Refactor overflow handling in traits::select to propagate overflow instead of aborting eagerly

We store the obligation that caused the overflow as part of the OverflowError, and report it at the public API endpoints (rather than in the implementation internals).

Author: aravind-pg

src/librustc/traits/error_reporting.rs

@@ -24,6 +24,7 @@ use super::{
     SelectionContext,
     SelectionError,
     ObjectSafetyViolation,
+    Overflow,
 };
 
 use errors::DiagnosticBuilder;
@@ -830,6 +831,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 }
                 err.struct_error(self.tcx, span, "constant expression")
             }
+
+            Overflow(_) => {
+                bug!("overflow should be handled before the `report_selection_error` path");
+            }
         };
         self.note_obligation_cause(&mut err, obligation);
         err.emit();

src/librustc/traits/mod.rs

@@ -349,6 +349,8 @@ pub enum SelectionError<'tcx> {
                                 ty::error::TypeError<'tcx>),
     TraitNotObjectSafe(DefId),
     ConstEvalFailure(ConstEvalErr<'tcx>),
+    // upon overflow, stores the obligation that hit the recursion limit
+    Overflow(TraitObligation<'tcx>),
 }
 
 pub struct FulfillmentError<'tcx> {