checker: fix stack var outside usage when var is a mutable param (#24249)

Author: felipensp

vlib/v/checker/checker.v

@@ -5594,6 +5594,11 @@ fn (mut c Checker) fail_if_stack_struct_action_outside_unsafe(mut ident ast.Iden
 			sym := c.table.sym(obj.typ.set_nr_muls(0))
 			is_heap := sym.is_heap()
 			if (!is_heap || !obj.typ.is_ptr()) && !c.pref.translated && !c.file.is_translated {
+				is_mut_param := c.table.cur_fn != unsafe { nil }
+					&& c.table.cur_fn.params.filter(it.name == ident.name && it.is_mut).len > 0
+				if is_mut_param {
+					return
+				}
 				suggestion := if !is_heap && sym.kind == .struct {
 					'declaring `${sym.name}` as `@[heap]`'
 				} else if !is_heap {

vlib/v/checker/tests/no_heap_struct.out

@@ -5,13 +5,6 @@ vlib/v/checker/tests/no_heap_struct.vv:13:6: error: `x` cannot be assigned outsi
       |            ^
    14 |     }
    15 |     return s
-vlib/v/checker/tests/no_heap_struct.vv:19:9: error: `x` cannot be returned outside `unsafe` blocks as it might refer to an object stored on stack. Consider declaring `Abc` as `@[heap]`.
-   17 | 
-   18 | fn g(mut x Abc) &Abc {
-   19 |     return x
-      |            ^
-   20 | }
-   21 |
 vlib/v/checker/tests/no_heap_struct.vv:23:7: error: `x` cannot be assigned outside `unsafe` blocks as it might refer to an object stored on stack. Consider declaring `Abc` as `@[heap]`.
    21 | 
    22 | fn h(x &Abc) &Abc {

vlib/v/checker/tests/stack_var_check_err.out

@@ -0,0 +1,42 @@
+module main
+
+@[heap]
+interface IGameObject {
+mut:
+	name       string
+	parent     ?&IGameObject
+	next       ?&IGameObject
+	child      ?&IGameObject
+	last_child ?&IGameObject
+	add_child(mut o IGameObject)
+}
+
+@[heap]
+struct GameObject implements IGameObject {
+mut:
+	name       string
+	parent     ?&IGameObject
+	next       ?&IGameObject
+	child      ?&IGameObject
+	last_child ?&IGameObject
+}
+
+fn (mut gameobject GameObject) add_child(mut o IGameObject) {
+	o.parent = gameobject
+	if mut last_child := gameobject.last_child {
+		last_child.next = o
+	} else {
+		gameobject.child = o
+	}
+	gameobject.last_child = o
+}
+
+fn main() {
+	mut v1 := &GameObject{
+		name: 'v1'
+	}
+	mut v2 := &GameObject{
+		name: 'v2'
+	}
+	v1.add_child(mut v2)
+}