Add HamlLint::Linter::NoPlainNodes linter

Add a simple haml_lint linter to report all plain nodes.
"Plain nodes" in HAML are scritpless plaintext leaf nodes.

Add haml-lint_todo to onboard new haml linters.
Excludes existing NoPlainNodes lints.

.haml-lint.yml

+inherits_from:
+  - .haml-lint_todo.yml
 # Whether to ignore frontmatter at the beginning of HAML documents for
 # frameworks such as Jekyll/Middleman
 skip_frontmatter: false
 exclude:
   - 'vendor/**/*'
   - 'spec/**/*'
+require:
+  - './lib/haml_lint/linter/no_plain_nodes.rb'
 
 linters:
   AltText:

lib/haml_lint/linter/no_plain_nodes.rb

+# frozen_string_literal: true
+
+require 'active_support/core_ext/array/grouping'
+
+module HamlLint
+  class Linter
+    class NoPlainNodes < Linter
+      include LinterRegistry
+
+      def visit_tag(node)
+        if inline_plain_node?(node)
+          check_inline(node)
+        elsif !node.script.empty?
+          check_script(node)
+        else
+          check(node)
+        end
+      end
+
+      private
+
+      def check(node)
+        text_in_node(node).each { |string| record(node, string) }
+      end
+
+      def check_inline(node)
+        text = inline_text(node)
+        record(node, text) unless text.empty?
+      end
+
+      def check_script(node)
+        text = inline_text(node)
+        record(node, text) unless text.start_with?('=') || text.empty?
+      end
+
+      # Build an array of all strings in child text nodes.
+      # non text nodes are nil, where we'll split the sentences.
+      def text_in_node(node)
+        texts = node.children.map do |child|
+          child.text.strip if text_node?(child)
+        end
+
+        texts.split(nil).map { |sentence| sentence.join(' ') unless sentence.empty? }.compact
+      end
+
+      # Removes a node's attributes and tag from the source code,
+      # returning the inline text of a node.
+      def inline_text(node)
+        text = node.source_code.gsub("%#{node.tag_name}", '')
+
+        attributes = node.attributes_source.map(&:last)
+        attributes.each { |attribute| text = text.gsub(attribute, '') }
+
+        text.strip
+      end
+
+      def record(node, string)
+        record_lint(node, message(string))
+      end
+
+      def message(string)
+        "`#{string}` is a plain node. Please use an i18n method like `#{fixed(string)}`"
+      end
+
+      def fixed(string)
+        "= _('#{string}')"
+      end
+
+      def inline_plain_node?(node)
+        node.children.empty? && node.script.empty?
+      end
+
+      def plain_node?(node)
+        node.is_a?(::HamlLint::Tree::PlainNode)
+      end
+
+      def text_node?(node)
+        return false unless plain_node?(node)
+
+        !node.text.empty?
+      end
+    end
+  end
+end

spec/lib/haml_lint/linter/no_plain_nodes_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'haml_lint'
+require 'haml_lint/spec'
+
+describe HamlLint::Linter::NoPlainNodes do
+  include_context 'linter'
+
+  context 'reports when a tag has an inline plain node' do
+    let(:haml) { '%tag Hello Tanuki' }
+    let(:message) { "`Hello Tanuki` is a plain node. Please use an i18n method like `= _('Hello Tanuki')`" }
+
+    it { is_expected.to report_lint message: message }
+  end
+
+  context 'reports when a tag has multiline plain nodes' do
+    let(:haml) { <<-HAML }
+      %tag
+        Hello
+        Tanuki
+    HAML
+
+    it { is_expected.to report_lint count: 1 }
+  end
+
+  context 'reports when a tag has an inline plain node with interpolation' do
+    let(:haml) { '%tag Hello #{"Tanuki"}!' } # rubocop:disable Lint/InterpolationCheck
+
+    it { is_expected.to report_lint }
+  end
+
+  context 'does not report when a tag has an inline script' do
+    let(:haml) { '%tag= "Hello Tanuki"' }
+
+    it { is_expected.not_to report_lint }
+  end
+
+  context 'does not report when a tag is empty' do
+    let(:haml) { '%tag' }
+
+    it { is_expected.not_to report_lint }
+  end
+
+  context 'reports multiple when a tag has multiline plain nodes split by non-text nodes' do
+    let(:haml) { <<-HAML }
+      %tag
+        Hello
+        .split-node There
+        Tanuki
+    HAML
+
+    it { is_expected.to report_lint count: 3 }
+  end
+end