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

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -16,6 +16,7 @@ use rustc_middle::ty::{self, SymbolName, TyCtxt};
 use rustc_middle::ty::{GenericArgKind, GenericArgsRef};
 use rustc_middle::util::Providers;
 use rustc_session::config::{CrateType, OomStrategy};
+use rustc_span::sym;
 use rustc_target::spec::{SanitizerSet, TlsModel};
 
 pub fn threshold(tcx: TyCtxt<'_>) -> SymbolExportLevel {
@@ -81,6 +82,10 @@ fn reachable_non_generics_provider(tcx: TyCtxt<'_>, _: LocalCrate) -> DefIdMap<S
                 return library.kind.is_statically_included().then_some(def_id);
             }
 
+            if tcx.has_attr(def_id, sym::rustc_intrinsic_must_be_overridden) {
+                return None;
+            }
+
             // Only consider nodes that actually have exported symbols.
             match tcx.def_kind(def_id) {
                 DefKind::Fn | DefKind::Static(_) => {}

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -903,7 +903,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             MergingSucc::False
                         };
                     }
-                    Err(instance) => Some(instance),
+                    Err(instance) => {
+                        if intrinsic.must_be_overridden {
+                            span_bug!(
+                                span,
+                                "intrinsic {} must be overridden by codegen backend, but isn't",
+                                intrinsic.name,
+                            );
+                        }
+                        Some(instance)
+                    }
                 }
             }
         };