feat: add new rule to check comparisons with empty list

docs/rules/README.md

@@ -11,6 +11,7 @@ Below is a list of available rules.
 |[terraform_deprecated_interpolation](terraform_deprecated_interpolation.md)|Disallow deprecated (0.11-style) interpolation|✔|
 |[terraform_documented_outputs](terraform_documented_outputs.md)|Disallow `output` declarations without description||
 |[terraform_documented_variables](terraform_documented_variables.md)|Disallow `variable` declarations without description||
+|[terraform_empty_list_check](terraform_empty_list_check.md)|Disallow comparisons with `[]` when checking if a list is empty||
 |[terraform_module_pinned_source](terraform_module_pinned_source.md)|Disallow specifying a git or mercurial repository as a module source without pinning to a version|✔|
 |[terraform_module_version](terraform_module_version.md)|Checks that Terraform modules sourced from a registry specify a version|✔|
 |[terraform_naming_convention](terraform_naming_convention.md)|Enforces naming conventions for resources, data sources, etc||
@@ -21,3 +22,4 @@ Below is a list of available rules.
 |[terraform_unused_declarations](terraform_unused_declarations.md)|Disallow variables, data sources, and locals that are declared but never used||
 |[terraform_unused_required_providers](terraform_unused_required_providers.md)|Check that all `required_providers` are used in the module||
 |[terraform_workspace_remote](terraform_workspace_remote.md)|`terraform.workspace` should not be used with a "remote" backend with remote execution|✔|
+

docs/rules/terraform_empty_list_check.md

@@ -0,0 +1,36 @@
+# terraform_empty_list_check
+
+Disallow comparisons with `[]` when checking if a list is empty.
+
+## Example
+
+```hcl
+variable "my_list" {
+	type = list(string)
+}
+resource "aws_db_instance" "mysql" {
+	count = var.my_list == [] ? 0 : 1
+    instance_class = "m4.2xlarge"
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Warning: List is compared with [] instead of checking if length is 0. (terraform_empty_list_check)
+
+  on test.tf line 5:
+   5:   count = var.my_list == [] ? 0 : 1
+
+Reference: https://github.com/terraform-linters/tflint/blob/master/docs/rules/terraform_empty_list_check.md
+ 
+```
+
+## Why
+
+The `==` operator can only return true when the two operands have identical types, and the type of `[]` alone (without any further type conversions) is an empty tuple rather than a list of objects, strings, numbers or any other type. Therefore, a comparison with a single `[]` with the goal of checking if a list is empty, will always return false.
+
+## How To Fix
+
+Check if a list is empty by checking its length instead. For example: `length(var.my_list) == 0`.

rules/provider.go

@@ -21,6 +21,7 @@ var DefaultRules = []Rule{
 	terraformrules.NewTerraformDeprecatedInterpolationRule(),
 	terraformrules.NewTerraformDocumentedOutputsRule(),
 	terraformrules.NewTerraformDocumentedVariablesRule(),
+	terraformrules.NewTerraformEmptyListCheckRule(),
 	terraformrules.NewTerraformModulePinnedSourceRule(),
 	terraformrules.NewTerraformModuleVersionRule(),
 	terraformrules.NewTerraformNamingConventionRule(),

rules/terraformrules/terraform_empty_list_check.go

@@ -0,0 +1,88 @@
+package terraformrules
+
+import (
+	"log"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/terraform-linters/tflint/tflint"
+)
+
+// TerraformEmptyListCheckRule checks whether is there a comparison with an empty list
+type TerraformEmptyListCheckRule struct{}
+
+// NewTerraformCommentSyntaxRule returns a new rule
+func NewTerraformEmptyListCheckRule() *TerraformEmptyListCheckRule {
+	return &TerraformEmptyListCheckRule{}
+}
+
+// Name returns the rule name
+func (r *TerraformEmptyListCheckRule) Name() string {
+	return "terraform_empty_list_check"
+}
+
+// Enabled returns whether the rule is enabled by default
+func (r *TerraformEmptyListCheckRule) Enabled() bool {
+	return true
+}
+
+// Severity returns the rule severity
+func (r *TerraformEmptyListCheckRule) Severity() tflint.Severity {
+	return tflint.WARNING
+}
+
+// Link returns the rule reference link
+func (r *TerraformEmptyListCheckRule) Link() string {
+	return tflint.ReferenceLink(r.Name())
+}
+
+// Check checks whether the list is being compared with static empty list
+func (r *TerraformEmptyListCheckRule) Check(runner *tflint.Runner) error {
+	if !runner.TFConfig.Path.IsRoot() {
+		// This rule does not evaluate child modules.
+		return nil
+	}
+
+	log.Printf("[TRACE] Check `%s` rule for `%s` runner", r.Name(), runner.TFConfigPath())
+
+	files := make(map[string]*struct{})
+	for _, variable := range runner.TFConfig.Module.Variables {
+		files[variable.DeclRange.Filename] = nil
+	}
+
+	for filename := range files {
+		if err := r.checkEmptyList(runner, filename); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (r *TerraformEmptyListCheckRule) checkEmptyList(runner *tflint.Runner, filename string) error {
+	return runner.WalkExpressions(func(expr hcl.Expression) error {
+		if conditionalExpr, ok := expr.(*hclsyntax.ConditionalExpr); ok {
+			if binaryOpExpr, ok := conditionalExpr.Condition.(*hclsyntax.BinaryOpExpr); ok {
+				if binaryOpExpr.Op.Type.FriendlyName() == "bool" {
+					if right, ok := binaryOpExpr.RHS.(*hclsyntax.TupleConsExpr); ok {
+						checkEmptyList(right, runner, r, binaryOpExpr)
+					}
+					if left, ok := binaryOpExpr.LHS.(*hclsyntax.TupleConsExpr); ok {
+						checkEmptyList(left, runner, r, binaryOpExpr)
+					}
+				}
+			}
+		}
+		return nil
+	})
+}
+
+func checkEmptyList(tupleConsExpr *hclsyntax.TupleConsExpr, runner *tflint.Runner, r *TerraformEmptyListCheckRule, binaryOpExpr *hclsyntax.BinaryOpExpr) {
+	if len(tupleConsExpr.Exprs) == 0 {
+		runner.EmitIssue(
+			r,
+			"List is compared with [] instead of checking if length is 0.",
+			binaryOpExpr.Range(),
+		)
+	}
+}

rules/terraformrules/terraform_empty_list_check_test.go

@@ -0,0 +1,85 @@
+package terraformrules
+
+import (
+	"testing"
+
+	hcl "github.com/hashicorp/hcl/v2"
+	"github.com/terraform-linters/tflint/tflint"
+)
+
+func Test_TerraformEmptyListCheckRule(t *testing.T) {
+	cases := []struct {
+		Name     string
+		Content  string
+		Expected tflint.Issues
+	}{
+		{
+			Name: "comparing with [] is not recommended",
+			Content: `
+variable "my_list" {
+	type = list(string)
+}
+resource "aws_db_instance" "mysql" {
+	count = var.my_list == [] ? 0 : 1
+    instance_class = "m4.2xlarge"
+}`,
+			Expected: tflint.Issues{
+				{
+					Rule:    NewTerraformEmptyListCheckRule(),
+					Message: "List is compared with [] instead of checking if length is 0.",
+					Range: hcl.Range{
+						Filename: "resource.tf",
+						Start:    hcl.Pos{Line: 6, Column: 10},
+						End:      hcl.Pos{Line: 6, Column: 27},
+					},
+				},
+			},
+		},
+		{
+			Name: "negatively comparing with [] is not recommended",
+			Content: `
+variable "my_list" {
+	type = list(string)
+}
+resource "aws_db_instance" "mysql" {
+	count = var.my_list != [] ? 1 : 0
+    instance_class = "m4.2xlarge"
+}`,
+			Expected: tflint.Issues{
+				{
+					Rule:    NewTerraformEmptyListCheckRule(),
+					Message: "List is compared with [] instead of checking if length is 0.",
+					Range: hcl.Range{
+						Filename: "resource.tf",
+						Start:    hcl.Pos{Line: 6, Column: 10},
+						End:      hcl.Pos{Line: 6, Column: 27},
+					},
+				},
+			},
+		},
+		{
+			Name: "checking if length is 0 is recommended",
+			Content: `
+variable "my_list" {
+	type = list(string)
+}
+resource "aws_db_instance" "mysql" {
+	count = length(var.my_list) == 0 ? 1 : 0
+	instance_class = "m4.2xlarge"
+}`,
+			Expected: tflint.Issues{},
+		},
+	}
+
+	rule := NewTerraformEmptyListCheckRule()
+
+	for _, tc := range cases {
+		runner := tflint.TestRunner(t, map[string]string{"resource.tf": tc.Content})
+
+		if err := rule.Check(runner); err != nil {
+			t.Fatalf("Unexpected error occurred: %s", err)
+		}
+
+		tflint.AssertIssues(t, tc.Expected, runner.Issues)
+	}
+}

test.tf

@@ -0,0 +1,7 @@
+variable "my_list" {
+	type = list(string)
+}
+resource "aws_db_instance" "mysql" {
+	count = var.my_list == [] ? 0 : 1
+    instance_class = "m4.2xlarge"
+}