surprising error observing a fallible using closure - #283

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 1.76%. Comparing base (0e66079) to head (1e312e5).

@@            Coverage Diff             @@
##           master    #283       +/-   ##
==========================================
- Coverage   82.47%   1.76%   -80.72%     
==========================================
  Files          84      83        -1     
  Lines       11887   11471      -416     
==========================================
- Hits         9804     202     -9602     
- Misses       2083   11269     +9186     

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Sorry for the very late reply, and thanks for reporting this.

The failure itself is expected, but the RcDerefMut part of the diagnostic was indeed unrelated to anything the caller should implement.

subscribe(|...| ...) wraps the callback in FnMutObserver, which implements Observer<_, Infallible>. throw_err("some err"), however, requires an Observer<(), &str>.

While proving that bound, rustc picked our generic forwarding implementation for RcDerefMut<Target = Option<O>> as a diagnostic candidate. That is where the surprising RcDerefMut requirement came from.

#285 fixes this without removing or narrowing the forwarding implementation:

• The forwarding implementation is excluded from trait recommendations.
• on_error now correctly exposes Err = Infallible, since it consumes the terminal error instead of forwarding it.
• The subscribe documentation clarifies that next-only subscriptions are for infallible observables.

The updated diagnostic now points directly to the real mismatch:

error[E0277]: the trait bound `FnMutObserver<_>: Observer<(), &str>` is not satisfied
...
help: the trait `Observer<(), &str>` is not implemented for `FnMutObserver<_>`
      but trait `Observer<(), Infallible>` is implemented for it
...
  = help: for that trait implementation, expected `Infallible`, found `&str`

For a fallible source, the intended next-only usage is:

Shared::throw_err("some err")
  .on_error(|err| eprintln!("{err}"))
  .subscribe(|_| unreachable!());

This now reports the actual &str versus Infallible mismatch and no longer mentions RcDerefMut.

Thanks!