Add a scheme for moving away from extern "rust-intrinsic" entirely

compiler/rustc_middle/src/ty/intrinsic.rs

@@ -5,6 +5,8 @@ use super::TyCtxt;
 #[derive(Copy, Clone, Debug, Decodable, Encodable, HashStable)]
 pub struct IntrinsicDef {
     pub name: Symbol,
+    /// Whether the intrinsic has no meaningful body and all backends need to shim all calls to it.
+    pub must_be_overridden: bool,
 }
 
 impl TyCtxt<'_> {

compiler/rustc_middle/src/ty/util.rs

@@ -1646,7 +1646,10 @@ pub fn intrinsic(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::IntrinsicDef
     if matches!(tcx.fn_sig(def_id).skip_binder().abi(), Abi::RustIntrinsic)
         || tcx.has_attr(def_id, sym::rustc_intrinsic)
     {
-        Some(ty::IntrinsicDef { name: tcx.item_name(def_id.into()) })
+        Some(ty::IntrinsicDef {
+            name: tcx.item_name(def_id.into()),
+            must_be_overridden: tcx.has_attr(def_id, sym::rustc_intrinsic_must_be_overridden),
+        })
     } else {
         None
     }