|as_promise| converts a FIDL asynchronous call in the new C++ bindings

into a promise. Example usage:

// Let's say an async FIDL call originally uses a callback.

fidl::Client

<MyProtocol

> client(...);

client->Foo({ ... }).Then([] (fidl::Result

<MyProtocol

::Foo>

&

result) {

assert(result.is_ok());

});

// It can be turned into a promise by wrapping the call with |as_promise|

// as opposed to attaching a callback via |Then|:

auto p1 = fidl_fpromise::as_promise(client->Foo({ ... }));

// And used like any other regular promise:

auto p2 = p1.then([] (auto

&

result) {

assert(result.is_ok());

});

some_executor.schedule_task(std::move(p2));

The signature of the resulting promise is akin to

fpromise::promise

<SuccessType

, ErrorType>

where |SuccessType| is the payload type for when the FIDL call succeeds, or

|void| if the FIDL call has an empty/zero-argument payload.

where |ErrorType| is |fidl::Error| if the FIDL call does not use application

errors, and |fidl::ErrorsIn

<MyProtocol

::FidlMethod>| otherwise. |MyProtocol|

and |FidlMethod| are all placeholders to be replaced by the actual protocol

and method names.

This is alternate overload for use with WireClients.

Notably, this takes a callback to directly handle the WireUnownedResult, as its lifetime does not

extend beyond the scope of WireThenableImpl.

Additionally, the caller may specify the success and error types of the resulting promise via

the type arguments V and E.