Address code reviews

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -10,6 +10,7 @@ use rustc_middle::ty::{self, Ty};
 use rustc_mir_dataflow::move_paths::{LookupResult, MovePathIndex};
 use rustc_span::{BytePos, ExpnKind, MacroKind, Span};
 use rustc_trait_selection::error_reporting::traits::FindExprBySpan;
+use rustc_trait_selection::infer::InferCtxtExt;
 use tracing::debug;
 
 use crate::MirBorrowckCtxt;
@@ -270,6 +271,10 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             if self.maybe_copy(original_path.ty(self.body, self.infcx.tcx).ty) {
                 // If the type may implement Copy, skip the error.
                 // It's an error with the Copy implementation (e.g. duplicate Copy) rather than borrow check
+                self.span_delayed_bug(
+                    span,
+                    "Type may implement copy, but there is no other error.",
+                );
                 return;
             }
             (
@@ -297,8 +302,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
     }
 
     fn maybe_copy(&mut self, ty: Ty<'tcx>) -> bool {
-        use rustc_trait_selection::infer::InferCtxtExt;
         let Some(copy_trait_def) = self.infcx.tcx.lang_items().copy_trait() else { return false };
+        // This is only going to be ambiguous if there are incoherent impls, because otherwise
+        // ambiguity should never happen in MIR.
         self.infcx.type_implements_trait(copy_trait_def, [ty], self.param_env).may_apply()
     }
 

tests/ui/derives/duplicate-derive-copy-clone-diagnostics.rs

@@ -3,4 +3,4 @@
 //~^ ERROR conflicting implementations of trait `Copy`
 enum E {}
 
-fn main() {}
+fn main() {}