Remove dependency tracking for variance computation

[SimonSapin]

This custom tracking is now replaced by the red/green algorithm.

Fix #45471

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[SimonSapin]

r? @nikomatsakis

I made the changes described in #45471, then kept removing things until everything compiled.

Does src/librustc_typeck/variance/README.md also need changes?

[nikomatsakis]

@SimonSapin Yeah. Let's replace this paragraph:

Eventually, the goal is to rely on the red-green dependency management algorithm. At the moment, however, we rely instead on a hack, where variances_of ignores the dependencies of accessing crate_variances and instead computes the correct dependencies itself. To this end, when we build up the constraints in the system, we also built up a transitive dependencies relation as part of the crate map. A (X, Y) pair is added to the map each time we have a constraint that the variance of some inferred for the item X depends on the variance of some element of Y. This is to some extent a mirroring of the inference graph in the dependency graph. This means we can just completely ignore the fixed-point iteration, since it is just shuffling values along this graph.

with

Ultimately, this setup relies on the red-green algorithm. In particular, every variance query ultimately depends on -- effectively -- all type definitions in the entire crate (through crate_variances), but since most changes will not result in a change to the actual results from variance inference, the variances_of query will wind up being considered green after it is re-evaluated.

[SimonSapin]

Done.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 94edd8f has been approved by nikomatsakis

[bors]

⌛ Testing commit 94edd8f with merge b247805...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing b247805 to master...