Introduce new inline snippets tag

Inline snippets will reduce code duplication and
improve the developer experience, eliminating the
need for one-off snippet files

lib/liquid/locales/en.yml

@@ -5,6 +5,7 @@
       block_tag_unexpected_args: "Syntax Error in '%{tag}' - Valid syntax: {% %{tag} %}{% end%{tag} %}"
       assign: "Syntax Error in 'assign' - Valid syntax: assign [var] = [source]"
       capture: "Syntax Error in 'capture' - Valid syntax: capture [var]"
+      snippet: "Syntax Error in 'snippet' - Valid syntax: snippet [quoted string]"
       case: "Syntax Error in 'case' - Valid syntax: case [condition]"
       case_invalid_when: "Syntax Error in tag 'case' - Valid when condition: {% when [condition] [or condition2...] %}"
       case_invalid_else: "Syntax Error in tag 'case' - Valid else condition: {% else %} (no parameters) "

lib/liquid/tags.rb

@@ -20,6 +20,7 @@ require_relative "tags/raw"
 require_relative "tags/render"
 require_relative "tags/cycle"
 require_relative "tags/doc"
+require_relative "tags/snippet"
 
 module Liquid
   module Tags
@@ -44,6 +45,7 @@ module Liquid
       'echo' => Echo,
       'tablerow' => TableRow,
       'doc' => Doc,
+      'snippet' => Snippet,
     }.freeze
   end
 end

lib/liquid/tags/render.rb

@@ -51,6 +51,24 @@ module Liquid
       template_name = @template_name_expr
       raise ::ArgumentError unless template_name.is_a?(String)
 
+      # Inline snippets take precedence over external snippets
+      if (inline_snippet = context.registers[:inline_snippet][template_name])
+        inner_context = context.new_isolated_subcontext
+
+        snippet_body = inline_snippet[:body]
+        snippet_args = inline_snippet[:args]
+        # Validate and set the arguments in the inner context
+        @attributes.each do |key, value|
+          unless snippet_args.include?(key)
+            raise Liquid::ArgumentError, "Invalid argument `#{key}` for snippet `#{template_name}`"
+          end
+
+          inner_context[key] = context.evaluate(value)
+        end
+
+        return output << snippet_body.render(inner_context)
+      end
+
       partial = PartialCache.load(
         template_name,
         context: context,

lib/liquid/tags/snippet.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module Liquid
+  # @liquid_public_docs
+  # @liquid_type tag
+  # @liquid_category theme
+  # @liquid_name snippet
+  # @liquid_summary
+  #   Creates a new inline snippet using a string value as the identifier.
+  # @liquid_description
+  #   You can create inline snippets to make your Liquid code more modular.
+  # @liquid_syntax
+  #   {% snippet "input" %}
+  #     value
+  #   {% endsnippet %}
+  class Snippet < Block
+    SYNTAX = /(#{QuotedString})(?:\s*\|\s*([\w\s,]+)\s*\|)?/o
+    def initialize(tag_name, markup, options)
+      super
+
+      if markup =~ SYNTAX
+        @to = Regexp.last_match(1)
+        args = Regexp.last_match(2)
+
+        @args = args ? args.split(/\s*,\s*/) : []
+      else
+        raise SyntaxError, options[:locale].t("errors.syntax.snippet")
+      end
+    end
+
+    def render(context)
+      context.registers[:inline_snippet] ||= {}
+      context.registers[:inline_snippet][snippet_id] = {
+        body: snippet_body,
+        args: @args,
+      }
+      ''
+    end
+
+    private
+
+    def snippet_id
+      @to[1, @to.size - 2]
+    end
+
+    def snippet_body
+      body = @body
+      body
+    end
+  end
+end