checker: check cast to reference struct (fix #15590) (#15601)

vlib/v/checker/checker.v

@@ -2430,6 +2430,16 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			ft := c.table.type_to_str(from_type)
 			c.error('cannot cast `$ft` to struct', node.pos)
 		}
+	} else if to_sym.kind == .struct_ && to_type.is_ptr() {
+		if from_sym.kind == .alias {
+			from_type = (from_sym.info as ast.Alias).parent_type.derive_add_muls(from_type)
+		}
+		if !from_type.is_int() && final_from_sym.kind != .enum_ && !from_type.is_pointer()
+			&& !from_type.is_ptr() {
+			ft := c.table.type_to_str(from_type)
+			tt := c.table.type_to_str(to_type)
+			c.error('cannot cast `$ft` to `$tt`', node.pos)
+		}
 	} else if to_sym.kind == .interface_ {
 		if c.type_implements(from_type, to_type, node.pos) {
 			if !from_type.is_ptr() && !from_type.is_pointer() && from_sym.kind != .interface_

vlib/v/checker/tests/cast_to_ref_struct_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/cast_to_ref_struct_err.vv:9:16: error: cannot cast `string` to `&SomeError`
+    7 |
+    8 | fn main() {
+    9 |     a := unsafe { &SomeError('yo') }
+      |                   ~~~~~~~~~~~~~~~~
+   10 |     println(a)
+   11 | }

vlib/v/checker/tests/cast_to_ref_struct_err.vv

@@ -0,0 +1,11 @@
+module main
+
+pub struct SomeError {
+mut:
+	msg string
+}
+
+fn main() {
+	a := unsafe { &SomeError('yo') }
+	println(a)
+}