Refactor var() parsing logic for improved clarity (#66)

## Summary
Refactored the `collect_var_usages` function to simplify the logic for
detecting CSS custom property usages within `var()` functions. The new
implementation is more concise and easier to understand while
maintaining the same functionality.

## Key Changes
- **Simplified var() detection logic**: Replaced nested loops and
conditional checks with early returns, reducing cognitive complexity
- **Direct property access**: Changed from iterating through
`node.children` to directly accessing `first_child` and `next_sibling`
properties
- **Cleaner fallback detection**: Simplified `has_fallback` detection by
checking if `first_sibling` exists rather than iterating through
children
- **Added test coverage**: Added comprehensive tests for `var()` usages
inside `light-dark()` function in both rule test files and utility test
file

## Implementation Details
The refactored code maintains identical behavior:
- Still correctly identifies custom property names (those starting with
`--`)
- Still detects whether a fallback value is present
- Still works with nested functions like `light-dark(var(...),
var(...))`
- Early return pattern improves readability and reduces nesting depth

All existing tests pass, and new tests verify the functionality works
correctly with the `light-dark()` CSS function.

https://claude.ai/code/session_012pu1Bv9AuopQ9ExG87ZSe5

Co-authored-by: Claude <noreply@anthropic.com>

Author: bartveneman

src/rules/no-unknown-custom-property/index.test.ts

@@ -399,6 +399,32 @@ test('should still error when var() uses a property not present in any importFro
 	)
 })
 
+test('should not error when declared properties are used inside light-dark()', async () => {
+	const config = {
+		plugins: [plugin],
+		rules: {
+			[rule_name]: true,
+		},
+	}
+
+	const {
+		results: [{ warnings, errored }],
+	} = await stylelint.lint({
+		code: `
+			:root {
+				--blue-800: blue;
+				--blue-700: navy;
+			}
+			a {
+				color: light-dark(var(--blue-800), var(--blue-700));
+			}`,
+		config,
+	})
+
+	expect(errored).toBe(false)
+	expect(warnings).toStrictEqual([])
+})
+
 test('should still detect unknown custom property when input.css offsets do not match (Svelte embedded CSS)', async () => {
 	const css = 'a { color: var(--undefined); }'
 	const config = {

src/rules/no-unused-custom-properties/index.test.ts

@@ -337,6 +337,32 @@ test('should still error when a declared property is not used in the current fil
 	)
 })
 
+test('should not error when a declared property is used inside light-dark()', async () => {
+	const config = {
+		plugins: [plugin],
+		rules: {
+			[rule_name]: true,
+		},
+	}
+
+	const {
+		results: [{ warnings, errored }],
+	} = await stylelint.lint({
+		code: `
+			:root {
+				--blue-800: blue;
+				--blue-700: navy;
+			}
+			a {
+				color: light-dark(var(--blue-800), var(--blue-700));
+			}`,
+		config,
+	})
+
+	expect(errored).toBe(false)
+	expect(warnings).toStrictEqual([])
+})
+
 test('should still detect unused custom property when input.css offsets do not match (Svelte embedded CSS)', async () => {
 	const css = ':root { --unused: red; }'
 	const config = {

src/utils/custom-properties.test.ts

@@ -105,6 +105,16 @@ test('collect_var_usages: finds var() usages across different selectors', () =>
 	expect(names).toContain('--space-3')
 })
 
+test('collect_var_usages: finds var() usages inside light-dark()', () => {
+	const root = parse('a { color: light-dark(var(--blue-800), var(--blue-700)); }')
+	const usages = collect_var_usages(root)
+	expect(usages.length).toBe(2)
+	const blue_800 = usages.find((u) => u.name === '--blue-800')
+	const blue_700 = usages.find((u) => u.name === '--blue-700')
+	expect(blue_800?.has_fallback).toBe(false)
+	expect(blue_700?.has_fallback).toBe(false)
+})
+
 test('collect_var_usages: finds var() usages even when input.css offsets do not match (Svelte embedded CSS)', () => {
 	// Simulate what happens with Svelte: stylelint extracts CSS from <style>...</style>
 	// but root.source.input.css may contain the full Svelte file while

src/utils/custom-properties.ts

@@ -36,23 +36,16 @@ export function collect_var_usages(root: Root): VarUsage[] {
 		const parsed = parse_declaration(decl_source)
 
 		walk(parsed, (node) => {
-			if (node.type === FUNCTION && node.name === 'var') {
-				let found_name: string | null = null
-				let has_fallback = false
-
-				for (const child of node.children) {
-					if (found_name === null && child.type === IDENTIFIER && child.text.startsWith('--')) {
-						found_name = child.text
-					} else if (found_name !== null) {
-						has_fallback = true
-						break
-					}
-				}
-
-				if (found_name !== null) {
-					usages.push({ name: found_name, has_fallback, node: declaration })
-				}
-			}
+			if (node.type !== FUNCTION || node.name !== 'var') return
+
+			const first = node.first_child
+			if (first === null || first.type !== IDENTIFIER || !first.text.startsWith('--')) return
+
+			usages.push({
+				name: first.text,
+				has_fallback: first.next_sibling !== null,
+				node: declaration,
+			})
 		})
 	})