terraform_deprecated_index: improve perf for files with many expres…

…sions (#49)

rules/terraform_deprecated_index.go

@@ -48,20 +48,16 @@ func (r *TerraformDeprecatedIndexRule) Check(runner tflint.Runner) error {
 		return nil
 	}
 
-	diags := runner.WalkExpressions(tflint.ExprWalkFunc(func(expr hcl.Expression) hcl.Diagnostics {
-		filename := expr.Range().Filename
-		file, err := runner.GetFile(filename)
-		if err != nil {
-			return hcl.Diagnostics{
-				{
-					Severity: hcl.DiagError,
-					Summary:  "failed to call GetFile()",
-					Detail:   err.Error(),
-				},
-			}
-		}
+	files, err := runner.GetFiles()
+	if err != nil {
+		return err
+	}
 
+	diags := runner.WalkExpressions(tflint.ExprWalkFunc(func(expr hcl.Expression) hcl.Diagnostics {
 		for _, variable := range expr.Variables() {
+			filename := expr.Range().Filename
+			file := files[filename]
+
 			bytes := expr.Range().SliceBytes(file.Bytes)
 
 			tokens, diags := hclsyntax.LexExpression(bytes, filename, variable.SourceRange().Start)