checker: disallow struct{...val.$(field.name)} (#16852)

vlib/v/ast/ast.v

@@ -459,6 +459,7 @@ pub mut:
 	typ                  Type   // the type of this struct
 	update_expr          Expr   // `a` in `...a`
 	update_expr_type     Type
+	update_expr_pos      token.Pos
 	update_expr_comments []Comment
 	is_update_embed      bool
 	has_update_expr      bool // has `...a`

vlib/v/checker/struct.v

@@ -334,8 +334,9 @@ fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 		}
 	}
 	// allow init structs from generic if they're private except the type is from builtin module
-	if !type_sym.is_pub && type_sym.kind != .placeholder && type_sym.language != .c
-		&& (type_sym.mod != c.mod && !(node.typ.has_flag(.generic) && type_sym.mod != 'builtin')) {
+	if !node.has_update_expr && !type_sym.is_pub && type_sym.kind != .placeholder
+		&& type_sym.language != .c && (type_sym.mod != c.mod && !(node.typ.has_flag(.generic)
+		&& type_sym.mod != 'builtin')) {
 		c.error('type `${type_sym.name}` is private', node.pos)
 	}
 	if type_sym.kind == .struct_ {
@@ -591,6 +592,9 @@ fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 	if node.has_update_expr {
 		update_type := c.expr(node.update_expr)
 		node.update_expr_type = update_type
+		if node.update_expr is ast.ComptimeSelector {
+			c.error('cannot use struct update syntax in compile time expressions', node.update_expr_pos)
+		}
 		if c.table.final_sym(update_type).kind != .struct_ {
 			s := c.table.type_to_str(update_type)
 			c.error('expected struct, found `${s}`', node.update_expr.pos())

vlib/v/checker/tests/struct_update_comptime_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_update_comptime_err.vv:18:4: error: cannot use struct update syntax in compile time expressions
+   16 |     $for field in U.fields {
+   17 |         _ = struct {
+   18 |             ...val.$(field.name)
+      |             ~~~
+   19 |         }
+   20 |     }

vlib/v/checker/tests/struct_update_comptime_err.vv

@@ -0,0 +1,32 @@
+struct Aa {
+	sub Bb
+}
+
+struct AaWithAliasType {
+	sub Bb
+}
+
+type AliasType = Bb
+
+struct Bb {
+	a int
+}
+
+fn encode_struct[U](val U) {
+	$for field in U.fields {
+		_ = struct {
+			...val.$(field.name)
+		}
+	}
+}
+
+fn main() {
+	aa := Aa{}
+	bb := Bb{}
+	aa_with_alias_type := AaWithAliasType{}
+
+	encode_struct(aa)
+	encode_struct(aa_with_alias_type)
+
+	encode_struct(bb)
+}

vlib/v/parser/struct.v

@@ -408,6 +408,7 @@ fn (mut p Parser) struct_init(typ_str string, kind ast.StructInitKind) ast.Struc
 	mut update_expr := ast.empty_expr
 	mut update_expr_comments := []ast.Comment{}
 	mut has_update_expr := false
+	mut update_expr_pos := token.Pos{}
 	for p.tok.kind !in [.rcbr, .rpar, .eof] {
 		mut field_name := ''
 		mut expr := ast.empty_expr
@@ -424,6 +425,7 @@ fn (mut p Parser) struct_init(typ_str string, kind ast.StructInitKind) ast.Struc
 			comments = p.eat_comments(same_line: true)
 		} else if is_update_expr {
 			// struct updating syntax; f2 := Foo{ ...f, name: 'f2' }
+			update_expr_pos = p.tok.pos()
 			p.check(.ellipsis)
 			update_expr = p.expr(0)
 			update_expr_comments << p.eat_comments(same_line: true)
@@ -475,6 +477,7 @@ fn (mut p Parser) struct_init(typ_str string, kind ast.StructInitKind) ast.Struc
 		typ: typ
 		fields: fields
 		update_expr: update_expr
+		update_expr_pos: update_expr_pos
 		update_expr_comments: update_expr_comments
 		has_update_expr: has_update_expr
 		name_pos: first_pos