add some better debugs

src/librustc/infer/higher_ranked/mod.rs

@@ -155,7 +155,10 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
                                .filter(|&&r| !skol_resolution_map.contains_key(r))
                                .cloned()
                                .next()
-                               .expect("no representative region");
+                               .unwrap_or_else(|| {
+                                   bug!("no representative region for `{:?}` in `{:?}`",
+                                        skol, regions)
+                               });
 
                     (skol, representative)
                 })

src/librustc_typeck/astconv.rs

@@ -1226,7 +1226,12 @@ impl<'o, 'gcx: 'tcx, 'tcx> AstConv<'gcx, 'tcx>+'o {
                     None
                 }
             });
-            self.ty_of_arg(a, expected_arg_ty)
+
+            let input_ty = self.ty_of_arg(a, expected_arg_ty);
+            debug!("ty_of_closure: i={} input_ty={:?} expected_arg_ty={:?}",
+                   i, input_ty, expected_arg_ty);
+
+            input_ty
         });
 
         let expected_ret_ty = expected_sig.as_ref().map(|e| e.output());

src/librustc_typeck/check/closure.rs

@@ -51,7 +51,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                      body: &'gcx hir::Body,
                      expected_sig: Option<ty::FnSig<'tcx>>)
                      -> Ty<'tcx> {
-        debug!("check_closure opt_kind={:?} expected_sig={:?}",
+        debug!("check_closure(opt_kind={:?}, expected_sig={:?})",
                opt_kind,
                expected_sig);
 
@@ -61,10 +61,15 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                          decl,
                                          Abi::RustCall,
                                          expected_sig);
+
+        debug!("check_closure: ty_of_closure returns {:?}", sig);
+
         // `deduce_expectations_from_expected_type` introduces late-bound
         // lifetimes defined elsewhere, which we need to anonymize away.
         let sig = self.tcx.anonymize_late_bound_regions(&sig);
 
+        debug!("check_closure: anonymized sig={:?}", sig);
+
         // Create type variables (for now) to represent the transformed
         // types of upvars. These will be unified during the upvar
         // inference phase (`upvar.rs`).
@@ -104,7 +109,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             sig.abi
         ));
 
-        debug!("closure for {:?} --> sig={:?} opt_kind={:?}",
+        debug!("check_closure: expr_def_id={:?}, sig={:?}, opt_kind={:?}",
                expr_def_id,
                sig,
                opt_kind);