checker: fix c error on improper string to rune cast (#13197) (#13257)

vlib/v/checker/checker.v

@@ -2732,6 +2732,11 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			node.pos)
 	}
 
+	if to_sym.kind == .rune && from_sym.is_string() {
+		snexpr := node.expr.str()
+		c.error('cannot cast `$from_sym.name` to rune, use `${snexpr}.runes()` instead.', node.pos)
+	}
+
 	if to_type == ast.string_type {
 		if from_type in [ast.byte_type, ast.bool_type] {
 			snexpr := node.expr.str()

vlib/v/checker/tests/cast_string_to_rune_err.out

@@ -0,0 +1,26 @@
+vlib/v/checker/tests/cast_string_to_rune_err.vv:2:7: error: cannot cast `string` to rune, use `'0'.runes()` instead.
+    1 | fn main() {
+    2 |     _ := rune('0')
+      |          ~~~~~~~~~
+    3 |     _ := rune('012345')
+    4 |     _ := rune('v')
+vlib/v/checker/tests/cast_string_to_rune_err.vv:3:7: error: cannot cast `string` to rune, use `'012345'.runes()` instead.
+    1 | fn main() {
+    2 |     _ := rune('0')
+    3 |     _ := rune('012345')
+      |          ~~~~~~~~~~~~~~
+    4 |     _ := rune('v')
+    5 |     _ := rune('abcd')
+vlib/v/checker/tests/cast_string_to_rune_err.vv:4:7: error: cannot cast `string` to rune, use `'v'.runes()` instead.
+    2 |     _ := rune('0')
+    3 |     _ := rune('012345')
+    4 |     _ := rune('v')
+      |          ~~~~~~~~~
+    5 |     _ := rune('abcd')
+    6 | }
+vlib/v/checker/tests/cast_string_to_rune_err.vv:5:7: error: cannot cast `string` to rune, use `'abcd'.runes()` instead.
+    3 |     _ := rune('012345')
+    4 |     _ := rune('v')
+    5 |     _ := rune('abcd')
+      |          ~~~~~~~~~~~~
+    6 | }

vlib/v/checker/tests/cast_string_to_rune_err.vv

@@ -0,0 +1,6 @@
+fn main() {
+	_ := rune('0')
+	_ := rune('012345')
+	_ := rune('v')
+	_ := rune('abcd')
+}