Provide a better error message when mocking fails and improve ergonomics

[Michael-F-Bryan]

PR Type

Refactor. Optionally with some additional functionality.

PR Checklist

Check your PR fulfills the following:

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt

Overview

The Mocker type passes around Box<dyn Any> for both messages and message results. That means it's not uncommon for your mock function to accidentally return the wrong type and panic with a rather cryptic "wrong return type for message".

The main feature of this PR is to adjust the panic message to mention the type M: Message the Mocker<T> is handling, and the type M::Result we expected the mock function to return.

As an addition (which can be cherry picked out if we don't want to include it), the implementation of Handler<M> required that instead of returning a M::Result directly, the mock function returns a Option<M::Result> so we can do downcast_mut() and a mem::replace() to extract the M::Result. 69f4135 switches to using ret.downcast::<M::Result>() instead of downcast_mut() so we can take the M::Result by value, chaining on a ret.downcast::<Option<M::Result>>() so we retain the original behaviour.

A second optional addition, 5d47d1f, adds a Mocker::mock_one() constructor for when your Mocker will only need to mock a single message. This lets the user avoid writing code for manually boxing and downcasting.

This change introduces the use of std::any::type_name() which only became stable in Rust 1.38.0. The MSRV for Actix is 1.40, so this PR shouldn't introduce any backwards compatibility issues.

[Michael-F-Bryan]

As a note, I couldn't quite get the #[should_panic(expected = "...")] part of the test to work. It looks like the panic occurs on a tokio thread and the original thread only gets a "mailbox closed" error.

---- actors::mocker::tests::users_get_a_useful_panic_message_when_mocking_fails stdout ----
thread 'actors::mocker::tests::users_get_a_useful_panic_message_when_mocking_fails' panicked at 'Wrong return type for message. Expected a actix::actors::mocker::tests::Pong when responding to actix::actors::mocker::tests::Ping', src\actors\mocker.rs:101:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'actors::mocker::tests::users_get_a_useful_panic_message_when_mocking_fails' panicked at 'called `Result::unwrap()` on an `Err` value: MailboxError(Mailbox has closed)', src\actors\mocker.rs:158:60
Panic in Arbiter thread.
note: panic did not contain expected string
      panic message: `"called `Result::unwrap()` on an `Err` value: MailboxError(Mailbox has closed)"`,
 expected substring: `"Wrong return type for message. Expected a actix::actors::mocker::tests::Pong when responding to actix::actors::mocker::tests::Ping"`

failures:
    actors::mocker::tests::users_get_a_useful_panic_message_when_mocking_fails

test result: FAILED. 4 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

Is there any way to get the panic message back so we can std::panic::resume_unwind()? Preferably without using std::panic::set_hook() and std::panic::take_hook() to temporarily hijack the global panic handler because it could affect tests running on other threads.

[robjtede]

Marking as draft due to CI failures. LMK if you're still interested in pursuing this PR.