Avoid silently writing to a file when the involved ty is long

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -1054,6 +1054,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     bound_list.into_iter().map(|(_, path)| path).collect::<Vec<_>>().join("\n");
                 let actual_prefix = rcvr_ty.prefix_string(self.tcx);
                 info!("unimplemented_traits.len() == {}", unimplemented_traits.len());
+                let mut long_ty_file = None;
                 let (primary_message, label) = if unimplemented_traits.len() == 1
                     && unimplemented_traits_only
                 {
@@ -1066,8 +1067,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 // Avoid crashing.
                                 return (None, None);
                             }
-                            let OnUnimplementedNote { message, label, .. } =
-                                self.err_ctxt().on_unimplemented_note(trait_ref, &obligation);
+                            let OnUnimplementedNote { message, label, .. } = self
+                                .err_ctxt()
+                                .on_unimplemented_note(trait_ref, &obligation, &mut long_ty_file);
                             (message, label)
                         })
                         .unwrap()
@@ -1081,6 +1083,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     )
                 });
                 err.primary_message(primary_message);
+                if let Some(file) = long_ty_file {
+                    err.note(format!(
+                        "the full name for the type has been written to '{}'",
+                        file.display(),
+                    ));
+                    err.note(
+                        "consider using `--verbose` to print the full type name to the console",
+                    );
+                }
                 if let Some(label) = label {
                     custom_span_label = true;
                     err.span_label(span, label);

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -111,9 +111,8 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         &self,
         trait_ref: ty::PolyTraitRef<'tcx>,
         obligation: &PredicateObligation<'tcx>,
+        long_ty_file: &mut Option<PathBuf>,
     ) -> OnUnimplementedNote {
-        let mut long_ty_file = None;
-
         let (def_id, args) = self
             .impl_similar_to(trait_ref, obligation)
             .unwrap_or_else(|| (trait_ref.def_id(), trait_ref.skip_binder().args));
@@ -268,7 +267,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }));
 
         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(self.tcx, def_id) {
-            command.evaluate(self.tcx, trait_ref, &flags, &mut long_ty_file)
+            command.evaluate(self.tcx, trait_ref, &flags, long_ty_file)
         } else {
             OnUnimplementedNote::default()
         }

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -445,13 +445,16 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             file.display(),
                         ));
 
+                        let mut long_ty_file = None;
+
                         let OnUnimplementedNote {
                             message,
                             label,
                             notes,
                             parent_label,
                             append_const_msg,
-                        } = self.on_unimplemented_note(trait_ref, &obligation);
+                        } = self.on_unimplemented_note(trait_ref, &obligation, &mut long_ty_file);
+
                         let have_alt_message = message.is_some() || label.is_some();
                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
                         let is_unsize =
@@ -506,6 +509,13 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
                         let mut err = struct_span_code_err!(self.dcx(), span, E0277, "{}", err_msg);
 
+                        if let Some(long_ty_file) = long_ty_file {
+                            err.note(format!(
+                                "the full name for the type has been written to '{}'",
+                                long_ty_file.display(),
+                            ));
+                            err.note("consider using `--verbose` to print the full type name to the console");
+                        }
                         let mut suggested = false;
                         if is_try_conversion {
                             suggested = self.try_conversion_context(&obligation, trait_ref.skip_binder(), &mut err);
@@ -753,6 +763,8 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             return err.emit();
                         }
 
+
+
                         err
                     }
 

tests/ui/traits/on_unimplemented_long_types.stderr

@@ -13,6 +13,8 @@ LL | |         ))))))))))),
 LL | |     )))))))))))
    | |_______________- return type was inferred to be `Option<Option<Option<...>>>` here
    |
+   = note: the full name for the type has been written to '$TEST_BUILD_DIR/traits/on_unimplemented_long_types/on_unimplemented_long_types.long-type-hash.txt'
+   = note: consider using `--verbose` to print the full type name to the console
    = help: the trait `std::fmt::Display` is not implemented for `Option<Option<Option<...>>>`
    = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
    = note: the full name for the type has been written to '$TEST_BUILD_DIR/traits/on_unimplemented_long_types/on_unimplemented_long_types.long-type-hash.txt'