Rewrite HIR->MIR lowering to use Places

[arora-aman]

[nikomatsakis]

An idea for how we might approach this (but not the only one)

let c = || use(foo.bar.baz, foo.bar.baz2);

This results in a HIR like:

• use()
  • field:baz // today: self.0.bar.baz
    • field:bar // today: self.0.bar
      • variable:foo // today returns a place like self.0

under new system:

• when we reach foo, we would look up in the map and get a set of minimum captured places
• the variable:foo nodes must appear as a child of some subtree that is in that set
• whereas today MIR construction returns (HIR node -> MIR place)
  • it would return (HIR node -> (MIR place | {upvar captures})
• the field:bar node would get back a set of captures that is not a singleton set, narrow it down, and return it up
• the field:baz node would return the MIR place self.0

[arora-aman]

https://hackmd.io/aD41lqdMSmG7mvHJYt-mtg