Out of interest, is there any canonical documentation on this? In particular, how the xpidl types map to C++ types?
There are a few hints here and there (eg https://bugzilla.mozilla.org/show_bug.cgi?id=1551704#c1), but mostly I've been looking at patches on Bug 1509981 to try and work it out.

(In reply to Ben Campbell from comment #1)

Out of interest, is there any canonical documentation on this? In particular, how the xpidl types map to C++ types?

https://developer.mozilla.org/en-US/docs/Archive/Mozilla/XPIDL is apparently kept relatively fresh. The information you're looking for is primarily tables 1 and 2 (and probably should be adjusted to add Rust notes too).

Thanks Joshua.
One thing that caused me a few minutes head-scratching just now is that an .idl definiton of:

Array<AString> getFoo()

comes out in C++ as:

NS_IMETHOD GetFoo(nsTArray<nsString >& _retval)

Rather than what I was expecting:

NS_IMETHOD GetFoo(nsTArray<nsAString >& _retval)

ie different to the AString -> nsAString mapping for non-array strings.
It's obvious in hindsight - nsAString is abstract, but the nsTArray needs to hold a concrete class.

Not suggesting any action on this, just wanted to leave breadcrumbs in case anyone else got confused too :-)