Fix dead_code lint

Author: fee1-dead

compiler/rustc_passes/src/dead.rs

@@ -268,6 +268,11 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                     }
                     return true;
                 }
+
+                // ignore generated `DerivedClone` but do not expose this to users.
+                if Some(trait_of) == self.tcx.get_diagnostic_item(sym::DerivedClone) {
+                    return true;
+                }
             }
         }
 

library/core/src/clone.rs

@@ -178,6 +178,7 @@ pub struct AssertParamIsCopy<T: Copy + ?Sized> {
     reason = "deriving hack, should not be public",
     issue = "none"
 )]
+#[rustc_diagnostic_item = "DerivedClone"]
 pub trait DerivedClone: Sized {
     fn clone(&self) -> Self;
 }