Add warnings support to render output

[osteele]

Summary

Add a mechanism for the liquid engine to report warnings (non-fatal issues) alongside rendered output. This enables callers to detect and report problematic template patterns without failing the build.

Motivation

The immediate use case is dot-notation in assign tags ({% assign page.canonical_url = page.url %}). Ruby Jekyll/Liquid accepts this syntax without error, but it's effectively a no-op — the value is stored under the literal key "page.canonical_url" in assigns, but {{ page.canonical_url }} resolves by looking up page then .canonical_url on the page drop, so the assigned value is never accessible.

Currently, the liquid engine either:

• Errors on this syntax (when JekyllExtensions is disabled) — breaking compatibility with Jekyll sites that use it
• Silently accepts it (when JekyllExtensions is enabled) — hiding broken code

Neither is ideal. A warnings mechanism would let callers accept the syntax for compatibility while informing users their code doesn't do what they expect.

Proposed approach

Add a ParseAndRenderDetailed method (or similar) that returns a result struct including warnings:

type RenderResult struct {
    Output   []byte
    Warnings []Warning
}

type Warning struct {
    Loc     SourceLoc
    Message string
}

The existing ParseAndRender signature stays unchanged (discards warnings) for backward compatibility.

For the dot-notation assign case specifically:

• When JekyllExtensions is disabled: treat as a warning (not an error), execute as a no-op
• The warning message should explain that the syntax has no effect, e.g.: "{% assign page.custom_var = ... %} has no effect. Dot notation in assign creates an inaccessible variable."

Why return values rather than a callback

• Fits naturally with callers that collect results before reporting (e.g., gojekyll collects errors across all pages before reporting)
• Enables strict mode: render first, then check if len(warnings) > 0 && strict { return error }
• Cleaner API — pure data in, data out, no threading concerns
• No breaking changes to existing API

Related

• Feature request: object attribute assign gojekyll#60 — original feature request for dot-notation assign
• feat: enable Jekyll-compatible dot notation in assign tags gojekyll#120 — PR that enables EnableJekyllExtensions() (to be superseded by this approach)