Improve errors for missing Debug and Display impls

compiler/rustc_span/src/symbol.rs

@@ -479,6 +479,7 @@ symbols! {
         discriminant_type,
         discriminant_value,
         dispatch_from_dyn,
+        display_trait,
         div,
         div_assign,
         doc,

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

@@ -514,6 +514,30 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                             }
                         }
 
+                        // Return early if the trait is Debug or Display and the invocation
+                        // originates within a standard library macro, because the output
+                        // is otherwise overwhelming and unhelpful (see #85844 for an
+                        // example).
+
+                        let trait_is_debug =
+                            self.tcx.is_diagnostic_item(sym::debug_trait, trait_ref.def_id());
+                        let trait_is_display =
+                            self.tcx.is_diagnostic_item(sym::display_trait, trait_ref.def_id());
+
+                        let in_std_macro =
+                            match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
+                                Some(macro_def_id) => {
+                                    let crate_name = tcx.crate_name(macro_def_id.krate);
+                                    crate_name == sym::std || crate_name == sym::core
+                                }
+                                None => false,
+                            };
+
+                        if in_std_macro && (trait_is_debug || trait_is_display) {
+                            err.emit();
+                            return;
+                        }
+
                         err
                     }
 

library/core/src/fmt/mod.rs

@@ -564,7 +564,7 @@ impl Display for Arguments<'_> {
     on(
         crate_local,
         label = "`{Self}` cannot be formatted using `{{:?}}`",
-        note = "add `#[derive(Debug)]` or manually implement `{Debug}`"
+        note = "add `#[derive(Debug)]` to `{Self}` or manually implement `{Debug}` for `{Self}`"
     ),
     message = "`{Self}` doesn't implement `{Debug}`",
     label = "`{Self}` cannot be formatted using `{{:?}}` because it doesn't implement `{Debug}`"
@@ -662,6 +662,7 @@ pub use macros::Debug;
     note = "in format strings you may be able to use `{{:?}}` (or {{:#?}} for pretty-print) instead"
 )]
 #[doc(alias = "{}")]
+#[rustc_diagnostic_item = "display_trait"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Display {
     /// Formats the value using the given formatter.