Allow backticks to be quoted in string literals (#820)

* allow backticks to be quoted in string literals

* remove redundant empty string

parser/lexer/lexer.go

@@ -242,15 +242,47 @@ func (l *Lexer) scanString(quote rune) (n int) {
 }
 
 func (l *Lexer) scanRawString(quote rune) (n int) {
-	ch := l.next() // read character after back tick
-	for ch != quote {
-		if ch == eof {
+	var escapedQuotes int
+loop:
+	for {
+		ch := l.next()
+		for ch == quote && l.peek() == quote {
+			// skip current and next char which are the quote escape sequence
+			l.next()
+			ch = l.next()
+			escapedQuotes++
+		}
+		switch ch {
+		case quote:
+			break loop
+		case eof:
 			l.error("literal not terminated")
 			return
 		}
-		ch = l.next()
 		n++
 	}
-	l.emitValue(String, l.source.String()[l.start.byte+1:l.end.byte-1])
+	str := l.source.String()[l.start.byte+1 : l.end.byte-1]
+
+	// handle simple case where no quoted backtick was found, then no allocation
+	// is needed for the new string
+	if escapedQuotes == 0 {
+		l.emitValue(String, str)
+		return
+	}
+
+	var b strings.Builder
+	var skipped bool
+	b.Grow(len(str) - escapedQuotes)
+	for _, r := range str {
+		if r == quote {
+			if !skipped {
+				skipped = true
+				continue
+			}
+			skipped = false
+		}
+		b.WriteRune(r)
+	}
+	l.emitValue(String, b.String())
 	return
 }

parser/lexer/lexer_test.go

@@ -68,6 +68,22 @@ func TestLex(t *testing.T) {
 				{Kind: EOF},
 			},
 		},
+		{
+			"`escaped backticks` `` `a``b` ```` `a``` ```b` ```a````b``` ```````` ```a````` `````b```",
+			[]Token{
+				{Kind: String, Value: "escaped backticks"},
+				{Kind: String, Value: ""},
+				{Kind: String, Value: "a`b"},
+				{Kind: String, Value: "`"},
+				{Kind: String, Value: "a`"},
+				{Kind: String, Value: "`b"},
+				{Kind: String, Value: "`a``b`"},
+				{Kind: String, Value: "```"},
+				{Kind: String, Value: "`a``"},
+				{Kind: String, Value: "``b`"},
+				{Kind: EOF},
+			},
+		},
 		{
 			"a and orb().val #.",
 			[]Token{
@@ -332,6 +348,26 @@ literal not terminated (1:10)
  | id "hello
  | .........^
 
+id ` + "`" + `hello
+literal not terminated (1:10)
+ | id ` + "`" + `hello
+ | .........^
+
+id ` + "`" + `hello` + "``" + `
+literal not terminated (1:12)
+ | id ` + "`" + `hello` + "``" + `
+ | ...........^
+
+id ` + "```" + `hello
+literal not terminated (1:12)
+ | id ` + "```" + `hello
+ | ...........^
+
+id ` + "`" + `hello` + "``" + ` world
+literal not terminated (1:18)
+ | id ` + "`" + `hello` + "``" + ` world
+ | .................^
+
 früh ♥︎
 unrecognized character: U+2665 '♥' (1:6)
  | früh ♥︎