Add Strict locals missing linter (#175)

And here I propose another linter that we would love to use in our
project. It checks for [Strict
locals](https://guides.rubyonrails.org/action_view_overview.html#strict-locals)
magic line and reports when the magic line is missing from the template.
Strict locals are encouraged by some in Rails partials as they allow to
define an explicit "API" for the templates showing clearly which
variables should be passed into the template when rendering it vs. which
ones are purely local for the template. They also allow to define
default values. I wrote a little
[post](https://dev.to/nejremeslnici/strict-locals-in-slim-haml-partials-in-rails-2f73)
about them recently. Thanks for your considering this!


![image](https://github.com/sds/slim-lint/assets/462701/c34333ec-4dbe-42f4-8e34-fabf879ecc69)


![image](https://github.com/sds/slim-lint/assets/462701/96c491ee-4ccd-44fe-8bcb-4227770af5c5)

config/default.yml

@@ -89,6 +89,12 @@ linters:
       - Style/WhileUntilDo
       - Style/WhileUntilModifier
 
+  StrictLocalsMissing:
+    enabled: false
+    include:
+      # Include only Rails partial templates by default
+      - app/views/**/_*.html.slim
+
   Tab:
     enabled: true
 

lib/slim_lint/linter/README.md

@@ -13,6 +13,7 @@ Below is a list of linters supported by `slim-lint`, ordered alphabetically.
 * [LineLength](#linelength)
 * [RedundantDiv](#redundantdiv)
 * [RuboCop](#rubocop)
+* [StrictLocalsMissing](#strictlocalsmissing)
 * [Tab](#tab)
 * [TagCase](#tagcase)
 * [TrailingBlankLines](#trailingblanklines)
@@ -281,6 +282,40 @@ AllCops:
   DisplayCopNames: true
 ```
 
+## StrictLocalsMissing
+
+Reports on missing [strict locals magic comment](https://guides.rubyonrails.org/action_view_overview.html#strict-locals)
+in Slim templates. Use the `include` configuration option to narrow down
+the files to e.g. only partial view templates in Rails:
+
+```yaml
+linters:
+  StrictLocalsMissing:
+    enabled: true
+    include:
+      - app/views/**/_*.html.slim
+```
+
+**Bad for the above configuration**
+
+In `app/views/somewhere/_partial.html.slim`:
+
+```slim
+= some_helper(foo, bar)
+```
+**Good for the above configuration**
+
+In `app/views/somewhere/_partial.html.slim`:
+
+```slim
+/# locals: (foo:, bar: 'default')
+= some_helper(foo, bar)
+```
+
+By default, Rails partial templates accept any local variables.
+Strict locals, on the other hand, help define an explicit interface
+for the template that shows which local variables it accepts.
+
 ## Tab
 
 Reports detection of tabs used for indentation.

lib/slim_lint/linter/strict_locals_missing.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module SlimLint
+  # Reports on missing strict locals magic line in Slim templates.
+  class Linter::StrictLocalsMissing < Linter
+    include LinterRegistry
+
+    on_start do |_sexp|
+      unless document.source =~ %r{/#\s+locals:\s+\(.*\)}
+        dummy_node = Struct.new(:line)
+        report_lint(dummy_node.new(1), 'Strict locals magic line is missing')
+      end
+    end
+  end
+end

spec/slim_lint/linter/strict_locals_missing_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe SlimLint::Linter::StrictLocalsMissing do
+  include_context 'linter'
+
+  context 'when magic line defines strict locals' do
+    let(:slim) { '/# locals: (foo:, bar:)' }
+
+    it { should_not report_lint }
+  end
+
+  context 'when magic line defines empty strict locals' do
+    let(:slim) { '/# locals: ()' }
+
+    it { should_not report_lint }
+  end
+
+  context 'when magic line is not at the top of the file' do
+    let(:slim) { <<-SLIM }
+      h1 Hello
+      = call(foo, bar)
+      /# locals: (foo:, bar:)
+    SLIM
+
+    it { should_not report_lint }
+  end
+
+  context 'when template has no strict locals definition' do
+    let(:slim) { 'h1 hello' }
+
+    it { should report_lint }
+  end
+end