Support using rulesets as default values of a mixin parameter

Fix parser error:
> could not understand value for named argument
> in detached-rulesets.less on line 105, column 24

Less.js creates Detached rulesets even for empty arrays.

The NameValue optimization was out of sync with parseRule(), failing
to remove its position from the stack. This meant that when a MixinCall
appears in the input, and the parser tries parseMixinDefinition (which
in turn tries parsePrimary/parseNameValue/parseRule), it would stay
"stuck" inside that parseMixinDefinition attempt instead of restoring
fully back to the top-level parsePrimary and trying a parseMixinCall.

Bug: T353143
Change-Id: Ifca743b916c53c7046bb7e764dcd0c16c0ebfcbb

Author: anny21

lib/Less/Parser.php

@@ -1516,13 +1516,7 @@ private function parseMixinArgs( $isCall ) {
 					// we do not support setting a ruleset as a default variable - it doesn't make sense
 					// However if we do want to add it, there is nothing blocking it, just don't error
 					// and remove isCall dependency below
-					$value = null;
-					if ( $isCall ) {
-						$value = $this->parseDetachedRuleset();
-					}
-					if ( !$value ) {
-						$value = $this->parseExpression();
-					}
+					$value = $this->parseDetachedRuleset() ?? $this->parseExpression();
 
 					if ( !$value ) {
 						if ( $isCall ) {
@@ -1899,12 +1893,9 @@ private function parseBlock() {
 
 	private function parseBlockRuleset() {
 		$block = $this->parseBlock();
-
-		if ( $block ) {
-			$block = new Less_Tree_Ruleset( null, $block );
+		if ( $block !== null ) {
+			return new Less_Tree_Ruleset( null, $block );
 		}
-
-		return $block;
 	}
 
 	/** @return Less_Tree_DetachedRuleset|null */
@@ -1983,7 +1974,7 @@ private function parseNameValue() {
 			if ( $match[3] ) {
 				$match[2] .= $match[3];
 			}
-
+			$this->forget();
 			return new Less_Tree_NameValue( $match[1], $match[2], $index, $this->env->currentFileInfo );
 		}
 

test/phpunit/FixturesTest.php

@@ -30,7 +30,6 @@ class phpunit_FixturesTest extends phpunit_bootstrap {
 			'comments2' => true, // T353131 & T353132
 			'css' => true, // T352911 & T352866
 			'css-guards' => true, // T353144
-			'detached-rulesets' => true, // T353143
 			'import' => true, // T353146
 			'import-interpolation' => true, // TODO
 			'import-reference' => true, // T352862