checker: check struct casting (#5864)

vlib/v/checker/checker.v

@@ -2328,6 +2328,13 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 				to_type_sym.kind == .byte {
 				type_name := c.table.type_to_str(node.expr_type)
 				c.error('cannot cast type `$type_name` to `byte`', node.pos)
+			} else if to_type_sym.kind == .struct_ && !node.typ.is_ptr() && !(to_type_sym.info as table.Struct).is_typedef {
+				from_type_info := from_type_sym.info as table.Struct
+				to_type_info := to_type_sym.info as table.Struct
+				if !c.check_struct_signature(from_type_info, to_type_info) {
+					c.error('cannot convert struct `$from_type_sym.name` to struct `$to_type_sym.name`',
+						node.pos)
+				}
 			}
 			if node.has_arg {
 				c.expr(node.arg)
@@ -3257,6 +3264,34 @@ pub fn (mut c Checker) error(message string, pos token.Position) {
 	c.warn_or_error(msg, pos, false)
 }
 
+// check_struct_signature checks if both structs has the same signature / fields for casting
+fn (c Checker) check_struct_signature(from, to table.Struct) bool {
+	if from.fields.len != to.fields.len {
+		return false
+	}
+	for _, field in from.fields {
+		filtered := to.fields.filter(it.name == field.name)
+		if filtered.len != 1 {
+			// field doesn't exist
+			return false
+		}
+		counterpart := filtered[0]
+		if field.typ != counterpart.typ {
+			// field has different tye
+			return false
+		}
+		if field.is_pub != counterpart.is_pub {
+			// field is not public while the other one is
+			return false
+		}
+		if field.is_mut != counterpart.is_mut {
+			// field is not mutable while the other one is
+			return false
+		}
+	}
+	return true
+}
+
 fn (mut c Checker) warn_or_error(message string, pos token.Position, warn bool) {
 	// add backtrace to issue struct, how?
 	// if c.pref.is_verbose {

vlib/v/checker/tests/struct_cast_to_struct_generic_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_cast_to_struct_generic_err.v:11:11: error: cannot convert struct `Abc<int>` to struct `Xyz`
+    9 | fn main() {
+   10 |     abc := Abc<int>{}
+   11 |     _ := Xyz(abc)
+      |              ~~~
+   12 | }

vlib/v/checker/tests/struct_cast_to_struct_generic_err.vv

@@ -0,0 +1,12 @@
+struct Abc<T> {
+	name T
+}
+
+struct Xyz {
+	name string
+}
+
+fn main() {
+	abc := Abc<int>{}
+	_ := Xyz(abc)
+}

vlib/v/checker/tests/struct_cast_to_struct_mut_err_a.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_cast_to_struct_mut_err_a.v:12:11: error: cannot convert struct `Abc` to struct `Xyz`
+   10 | fn main() {
+   11 |     abc := Abc{}
+   12 |     _ := Xyz(abc)
+      |              ~~~
+   13 | }

vlib/v/checker/tests/struct_cast_to_struct_mut_err_a.vv

@@ -0,0 +1,13 @@
+struct Abc {
+mut:
+	name string
+}
+
+struct Xyz {
+	name string
+}
+
+fn main() {
+	abc := Abc{}
+	_ := Xyz(abc)
+}

vlib/v/checker/tests/struct_cast_to_struct_mut_err_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_cast_to_struct_mut_err_b.v:12:11: error: cannot convert struct `Abc` to struct `Xyz`
+   10 | fn main() {
+   11 |     abc := Abc{}
+   12 |     _ := Xyz(abc)
+      |              ~~~
+   13 | }

vlib/v/checker/tests/struct_cast_to_struct_mut_err_b.vv

@@ -0,0 +1,13 @@
+struct Abc {
+	name string
+}
+
+struct Xyz {
+mut:
+	name string
+}
+
+fn main() {
+	abc := Abc{}
+	_ := Xyz(abc)
+}

vlib/v/checker/tests/struct_cast_to_struct_pub_err_a.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_cast_to_struct_pub_err_a.v:12:11: error: cannot convert struct `Abc` to struct `Xyz`
+   10 | fn main() {
+   11 |     abc := Abc{}
+   12 |     _ := Xyz(abc)
+      |              ~~~
+   13 | }

vlib/v/checker/tests/struct_cast_to_struct_pub_err_a.vv

@@ -0,0 +1,13 @@
+struct Abc {
+pub:
+	name string
+}
+
+struct Xyz {
+	name string
+}
+
+fn main() {
+	abc := Abc{}
+	_ := Xyz(abc)
+}

vlib/v/checker/tests/struct_cast_to_struct_pub_err_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_cast_to_struct_pub_err_b.v:12:11: error: cannot convert struct `Abc` to struct `Xyz`
+   10 | fn main() {
+   11 |     abc := Abc{}
+   12 |     _ := Xyz(abc)
+      |              ~~~
+   13 | }

vlib/v/checker/tests/struct_cast_to_struct_pub_err_b.vv

@@ -0,0 +1,13 @@
+struct Abc {
+	name string
+}
+
+struct Xyz {
+pub:
+	name string
+}
+
+fn main() {
+	abc := Abc{}
+	_ := Xyz(abc)
+}

vlib/v/fmt/tests/struct_init_with_custom_len_keep.vv

@@ -5,7 +5,7 @@ struct Foo {
 
 struct Bar {
 	f &Foo = &Foo(0)
-	d Foo = Foo(0)
+	d Foo = Foo{0}
 }
 
 fn main() {