checker: fix missing struct cast validation (fix #23748) (#23788)

vlib/v/checker/checker.v

@@ -3568,12 +3568,16 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 		tt := c.table.type_to_str(to_type)
 		c.error('cannot cast incompatible option ${final_to_sym.name} `${ft}` to `${tt}`',
 			node.pos)
-	}
-
-	if to_sym.kind == .rune && from_sym.is_string() {
+	} else if to_sym.kind == .rune && from_sym.is_string() {
 		snexpr := node.expr.str()
 		ft := c.table.type_to_str(from_type)
 		c.error('cannot cast `${ft}` to rune, use `${snexpr}.runes()` instead.', node.pos)
+	} else if !from_type.is_ptr() && from_type != ast.string_type
+		&& final_from_sym.info is ast.Struct && !final_from_sym.info.is_empty_struct()
+		&& (final_to_sym.is_int() || final_to_sym.is_float()) {
+		ft := c.table.type_to_str(from_type)
+		tt := c.table.type_to_str(to_type)
+		c.error('cannot cast type `${ft}` to `${tt}`', node.pos)
 	}
 
 	if to_sym.kind == .enum && !(c.inside_unsafe || c.file.is_translated) && from_sym.is_int() {

vlib/v/checker/tests/c_struct_cast.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/c_struct_cast.vv:18:7: error: cannot cast type `Vector3` to `int`
+   16 |     }
+   17 |     dump(v)
+   18 |     dump(int(v))
+      |          ~~~~~~
+   19 | }

vlib/v/checker/tests/c_struct_cast.vv

@@ -0,0 +1,19 @@
+module main
+
+pub struct C.Vector3 {
+	x f32
+	y f32
+	z f32
+}
+
+pub type Vector3 = C.Vector3
+
+fn main() {
+	v := Vector3{
+		x: 0.0
+		y: 1.7
+		z: 0.0
+	}
+	dump(v)
+	dump(int(v))
+}