add WillIterateCycle event

[carljm]

I would like to write tests asserting about cycles iterated and log cycles iterated as a metric. Add a WillIterateCycle event and fire it whenever a cycle head is about to iterate the cycle again. Update some cycle tests accordingly.

[netlify]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
🔨 Latest commit	05b8de1
🔍 Latest deploy log	https://app.netlify.com/sites/salsa-rs/deploys/67f7c1c1a46cb20008fe3d04

[codspeed-hq]

CodSpeed Performance Report

Merging #790 will not alter performance

_{Comparing carljm:cycle-event (05b8de1) with master (b165ba7)}

Summary

✅ 12 untouched benchmarks

[MichaReiser]

I think that's not just useful for third-party tests but also for our own. We don't have good infrastructure to set up tracing in tests, which is why I mainly use the db events to debug test errors (with occasional println sprinkled in every now and there)

[MichaReiser]

I think that's not just useful for third-party tests but also for our own. We don't have good infrastructure to set up tracing in tests, which is why I mainly use the db events to debug test errors (with occasional println sprinkled in every now and there). In fact, I had to add multiple println expressions to verify that my test used the cycle head that I intended.

[carljm]

We don't have good infrastructure to set up tracing in tests

Side note from this PR, but if you replace #[test] with [#test_log::test] on a Salsa test and then run with RUST_LOG set, you get tracing output -- I use this a lot.

[MichaReiser]

We don't have good infrastructure to set up tracing in tests

Side note from this PR, but if you replace #[test] with [#test_log::test] on a Salsa test and then run with RUST_LOG set, you get tracing output -- I use this a lot.

Ohhh nice, I didn't know about that.