Overhaul RegionKind and Region.

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.

Author: nnethercote

compiler/rustc_borrowck/src/constraint_generation.rs

@@ -60,8 +60,8 @@ impl<'cg, 'cx, 'tcx> Visitor<'tcx> for ConstraintGeneration<'cg, 'cx, 'tcx> {
 
     /// We sometimes have `region` within an rvalue, or within a
     /// call. Make them live at the location where they appear.
-    fn visit_region(&mut self, region: &ty::Region<'tcx>, location: Location) {
-        self.add_regular_live_constraint(*region, location);
+    fn visit_region(&mut self, region: ty::Region<'tcx>, location: Location) {
+        self.add_regular_live_constraint(region, location);
         self.super_region(region);
     }
 

compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs

@@ -356,8 +356,8 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
     })?;
 
     debug!(?sub_region, "cause = {:#?}", cause);
-    let nice_error = match (error_region, sub_region) {
-        (Some(error_region), &ty::ReVar(vid)) => NiceRegionError::new(
+    let nice_error = match (error_region, *sub_region) {
+        (Some(error_region), ty::ReVar(vid)) => NiceRegionError::new(
             infcx,
             RegionResolutionError::SubSupConflict(
                 vid,
@@ -374,7 +374,7 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
             RegionResolutionError::ConcreteFailure(cause.clone(), error_region, placeholder_region),
         ),
         // Note universe here is wrong...
-        (None, &ty::ReVar(vid)) => NiceRegionError::new(
+        (None, ty::ReVar(vid)) => NiceRegionError::new(
             infcx,
             RegionResolutionError::UpperBoundUniverseConflict(
                 vid,

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -497,14 +497,14 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         // We need to add synthesized lifetimes where appropriate. We do
         // this by hooking into the pretty printer and telling it to label the
         // lifetimes without names with the value `'0`.
-        match ty.kind() {
-            ty::Ref(
-                ty::RegionKind::ReLateBound(_, ty::BoundRegion { kind: br, .. })
-                | ty::RegionKind::RePlaceholder(ty::PlaceholderRegion { name: br, .. }),
-                _,
-                _,
-            ) => printer.region_highlight_mode.highlighting_bound_region(*br, counter),
-            _ => {}
+        if let ty::Ref(region, ..) = ty.kind() {
+            match **region {
+                ty::ReLateBound(_, ty::BoundRegion { kind: br, .. })
+                | ty::RePlaceholder(ty::PlaceholderRegion { name: br, .. }) => {
+                    printer.region_highlight_mode.highlighting_bound_region(br, counter)
+                }
+                _ => {}
+            }
         }
 
         let _ = ty.print(printer);
@@ -517,19 +517,17 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let mut s = String::new();
         let mut printer = ty::print::FmtPrinter::new(self.infcx.tcx, &mut s, Namespace::TypeNS);
 
-        let region = match ty.kind() {
-            ty::Ref(region, _, _) => {
-                match region {
-                    ty::RegionKind::ReLateBound(_, ty::BoundRegion { kind: br, .. })
-                    | ty::RegionKind::RePlaceholder(ty::PlaceholderRegion { name: br, .. }) => {
-                        printer.region_highlight_mode.highlighting_bound_region(*br, counter)
-                    }
-                    _ => {}
+        let region = if let ty::Ref(region, ..) = ty.kind() {
+            match **region {
+                ty::ReLateBound(_, ty::BoundRegion { kind: br, .. })
+                | ty::RePlaceholder(ty::PlaceholderRegion { name: br, .. }) => {
+                    printer.region_highlight_mode.highlighting_bound_region(br, counter)
                 }
-
-                region
+                _ => {}
             }
-            _ => bug!("ty for annotation of borrow region is not a reference"),
+            region
+        } else {
+            bug!("ty for annotation of borrow region is not a reference");
         };
 
         let _ = region.print(printer);

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -139,7 +139,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
     /// Returns `true` if a closure is inferred to be an `FnMut` closure.
     fn is_closure_fn_mut(&self, fr: RegionVid) -> bool {
-        if let Some(ty::ReFree(free_region)) = self.to_error_region(fr) {
+        if let Some(ty::ReFree(free_region)) = self.to_error_region(fr).as_deref() {
             if let ty::BoundRegionKind::BrEnv = free_region.bound_region {
                 if let DefiningTy::Closure(_, substs) =
                     self.regioncx.universal_regions().defining_ty
@@ -628,8 +628,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         fr_name: RegionName,
         outlived_fr: RegionVid,
     ) {
-        if let (Some(f), Some(ty::RegionKind::ReStatic)) =
-            (self.to_error_region(fr), self.to_error_region(outlived_fr))
+        if let (Some(f), Some(ty::ReStatic)) =
+            (self.to_error_region(fr), self.to_error_region(outlived_fr).as_deref())
         {
             if let Some(&ty::Opaque(did, substs)) = self
                 .infcx
@@ -652,7 +652,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                             bound.kind().skip_binder()
                         {
                             let r = r.subst(self.infcx.tcx, substs);
-                            if let ty::RegionKind::ReStatic = r {
+                            if r.is_static() {
                                 found = true;
                                 break;
                             } else {

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -264,7 +264,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
         let tcx = self.infcx.tcx;
 
         debug!("give_region_a_name: error_region = {:?}", error_region);
-        match error_region {
+        match *error_region {
             ty::ReEarlyBound(ebr) => {
                 if ebr.has_name() {
                     let span = tcx.hir().span_if_local(ebr.def_id).unwrap_or(DUMMY_SP);
@@ -433,7 +433,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
         span: Span,
         counter: usize,
     ) -> RegionNameHighlight {
-        let mut highlight = RegionHighlightMode::default();
+        let mut highlight = RegionHighlightMode::new(self.infcx.tcx);
         highlight.highlighting_region_vid(needle_fr, counter);
         let type_name =
             self.infcx.extract_inference_diagnostics_data(ty.into(), Some(highlight)).name;
@@ -818,7 +818,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
             return None;
         }
 
-        let mut highlight = RegionHighlightMode::default();
+        let mut highlight = RegionHighlightMode::new(tcx);
         highlight.highlighting_region_vid(fr, *self.next_region_name.try_borrow().unwrap());
         let type_name =
             self.infcx.extract_inference_diagnostics_data(yield_ty.into(), Some(highlight)).name;

compiler/rustc_borrowck/src/nll.rs

@@ -8,7 +8,7 @@ use rustc_middle::mir::{
     BasicBlock, Body, ClosureOutlivesSubject, ClosureRegionRequirements, LocalKind, Location,
     Promoted,
 };
-use rustc_middle::ty::{self, OpaqueTypeKey, RegionKind, RegionVid, Ty};
+use rustc_middle::ty::{self, OpaqueTypeKey, Region, RegionVid, Ty};
 use rustc_span::symbol::sym;
 use std::env;
 use std::fmt::Debug;
@@ -443,9 +443,9 @@ pub trait ToRegionVid {
     fn to_region_vid(self) -> RegionVid;
 }
 
-impl<'tcx> ToRegionVid for &'tcx RegionKind {
+impl<'tcx> ToRegionVid for Region<'tcx> {
     fn to_region_vid(self) -> RegionVid {
-        if let ty::ReVar(vid) = self { *vid } else { bug!("region is not an ReVar: {:?}", self) }
+        if let ty::ReVar(vid) = *self { vid } else { bug!("region is not an ReVar: {:?}", self) }
     }
 }
 

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -1178,7 +1178,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             }
 
             VerifyBound::OutlivedBy(r) => {
-                let r_vid = self.to_region_vid(r);
+                let r_vid = self.to_region_vid(*r);
                 self.eval_outlives(r_vid, lower_bound)
             }
 

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -133,7 +133,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                         for vid in self.rev_scc_graph.as_ref().unwrap().upper_bounds(scc) {
                             match self.definitions[vid].external_name {
                                 None => {}
-                                Some(&ty::ReStatic) => {}
+                                Some(region) if region.is_static() => {}
                                 Some(region) => return region,
                             }
                         }
@@ -183,7 +183,7 @@ fn check_opaque_type_parameter_valid(
     for (i, arg) in opaque_type_key.substs.iter().enumerate() {
         let arg_is_param = match arg.unpack() {
             GenericArgKind::Type(ty) => matches!(ty.kind(), ty::Param(_)),
-            GenericArgKind::Lifetime(ty::ReStatic) => {
+            GenericArgKind::Lifetime(lt) if lt.is_static() => {
                 tcx.sess
                     .struct_span_err(span, "non-defining opaque type use in defining scope")
                     .span_label(
@@ -196,7 +196,7 @@ fn check_opaque_type_parameter_valid(
                 return false;
             }
             GenericArgKind::Lifetime(lt) => {
-                matches!(lt, ty::ReEarlyBound(_) | ty::ReFree(_))
+                matches!(*lt, ty::ReEarlyBound(_) | ty::ReFree(_))
             }
             GenericArgKind::Const(ct) => matches!(ct.val, ty::ConstKind::Param(_)),
         };

compiler/rustc_borrowck/src/renumber.rs

@@ -72,7 +72,7 @@ impl<'a, 'tcx> MutVisitor<'tcx> for NllVisitor<'a, 'tcx> {
     #[instrument(skip(self), level = "debug")]
     fn visit_region(&mut self, region: &mut ty::Region<'tcx>, location: Location) {
         let old_region = *region;
-        *region = self.renumber_regions(&old_region);
+        *region = self.renumber_regions(old_region);
 
         debug!(?region);
     }

compiler/rustc_borrowck/src/type_check/constraint_conversion.rs

@@ -142,8 +142,8 @@ impl<'a, 'tcx> ConstraintConversion<'a, 'tcx> {
     }
 
     fn to_region_vid(&mut self, r: ty::Region<'tcx>) -> ty::RegionVid {
-        if let ty::RePlaceholder(placeholder) = r {
-            self.constraints.placeholder_region(self.infcx, *placeholder).to_region_vid()
+        if let ty::RePlaceholder(placeholder) = *r {
+            self.constraints.placeholder_region(self.infcx, placeholder).to_region_vid()
         } else {
             self.universal_regions.to_region_vid(r)
         }