Tracking issue for type checking refactor

[jroesch]

This a placeholder issue to contain our conversation and set of pull requests around the type checking refactor. The goal of the work is to unify our type checking contexts into a single piece of state. We hope to eventually have a single context instead of the fractured situation we currently have.

• unify ParameterEnvironment + InferCtxt (Initial refactor of InferCtxt towards unified type checking state and per item side tables #26582)
• move FulfillmentContext into InferCtxt
• modify SelectionContext to only take a single context argument
• remove remaining typer impls
• remove the ClosureTyper + Typer trait
• do some renaming to make these things more applicable to their new roles

[eddyb]

cc @rust-lang/compiler

[steveklabnik]

@jroesch bump! You only have one box not checked here, can this be closed?

[Mark-Simulacrum]

I'd be happy to do the work renaming things if given a list (or even an approximate list, general idea).

[eddyb]

I'm not even sure what we should do now, it's likely time for a cache-reuse-targetted redesign.

[nikomatsakis]

I think we can close this issue, since I think this path of changes is dead. I have some new plans I hope to write up in the next few days (or weeks...) though.