Highlight Engine Cleanup

Status: complete

The cleanup originally planned in this document has been carried out and then superseded by the unified grammar catalog refactor (see GrammarRegistry in crates/fresh-editor/src/primitives/grammar/types.rs).

What the module looks like now

HighlightEngine in crates/fresh-editor/src/primitives/highlight_engine.rs exposes exactly three public constructors:

• from_entry(&GrammarEntry, &GrammarRegistry) -> Self — the canonical path. Picks syntect if the entry has a syntect index, else tree-sitter if it has a Language, else HighlightEngine::None. This is the single place the "prefer syntect, fall back to tree-sitter" fallback lives.
• for_file(path, registry) -> Self — thin wrapper: registry.find_by_path + from_entry.
• for_syntax_name(name, registry) -> Self — thin wrapper: find_by_name + from_entry.

DetectedLanguage::from_entry consumes the same catalog entries, so the highlighter and the per-buffer language state stay in sync through one type.

What's gone

• HighlighterPreference enum (the Auto/TextMate/TreeSitter dead preference system).
• for_language() constructor.
• for_file_with_languages, for_file_with_preference, for_file_with_languages_and_preference — collapsed into for_file.
• Duplicated textmate_for_file helpers.
• Double detection of ts_language: the catalog entry carries it, so Language::from_path runs at most once per resolution.

How to add a language now

Extend the catalog, not the engine. See GrammarRegistry::rebuild_catalog and fresh_languages::Language::extensions for the shape. Plugin authors use GrammarRegistry::with_additional_grammars.