fix #4232: support empty :is() and :where()

Author: evanw

CHANGELOG.md

@@ -6,6 +6,10 @@
 
     Calling `rebuild()` followed by `cancel()` in rapid succession could previously leak memory. The bundler uses a producer/consumer model internally, and the resource leak was caused by the consumer being termianted while there were still remaining unreceived results from a producer. To avoid the leak, the consumer now waits for all producers to finish before terminating.
 
+* Support empty `:is()` and `:where()` syntax in CSS ([#4232](https://github.com/evanw/esbuild/issues/4232))
+
+    Previously using these selectors with esbuild would generate a warning. That warning has been removed in this release for these cases.
+
 * Consider negated bigints to have no side effects
 
     While esbuild currently considers `1`, `-1`, and `1n` to all have no side effects, it didn't previously consider `-1n` to have no side effects. This is because esbuild does constant folding with numbers but not bigints. However, it meant that unused negative bigint constants were not tree-shaken. With this release, esbuild will now consider these expressions to also be side-effect free:

internal/css_parser/css_parser_selector.go

@@ -10,15 +10,22 @@ import (
 )
 
 type parseSelectorOpts struct {
-	composesContext        *composesContext
-	pseudoClassKind        css_ast.PseudoClassKind
-	isDeclarationContext   bool
-	stopOnCloseParen       bool
-	onlyOneComplexSelector bool
-	noLeadingCombinator    bool
+	composesContext         *composesContext
+	pseudoClassKind         css_ast.PseudoClassKind
+	isDeclarationContext    bool
+	stopOnCloseParen        bool
+	onlyOneComplexSelector  bool
+	isForgivingSelectorList bool
+	noLeadingCombinator     bool
 }
 
 func (p *parser) parseSelectorList(opts parseSelectorOpts) (list []css_ast.ComplexSelector, ok bool) {
+	// Potentially parse an empty list for ":is()" and ":where()"
+	if opts.isForgivingSelectorList && opts.stopOnCloseParen && p.current().Kind == css_lexer.TCloseParen {
+		ok = true
+		return
+	}
+
 	// Parse the first selector
 	sel, good := p.parseComplexSelector(parseComplexSelectorOpts{
 		parseSelectorOpts: opts,
@@ -699,9 +706,10 @@ func (p *parser) parsePseudoClassSelector(loc logger.Loc, isElement bool) (css_a
 					oldLocal := p.makeLocalSymbols
 					p.makeLocalSymbols = local
 					selectors, ok := p.parseSelectorList(parseSelectorOpts{
-						pseudoClassKind:        kind,
-						stopOnCloseParen:       true,
-						onlyOneComplexSelector: kind == css_ast.PseudoClassGlobal || kind == css_ast.PseudoClassLocal,
+						pseudoClassKind:         kind,
+						stopOnCloseParen:        true,
+						onlyOneComplexSelector:  kind == css_ast.PseudoClassGlobal || kind == css_ast.PseudoClassLocal,
+						isForgivingSelectorList: kind == css_ast.PseudoClassIs || kind == css_ast.PseudoClassWhere,
 					})
 					p.makeLocalSymbols = oldLocal
 

internal/css_parser/css_parser_test.go

@@ -986,6 +986,13 @@ func TestSelector(t *testing.T) {
 	expectPrinted(t, "a:is( c ) {}", "a:is(c) {\n}\n", "")
 	expectPrinted(t, "a:is( c , d ) {}", "a:is(c, d) {\n}\n", "")
 
+	// Check an empty <forgiving-selector-list> (see https://github.com/evanw/esbuild/issues/4232)
+	expectPrinted(t, ":is() {}", ":is() {\n}\n", "")
+	expectPrinted(t, ":where() {}", ":where() {\n}\n", "")
+	expectPrinted(t, ":not(:is()) {}", ":not(:is()) {\n}\n", "")
+	expectPrinted(t, ":not(:where()) {}", ":not(:where()) {\n}\n", "")
+	expectPrinted(t, ":not() {}", ":not() {\n}\n", "<stdin>: WARNING: Unexpected \")\"\n")
+
 	// These test cases previously caused a hang (see https://github.com/evanw/esbuild/issues/2276)
 	expectPrinted(t, ":x(", ":x() {\n}\n", "<stdin>: WARNING: Unexpected end of file\n")
 	expectPrinted(t, ":x( {}", ":x({}) {\n}\n", "<stdin>: WARNING: Expected \")\" to go with \"(\"\n<stdin>: NOTE: The unbalanced \"(\" is here:\n")