fix #1965: remove css "//" comment error recovery

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## Unreleased
+
+* Remove error recovery for invalid `//` comments in CSS ([#1965](https://github.com/evanw/esbuild/issues/1965))
+
+    Previously esbuild treated `//` as a comment in CSS and generated a warning, even though comments in CSS use `/* ... */` instead. This allowed you to run esbuild on CSS intended for certain CSS preprocessors that support single-line comments.
+
+    However, some people are changing from another build tool to esbuild and have a code base that relies on `//` being preserved even though it's nonsense CSS and causes the entire surrounding rule to be discarded by the browser. Presumably this nonsense CSS ended up there at some point due to an incorrectly-configured build pipeline and the site now relies on that entire rule being discarded. If esbuild interprets `//` as a comment, it could cause the rule to no longer be discarded or even cause something else to happen.
+
+    With this release, esbuild no longer treats `//` as a comment in CSS. It still warns about it but now passes it through unmodified. This means it's no longer possible to run esbuild on CSS code containing single-line comments but it means that esbuild's behavior regarding these nonsensical CSS rules more accurately represents what happens in a browser.
+
 ## 0.14.14
 
 * Fix bug with filename hashes and the `file` loader ([#1957](https://github.com/evanw/esbuild/issues/1957))

internal/css_lexer/css_lexer.go

@@ -107,14 +107,21 @@ func (t T) IsNumeric() bool {
 	return t == TNumber || t == TPercentage || t == TDimension
 }
 
+type TokenFlags uint8
+
+const (
+	IsID TokenFlags = 1 << iota
+	DidWarnAboutSingleLineComment
+)
+
 // This token struct is designed to be memory-efficient. It just references a
 // range in the input file instead of directly containing the substring of text
 // since a range takes up less memory than a string.
 type Token struct {
 	Range      logger.Range // 8 bytes
 	UnitOffset uint16       // 2 bytes
 	Kind       T            // 1 byte
-	IsID       bool         // 1 byte
+	Flags      TokenFlags   // 1 byte
 }
 
 func (token Token) DecodedText(contents string) string {
@@ -159,6 +166,7 @@ type lexer struct {
 	tracker                 logger.LineColumnTracker
 	approximateNewlineCount int
 	current                 int
+	oldSingleLineCommentEnd logger.Loc
 	codePoint               rune
 	Token                   Token
 }
@@ -264,9 +272,19 @@ func (lexer *lexer) next() {
 				lexer.consumeToEndOfMultiLineComment(lexer.Token.Range)
 				continue
 			case '/':
-				lexer.step()
-				lexer.consumeToEndOfSingleLineComment()
-				continue
+				// Warn when people use "//" comments, which are invalid in CSS
+				loc := lexer.Token.Range.Loc
+				if loc.Start >= lexer.oldSingleLineCommentEnd.Start {
+					contents := lexer.source.Contents
+					end := lexer.current
+					for end < len(contents) && !isNewline(rune(contents[end])) {
+						end++
+					}
+					lexer.log.Add(logger.Warning, &lexer.tracker, logger.Range{Loc: loc, Len: 2},
+						"Comments in CSS use \"/* ... */\" instead of \"//\"")
+					lexer.oldSingleLineCommentEnd.Start = int32(end)
+					lexer.Token.Flags |= DidWarnAboutSingleLineComment
+				}
 			}
 			lexer.Token.Kind = TDelimSlash
 
@@ -294,7 +312,7 @@ func (lexer *lexer) next() {
 			if IsNameContinue(lexer.codePoint) || lexer.isValidEscape() {
 				lexer.Token.Kind = THash
 				if lexer.wouldStartIdentifier() {
-					lexer.Token.IsID = true
+					lexer.Token.Flags |= IsID
 				}
 				lexer.consumeName()
 			} else {
@@ -514,13 +532,6 @@ func containsAtPreserveOrAtLicense(text string) bool {
 	return false
 }
 
-func (lexer *lexer) consumeToEndOfSingleLineComment() {
-	for !isNewline(lexer.codePoint) && lexer.codePoint != eof {
-		lexer.step()
-	}
-	lexer.log.Add(logger.Warning, &lexer.tracker, lexer.Token.Range, "Comments in CSS use \"/* ... */\" instead of \"//\"")
-}
-
 func (lexer *lexer) isValidEscape() bool {
 	if lexer.codePoint != '\\' {
 		return false

internal/css_parser/css_parser.go

@@ -117,6 +117,9 @@ func (p *parser) expect(kind css_lexer.T) bool {
 		return true
 	}
 	t := p.current()
+	if (t.Flags & css_lexer.DidWarnAboutSingleLineComment) != 0 {
+		return false
+	}
 
 	var text string
 	var suggestion string
@@ -152,7 +155,7 @@ func (p *parser) expect(kind css_lexer.T) bool {
 }
 
 func (p *parser) unexpected() {
-	if t := p.current(); t.Range.Loc.Start > p.prevError.Start {
+	if t := p.current(); t.Range.Loc.Start > p.prevError.Start && (t.Flags&css_lexer.DidWarnAboutSingleLineComment) == 0 {
 		var text string
 		switch t.Kind {
 		case css_lexer.TEndOfFile, css_lexer.TWhitespace:

internal/css_parser/css_parser_selector.go

@@ -154,7 +154,7 @@ subclassSelectors:
 	for {
 		switch p.current().Kind {
 		case css_lexer.THash:
-			if !p.current().IsID {
+			if (p.current().Flags & css_lexer.IsID) == 0 {
 				break subclassSelectors
 			}
 			name := p.decoded()

internal/css_parser/css_parser_test.go

@@ -93,6 +93,17 @@ func expectPrintedMangleMinify(t *testing.T, contents string, expected string) {
 	})
 }
 
+func TestSingleLineComment(t *testing.T) {
+	expectParseError(t, "a, // a\nb // b\n{}",
+		"<stdin>: WARNING: Comments in CSS use \"/* ... */\" instead of \"//\"\n"+
+			"<stdin>: WARNING: Comments in CSS use \"/* ... */\" instead of \"//\"\n")
+	expectParseError(t, "a, ///// a /////\n{}",
+		"<stdin>: WARNING: Comments in CSS use \"/* ... */\" instead of \"//\"\n")
+
+	expectPrinted(t, "a, // a\nb // b\n{}", "a, // a b // b {\n}\n")
+	expectPrinted(t, "a, ///// a /////\n{}", "a, ///// a ///// {\n}\n")
+}
+
 func TestEscapes(t *testing.T) {
 	// TIdent
 	expectPrinted(t, "a { value: id\\65nt }", "a {\n  value: ident;\n}\n")