From f1f1c9b25b1046baecd271963739a8fef7c358e1 Mon Sep 17 00:00:00 2001
From: Aris Merchant <22333129+inquisitivecrystal@users.noreply.github.com>
Date: Sun, 13 Jun 2021 01:36:53 -0700
Subject: Improve errors for missing Debug and Display impls

---
 compiler/rustc_span/src/symbol.rs                  |  1 +
 .../src/traits/error_reporting/mod.rs              | 24 ++++++++++++++++++++++
 2 files changed, 25 insertions(+)

(limited to 'compiler')

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index fb37c5e9c1e..45c35959380 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -479,6 +479,7 @@ symbols! {
         discriminant_type,
         discriminant_value,
         dispatch_from_dyn,
+        display_trait,
         div,
         div_assign,
         doc,
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 19c3385dd4c..e276d92bf5a 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/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
                     }
 
-- 
cgit 1.4.1-3-g733a5