Handle error case safely with a fallback

Author: Nashenas88

src/librustc_typeck/check/method/suggest.rs

@@ -73,33 +73,42 @@ pub fn report_error<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                         // snippet
                     };
 
-                    // Determine if the field can be used as a function in some way
-                    let fn_once_trait_did = match cx.lang_items.require(FnOnceTraitLangItem) {
-                        Ok(trait_did) => trait_did,
-                        Err(err) => cx.sess.fatal(&err[..])
-                    };
+                    fn span_stored_function() {
+                        cx.sess.span_note(span, &format!("use `({0}.{1})(...)` if you meant to call \
+                                                          the function stored in the `{1}` field",
+                                                         expr_string, item_name));
+                    }
 
-                    let field_ty = ty::lookup_field_type(cx, did, field.id, substs);
-                    let field_ty_substs = Substs::new_trait(vec![fcx.inh.infcx.next_ty_var()],
-                                                            Vec::new(),
-                                                            field_ty);
-                    let trait_ref = ty::TraitRef::new(fn_once_trait_did,
-                                                      cx.mk_substs(field_ty_substs));
-                    let poly_trait_ref = trait_ref.to_poly_trait_ref();
-                    let obligation = Obligation::misc(span,
-                                                      fcx.body_id,
-                                                      poly_trait_ref.as_predicate());
-                    let mut selcx = SelectionContext::new(fcx.infcx(), fcx);
-
-                    if selcx.evaluate_obligation(&obligation) {
-                        cx.sess.span_note(span,
-                            &format!("use `({0}.{1})(...)` if you meant to call the \
-                                      function stored in the `{1}` field",
-                                expr_string, item_name));
-                    } else {
+                    fn span_did_you_mean() {
                         cx.sess.span_note(span, &format!("did you mean to write `{0}.{1}`?",
                                                          expr_string, item_name));
                     }
+
+                    // Determine if the field can be used as a function in some way
+                    let field_ty = ty::lookup_field_type(cx, did, field.id, substs);
+                    if let Ok(fn_once_trait_did) = cx.lang_items.require(FnOnceTraitLangItem) {
+                        let fn_once_substs = Substs::new_trait(vec![fcx.inh.infcx.next_ty_var()],
+                                                               Vec::new(),
+                                                               field_ty);
+                        let trait_ref = ty::TraitRef::new(fn_once_trait_did,
+                                                          cx.mk_substs(fn_once_substs));
+                        let poly_trait_ref = trait_ref.to_poly_trait_ref();
+                        let obligation = Obligation::misc(span,
+                                                          fcx.body_id,
+                                                          poly_trait_ref.as_predicate());
+                        let mut selcx = SelectionContext::new(fcx.infcx(), fcx);
+
+                        if selcx.evaluate_obligation(&obligation) {
+                            span_stored_function();
+                        } else {
+                            span_did_you_mean();
+                        }
+                    } else {
+                        match field_ty.sty {
+                            ty::TyClosure(_,_) | ty::TyFnPtr(_,_) => span_stored_function(),
+                            _ => span_did_you_mean(),
+                        }
+                    }
                 }
             }