Use question_mark feature in librustc_const_eval.

src/librustc_const_eval/eval.rs

@@ -278,9 +278,9 @@ pub fn const_expr_to_pat<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
     let pat = match expr.node {
         hir::ExprTup(ref exprs) =>
-            PatKind::Tuple(try!(exprs.iter()
-                                     .map(|expr| const_expr_to_pat(tcx, &expr, pat_id, span))
-                                     .collect()), None),
+            PatKind::Tuple(exprs.iter()
+                                .map(|expr| const_expr_to_pat(tcx, &expr, pat_id, span))
+                                .collect::<Result<_, _>>()?, None),
 
         hir::ExprCall(ref callee, ref args) => {
             let def = tcx.expect_def(callee.id);
@@ -297,34 +297,31 @@ pub fn const_expr_to_pat<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 })),
                 _ => bug!()
             };
-            let pats = try!(args.iter()
-                                .map(|expr| const_expr_to_pat(tcx, &**expr,
-                                                              pat_id, span))
-                                .collect());
+            let pats = args.iter()
+                           .map(|expr| const_expr_to_pat(tcx, &**expr, pat_id, span))
+                           .collect::<Result<_, _>>()?;
             PatKind::TupleStruct(path, pats, None)
         }
 
         hir::ExprStruct(ref path, ref fields, None) => {
             let field_pats =
-                try!(fields.iter()
-                           .map(|field| Ok(codemap::Spanned {
-                               span: syntax_pos::DUMMY_SP,
-                               node: hir::FieldPat {
-                                   name: field.name.node,
-                                   pat: try!(const_expr_to_pat(tcx, &field.expr,
-                                                               pat_id, span)),
-                                   is_shorthand: false,
-                               },
-                           }))
-                           .collect());
+                fields.iter()
+                      .map(|field| Ok(codemap::Spanned {
+                          span: syntax_pos::DUMMY_SP,
+                          node: hir::FieldPat {
+                              name: field.name.node,
+                              pat: const_expr_to_pat(tcx, &field.expr, pat_id, span)?,
+                              is_shorthand: false,
+                          },
+                      }))
+                      .collect::<Result<_, _>>()?;
             PatKind::Struct(path.clone(), field_pats, false)
         }
 
         hir::ExprVec(ref exprs) => {
-            let pats = try!(exprs.iter()
-                                 .map(|expr| const_expr_to_pat(tcx, &expr,
-                                                               pat_id, span))
-                                 .collect());
+            let pats = exprs.iter()
+                            .map(|expr| const_expr_to_pat(tcx, &expr, pat_id, span))
+                            .collect::<Result<_, _>>()?;
             PatKind::Vec(pats, None, hir::HirVec::new())
         }