Region inference: Use outlives-static constraints in constraint search

Revise the extra `r: 'static` constraints added upon universe issues
to add an explanation, and use that explanation during constraint blame
search. This greatly simplifies the region inference logic, which
now does not need to reverse-engineer the event that caused a region
to outlive 'static.

Author: amandasystems

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -565,7 +565,7 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
         let (blame_constraint, path) = self.regioncx.best_blame_constraint(
             borrow_region,
             NllRegionVariableOrigin::FreeRegion,
-            |r| self.regioncx.provides_universal_region(r, borrow_region, outlived_region),
+            outlived_region,
         );
         let BlameConstraint { category, from_closure, cause, .. } = blame_constraint;
 

compiler/rustc_borrowck/src/diagnostics/opaque_suggestions.rs

@@ -174,10 +174,9 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for FindOpaqueRegion<'_, 'tcx> {
                 let opaque_region_vid = self.regioncx.to_region_vid(opaque_region);
 
                 // Find a path between the borrow region and our opaque capture.
-                if let Some((path, _)) =
-                    self.regioncx.find_constraint_paths_between_regions(self.borrow_region, |r| {
-                        r == opaque_region_vid
-                    })
+                if let Some(path) = self
+                    .regioncx
+                    .constraint_path_between_regions(self.borrow_region, opaque_region_vid)
                 {
                     for constraint in path {
                         // If we find a call in this path, then check if it defines the opaque.

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -62,7 +62,7 @@ impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
             | ConstraintCategory::Boring
             | ConstraintCategory::BoringNoLocation
             | ConstraintCategory::Internal
-            | ConstraintCategory::IllegalUniverse => "",
+            | ConstraintCategory::IllegalPlaceholder(..) => "",
         }
     }
 }
@@ -456,9 +456,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
     ) {
         debug!("report_region_error(fr={:?}, outlived_fr={:?})", fr, outlived_fr);
 
-        let (blame_constraint, path) = self.regioncx.best_blame_constraint(fr, fr_origin, |r| {
-            self.regioncx.provides_universal_region(r, fr, outlived_fr)
-        });
+        let (blame_constraint, path) =
+            self.regioncx.best_blame_constraint(fr, fr_origin, outlived_fr);
         let BlameConstraint { category, cause, variance_info, .. } = blame_constraint;
 
         debug!("report_region_error: category={:?} {:?} {:?}", category, cause, variance_info);