markused: fix printing smartcasted interface values (fix #24579) (#24583)

vlib/v/checker/used_features.v

@@ -138,6 +138,16 @@ fn (mut c Checker) markused_fn_call(mut node ast.CallExpr) {
 				c.table.used_features.option_or_result = true
 			}
 			c.table.used_features.print_types[node.args[0].typ.idx()] = true
+			if !c.table.used_features.auto_str_ptr && node.args[0].expr is ast.Ident {
+				var_obj := node.args[0].expr.obj
+				if var_obj is ast.Var {
+					if var_obj.orig_type != 0
+						&& c.table.final_sym(var_obj.orig_type).kind == .interface {
+						c.table.used_features.auto_str_ptr = true
+						return
+					}
+				}
+			}
 		}
 		if node.args[0].typ.is_ptr() {
 			c.table.used_features.auto_str_ptr = true

vlib/v/tests/skip_unused/print_interface_smartcast.run.out

@@ -0,0 +1 @@
+&xxx 1:1:1

vlib/v/tests/skip_unused/print_interface_smartcast.skip_unused.run.out

@@ -0,0 +1 @@
+&xxx 1:1:1

vlib/v/tests/skip_unused/print_interface_smartcast.vv

@@ -0,0 +1,42 @@
+module main
+
+pub struct MyError implements IError {
+pub:
+	msg    string
+	code   int
+	domain int
+	level  int
+}
+
+pub fn (e MyError) str() string {
+	return '${e.msg} ${e.code}:${e.domain}:${e.level}'
+}
+
+pub fn (e MyError) msg() string {
+	return e.msg
+}
+
+pub fn (e MyError) code() int {
+	return e.code
+}
+
+fn MyError.new(msg string, code int, domain int, level int) MyError {
+	return MyError{
+		msg:    msg
+		code:   code
+		domain: domain
+		level:  level
+	}
+}
+
+fn foo() ! {
+	return MyError.new('xxx', 1, 1, 1)
+}
+
+fn main() {
+	foo() or {
+		if err is MyError {
+			eprintln(err)
+		}
+	}
+}