comments for defining_ty and compute_indices

Plus an extra assertion.

src/librustc_mir/borrow_check/nll/universal_regions.rs

@@ -456,6 +456,21 @@ impl<'cx, 'gcx, 'tcx> UniversalRegionsBuilder<'cx, 'gcx, 'tcx> {
         }
     }
 
+    /// Returns the "defining type" of the current MIR:
+    ///
+    /// - for functions, this is the `TyFnDef`;
+    /// - for closures, this is the `TyClosure`;
+    /// - for generators, this is the `TyGenerator`;
+    /// - for constants, this is the type of value that gets produced.
+    ///   - FIXME. Constants are handled somewhat inelegantly; this gets
+    ///     patched in a later PR that has already landed on nll-master.
+    ///
+    /// The key feature of the "defining type" is that it contains the
+    /// information needed to derive all the universal regions that
+    /// are in scope as well as the types of the inputs/output from
+    /// the MIR. In general, early-bound universal regions appear free
+    /// in the defining type and late-bound regions appear bound in
+    /// the signature.
     fn defining_ty(&self) -> ty::Ty<'tcx> {
         let tcx = self.infcx.tcx;
         let closure_base_def_id = tcx.closure_base_def_id(self.mir_def_id);
@@ -471,6 +486,10 @@ impl<'cx, 'gcx, 'tcx> UniversalRegionsBuilder<'cx, 'gcx, 'tcx> {
             .replace_free_regions_with_nll_infer_vars(FR, &defining_ty)
     }
 
+    /// Builds a hashmap that maps from the universal regions that are
+    /// in scope (as a `ty::Region<'tcx>`) to their indices (as a
+    /// `RegionVid`). The map returned by this function contains only
+    /// the early-bound regions.
     fn compute_indices(
         &self,
         fr_static: RegionVid,
@@ -490,6 +509,7 @@ impl<'cx, 'gcx, 'tcx> UniversalRegionsBuilder<'cx, 'gcx, 'tcx> {
                 // that correspond to early-bound regions declared on
                 // the `closure_base_def_id`.
                 assert!(substs.substs.len() >= identity_substs.len());
+                assert_eq!(substs.substs.regions().count(), identity_substs.regions().count());
                 substs.substs
             }
             ty::TyFnDef(_, substs) => substs,