scanner: fix escape `\u` (fix #6849) (#7012)

vlib/v/checker/tests/string_escape_u_err_a.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/string_escape_u_err_a.vv:2:15: error: `\u` incomplete unicode character value
+    1 | fn main() {
+    2 |     println('\u')
+      |               ^
+    3 | }

vlib/v/checker/tests/string_escape_u_err_a.vv

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

vlib/v/checker/tests/string_escape_u_err_b.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/string_escape_u_err_b.vv:2:15: error: `\u` incomplete unicode character value
+    1 | fn main() {
+    2 |     println('\u345')
+      |               ^
+    3 | }

vlib/v/checker/tests/string_escape_u_err_b.vv

@@ -0,0 +1,3 @@
+fn main() {
+    println('\u345')
+}

vlib/v/scanner/scanner.v

@@ -1039,11 +1039,19 @@ 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')
+		// Escape `\x` `\u`
+		if prevc == slash && !is_raw && !is_cstr && s.count_symbol_before(s.pos - 2, slash) % 2 == 0 {
+			// Escape `\x`
+			if c == `x` && (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')
+			}
+			// Escape `\u`
+			if c == `u` && (s.text[s.pos + 1] == s.quote ||
+				s.text[s.pos + 2] == s.quote || s.text[s.pos + 3] == s.quote || s.text[s.pos + 4] == s.quote ||
+				!s.text[s.pos + 1].is_hex_digit() || !s.text[s.pos + 2].is_hex_digit() || !s.text[s.pos + 3].is_hex_digit() ||
+				!s.text[s.pos + 4].is_hex_digit()) {
+				s.error(r'`\u` incomplete unicode character value')
+			}
 		}
 		// ${var} (ignore in vfmt mode) (skip \$)
 		if prevc == `$` && c == `{` && !is_raw && s.count_symbol_before(s.pos - 2, slash) % 2 == 0 {