Parser: Introduce two-pass algorithm for matching HTML Elements #795
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This pull request introduces a two-pass algorithm for HTML tag matching to better handle ERB control flow boundaries.
Previously, HTML tags were matched during parsing (single-pass), which caused issues when tags appeared across ERB control flow boundaries:
```erb
<div>
<% if true %>
<h1>Title
<% end %>
</h1>
</div>
```
This would incorrectly report errors or miss mismatched tags because the parser couldn't understand that the `<% if %>` creates a scope boundary.
---
In the first pass during parsing, the parser collects `HTMLOpenTagNode` and `HTMLCloseTagNode` nodes separately without attempting to match them or perform any validation.
In the second pass after the ERB structure has been fully analyzed, the new `herb_parser_match_html_tags_post_analyze()` function matches tags while respecting ERB control flow scope boundaries.
This function recursively processes all ERB control structures including `if`/`elsif`/`else`, `case`/`when`, `begin`/`rescue`/`ensure`, and others, attaching any mismatch errors directly to the relevant stray open or close tag
nodes.
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This was referenced Nov 7, 2025
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This pull request fixes a critical performance regression in the two-pass tag matching algorithm introduced in #795. Documents that previously parsed in ~75ms were taking ~11 seconds (and in some cases didn't even complete in a reasonable timeframe, see #828). The `match_tags_visitor` function was causing double recursion by manually processing array fields through calls to `match_tags_in_node_array()`, but then returning `true` which instructed `herb_visit_node` to also automatically traverse those same children a second time. This created an exponential explosion where each level of nesting squared the number of visits. Resolves #828
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This pull request introduces a two-pass algorithm for HTML tag matching to better handle ERB control flow boundaries and provide more accurate and actionable error messages.
Previously, HTML tags were matched during parsing (single-pass), which caused issues when tags appeared across ERB control flow boundaries. This would incorrectly report errors or miss mismatched tags because the parser couldn't understand that the
<% if %>creates a scope boundary.Now with this pull request:
In the first pass during parsing, the parser collects
HTMLOpenTagNodeandHTMLCloseTagNodenodes separately without attempting to match them or perform any validation, so it doesn't built anyHTMLElementNodeanymore during this pass.In the second pass after the ERB structure has been fully analyzed, the new
herb_parser_match_html_tags_post_analyze()function matches tags while respecting ERB control flow scope boundaries.This function recursively processes all ERB control structures including
if/elsif/else,case/when,begin/rescue/ensure, and others, attaching any mismatch errors directly to the relevant stray open or close tag nodes.Improved HTML Element Matching
Before
After
ERB Control Error Improvements
Before
After