checker: add check for recursive array init on struct (fix #21195) (#24278)

vlib/v/checker/struct.v

@@ -96,7 +96,7 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 							symfield.is_recursive = true
 						} else {
 							c.error('recursive struct is only possible with optional pointer (e.g. ?&${c.table.type_to_str(field.typ.clear_flag(.option))})',
-								node.pos)
+								field.pos)
 						}
 					}
 				}
@@ -249,6 +249,13 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 						}
 					}
 				}
+				if field.default_expr is ast.ArrayInit {
+					if c.table.final_sym(field.default_expr_typ).kind in [.array, .array_fixed]
+						&& c.table.value_type(field.default_expr_typ) == struct_typ_idx {
+						c.error('cannot initialize array of same struct type that is being defined (recursion detected)',
+							field.pos)
+					}
+				}
 				interface_implemented := sym.kind == .interface
 					&& c.type_implements(field.default_expr_typ, field.typ, field.pos)
 				c.check_expected(field.default_expr_typ, field.typ) or {

vlib/v/checker/tests/invalid_recursive_struct_err.out

@@ -1,5 +1,6 @@
-vlib/v/checker/tests/invalid_recursive_struct_err.vv:1:1: error: recursive struct is only possible with optional pointer (e.g. ?&Node)
+vlib/v/checker/tests/invalid_recursive_struct_err.vv:3:2: error: recursive struct is only possible with optional pointer (e.g. ?&Node)
     1 | struct Node {
-      | ~~~~~~~~~~~
     2 | mut:
     3 |     next ?Node
+      |     ~~~~~~~~~~
+    4 | }

vlib/v/checker/tests/recursive_struct_inited_array_err.out

@@ -0,0 +1,19 @@
+vlib/v/checker/tests/recursive_struct_inited_array_err.vv:10:2: warning: unused variable: `buggy`
+    8 | 
+    9 | fn main() {
+   10 |     buggy := Node{}
+      |     ~~~~~
+   11 |     normal := BetterNode{}
+   12 | }
+vlib/v/checker/tests/recursive_struct_inited_array_err.vv:11:2: warning: unused variable: `normal`
+    9 | fn main() {
+   10 |     buggy := Node{}
+   11 |     normal := BetterNode{}
+      |     ~~~~~~
+   12 | }
+vlib/v/checker/tests/recursive_struct_inited_array_err.vv:2:2: error: cannot initialize array of same struct type that is being defined (recursion detected)
+    1 | struct Node {
+    2 |     next []Node = []Node{len: 1}
+      |     ~~~~~~~~~~~
+    3 | }
+    4 |

vlib/v/checker/tests/recursive_struct_inited_array_err.vv

@@ -0,0 +1,12 @@
+struct Node {
+	next []Node = []Node{len: 1}
+}
+
+struct BetterNode {
+	next []BetterNode
+}
+
+fn main() {
+	buggy := Node{}
+	normal := BetterNode{}
+}