fn foo(&must self) {
  let a = &mut self;
}

let outer = SomeFutureCollector::new();
let callback = || {
  let fut = tokio::runtime::Handle::current().spawn(/* async block or function here */);
  outer.push(fut);
};

error[E0277]: the size for values of type `(dyn std::error::Error + 'static)` cannot be known at compilation time
   --> src\matcher.rs:135:33
    |
89  |     pub trait MatchStrategy {
    |               ------------- required by a bound in this
...
92  |         type MatchErr: std::error::Error;
    |                        ----------------- required by this bound in `matcher::strategies::MatchStrategy`
...
135 |                 type MatchErr = Box<dyn Error>;
    |                                 ^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `(dyn std::error::Error + 'static)`
    = note: required because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<(dyn std::error::Error + 'static)>`