Bindings to std::function produce poor error messages

[mukunth1987]

Bindings to std::function produce poor error messages

[adetaylor]

Thanks - this is, I think, likely to be a limitation for a long time (I believe bindgen doesn't give us the right information, and even if it did, cxx does not yet support std::function. However these diagnostics are not OK, and I regard that as a bug.

[adetaylor]

FWIW when testing this by adding your code into the demo, I get something different. The build process says:

  Error:   × cxx couldn't handle our generated bindings - could be a bug in autocxx:
    │ unsupported type

This is because we are including this in the #[cxx::bridge] mod:

pub fn sendRequest(self: Pin<&mut A>, handler: [root::u128; 3usize]);

Solutions to improve this, then, would be one or several of the following:

• In bindgen, output something more useful than an array when it encounters such a type.
• In autocxx, during the function analysis phase, if we find a function has an array argument, then reject that function with a marginally more useful message. ("Arrays are not yet supported by cxx. Sometimes bindgen can generate arrays if it's not sure how to interpret a type, so the problem might be some other type not yet supported by bindgen.")
• Either way, include span information from bindgen so that we can point to a more useful location in our error messages