Make Clone::clone a lang item

Author: saethlin

compiler/rustc_hir/src/lang_items.rs

@@ -413,6 +413,7 @@ language_item_table! {
     EffectsIntersectionOutput, sym::EffectsIntersectionOutput, effects_intersection_output, Target::AssocTy, GenericRequirement::None;
     EffectsCompat,           sym::EffectsCompat,       effects_compat,             Target::Trait,          GenericRequirement::Exact(1);
     EffectsTyCompat,         sym::EffectsTyCompat,     effects_ty_compat,          Target::Trait,          GenericRequirement::Exact(1);
+    CloneFn,                 sym::clone_fn,            clone_fn,                   Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
 }
 
 pub enum GenericRequirement {

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -271,8 +271,7 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
             return;
         }
 
-        let trait_def_id = self.tcx.trait_of_item(fn_def_id);
-        if trait_def_id.is_none() || trait_def_id != self.tcx.lang_items().clone_trait() {
+        if Some(fn_def_id) != self.tcx.lang_items().clone_fn() {
             return;
         }
 

compiler/rustc_span/src/symbol.rs

@@ -557,6 +557,7 @@ symbols! {
         clobber_abi,
         clone,
         clone_closures,
+        clone_fn,
         clone_from,
         closure,
         closure_lifetime_binder,

library/core/src/clone.rs

@@ -160,6 +160,7 @@ pub trait Clone: Sized {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use = "cloning is often expensive and is not expected to have side effects"]
+    #[cfg_attr(not(bootstrap), lang = "clone_fn")]
     fn clone(&self) -> Self;
 
     /// Performs copy-assignment from `source`.