The code that broke is:

#[derive(JSTraceable)]
pub enum Filter {
    None,
    Native(fn (node: &Node) -> u16),
    JS(Rc<NodeFilter>)
}

/// A trait to allow tracing (only) DOM objects.
pub unsafe trait JSTraceable {
    /// Trace `self`.
    unsafe fn trace(&self, trc: *mut JSTracer);
}

unsafe impl<A, B> JSTraceable for fn(A) -> B {
    #[inline]
    unsafe fn trace(&self, _: *mut JSTracer) {
        // Do nothing
    }
}

#[proc_macro_derive(JSTraceable)]
pub fn expand_token_stream(input: proc_macro::TokenStream) -> proc_macro::TokenStream {

    // ...

    let style = synstructure::BindStyle::Ref.into();
    let match_body = synstructure::each_field(&mut type_, &style, |binding| {
        Some(quote! { #binding.trace(tracer); })
    });

    // ...

I think the generated code would look something like this:

unsafe impl JSTraceable for Filter {
    unsafe fn trace(&self, tracer: *mut JSTracer) {
        match *self {
            Filter::Native(ref field1) => {
                field1.trace(tracer);
            }
            // ...
        }
    }
}

The error message in rustc 1.21.0-nightly (7eeac1b 2017-08-30) was:

   Compiling script v0.0.1 (file:///home/simon/servo2/components/script)
error[E0599]: no method named `trace` found for type `&fn(&dom::node::Node) -> u16` in the current scope
   --> /home/simon/servo2/components/script/dom/treewalker.rs:464:10
    |
464 | #[derive(JSTraceable)]
    |          ^^^^^^^^^^^
    |
    = note: JSTraceable is a function, perhaps you wish to call it
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following trait defines an item `trace`, perhaps you need to implement it:
            candidate #1: `dom::bindings::trace::JSTraceable`

error: aborting due to previous error

error: Could not compile `script`.

The fix was adding a new impl for fn(&A) -> B, in addition to the one for fn(A) -> B. Isn’t the former a subset of the types covered by the latter?

unsafe impl<'a, A, B> JSTraceable for fn(&A) -> B {
    #[inline]
    unsafe fn trace(&self, _: *mut JSTracer) {
        // Do nothing
    }
}

regression in method dispatch #44224

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nikomatsakisopenedon Aug 31, 2017

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nikomatsakis
openedon Aug 31, 2017