Don't panic if a query branched on an untracked state

[MichaReiser]

After a change to an input (e.g., a file was modified), Salsa re-executes all queries that read that input or depend on any such query. To limit invalidation, Salsa can backdate a query. If a query computes to the same value as in the previous revision, Salsa considers it unchanged and skips re-executing dependent queries. This is called backdating a query. The reason it's called backdating is that Salsa tracks the revision in which a query was last changed; backdating sets the last-changed revision "back" to the revision from before the input change. During backdating, Salsa enforces that the revision is indeed going backwards. That is, the revision Salsa backdates to (or the last changed revision from the previous query run) must be smaller (older) than the last changed revision from the current run.

Under normal circumstances, this invariant should always hold because for a query result to change, the query has to read some input that was changed in the current revision (at least in a revision newer than the last run). This ensures that the last changed revision from the new run is never older than that from the previous run. However, this invariant can be violated if a query branches on a state that isn't tracked in Salsa. Salsa will rerun the query because one of its inputs changed, but the query branches on some untracked state before reading that input, so that the query's last changed revision might be earlier than when the input changed.

Reading an untracked state in a query is always a bug (unless you call db.untracked_read), and it can lead to stale results. But I don't think it's an error severe enough to warrant Salsa panicking.

Related discussion: #Contributing to Salsa > assertion failed: old_memo.revisions.changed_at <= revisions

Related ty PR that fixed a few unintentional untracked reads.

[netlify]

✅ Deploy Preview for salsa-rs canceled.

Name	Link
🔨 Latest commit	1699379
🔍 Latest deploy log	https://app.netlify.com/projects/salsa-rs/deploys/69bd2bcd0c9f1500088d0323

[MichaReiser]

Ugh, 1.85 doesn't like the format_args lifetime. I'll switch to format

[codspeed-hq]

Merging this PR will improve performance by 4.07%

⚡ 1 improved benchmark
✅ 12 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	amortized[SupertypeInput]	3 µs	2.9 µs	+4.07%

---

_{Comparing MichaReiser:backdate-untracked-read (1699379) with master (53421c2)}