Ability to opt into change detection on *ReadOnlyItem in mutable QueryData derive

[Themayu]

What problem does this solve or what need does it fill?

Currently, the QueryData derive macro transforms fields of the type &'static mut T to Mut<'w, T> when generating the item type for the query. However, the readonly item type continues to use &'w T as the reference type. This makes it impossible to use change detection on the readonly form of mutable queries.

As an example from my code:

#[derive(QueryData)]
#[query_data(mutable)]
pub struct TextBoxQuery {
    pub data: &'static mut TextBox,
    pub style: &'static mut TextBoxStyle,
    pub text_style: &'static mut TextBoxTextStyle,
}

// derive(QueryData) output
pub struct TextBoxQueryItem<'w> {
    pub data: Mut<'w, TextBox>,
    pub style: Mut<'w, TextBoxStyle>,
    pub text_style: Mut<'w, TextBoxTextStyle>,
}

pub struct TextBoxQueryReadOnlyItem<'w> {
    pub data: &'w TextBox,
    pub style: &'w TextBoxStyle,
    pub text_style: &'w TextBoxTextStyle,
}

What solution would you like?

I would like a way to inform the QueryData derive macro to use bevy_ecs::change_detection::Ref<'w, T> instead of &'w T. I believe it would be useful if we can choose between applying it to the whole struct or to individual fields, like so:

Example 1: Whole struct application

#[derive(QueryData)]
#[query_data(mutable, detect_changes)]
pub struct TextBoxQuery {
    pub data: &'static mut TextBox,
    pub style: &'static mut TextBoxStyle,
    pub text_style: &'static mut TextBoxTextStyle,
}

// derive(QueryData) output
pub struct TextBoxQueryItem<'w> {
    pub data: Mut<'w, TextBox>,
    pub style: Mut<'w, TextBoxStyle>,
    pub text_style: Mut<'w, TextBoxTextStyle>,
}

pub struct TextBoxQueryReadOnlyItem<'w> {
    pub data: Ref<'w, TextBox>,
    pub style: Ref<'w, TextBoxStyle>,
    pub text_style: Ref<'w, TextBoxTextStyle>,
}

Example 2: Per field application

#[derive(QueryData)]
#[query_data(mutable)]
pub struct TextBoxQuery {
    #[query_data(detect_changes)]
    pub data: &'static mut TextBox,

    pub style: &'static mut TextBoxStyle,
    pub text_style: &'static mut TextBoxTextStyle,
}

// derive(QueryData) output
pub struct TextBoxQueryItem<'w> {
    pub data: Mut<'w, TextBox>,
    pub style: Mut<'w, TextBoxStyle>,
    pub text_style: Mut<'w, TextBoxTextStyle>,
}

pub struct TextBoxQueryReadOnlyItem<'w> {
    pub data: Ref<'w, TextBox>,
    pub style: &'w TextBoxStyle,
    pub text_style: &'w TextBoxTextStyle,
}

What alternative(s) have you considered?

The only other option I can see is for me to manually implement WorldQuery on TextBoxQuery, and I do not wish to manually implement WorldQuery due to how brittle it is.

[Themayu]

The detect_changes option name is up for discussion if necessary. I just chose the most obvious option because I didn't want to spend hours bikeshedding my own feature request before even posting it.

[alice-i-cecile]

Seems like a very reasonable request. Mild preference for approach two, with per-field attributes since it's a bit more flexible.

[Themayu]

@alice-i-cecile I think you misunderstood. There aren't two separate approaches here - I am proposing supporting both forms of usage, in a similar way to how serde has both #[serde(rename_all = ...)] at the type level and #[serde(rename = ...)] at the individual field level.

[alice-i-cecile]

Ah, I'm fine with that as well.

[Themayu]

I've come up with a (in my opinion) nicer way of achieving the goal here, which has been laid out in #13329.

[Themayu]

Closed in favour of #13329.