servo_arc: bad drop in WASM target

[noahbald]

Sorry if this issue is a bit vague because I'm having a hard time finding the exact cause.
I'm compiling to WASM using wasm-bindgen 0.2.100 and when running I receive the following panic

panicked at /rust/deps/dlmalloc-0.2.7/src/dlmalloc.rs:1198:13:
assertion failed: psize <= size + max_overhead

And this seems to happening when I create a selector with a selector list, e.g.

pub struct Parser<E: crate::element::Element> {
    element: PhantomData<E>,
}

impl<'i, E: crate::element::Element> selectors::parser::Parser<'i> for Parser<E> {
    type Impl = E::Impl;
    type Error = SelectorParseErrorKind<'i>;
}

impl<E: crate::element::Element> Default for Parser<E> {
    fn default() -> Self {
        Self {
            element: PhantomData,
        }
    }
}

impl<E: crate::element::Element> Selector<E> {
    pub fn matches_with_scope_and_cache(
        &self,
        element: &E,
        scope: Option<E>,
        selector_caches: &mut SelectorCaches,
    ) -> bool {
        let context = &mut matching::MatchingContext::new(
            matching::MatchingMode::Normal,
            None,
            selector_caches,
            matching::QuirksMode::NoQuirks,
            matching::NeedsSelectorFlags::No,
            matching::MatchingForInvalidation::No,
        );
        context.scope_element = scope.map(|x| selectors::Element::opaque(&x));
        matching::matches_selector_list(&self.0, element, context)
    }
}

#[test]
fn bad_demalloc() {
  // This will panic in wasm
  Selector::new( "script,a" )
}

related: rustwasm/wasm-pack#1389

[noahbald]

https://github.com/noahbald/stylo-selector-fail-129
Minimal reproduction using scraper

[noahbald]

I've found this line to be the source of the panic

stylo/servo_arc/lib.rs

Line 367 in 8422412

std::ptr::drop_in_place(inner);

[noahbald]

I've found that adding the following to /selectors/Cargo.toml resolves the issue

[target.'cfg(target_arch = "wasm32")'.dependencies.servo_arc]
path = "../servo_arc"
features = ["track_alloc_size"]

Any maintainer know if this is a reasonable fix?

[noahbald]

Also, updating my crate's Cargo.toml to include servo_arc with track_alloc_size enabled fixes the issue, so a patch may not be needed. Perhaps should be added to docs?