checker: fix it variable marked as auto_heap

vlib/v/ast/ast.v

@@ -896,6 +896,7 @@ pub:
 	is_autofree_tmp bool
 	is_inherited    bool
 	has_inherited   bool
+	is_it_var       bool // is `it` var
 pub mut:
 	is_arg        bool // fn args should not be autofreed
 	is_auto_deref bool

vlib/v/checker/checker.v

@@ -298,7 +298,7 @@ pub fn (mut c Checker) check_scope_vars(sc &ast.Scope) {
 							}
 						}
 					}
-					if obj.is_mut && !obj.is_changed && !c.is_builtin_mod && obj.name != 'it' {
+					if obj.is_mut && !obj.is_changed && !c.is_builtin_mod && !obj.is_it_var {
 						// if obj.is_mut && !obj.is_changed && !c.is_builtin {  //TODO C error bad field not checked
 						// c.warn('`$obj.name` is declared as mutable, but it was never changed',
 						// obj.pos)
@@ -4591,7 +4591,7 @@ fn (mut c Checker) mark_as_referenced(mut node ast.Expr, as_interface bool) {
 					match type_sym.kind {
 						.struct {
 							info := type_sym.info as ast.Struct
-							if !info.is_heap {
+							if !info.is_heap && !node.obj.is_it_var {
 								node.obj.is_auto_heap = true
 							}
 						}

vlib/v/checker/fn.v

@@ -3749,10 +3749,11 @@ fn scope_register_a_b(mut s ast.Scope, pos token.Pos, typ ast.Type) {
 
 fn scope_register_var_name(mut s ast.Scope, pos token.Pos, typ ast.Type, name string) {
 	s.register(ast.Var{
-		name:    name
-		pos:     pos
-		typ:     typ
-		is_used: true
+		name:      name
+		pos:       pos
+		typ:       typ
+		is_used:   true
+		is_it_var: name == 'it'
 	})
 }
 

vlib/v/tests/builtin_arrays/array_map_cast_interface_test.v

@@ -0,0 +1,25 @@
+interface Rect {
+	width  u32
+	height u32
+}
+
+struct Square {
+	side   u32
+	width  u32
+	height u32
+}
+
+fn test_main() {
+	squares := [Square{
+		side:   5
+		width:  5
+		height: 5
+	}]
+	rects := squares.map(Rect(it))
+
+	assert rects.str() == '[Rect(Square{
+    side: 5
+    width: 5
+    height: 5
+})]'
+}