Add helper method for reusing an existing interned region

src/librustc_infer/infer/canonical/canonicalizer.rs

@@ -326,13 +326,7 @@ fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
                      opportunistically resolved to {:?}",
                     vid, r
                 );
-                // micro-optimize -- avoid an interner look-up if the vid
-                // hasn't changed.
-                let r = if vid == resolved_vid {
-                    r
-                } else {
-                    self.tcx.mk_region(ty::ReVar(resolved_vid))
-                };
+                let r = self.tcx.reuse_or_mk_region(r, ty::ReVar(resolved_vid));
                 self.canonicalize_region_mode.canonicalize_free_region(self, r)
             }
 

src/librustc_infer/infer/resolve.rs

@@ -85,7 +85,7 @@ fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
                     .borrow_mut()
                     .unwrap_region_constraints()
                     .opportunistic_resolve_var(rid);
-                if resolved == rid { r } else { self.tcx().mk_region(ty::ReVar(resolved)) }
+                self.tcx().reuse_or_mk_region(r, ty::ReVar(resolved))
             }
             _ => r,
         }

src/librustc_middle/ty/context.rs

@@ -2081,6 +2081,13 @@ pub fn signature_unclosure(
         })
     }
 
+    /// Same a `self.mk_region(kind)`, but avoids accessing the interners if
+    /// `*r == kind`.
+    #[inline]
+    pub fn reuse_or_mk_region(self, r: Region<'tcx>, kind: RegionKind) -> Region<'tcx> {
+        if *r == kind { r } else { self.mk_region(kind) }
+    }
+
     #[allow(rustc::usage_of_ty_tykind)]
     #[inline]
     pub fn mk_ty(&self, st: TyKind<'tcx>) -> Ty<'tcx> {