resource "aws_instance" "unknown" {
-  instance_type = "${var.unknown}"
+  instance_type = var.unknown
 }
 
 resource "aws_instance" "default" {
-  instance_type = "${var.default}"
+  instance_type = var.default
 }
 
 resource "aws_instance" "default_values_file" {
-  instance_type = "${var.default_values_file}"
+  instance_type = var.default_values_file
 }
 
 resource "aws_instance" "auto_values_file" {
-  instance_type = "${var.auto_values_file}"
+  instance_type = var.auto_values_file
 }
 
 resource "aws_instance" "values_file" {
-  instance_type = "${var.values_file}"
+  instance_type = var.values_file
 }
 
 resource "aws_instance" "var" {
-  instance_type = "${var.var}"
+  instance_type = var.var
 }

rules/provider.go

@@ -37,6 +37,7 @@ var manualDefaultRules = []Rule{
 	awsrules.NewAwsSpotFleetRequestInvalidExcessCapacityTerminationPolicyRule(),
 	terraformrules.NewTerraformDashInResourceNameRule(),
 	terraformrules.NewTerraformDashInOutputNameRule(),
+	terraformrules.NewTerraformDeprecatedInterpolationRule(),
 	terraformrules.NewTerraformDocumentedOutputsRule(),
 	terraformrules.NewTerraformDocumentedVariablesRule(),
 	terraformrules.NewTerraformModulePinnedSourceRule(),

rules/terraformrules/terraform_deprecated_interpolation.go

@@ -0,0 +1,84 @@
+package terraformrules
+
+import (
+	"log"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/terraform-linters/tflint/tflint"
+)
+
+// TerraformDeprecatedInterpolationRule warns of deprecated interpolation in Terraform v0.11 or earlier.
+type TerraformDeprecatedInterpolationRule struct{}
+
+// NewTerraformDeprecatedInterpolationRule return a new rule
+func NewTerraformDeprecatedInterpolationRule() *TerraformDeprecatedInterpolationRule {
+	return &TerraformDeprecatedInterpolationRule{}
+}
+
+// Name returns the rule name
+func (r *TerraformDeprecatedInterpolationRule) Name() string {
+	return "terraform_deprecated_interpolation"
+}
+
+// Enabled returns whether the rule is enabled by default
+func (r *TerraformDeprecatedInterpolationRule) Enabled() bool {
+	return true
+}
+
+// Severity returns the rule severity
+func (r *TerraformDeprecatedInterpolationRule) Severity() string {
+	return tflint.WARNING
+}
+
+// Link returns the rule reference link
+func (r *TerraformDeprecatedInterpolationRule) Link() string {
+	return tflint.ReferenceLink(r.Name())
+}
+
+// Check emits issues on the deprecated interpolation syntax.
+// This logic is equivalent to the warning logic implemented in Terraform.
+// See https://github.com/hashicorp/terraform/pull/23348
+func (r *TerraformDeprecatedInterpolationRule) Check(runner *tflint.Runner) error {
+	log.Printf("[TRACE] Check `%s` rule for `%s` runner", r.Name(), runner.TFConfigPath())
+
+	for _, resource := range runner.TFConfig.Module.ManagedResources {
+		r.checkForDeprecatedInterpolationsInBody(runner, resource.Config)
+	}
+	for _, provider := range runner.TFConfig.Module.ProviderConfigs {
+		r.checkForDeprecatedInterpolationsInBody(runner, provider.Config)
+	}
+
+	return nil
+}
+
+func (r *TerraformDeprecatedInterpolationRule) checkForDeprecatedInterpolationsInBody(runner *tflint.Runner, body hcl.Body) {
+	nativeBody, ok := body.(*hclsyntax.Body)
+	if !ok {
+		return
+	}
+
+	for _, attr := range nativeBody.Attributes {
+		r.checkForDeprecatedInterpolationsInExpr(runner, attr.Expr)
+	}
+
+	for _, block := range nativeBody.Blocks {
+		r.checkForDeprecatedInterpolationsInBody(runner, block.Body)
+	}
+
+	return
+}
+
+func (r *TerraformDeprecatedInterpolationRule) checkForDeprecatedInterpolationsInExpr(runner *tflint.Runner, expr hcl.Expression) {
+	if _, ok := expr.(*hclsyntax.TemplateWrapExpr); !ok {
+		return
+	}
+
+	runner.EmitIssue(
+		r,
+		"Interpolation-only expressions are deprecated in Terraform v0.12.14",
+		expr.Range(),
+	)
+
+	return
+}

rules/terraformrules/terraform_deprecated_interpolation_test.go

@@ -0,0 +1,109 @@
+package terraformrules
+
+import (
+	"testing"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/terraform-linters/tflint/tflint"
+)
+
+func Test_TerraformDeprecatedInterpolationRule(t *testing.T) {
+	cases := []struct {
+		Name     string
+		Content  string
+		Expected tflint.Issues
+	}{
+		{
+			Name: "deprecated single interpolation",
+			Content: `
+resource "null_resource" "a" {
+	triggers = "${var.triggers}"
+}`,
+			Expected: tflint.Issues{
+				{
+					Rule:    NewTerraformDeprecatedInterpolationRule(),
+					Message: "Interpolation-only expressions are deprecated in Terraform v0.12.14",
+					Range: hcl.Range{
+						Filename: "config.tf",
+						Start:    hcl.Pos{Line: 3, Column: 13},
+						End:      hcl.Pos{Line: 3, Column: 30},
+					},
+				},
+			},
+		},
+		{
+			Name: "deprecated single interpolation in provider block",
+			Content: `
+provider "null" {
+	foo = "${var.triggers["foo"]}"
+}`,
+			Expected: tflint.Issues{
+				{
+					Rule:    NewTerraformDeprecatedInterpolationRule(),
+					Message: "Interpolation-only expressions are deprecated in Terraform v0.12.14",
+					Range: hcl.Range{
+						Filename: "config.tf",
+						Start:    hcl.Pos{Line: 3, Column: 8},
+						End:      hcl.Pos{Line: 3, Column: 32},
+					},
+				},
+			},
+		},
+		{
+			Name: "deprecated single interpolation in nested block",
+			Content: `
+resource "null_resource" "a" {
+	provisioner "local-exec" {
+		single = "${var.triggers["greeting"]}"
+	}
+}`,
+			Expected: tflint.Issues{
+				{
+					Rule:    NewTerraformDeprecatedInterpolationRule(),
+					Message: "Interpolation-only expressions are deprecated in Terraform v0.12.14",
+					Range: hcl.Range{
+						Filename: "config.tf",
+						Start:    hcl.Pos{Line: 4, Column: 12},
+						End:      hcl.Pos{Line: 4, Column: 41},
+					},
+				},
+			},
+		},
+		{
+			Name: "interpolation as template",
+			Content: `
+resource "null_resource" "a" {
+	triggers = "${var.triggers} "
+}`,
+			Expected: tflint.Issues{},
+		},
+		{
+			Name: "interpolation in array",
+			Content: `
+resource "null_resource" "a" {
+	triggers = ["${var.triggers}"]
+}`,
+			Expected: tflint.Issues{},
+		},
+		{
+			Name: "new interpolation syntax",
+			Content: `
+resource "null_resource" "a" {
+	triggers = var.triggers
+}`,
+			Expected: tflint.Issues{},
+		},
+	}
+
+	rule := NewTerraformDeprecatedInterpolationRule()
+
+	for _, tc := range cases {
+		runner := tflint.TestRunner(t, map[string]string{"config.tf": tc.Content})
+
+		if err := rule.Check(runner); err != nil {
+			t.Fatalf("Unexpected error occurred: %s", err)
+		}
+
+		tflint.AssertIssues(t, tc.Expected, runner.Issues)
+	}
+}