Remove nodes_in_current_session field and related assertions

[Zoxc]

This removes the nodes_in_current_session field and related assertions. These are enabled if -Z incremental-verify-ich is passed or debug_assertions is on. Historically these have been useful to catch query keys with improper HashStable impls which lead to collisions.

We currently also check for duplicate nodes when loading the dep graph. This check is more complete as it covers the entire dep graph and is enabled by default. It doesn't provide a query key for a collision however. This check is also delayed to the next incremental session.

We also have the verify_query_key_hashes which is also enabled if -Z incremental-verify-ich is passed or debug_assertions is on. This checks for dep node conflicts in each query cache and provides 2 conflicting keys if present.

I think these remaining checks are sufficient and so we can remove nodes_in_current_session.

[rustbot]

r? @jdonszelmann

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[Zoxc]

cc @nnethercote @Zalathar @zetanumbers

[jdonszelmann]

This looks reasonable to me, but I'm no expert on the query system. Going to defer to @nnethercote (or someone else if that's better)

r? @nnethercote

[nnethercote]

Looks good, just one nit below.

View changes since this review

[nnethercote]

I'd prefer to keep this entire function cfg(debug_assertions), and then likewise at the call sites. It makes it clearer that it's all debug-only, and avoids the need for the underscores on the parameter names.

[Zoxc]

I was thinking of just converting this to cfg!.

[nnethercote]

#[cfg] makes the call sites clearer.

[petrochenkov]

Why?
I basically always prefer if cfg! to #[cfg] when technically possible because it doesn't hide the configured-out code from type checker and other later passes.

[nnethercote]

Fine, whatever, just do cfg! then.