Decentralize queries

[cjgillot]

TL;DR

Allow each compiler crate to define its own queries, instead of declaring everything in librustc_middle.
This would remove the need for datatype definitions in librustc_middle.

Links and Details

Exploratory work: moving the query system out of librustc_middle -> rust-lang/rust#70951

Mentors or Reviewers

I have not looked for people yet.
@eddyb might be interested because of rust-lang/rust#70951 (comment)

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Note: Discussion of the proposal should happen on the associated zulip topic (read access available at associated zulip archive). This issue is meant to summarize the high level points and to track status of the proposal.

[spastorino]

Added to our compiler team meeting agenda, removing to-announce label.

[nikomatsakis]

@cjgillot heh I can't find the zulip stream ... so I like the idea here but I feel like we're missing some details of how you plan to go about it. It's a big PR -- can you maybe update the description with some details of how you are decentralizing exactly?

[nikomatsakis]

Closing per @cjgillot's comment on Zulip.