Extending selectors from other modules using @use

[vikigenius]

I am going through the codebase and looking at how the extend algorithm works

dart-sass/lib/src/extend/extension_store.dart

Lines 226 to 276 in 89a5394

	void addExtension(
	SelectorList extender, SimpleSelector target, ExtendRule extend,
	[List<CssMediaQuery>? mediaContext]) {
	var selectors = _selectors[target];
	var existingExtensions = _extensionsByExtender[target];
	Map<ComplexSelector, Extension>? newExtensions;
	var sources = _extensions.putIfAbsent(target, () => {});
	for (var complex in extender.components) {
	if (complex.isUseless) continue;
	var extension = Extension(complex, target, extend.span,
	mediaContext: mediaContext, optional: extend.isOptional);
	if (sources[complex] case var existingExtension?) {
	// If there's already an extend from [extender] to [target], we don't need
	// to re-run the extension. We may need to mark the extension as
	// mandatory, though.
	sources[complex] = MergedExtension.merge(existingExtension, extension);
	continue;
	}
	sources[complex] = extension;
	for (var simple in _simpleSelectors(complex)) {
	_extensionsByExtender.putIfAbsent(simple, () => []).add(extension);
	// Only source specificity for the original selector is relevant.
	// Selectors generated by `@extend` don't get new specificity.
	_sourceSpecificity.putIfAbsent(simple, () => complex.specificity);
	}
	if (selectors != null || existingExtensions != null) {
	newExtensions ??= {};
	newExtensions[complex] = extension;
	}
	}
	if (newExtensions == null) return;
	var newExtensionsByTarget = {target: newExtensions};
	if (existingExtensions != null) {
	var additionalExtensions =
	_extendExistingExtensions(existingExtensions, newExtensionsByTarget);
	if (additionalExtensions != null) {
	mapAddAll2(newExtensionsByTarget, additionalExtensions);
	}
	}
	if (selectors != null) {
	_extendExistingSelectors(selectors, newExtensionsByTarget);
	}
	}

It's not very clear to me how this handles extending selectors from other modules.

If someone can point me in the right direction it would be very helpful

[nex3]

Cross-module extensions are handled just before the CSS for the final stylesheet is combined and produced, in the AsyncEvaluator._extendModules() function.