[WIP] Never type experiments #84573

We now fallback type variables using the following rules: * Construct a coercion graph `A -> B` where `A` and `B` are unresolved type variables or the `!` type. * Let D be those variables that are reachable from `!`. * Let N be those variables that are reachable from a variable not in D. * All variables in (D \ N) fallback to `!`. * All variables in (D & N) fallback to `()`.

The comment seems incorrect. Testing revealed that the examples in question still work (as well as some variants) even without the special casing here.

Instead, we now record those type variables that are the target of a `NeverToAny` adjustment and consider those to be the "diverging" type variables. This allows us to remove the special case logic that creates a type variable for `!` in coercion.

Instead, we now rely on the code that looks for a NeverToAny adjustment.

This could cause us to visit the start node twice.

Extend the `DepthFirstSearch` iterator so that it can be re-used and extended with add'l start nodes. Then replace the FxHashSets of nodes we were using in the fallback analysis with a single iterator. This way we won't re-walk portions of the graph that are reached more than once, and we also do less allocation etc.

This avoids select_obligations_where_possible having an effect, which future proofs the algorithm.

This doesn't yet fully work -- running coercions seems to be prone to causing errors in the environment, which creates a little noise in some cases. It should be enough to let us re-run crater, though, I think.

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[WIP] Never type experiments #84573

[WIP] Never type experiments #84573

Commits on Aug 30, 2021