Feature/assign dot notation

[osteele]

Checklist

• I have read the contribution guidelines.
• make test passes.
• make lint passes.
• New and changed code is covered by tests.
• Performance improvements include benchmarks.
• Changes match the documented (not just the implemented) behavior of Shopify. -- yes, because new behavior is hidden behind a flag; also, new behavior matches the implemented behavior of Jekyll (which should probably be added to this list)

Description

This PR adds support for Jekyll-compatible dot notation in the {% assign %} tag, allowing nested property assignment like {% assign page.canonical_url = "/about/" %}. This feature is gated behind the JekyllExtensions configuration flag to maintain backward compatibility.

Changes

Core Implementation

• Extended assign tag syntax: Added support for dot notation in assign statements (e.g., obj.property = value)
• Grammar updates: Modified the expression parser grammar to support property paths in assignments
• Context API enhancement: Added SetPath() method to the render context for setting nested values
• Backward compatibility: Feature is only enabled when JekyllExtensions flag is set to true

Parser Changes

• Updated expressions.y grammar to parse dot notation in assign targets
• Modified Assignment struct to include both Variable (for simple assignments) and Path (for dot notation)
• Regenerated parser files using ragel/goyacc

Safety & Validation

• Returns clear error message when dot notation is used without Jekyll extensions enabled
• Automatically creates intermediate objects when setting nested properties
• Prevents setting properties on non-object types with appropriate error handling

Testing

Comprehensive test coverage has been added in TestAssignTag_JekyllExtensions:

• ✅ Dot notation works correctly with Jekyll extensions enabled
• ✅ Dot notation properly fails with clear error message when Jekyll extensions are disabled
• ✅ Simple assignments continue to work in both modes for backward compatibility
• ✅ Nested object creation works as expected (e.g., creating new_obj.prop when new_obj doesn't exist)
• ✅ All existing tests continue to pass

Examples

With Jekyll extensions enabled:

{% assign page.canonical_url = "/about/" %}
{% assign page.meta.description = "Test description" %}
{% assign new_obj.nested.deep = "value" %}

Without Jekyll extensions (standard mode):

{% assign simple_var = "value" %}  // ✅ Works
{% assign obj.prop = "value" %}    // ❌ Error: requires Jekyll extensions

Breaking Changes

None - the feature is opt-in via the JekyllExtensions configuration flag.

[Copilot]

Pull Request Overview

This PR adds Jekyll-compatible dot notation support for assign tags, allowing syntax like {% assign page.canonical_url = "/about/" %} when Jekyll extensions are enabled. This feature is hidden behind a configuration flag to maintain backward compatibility with standard Shopify Liquid.

Key changes:

• Added JekyllExtensions configuration option to control dot notation support
• Modified assign tag parser to handle nested property paths
• Added SetPath method to render context for setting nested object properties

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
engine.go	Added EnableJekyllExtensions() method to configure Jekyll compatibility
render/config.go	Added JekyllExtensions boolean field to Config struct
render/context.go	Implemented SetPath method for setting values at nested object paths
expressions/statements.go	Extended Assignment struct to support property paths alongside simple variables
expressions/expressions.y	Modified grammar to parse dot notation in assign targets
expressions/y.go	Generated parser code reflecting grammar changes
tags/standard_tags.go	Refactored assign tag to use configuration-aware factory function
tags/standard_tags_test.go	Added comprehensive tests for Jekyll extensions functionality
README.md	Added documentation for Jekyll compatibility features
Multiple test files	Updated function calls to match new AddStandardTags signature

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