scanner: fix error of backslash escaping x in ident_string(fix #6850) (#6994)

vlib/v/checker/tests/string_escape_x_err_a.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/string_escape_x_err_a.vv:2:15: error: `\x` used with no following hex digits
+    1 | fn main() {
+    2 |     println('\x')
+      |               ^
+    3 | }

vlib/v/checker/tests/string_escape_x_err_a.vv

@@ -0,0 +1,3 @@
+fn main() {
+    println('\x')
+}

vlib/v/checker/tests/string_escape_x_err_b.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/string_escape_x_err_b.vv:2:15: error: `\x` used with no following hex digits
+    1 | fn main() {
+    2 |     println('\xhh')
+      |               ^
+    3 | }

vlib/v/checker/tests/string_escape_x_err_b.vv

@@ -0,0 +1,3 @@
+fn main() {
+    println('\xhh')
+}

vlib/v/scanner/scanner.v

@@ -1039,6 +1039,12 @@ fn (mut s Scanner) ident_string() string {
 				s.error(r'cannot use `\x00` (NULL character) in the string literal')
 			}
 		}
+		// Escape `\x`
+		if prevc == slash &&
+			c == `x` && s.count_symbol_before(s.pos - 2, slash) % 2 == 0 && !is_raw && !is_cstr &&
+			(s.text[s.pos + 1] == s.quote || !s.text[s.pos + 1].is_hex_digit()) {
+			s.error(r'`\x` used with no following hex digits')
+		}
 		// ${var} (ignore in vfmt mode) (skip \$)
 		if prevc == `$` && c == `{` && !is_raw && s.count_symbol_before(s.pos - 2, slash) % 2 == 0 {
 			s.is_inside_string = true

vlib/v/scanner/scanner_test.v

@@ -136,3 +136,8 @@ fn test_ref_ref_array_ref_ref_foo() {
 	assert result[5] == .amp
 	assert result[6] == .name
 }
+
+fn test_escape_string() {
+	assert '\x61' == 'a'
+	assert '\x62' == 'b'
+}