Rollup merge of #104320 - fee1-dead-contrib:use-derive-const-in-std, r=oli-obk

Use `derive_const` and rm manual StructuralEq impl

This does not change any semantics of the impl except for the const stability. It should be fine because trait methods and const bounds can never be used in stable without enabling `const_trait_impl`.

cc `@oli-obk`

compiler/rustc_passes/src/stability.rs

@@ -536,6 +536,14 @@ impl<'tcx> MissingStabilityAnnotations<'tcx> {
             return;
         }
 
+        // if the const impl is derived using the `derive_const` attribute,
+        // then it would be "stable" at least for the impl.
+        // We gate usages of it using `feature(const_trait_impl)` anyways
+        // so there is no unstable leakage
+        if self.tcx.is_builtin_derive(def_id.to_def_id()) {
+            return;
+        }
+
         let is_const = self.tcx.is_const_fn(def_id.to_def_id())
             || self.tcx.is_const_trait_impl_raw(def_id.to_def_id());
         let is_stable = self