Make Clone::clone a lang item

compiler/rustc_hir/src/lang_items.rs

@@ -162,6 +162,7 @@ language_item_table! {
     StructuralPeq,           sym::structural_peq,      structural_peq_trait,       Target::Trait,          GenericRequirement::None;
     Copy,                    sym::copy,                copy_trait,                 Target::Trait,          GenericRequirement::Exact(0);
     Clone,                   sym::clone,               clone_trait,                Target::Trait,          GenericRequirement::None;
+    CloneFn,                 sym::clone_fn,            clone_fn,                   Target::Method(MethodKind::Trait { body: false }), GenericRequirement::None;
     Sync,                    sym::sync,                sync_trait,                 Target::Trait,          GenericRequirement::Exact(0);
     DiscriminantKind,        sym::discriminant_kind,   discriminant_kind_trait,    Target::Trait,          GenericRequirement::None;
     /// The associated item of the `DiscriminantKind` trait.

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -3,6 +3,7 @@
 use crate::simplify::simplify_duplicate_switch_targets;
 use crate::take_array;
 use rustc_ast::attr;
+use rustc_hir::LangItem;
 use rustc_middle::bug;
 use rustc_middle::mir::*;
 use rustc_middle::ty::layout;
@@ -271,8 +272,7 @@ 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 !self.tcx.is_lang_item(fn_def_id, LangItem::CloneFn) {
             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 +160,9 @@ 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"]
+    // Clone::clone is special because the compiler generates MIR to implement it for some types.
+    // See InstanceKind::CloneShim.
+    #[cfg_attr(not(bootstrap), lang = "clone_fn")]
     fn clone(&self) -> Self;
 
     /// Performs copy-assignment from `source`.