DepNodeIndex does not uniquely identify anonymous queries

[cjgillot]

The DepNodeIndex for an anonymous query is computed from its DepKind and from the list of dependencies, but does not depend on its key. As DepNodes are deduplicated before creation, the same DepNodeIndex can correspond to multiple anonymous query invocations.

Query self-profiling relies on DepNodeIndex to identify query invocations.

The consequences of this deduplication on self-profiling need further investigation.

cc @michaelwoerister

[jyn514]

What is an anonymous query?

[michaelwoerister]

@jyn514 See also https://rustc-dev-guide.rust-lang.org/queries/incremental-compilation-in-detail.html#query-modifiers

Right now it is also used as a way to speed up the erase_regions_ty query but I think it's debatable whether that's a good idea.