Overhaul interning. #93148

This will let us introduce a more widely-used `Interned` type in the next commit.

In particular, there's now more protection against incorrect usage, because you can only create one via `Interned::new_unchecked`, which makes it more obvious that you must be careful. There are also some tests.

Specifically, change `Ty` from this: ``` pub type Ty<'tcx> = &'tcx TyS<'tcx>; ``` to this ``` pub struct Ty<'tcx>(Interned<'tcx, TyS<'tcx>>); ``` There are two benefits to this. - It's now a first class type, so we can define methods on it. This means we can move a lot of methods away from `TyS`, leaving `TyS` as a barely-used type, which is appropriate given that it's not meant to be used directly. - The uniqueness requirement is now explicit, via the `Interned` type. E.g. the pointer-based `Eq` and `Hash` comes from `Interned`, rather than via `TyS`, which wasn't obvious at all. Much of this commit is boring churn. The interesting changes are in these files: - compiler/rustc_middle/src/arena.rs - compiler/rustc_middle/src/mir/visit.rs - compiler/rustc_middle/src/ty/context.rs - compiler/rustc_middle/src/ty/mod.rs Specifically: - Most mentions of `TyS` are removed. It's very much a dumb struct now; `Ty` has all the smarts. - `TyS` now has `crate` visibility instead of `pub`. - `TyS::make_for_test` is removed in favour of the static `BOOL_TY`, which just works better with the new structure. - The `Eq`/`Ord`/`Hash` impls are removed from `TyS`. `Interned`s impls of `Eq`/`Hash` now suffice. `Ord` is now partly on `Interned` (pointer-based, for the `Equal` case) and partly on `TyS` (contents-based, for the other cases). - There are many tedious sigil adjustments, i.e. adding or removing `*` or `&`. They seem to be unavoidable.

Specifically, change `Ty` from this: ``` pub struct Predicate<'tcx> { inner: &'tcx PredicateInner<'tcx> } ``` to this: ``` pub struct Predicate<'tcx>(&'tcx Interned<PredicateS<'tcx>>) ``` where `PredicateInner` is renamed as `PredicateS`. This (plus a few other minor changes) makes the parallels with `Ty` and `TyS` much clearer, and makes the uniqueness more explicit.

Specifically, change `Region` from this: ``` pub type Region<'tcx> = &'tcx RegionKind; ``` to this: ``` pub struct Region<'tcx>(&'tcx Interned<RegionKind>); ``` This now matches `Ty` and `Predicate` more closely. Things to note - Regions have always been interned, but we haven't been using pointer-based `Eq` and `Hash`. This is now happening. - I chose to impl `Deref` for `Region` because it makes pattern matching a lot nicer, and `Region` can be viewed as just a smart wrapper for `RegionKind`. - Various methods are moved from `RegionKind` to `Region`. - There is a lot of tedious sigil changes. - A couple of types like `HighlightBuilder`, `RegionHighlightMode` now have a `'tcx` lifetime because they hold a `Ty<'tcx>`, so they can call `mk_region`. - A couple of test outputs change slightly, I'm not sure why, but the new outputs are a little better.

The variant names are exported, so we can use them directly (possibly with a `ty::` qualifier). Lots of places already do this, this commit just increases consistency.

Specifically, rename the `Const` struct as `ConstS` and re-introduce `Const` as this: ``` pub struct Const<'tcx>(&'tcx Interned<ConstS>); ``` This now matches `Ty` and `Predicate` more closely, including using pointer-based `eq` and `hash`. Notable changes: - `mk_const` now takes a `ConstS`. - `Const` was copy, despite being 48 bytes. Now `ConstS` is not, so need a we need separate arena for it, because we can't use the `Dropless` one any more. - Many `&'tcx Const<'tcx>`/`&Const<'tcx>` to `Const<'tcx>` changes - Many `ct.ty` to `ct.ty()` and `ct.val` to `ct.val()` changes. - Lots of tedious sigil fiddling.

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Overhaul interning. #93148

Overhaul interning. #93148

Commits on Feb 15, 2022