checker: allow `Struct{...expr}` where `expr` is another struct type (#8495)

vlib/v/checker/checker.v

@@ -542,6 +542,7 @@ pub fn (mut c Checker) struct_init(mut struct_init ast.StructInit) table.Type {
 	match type_sym.kind {
 		.placeholder {
 			c.error('unknown struct: $type_sym.name', struct_init.pos)
+			return table.void_type
 		}
 		// string & array are also structs but .kind of string/array
 		.struct_, .string, .array, .alias {
@@ -684,10 +685,17 @@ pub fn (mut c Checker) struct_init(mut struct_init ast.StructInit) table.Type {
 			s := c.table.type_to_str(update_type)
 			c.error('expected struct, found `$s`', struct_init.update_expr.position())
 		} else if update_type != struct_init.typ {
-			sym := c.table.get_type_symbol(struct_init.typ)
-			update_sym := c.table.get_type_symbol(update_type)
-			c.error('expected struct `$sym.name`, found struct `$update_sym.name`', struct_init.update_expr.position())
-		} else if !struct_init.update_expr.is_lvalue() {
+			from_sym := c.table.get_type_symbol(update_type)
+			to_sym := c.table.get_type_symbol(struct_init.typ)
+			from_info := from_sym.info as table.Struct
+			to_info := to_sym.info as table.Struct
+			// TODO this check is too strict
+			if !c.check_struct_signature(from_info, to_info) {
+				c.error('struct `$from_sym.name` is not compatible with struct `$to_sym.name`',
+					struct_init.update_expr.position())
+			}
+		}
+		if !struct_init.update_expr.is_lvalue() {
 			// cgen will repeat `update_expr` for each field
 			// so enforce an lvalue for efficiency
 			c.error('expression is not an lvalue', struct_init.update_expr.position())
@@ -3653,6 +3661,8 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) table.Type {
 		&& !(to_type_sym.info as table.Struct).is_typedef {
 		// For now we ignore C typedef because of `C.Window(C.None)` in vlib/clipboard
 		if from_type_sym.kind == .struct_ && !node.expr_type.is_ptr() {
+			c.warn('casting to struct is deprecated, use e.g. `Struct{...expr}` instead',
+				node.pos)
 			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) {
@@ -5274,9 +5284,10 @@ 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
+// check `to` has all fields of `from`
 fn (c Checker) check_struct_signature(from table.Struct, to table.Struct) bool {
-	if from.fields.len != to.fields.len {
+	// Note: `to` can have extra fields
+	if from.fields.len == 0 {
 		return false
 	}
 	for _, field in from.fields {

vlib/v/checker/tests/cannot_cast_to_struct.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/cannot_cast_to_struct.vv:10:7: error: cannot convert struct `Abc` to struct `Test`
+vlib/v/checker/tests/cannot_cast_to_struct.vv:10:7: error: casting to struct is deprecated, use e.g. `Struct{...expr}` instead
     8 | 
     9 | fn main() {
    10 |     _ := Test(Abc{})

vlib/v/checker/tests/struct_cast_to_struct_generic_err.out

@@ -1,6 +1,6 @@
-vlib/v/checker/tests/struct_cast_to_struct_generic_err.vv:11:7: error: cannot convert struct `Abc<int>` to struct `Xyz`
+vlib/v/checker/tests/struct_cast_to_struct_generic_err.vv:11:7: error: casting to struct is deprecated, use e.g. `Struct{...expr}` instead
     9 | fn main() {
    10 |     abc := Abc<int>{}
    11 |     _ := Xyz(abc)
       |          ~~~~~~~~
-   12 | }
+   12 | }

vlib/v/checker/tests/struct_cast_to_struct_mut_err_a.out

@@ -1,6 +1,6 @@
-vlib/v/checker/tests/struct_cast_to_struct_mut_err_a.vv:12:7: error: cannot convert struct `Abc` to struct `Xyz`
+vlib/v/checker/tests/struct_cast_to_struct_mut_err_a.vv:12:7: error: casting to struct is deprecated, use e.g. `Struct{...expr}` instead
    10 | fn main() {
    11 |     abc := Abc{}
    12 |     _ := Xyz(abc)
       |          ~~~~~~~~
-   13 | }
+   13 | }

vlib/v/checker/tests/struct_cast_to_struct_mut_err_b.out

@@ -1,6 +1,6 @@
-vlib/v/checker/tests/struct_cast_to_struct_mut_err_b.vv:12:7: error: cannot convert struct `Abc` to struct `Xyz`
+vlib/v/checker/tests/struct_cast_to_struct_mut_err_b.vv:12:7: error: casting to struct is deprecated, use e.g. `Struct{...expr}` instead
    10 | fn main() {
    11 |     abc := Abc{}
    12 |     _ := Xyz(abc)
       |          ~~~~~~~~
-   13 | }
+   13 | }

vlib/v/checker/tests/struct_cast_to_struct_pub_err_a.out

@@ -1,6 +1,6 @@
-vlib/v/checker/tests/struct_cast_to_struct_pub_err_a.vv:12:7: error: cannot convert struct `Abc` to struct `Xyz`
+vlib/v/checker/tests/struct_cast_to_struct_pub_err_a.vv:12:7: error: casting to struct is deprecated, use e.g. `Struct{...expr}` instead
    10 | fn main() {
    11 |     abc := Abc{}
    12 |     _ := Xyz(abc)
       |          ~~~~~~~~
-   13 | }
+   13 | }

vlib/v/checker/tests/struct_cast_to_struct_pub_err_b.out

@@ -1,6 +1,6 @@
-vlib/v/checker/tests/struct_cast_to_struct_pub_err_b.vv:12:7: error: cannot convert struct `Abc` to struct `Xyz`
+vlib/v/checker/tests/struct_cast_to_struct_pub_err_b.vv:12:7: error: casting to struct is deprecated, use e.g. `Struct{...expr}` instead
    10 | fn main() {
    11 |     abc := Abc{}
    12 |     _ := Xyz(abc)
       |          ~~~~~~~~
-   13 | }
+   13 | }

vlib/v/checker/tests/struct_init_update_struct_err.out

@@ -1,7 +0,0 @@
-vlib/v/checker/tests/struct_init_update_struct_err.vv:11:6: error: expected struct `Foo`, found struct `Foo2`
-    9 |     f3 := Foo2{}
-   10 |     _ := Foo{
-   11 |         ...f3
-      |            ~~
-   12 |         name: 'f2'
-   13 |     }

vlib/v/checker/tests/struct_init_update_struct_err.vv

@@ -1,15 +0,0 @@
-struct Foo {
-	name string
-	age int
-}
-
-struct Foo2 {}
-
-fn main() {
-	f3 := Foo2{}
-	_ := Foo{
-		...f3
-		name: 'f2'
-	}
-}
-

vlib/v/checker/tests/struct_init_update_type_err.out

@@ -12,7 +12,7 @@ vlib/v/checker/tests/struct_init_update_type_err.vv:16:6: error: expected struct
       |            ^
    17 |     }
    18 |     f2 := Foo2{}
-vlib/v/checker/tests/struct_init_update_type_err.vv:20:6: error: expected struct `Foo`, found struct `Foo2`
+vlib/v/checker/tests/struct_init_update_type_err.vv:20:6: error: struct `Foo2` is not compatible with struct `Foo`
    18 |     f2 := Foo2{}
    19 |     _ = Foo{
    20 |         ...f2
@@ -26,3 +26,10 @@ vlib/v/checker/tests/struct_init_update_type_err.vv:23:6: error: expression is n
       |            ~~~~~
    24 |     }
    25 | }
+vlib/v/checker/tests/struct_init_update_type_err.vv:32:6: error: struct `Empty` is not compatible with struct `Foo`
+   30 |     e := Empty{}
+   31 |     _ = Foo{
+   32 |         ...e
+      |            ^
+   33 |     }
+   34 | }

vlib/v/checker/tests/struct_init_update_type_err.vv

@@ -24,3 +24,12 @@ fn main() {
 	}
 }
 
+struct Empty {}
+
+fn empty() {
+	e := Empty{}
+	_ = Foo{
+		...e
+	}
+}
+

vlib/v/gen/cgen.v

@@ -2570,6 +2570,7 @@ fn (mut g Gen) expr(node ast.Expr) {
 				g.expr_with_cast(node.expr, node.expr_type, node.typ)
 			} else if sym.kind == .struct_ && !node.typ.is_ptr()
 				&& !(sym.info as table.Struct).is_typedef {
+				// deprecated, replaced by Struct{...exr}
 				styp := g.typ(node.typ)
 				g.write('*(($styp *)(&')
 				g.expr(node.expr)

vlib/v/tests/struct_transmute_test.v

@@ -0,0 +1,42 @@
+struct Foo {
+	age int
+	name string
+}
+
+// different order
+struct Bar {
+	name string
+	age int
+}
+
+fn test_order() {
+	f := Foo{
+		age: 4
+		name: 'f'
+	}
+	b := Bar{
+		...f
+	}
+	assert b == Bar{'f',4}
+	b2 := Bar{
+		...f
+		name: 'b2'
+	}
+	assert b2.name == 'b2'
+	assert b2.age == 4
+}
+
+struct Qux {
+	name string
+	age int
+	extra bool
+}
+
+fn test_extra() {
+	f := Foo{4,'f'}
+	q := Qux{
+		...f
+		extra: true
+	}
+	assert q == Qux{'f', 4, true}
+}