Why mut borrow in vector::push_back won't work?

[0x-j]

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Describe your question in detail.

I have this code snippet.

    public entry fun post_message() {
        // my stuff
        let messages = borrow_global_mut<MessageBoard>(get_message_board_object_address()).messages;
        vector::push_back(&mut messages, object::object_from_constructor_ref(message_object_constructor_ref));
        debug::print(&string_utils::format1(&b"message_objects = {}", messages));
    }

    #[view]
    /// Get message objects, old messages come first
    public fun get_message_objects(limit: Option<u64>, offset: Option<u64>): vector<Object<Message>> acquires MessageBoard {
        let messages = borrow_global<MessageBoard>(get_message_board_object_address()).messages;
        debug::print(&string_utils::format1(&b"message_objects = {}", messages));
    }

What error, if any, are you getting?

messages is empty in the view function.

vector::push_back only actually save it when I write it with mut borrow first, is this intentional?

        let messages = &mut borrow_global_mut<MessageBoard>(get_message_board_object_address()).messages;
        vector::push_back(messages, object::object_from_constructor_ref(message_object_constructor_ref));

What have you tried or looked at? Or how can we reproduce the error?

No response

Which operating system are you using?

macOS

Which SDK or tool are you using? (if any)

TypeScript SDK

Describe your environment or tooling in detail

No response

[0x-j]

Yes it's intentional,, because reference.variable will always be not a reference by default, and instead a move.

This is the recommended approach.

// Retrieve an &mut reference of MessageBoard
let message_board = borrow_global_mut<MessageBoard>(address);

// Retrieve the reference to the messages, if it's not a reference, the compiler will complain, as it is moving it out of a mutable reference.
let messages = &mut message_board.messages;

// Using the mutable reference, push an object onto the vector
vector::push_back(messages, object);