Add an intital HIR and lowering step

[nrc]

r? @nikomatsakis

Trying to land this first stab, which basically just duplicates the AST. Will file issues for the various things I've got in mind to improve.

[bors]

☔️ The latest upstream changes (presumably #28136) made this pull request unmergeable. Please resolve the merge conflicts.

[nikomatsakis]

is it possible to separate out the ast => hir renamings and other "hyper-rote" fallback into a distinct commit?

[nikomatsakis]

ok so this seems fine, r+ modulo the various nits. Per IRC discussion, I think the duplication is ok as long as have a plan to remove it.

[jroesch]

Looked through the majority of this as well, the duplicated logic would be worrisome in the long term, but it sounds like you will relatively quickly refactor and remove it. I also like the approach of landing this chunk before it gets too big. 👍

[nrc]

@bors: r=nikomatsakis p=1

[bors]

📌 Commit facdf2e has been approved by nikomatsakis

[bors]

⌛️ Testing commit facdf2e with merge cd138dc...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt