xilem_web: Panic using BoxedView inside HTML element

[matthunz]

When trying to use a BoxedView in an html::div I get the following panic:

panicked at C:\Users\matth\.cargo\git\checkouts\xilem-031d790027f85a11\1e37164\crates\xilem_web\src\context.rs:304:14:
Element type has changed, this should never happen!

Reproducible by changing the counter example:

fn btn(
    label: &'static str,
    click_fn: impl Fn(&mut AppState, web_sys::MouseEvent) + 'static,
) ->  BoxedView<AppState> {
    Box::new(el::button(label).on_click(click_fn))
}

fn app_logic(state: &mut AppState) -> impl View<AppState> {
    el::div((
        btn("+1 click", |state, _| state.increment()),
        state.text.clone(),
    ))
}

Stack trace

Stack:

Error
    at imports.wbg.__wbg_new_abda76e883ba8a5f (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9.js:338:21)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.console_error_panic_hook::Error::new::h516a79c7e649e619 (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[2255]:0xa99ca)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.console_error_panic_hook::hook_impl::h5e4d6d7771b2c70d (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[432]:0x55f76)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.console_error_panic_hook::hook::h99fffc916c6dad72 (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[3139]:0xb8210)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.core::ops::function::Fn::call::h97eb8c787f0b3f7f (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[2707]:0xb2178)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.std::panicking::rust_panic_with_hook::h0833ff9dc94df5f1 (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[1002]:0x7eb17)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.std::panicking::begin_panic_handler::{{closure}}::hc3329d7ae0363c6f (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[1373]:0x90226)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.std::sys_common::backtrace::__rust_end_short_backtrace::h4268105044501476 (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[3596]:0xbc275)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.rust_begin_unwind (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[2830]:0xb4058)
    at lookbook_xilem_example-f6a974525f70f53f.wasm.core::panicking::panic_fmt::h765acaeb0182f5af (http://localhost:8080/lookbook-xilem-example-57aa3af25ad35cc9_bg.wasm:wasm-function[2970]:0xb5f83)


imports.wbg.__wbg_error_f851667af71bcfc6 @ lookbook-xilem-example-57aa3af25ad35cc9.js:332

This looks to be related to Pod::downcast_mut, where the underlying element type is incorrect.
I'm also not using any conditionals or a ViewSequence so this is definitely confusing to me, maybe AnyView is storing the wrong type?

[Philipp-M]

Hmm this is a tricky one, I think it's this issue: https://stackoverflow.com/questions/73807369/cant-downcast-from-mut-boxdyn-trait-as-mut-dyn-any

I.e. in the ViewSequence impl for impl View is coercing the inner element of the Pod which is already of type Box<dyn AnyNode> as &mut dyn Any (via .as_any_mut()) and then back to Box<dyn AnyNode>