lexer: Fix empty string lit in middle of line

Fix empty string lit in middle of line. As we advance at the beginning
of reading string and then again when peekRune is `"` we overshoot for
empty strings `""`. Fix this be doing the outer advance only at the end
of the iteration.

Author: juliaogris

pkg/lexer/lexer.go

@@ -193,7 +193,6 @@ func (l *Lexer) readString() (string, error) {
 	pos := l.pos
 	backslashCnt := 0
 	for {
-		l.advance()
 		if l.cur == '\\' {
 			backslashCnt++
 		} else {
@@ -207,6 +206,7 @@ func (l *Lexer) readString() (string, error) {
 		if pr == 0 || pr == '\n' {
 			break // error case
 		}
+		l.advance()
 	}
 	s := string(l.input[pos : l.pos+1])
 	r, err := strconv.Unquote(s)

pkg/lexer/lexer_test.go

@@ -353,3 +353,19 @@ func TestString(t *testing.T) {
 		})
 	}
 }
+
+func TestStringLit(t *testing.T) {
+	in := `fn "" 0`
+	wantTokens := []*Token{
+		{Type: IDENT, Literal: "fn", Offset: 0, Line: 1, Col: 1},
+		{Type: WS, Literal: "", Offset: 2, Line: 1, Col: 3},
+		{Type: STRING_LIT, Literal: "", Offset: 3, Line: 1, Col: 4},
+		{Type: WS, Literal: "", Offset: 5, Line: 1, Col: 6},
+		{Type: NUM_LIT, Literal: "0", Offset: 6, Line: 1, Col: 7},
+	}
+	l := New(in)
+	for _, want := range wantTokens {
+		got := l.Next()
+		assert.Equal(t, want, got)
+	}
+}

pkg/parser/parser_test.go

@@ -1190,6 +1190,18 @@ func TestCalledBuiltinFuncs(t *testing.T) {
 	assert.Equal(t, want, got)
 }
 
+func TestEmptyStringLitArg(t *testing.T) {
+	input := `
+fn "" 0
+
+func fn s:string n:num
+    print s n
+end`
+	parser := newParser(input, testBuiltins())
+	parser.Parse()
+	assertNoParseError(t, parser, input)
+}
+
 func TestDemo(t *testing.T) {
 	input := `
 move 10 10