Is this intentionally using != rather than <>?

No, that was a typo. Thanks for catching that.

Changing this test should allow you to get that remaining line fully covered. Indeed, I think in this form, this test doesn't add anything beyond the tests above.

Since t is still pending at this point, when you cancel r1, it cancels t, and the bind that creates r1 falls through bind's exception-forwarding case. So the function that puts 2 to y is simply never called, the put is never even started or tried, and this is only testing how bind responds to cancel.

To trigger that line, what this test should do is get rid of t, and define r1 as just that put y 2. It is put y 2 that has an on_cancel handler added to it in Lwt_mvar (and we currently never call it).

This will also be a pending promise, because y already has 1 at that moment. You'll want to cancel r1, take from y, check that it's 1, take again, and check that the taker is blocked the second time.

I don't think x is necessary.

The reader test below has a similar issue.

Thanks, that worked and is a lot simpler than the original.

This is optional (the test is correct regardless), but I believe you could do:

(* ... *)
let y = Lwt_mvar.put x 1 in
Lwt.return (Lwt.state y = Lwt.Sleep)

This makes the test slightly clearer, and it would be directly testing that y is pending. Actually, I would recommend always checking the state of promises with Lwt.state rather than comparing to another promise, as comparing with another promise will fail if the internal definition of promises ever changes to include, say, unique ids for tracing.

Sorry about the name Lwt.Sleep for "pending," that's a historical holdover :)

This is optional, but I recommend removing this (sorry if you saw it used in another test suite). Your code already nicely uses Lwt. in most places, removing it would, I think, just make the returns at the end of each test more explicit.

The above optional suggestion about Lwt.state would save a bind here as well, and the need for 2 :) Another way is to compare with Lwt.return (), but... Lwt.state :)

Lwt.state should make this final condition substantially clearer.