Display short types for unimplemented trait

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

@@ -669,6 +669,7 @@ impl<'tcx> OnUnimplementedDirective {
             options.iter().filter_map(|(k, v)| v.clone().map(|v| (*k, v))).collect();
 
         for command in self.subcommands.iter().chain(Some(self)).rev() {
+            debug!(?command);
             if let Some(ref condition) = command.condition
                 && !attr::eval_condition(condition, &tcx.sess, Some(tcx.features()), &mut |cfg| {
                     let value = cfg.value.map(|v| {
@@ -824,8 +825,12 @@ impl<'tcx> OnUnimplementedFormatString {
             .filter_map(|param| {
                 let value = match param.kind {
                     GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
+                        if let Some(ty) = trait_ref.args[param.index as usize].as_type() {
+                            tcx.short_ty_string(ty, &mut None)
+                        } else {
                             trait_ref.args[param.index as usize].to_string()
                         }
+                    }
                     GenericParamDefKind::Lifetime => return None,
                 };
                 let name = param.name;

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

@@ -3507,7 +3507,8 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             }
             ObligationCauseCode::OpaqueReturnType(expr_info) => {
                 if let Some((expr_ty, expr_span)) = expr_info {
-                    let expr_ty = with_forced_trimmed_paths!(self.ty_to_string(expr_ty));
+                    let expr_ty =
+                        with_forced_trimmed_paths!(self.tcx.short_ty_string(expr_ty, &mut None));
                     err.span_label(
                         expr_span,
                         with_forced_trimmed_paths!(format!(

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

@@ -389,6 +389,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     kind: _,
                 } = *obligation.cause.code()
                 {
+                    debug!("ObligationCauseCode::CompareImplItemObligation");
                     return self.report_extra_impl_obligation(
                         span,
                         impl_item_def_id,

tests/ui/traits/on_unimplemented_long_types.rs

@@ -0,0 +1,17 @@
+//@ compile-flags: --diagnostic-width=60 -Z write-long-types-to-disk=yes
+//@ normalize-stderr-test: "long-type-\d+" -> "long-type-hash"
+
+pub fn foo() -> impl std::fmt::Display {
+    //~^ ERROR doesn't implement `std::fmt::Display`
+    Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(
+        Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(
+            Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(
+                Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(Some(
+                    Some(Some(Some(Some(Some(Some(Some(Some(())))))))),
+                ))))))))))),
+            ))))))))))),
+        ))))))))))),
+    )))))))))))
+}
+
+fn main() {}

tests/ui/traits/on_unimplemented_long_types.stderr

@@ -0,0 +1,21 @@
+error[E0277]: `Option<Option<Option<...>>>` doesn't implement `std::fmt::Display`
+  --> $DIR/on_unimplemented_long_types.rs:4:17
+   |
+LL |   pub fn foo() -> impl std::fmt::Display {
+   |                   ^^^^^^^^^^^^^^^^^^^^^^ `Option<Option<Option<...>>>` cannot be formatted with the default formatter
+LL |
+LL | /     Some(Some(Some(Some(Some(Some(Some(Some(Some(S...
+LL | |         Some(Some(Some(Some(Some(Some(Some(Some(So...
+LL | |             Some(Some(Some(Some(Some(Some(Some(Som...
+LL | |                 Some(Some(Some(Some(Some(Some(Some...
+...  |
+LL | |         ))))))))))),
+LL | |     )))))))))))
+   | |_______________- return type was inferred to be `Option<Option<Option<...>>>` here
+   |
+   = 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
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.