Add InstanceVariables linter (#174)

Hello! I would like to add a linter to detect instance variables in Slim
templates. Replacing them with local variables is often encouraged in
Rails partial templates, but here I expect using the `include`
configuration option to define the targeted files so this linter can be
used together with not only partial templates but any glob pattern you
like.

The linter itself uses the work done on the Rubocop linter - namely the
Ruby code extractor. This linter parses the extracted ruby code and
searches for instance variables in there. It also uses the extracted
code source map to report proper line numbers.


![image](https://github.com/sds/slim-lint/assets/462701/6deb617c-5847-4087-8b96-7f3879e6958f)

This closes #155. Thanks!

.gitignore

@@ -2,3 +2,4 @@ Gemfile.lock
 /coverage
 /pkg
 .ruby-version
+/.vscode

config/default.yml

@@ -32,6 +32,12 @@ linters:
     enabled: false
     max: 300
 
+  InstanceVariables:
+    enabled: false
+    include:
+      # Include only partial templates by default
+      - app/views/**/_*.html.slim
+
   LineLength:
     enabled: true
     max: 80

lib/slim_lint/linter/README.md

@@ -9,6 +9,7 @@ Below is a list of linters supported by `slim-lint`, ordered alphabetically.
 * [EmptyControlStatement](#emptycontrolstatement)
 * [EmptyLines](#emptylines)
 * [FileLength](#filelength)
+* [InstanceVariables](#instancevariables)
 * [LineLength](#linelength)
 * [RedundantDiv](#redundantdiv)
 * [RuboCop](#rubocop)
@@ -157,6 +158,44 @@ linters:
 
 Long files are harder to read and usually indicative of complexity.
 
+## InstanceVariables
+
+Reports instance variables in Slim templates. Use the `include` configuration
+option to narrow down the files to e.g. only partial view templates in Rails:
+
+
+```yaml
+linters:
+  InstanceVariables:
+    enabled: true
+    include:
+      - app/views/**/_*.html.slim
+```
+
+**Bad for the above configuration**
+
+In `app/views/somewhere/_partial.html.slim`:
+
+```slim
+= @hello
+```
+**Good for the above configuration**
+
+In `app/views/somewhere/show.html.slim`:
+
+```slim
+= render 'partial', hello: @hello
+```
+In `app/views/somewhere/_partial.html.slim`:
+
+```slim
+= hello
+```
+
+The linter allows ensuring only local variables and/or helper methods are used
+in the configured set of Slim templates. This is often encouraged in Rails
+partial templates.
+
 ## LineLength
 
 Option | Description

lib/slim_lint/linter/instance_variables.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+module SlimLint
+  # Searches for instance variables in partial or other templates.
+  class Linter::InstanceVariables < Linter
+    include LinterRegistry
+
+    on_start do |_sexp|
+      processed_sexp = SlimLint::RubyExtractEngine.new.call(document.source)
+
+      extractor = SlimLint::RubyExtractor.new
+      extracted_source = extractor.extract(processed_sexp)
+      next if extracted_source.source.empty?
+
+      parsed_ruby = parse_ruby(extracted_source.source)
+      next unless parsed_ruby
+
+      report_instance_variables(parsed_ruby, extracted_source.source_map)
+    end
+
+    private
+
+    def report_instance_variables(parsed_ruby, source_map)
+      parsed_ruby.each_node do |node|
+        next unless node.ivar_type?
+
+        dummy_node = Struct.new(:line)
+        report_lint(dummy_node.new(source_map[node.loc.line]),
+                    'Avoid instance variables in the configured ' \
+                    "view templates (found `#{node.source}`)")
+      end
+    end
+  end
+end

spec/slim_lint/linter/instance_variables_spec.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe SlimLint::Linter::InstanceVariables do
+  include_context 'linter'
+
+  context 'with no instance variables' do
+    let(:slim) { <<-SLIM }
+      h1 Hello world!
+    SLIM
+
+    it { should_not report_lint }
+  end
+
+  context 'with instance variables in static contexts only' do
+    let(:slim) { <<-SLIM }
+      - if "@harmless_variable"
+        = "@another_harmless_variable"
+      p @and_another
+      ' \#{"@and_another"}
+    SLIM
+
+    it { should_not report_lint }
+  end
+
+  context 'with instance variables in control code' do
+    let(:slim) { <<-SLIM }
+      - if @world
+        ' hello world
+      - call(arg: @hello)
+    SLIM
+
+    it { should report_lint line: 1 }
+    it { should report_lint line: 3 }
+  end
+
+  context 'with instance variables in output code' do
+    let(:slim) { <<-SLIM }
+      p
+        ' hello
+        = @world
+      == call(arg: @hello)
+      span = @some_variable
+    SLIM
+
+    it { should report_lint line: 3 }
+    it { should report_lint line: 4 }
+    it { should report_lint line: 5 }
+  end
+
+  context 'with instance variables in interpolated code' do
+    let(:slim) { <<-SLIM }
+      ' \#{@hello}
+    SLIM
+
+    it { should report_lint line: 1 }
+  end
+end