new rule: deprecated_lookup

docs/rules/README.md

@@ -9,6 +9,7 @@ All rules are enabled by default, but by setting `preset = "recommended"`, you c
 |[terraform_comment_syntax](terraform_comment_syntax.md)|Disallow `//` comments in favor of `#`||
 |[terraform_deprecated_index](terraform_deprecated_index.md)|Disallow legacy dot index syntax|✔|
 |[terraform_deprecated_interpolation](terraform_deprecated_interpolation.md)|Disallow deprecated (0.11-style) interpolation|✔|
+|[terraform_deprecated_lookup](terraform_deprecated_lookup.md)|Disallow deprecated `lookup()` function with only 2 arguments.||
 |[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_equality](terraform_empty_list_equality.md)|Disallow comparisons with `[]` when checking if a collection is empty|✔|

docs/rules/terraform_deprecated_lookup.md

@@ -0,0 +1,50 @@
+# terraform_deprecated_lookup
+
+Disallow deprecated [`lookup` function](https://developer.hashicorp.com/terraform/language/functions/lookup) usage without a default.
+
+## Example
+
+```hcl
+locals {
+  map   = { a = 0 }
+  value = lookup(local.map, "a")
+}
+```
+
+```
+$ tflint
+1 issue(s) found:
+
+Warning: [Fixable] Lookup with 2 arguments is deprecated (terraform_deprecated_lookup)
+
+  on main.tf line 3:
+   3:   value = lookup(local.map, "a")
+
+Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.5.0/docs/rules/terraform_deprecated_lookup.md
+```
+
+## Why
+
+Calling [`lookup`](https://developer.hashicorp.com/terraform/language/functions/lookup) with 2 arguments has been deprecated since Terraform v0.7. `lookup(map, key)` is equivalent to the native index syntax `map[key]`. `lookup` should only be used with the third `default` argument, even though it is optional for backward compatiblity. 
+
+## How To Fix
+
+Use the native index syntax:
+
+Example:
+
+```hcl
+locals {
+  map   = { a = 0 }
+  value = lookup(local.map, "a")
+}
+```
+
+Change this to: 
+
+```hcl
+locals {
+  map   = { a = 0 }
+  value = local.map["a"]
+}
+```

rules/preset.go

@@ -20,6 +20,7 @@ var PresetRules = map[string][]tflint.Rule{
 		NewTerraformUnusedDeclarationsRule(),
 		NewTerraformUnusedRequiredProvidersRule(),
 		NewTerraformWorkspaceRemoteRule(),
+		NewTerraformDeprecatedLookupRule(),
 	},
 	"recommended": {
 		NewTerraformDeprecatedIndexRule(),

rules/terraform_deprecated_lookup.go

@@ -0,0 +1,81 @@
+package rules
+
+import (
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/terraform-linters/tflint-plugin-sdk/tflint"
+	"github.com/terraform-linters/tflint-ruleset-terraform/project"
+)
+
+// TerraformDeprecatedLookupRule warns about usage of the legacy dot syntax for indexes (foo.0)
+type TerraformDeprecatedLookupRule struct {
+	tflint.DefaultRule
+}
+
+// NewTerraformDeprecatedIndexRule return a new rule
+func NewTerraformDeprecatedLookupRule() *TerraformDeprecatedLookupRule {
+	return &TerraformDeprecatedLookupRule{}
+}
+
+// Name returns the rule name
+func (r *TerraformDeprecatedLookupRule) Name() string {
+	return "terraform_deprecated_lookup"
+}
+
+// Enabled returns whether the rule is enabled by default
+func (r *TerraformDeprecatedLookupRule) Enabled() bool {
+	return true
+}
+
+// Severity returns the rule severity
+func (r *TerraformDeprecatedLookupRule) Severity() tflint.Severity {
+	return tflint.WARNING
+}
+
+// Link returns the rule reference link
+func (r *TerraformDeprecatedLookupRule) Link() string {
+	return project.ReferenceLink(r.Name())
+}
+
+// Check walks all expressions and emit issues if deprecated index syntax is found
+func (r *TerraformDeprecatedLookupRule) Check(runner tflint.Runner) error {
+	path, err := runner.GetModulePath()
+	if err != nil {
+		return err
+	}
+	if !path.IsRoot() {
+		// This rule does not evaluate child modules.
+		return nil
+	}
+
+	diags := runner.WalkExpressions(tflint.ExprWalkFunc(func(e hcl.Expression) hcl.Diagnostics {
+		call, ok := e.(*hclsyntax.FunctionCallExpr)
+		if ok && call.Name == "lookup" && len(call.Args) == 2 {
+			if err := runner.EmitIssueWithFix(
+				r,
+				"Lookup with 2 arguments is deprecated",
+				call.Range(),
+				func(f tflint.Fixer) error {
+					return f.ReplaceText(call.Range(), f.TextAt(call.Args[0].Range()), "[", f.TextAt(call.Args[1].Range()), "]")
+				},
+			); err != nil {
+				return hcl.Diagnostics{
+					{
+						Severity: hcl.DiagError,
+						Summary:  "failed to call EmitIssueWithFix()",
+						Detail:   err.Error(),
+					},
+				}
+			}
+			return nil
+		}
+		return nil
+	}))
+	if diags.HasErrors() {
+		return diags
+	}
+
+	return nil
+}
+
+

rules/terraform_deprecated_lookup_test.go

@@ -0,0 +1,118 @@
+package rules
+
+import (
+	"testing"
+
+	"github.com/hashicorp/hcl/v2"
+	"github.com/terraform-linters/tflint-plugin-sdk/helper"
+)
+
+func Test_TerraformDeprecatedLookupRule(t *testing.T) {
+	cases := []struct {
+		Name     string
+		Content  string
+		Expected helper.Issues
+		Fixed    string
+	}{
+		{
+			Name: "deprecated lookup",
+			Content: `
+locals {
+  map   = { a = 0 }
+  value = lookup(local.map, "a")
+}
+`,
+			Expected: helper.Issues{
+				{
+					Rule:    NewTerraformDeprecatedLookupRule(),
+					Message: "Lookup with 2 arguments is deprecated",
+					Range: hcl.Range{
+						Filename: "config.tf",
+						Start: hcl.Pos{
+							Line:   4,
+							Column: 11,
+						},
+						End: hcl.Pos{
+							Line:   4,
+							Column: 33,
+						},
+					},
+				},
+			},
+			Fixed: `
+locals {
+  map   = { a = 0 }
+  value = local.map["a"]
+}
+`,
+		},
+		{
+			Name: "deprecated lookup nested",
+			Content: `
+locals {
+  map   = { a = { b = 0 } }
+  value = lookup(lookup(local.map, "a"), "b")
+}
+`,
+			Expected: helper.Issues{
+				{
+					Rule:    NewTerraformDeprecatedLookupRule(),
+					Message: "Lookup with 2 arguments is deprecated",
+					Range: hcl.Range{
+						Filename: "config.tf",
+						Start: hcl.Pos{
+							Line:   4,
+							Column: 11,
+						},
+						End: hcl.Pos{
+							Line:   4,
+							Column: 46,
+						},
+					},
+				},
+				{
+					Rule:    NewTerraformDeprecatedLookupRule(),
+					Message: "Lookup with 2 arguments is deprecated",
+					Range: hcl.Range{
+						Filename: "config.tf",
+						Start: hcl.Pos{
+							Line:   4,
+							Column: 18,
+						},
+						End: hcl.Pos{
+							Line:   4,
+							Column: 40,
+						},
+					},
+				},
+			},
+			Fixed: `
+locals {
+  map   = { a = { b = 0 } }
+  value = local.map["a"]["b"]
+}
+`,
+		},
+	}
+
+	rule := NewTerraformDeprecatedLookupRule()
+
+	for _, tc := range cases {
+		t.Run(tc.Name, func(t *testing.T) {
+			filename := "config.tf"
+
+			runner := helper.TestRunner(t, map[string]string{filename: tc.Content})
+
+			if err := rule.Check(runner); err != nil {
+				t.Fatalf("Unexpected error occurred: %s", err)
+			}
+
+			helper.AssertIssues(t, tc.Expected, runner.Issues)
+			want := map[string]string{}
+			if tc.Fixed != "" {
+				want[filename] = tc.Fixed
+			}
+			helper.AssertChanges(t, want, runner.Changes())
+		})
+	}
+}