Fix duplicate display of error E0502

Author: Josh Leeb-du Toit

src/librustc_borrowck/borrowck/check_loans.rs

@@ -395,18 +395,40 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
         assert!(self.bccx.region_scope_tree.scopes_intersect(old_loan.kill_scope,
                                                        new_loan.kill_scope));
 
-        self.report_error_if_loan_conflicts_with_restriction(
-            old_loan, new_loan, old_loan, new_loan) &&
-        self.report_error_if_loan_conflicts_with_restriction(
-            new_loan, old_loan, old_loan, new_loan)
+        let err_old_new = match self.report_error_if_loan_conflicts_with_restriction(
+            old_loan, new_loan, old_loan, new_loan
+        ) {
+            Err(err) => Some(err),
+            Ok(_) => None
+        };
+
+        let err_new_old = match self.report_error_if_loan_conflicts_with_restriction(
+            new_loan, old_loan, old_loan, new_loan
+        ) {
+            Err(err) => Some(err),
+            Ok(_) => None
+        };
+
+        if let Some(mut err_old) = err_old_new {
+            err_old.emit();
+            if let Some(mut err_new) = err_new_old {
+                err_new.cancel();
+            }
+        } else if let Some(mut err_new) = err_new_old {
+            err_new.emit();
+        } else {
+            return true;
+        }
+
+        false
     }
 
     pub fn report_error_if_loan_conflicts_with_restriction(&self,
                                                            loan1: &Loan<'tcx>,
                                                            loan2: &Loan<'tcx>,
                                                            old_loan: &Loan<'tcx>,
                                                            new_loan: &Loan<'tcx>)
-                                                           -> bool {
+                                                           -> Result<(), DiagnosticBuilder<'a>> {
         //! Checks whether the restrictions introduced by `loan1` would
         //! prohibit `loan2`. Returns false if an error is reported.
 
@@ -416,7 +438,7 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                loan2);
 
         if compatible_borrow_kinds(loan1.kind, loan2.kind) {
-            return true;
+            return Ok(());
         }
 
         let loan2_base_path = owned_ptr_base_path_rc(&loan2.loan_path);
@@ -520,11 +542,10 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                 _ => { }
             }
 
-            err.emit();
-            return false;
+            return Err(err);
         }
 
-        true
+        Ok(())
     }
 
     fn consume_common(&self,