diff --git a/compiler/rustc_codegen_cranelift/src/abi/mod.rs b/compiler/rustc_codegen_cranelift/src/abi/mod.rs
index fd1f081a0a8..6e846d721f2 100644
--- a/compiler/rustc_codegen_cranelift/src/abi/mod.rs
+++ b/compiler/rustc_codegen_cranelift/src/abi/mod.rs
@@ -396,9 +396,7 @@ pub(crate) fn codegen_terminator_call<'tcx>(
                     source_info,
                 ) {
                     Ok(()) => return,
-                    // Unimplemented intrinsics must have a fallback body. The fallback body is obtained
-                    // by converting the `InstanceDef::Intrinsic` to an `InstanceDef::Item`.
-                    Err(()) => Some(Instance::new(instance.def_id(), instance.args)),
+                    Err(instance) => Some(instance),
                 }
             }
             InstanceDef::DropGlue(_, None) => {
diff --git a/compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs b/compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs
index 5e64fec76e4..210a3da2c5d 100644
--- a/compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs
+++ b/compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs
@@ -268,7 +268,7 @@ pub(crate) fn codegen_intrinsic_call<'tcx>(
     destination: CPlace<'tcx>,
     target: Option<BasicBlock>,
     source_info: mir::SourceInfo,
-) -> Result<(), ()> {
+) -> Result<(), Instance<'tcx>> {
     let intrinsic = fx.tcx.item_name(instance.def_id());
     let instance_args = instance.args;
 
@@ -431,7 +431,7 @@ fn codegen_regular_intrinsic_call<'tcx>(
     ret: CPlace<'tcx>,
     destination: Option<BasicBlock>,
     source_info: mir::SourceInfo,
-) -> Result<(), ()> {
+) -> Result<(), Instance<'tcx>> {
     assert_eq!(generic_args, instance.args);
     let usize_layout = fx.layout_of(fx.tcx.types.usize);
 
@@ -1229,14 +1229,6 @@ fn codegen_regular_intrinsic_call<'tcx>(
             ret.write_cvalue(fx, CValue::by_val(cmp, ret.layout()));
         }
 
-        sym::const_allocate => {
-            intrinsic_args!(fx, args => (_size, _align); intrinsic);
-
-            // returns a null pointer at runtime.
-            let null = fx.bcx.ins().iconst(fx.pointer_type, 0);
-            ret.write_cvalue(fx, CValue::by_val(null, ret.layout()));
-        }
-
         sym::const_deallocate => {
             intrinsic_args!(fx, args => (_ptr, _size, _align); intrinsic);
             // nop at runtime.
@@ -1257,7 +1249,9 @@ fn codegen_regular_intrinsic_call<'tcx>(
             );
         }
 
-        _ => return Err(()),
+        // Unimplemented intrinsics must have a fallback body. The fallback body is obtained
+        // by converting the `InstanceDef::Intrinsic` to an `InstanceDef::Item`.
+        _ => return Err(Instance::new(instance.def_id(), instance.args)),
     }
 
     let ret_block = fx.get_block(destination.unwrap());
diff --git a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
index eac8cb43779..f162ef831b7 100644
--- a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
+++ b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
@@ -90,7 +90,7 @@ fn get_simple_intrinsic<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, name: Symbol) ->
 }
 
 impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
-    fn codegen_intrinsic_call(&mut self, instance: Instance<'tcx>, fn_abi: &FnAbi<'tcx, Ty<'tcx>>, args: &[OperandRef<'tcx, RValue<'gcc>>], llresult: RValue<'gcc>, span: Span) {
+    fn codegen_intrinsic_call(&mut self, instance: Instance<'tcx>, fn_abi: &FnAbi<'tcx, Ty<'tcx>>, args: &[OperandRef<'tcx, RValue<'gcc>>], llresult: RValue<'gcc>, span: Span) -> Result<(), Instance<'tcx>> {
         let tcx = self.tcx;
         let callee_ty = instance.ty(tcx, ty::ParamEnv::reveal_all());
 
@@ -137,7 +137,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                         args[2].immediate(),
                         llresult,
                     );
-                    return;
+                    return Ok(());
                 }
                 sym::breakpoint => {
                     unimplemented!();
@@ -166,12 +166,12 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                 sym::volatile_store => {
                     let dst = args[0].deref(self.cx());
                     args[1].val.volatile_store(self, dst);
-                    return;
+                    return Ok(());
                 }
                 sym::unaligned_volatile_store => {
                     let dst = args[0].deref(self.cx());
                     args[1].val.unaligned_volatile_store(self, dst);
-                    return;
+                    return Ok(());
                 }
                 sym::prefetch_read_data
                     | sym::prefetch_write_data
@@ -269,7 +269,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                             },
                             None => {
                                 tcx.dcx().emit_err(InvalidMonomorphization::BasicIntegerType { span, name, ty });
-                                return;
+                                return Ok(());
                             }
                         }
                     }
@@ -339,7 +339,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                     extended_asm.set_volatile_flag(true);
 
                     // We have copied the value to `result` already.
-                    return;
+                    return Ok(());
                 }
 
                 sym::ptr_mask => {
@@ -357,11 +357,12 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                 _ if name_str.starts_with("simd_") => {
                     match generic_simd_intrinsic(self, name, callee_ty, args, ret_ty, llret_ty, span) {
                         Ok(llval) => llval,
-                        Err(()) => return,
+                        Err(()) => return Ok(()),
                     }
                 }
 
-                _ => bug!("unknown intrinsic '{}'", name),
+                // Fall back to default body
+                _ => return Err(Instance::new(instance.def_id(), instance.args)),
             };
 
         if !fn_abi.ret.is_ignore() {
@@ -376,6 +377,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                     .store(self, result);
             }
         }
+        Ok(())
     }
 
     fn abort(&mut self) {
diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index e3e48ecb3aa..4415c51acf6 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -86,7 +86,7 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
         args: &[OperandRef<'tcx, &'ll Value>],
         llresult: &'ll Value,
         span: Span,
-    ) {
+    ) -> Result<(), ty::Instance<'tcx>> {
         let tcx = self.tcx;
         let callee_ty = instance.ty(tcx, ty::ParamEnv::reveal_all());
 
@@ -141,7 +141,7 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                     args[2].immediate(),
                     llresult,
                 );
-                return;
+                return Ok(());
             }
             sym::breakpoint => self.call_intrinsic("llvm.debugtrap", &[]),
             sym::va_copy => {
@@ -194,17 +194,17 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 if !result.layout.is_zst() {
                     self.store(load, result.llval, result.align);
                 }
-                return;
+                return Ok(());
             }
             sym::volatile_store => {
                 let dst = args[0].deref(self.cx());
                 args[1].val.volatile_store(self, dst);
-                return;
+                return Ok(());
             }
             sym::unaligned_volatile_store => {
                 let dst = args[0].deref(self.cx());
                 args[1].val.unaligned_volatile_store(self, dst);
-                return;
+                return Ok(());
             }
             sym::prefetch_read_data
             | sym::prefetch_write_data
@@ -305,7 +305,7 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                             name,
                             ty,
                         });
-                        return;
+                        return Ok(());
                     }
                 }
             }
@@ -387,7 +387,7 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 .unwrap_or_else(|| bug!("failed to generate inline asm call for `black_box`"));
 
                 // We have copied the value to `result` already.
-                return;
+                return Ok(());
             }
 
             _ if name.as_str().starts_with("simd_") => {
@@ -395,11 +395,15 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                     self, name, callee_ty, fn_args, args, ret_ty, llret_ty, span,
                 ) {
                     Ok(llval) => llval,
-                    Err(()) => return,
+                    Err(()) => return Ok(()),
                 }
             }
 
-            _ => bug!("unknown intrinsic '{}' -- should it have been lowered earlier?", name),
+            _ => {
+                debug!("unknown intrinsic '{}' -- falling back to default body", name);
+                // Call the fallback body instead of generating the intrinsic code
+                return Err(ty::Instance::new(instance.def_id(), instance.args));
+            }
         };
 
         if !fn_abi.ret.is_ignore() {
@@ -411,6 +415,7 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                     .store(self, result);
             }
         }
+        Ok(())
     }
 
     fn abort(&mut self) {
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index 0c4661ef16e..75d413dedad 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -837,8 +837,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             };
         }
 
-        match intrinsic {
-            None | Some(sym::drop_in_place) => {}
+        let instance = match intrinsic {
+            None | Some(sym::drop_in_place) => instance,
             Some(intrinsic) => {
                 let mut llargs = Vec::with_capacity(1);
                 let ret_dest = self.make_return_dest(
@@ -882,27 +882,24 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     })
                     .collect();
 
-                Self::codegen_intrinsic_call(
-                    bx,
-                    *instance.as_ref().unwrap(),
-                    fn_abi,
-                    &args,
-                    dest,
-                    span,
-                );
+                let instance = *instance.as_ref().unwrap();
+                match Self::codegen_intrinsic_call(bx, instance, fn_abi, &args, dest, span) {
+                    Ok(()) => {
+                        if let ReturnDest::IndirectOperand(dst, _) = ret_dest {
+                            self.store_return(bx, ret_dest, &fn_abi.ret, dst.llval);
+                        }
 
-                if let ReturnDest::IndirectOperand(dst, _) = ret_dest {
-                    self.store_return(bx, ret_dest, &fn_abi.ret, dst.llval);
+                        return if let Some(target) = target {
+                            helper.funclet_br(self, bx, target, mergeable_succ)
+                        } else {
+                            bx.unreachable();
+                            MergingSucc::False
+                        };
+                    }
+                    Err(instance) => Some(instance),
                 }
-
-                return if let Some(target) = target {
-                    helper.funclet_br(self, bx, target, mergeable_succ)
-                } else {
-                    bx.unreachable();
-                    MergingSucc::False
-                };
             }
-        }
+        };
 
         let mut llargs = Vec::with_capacity(arg_count);
         let destination = target.as_ref().map(|&target| {
diff --git a/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs b/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
index 8530bf9e2b3..d6a0dc6bbef 100644
--- a/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
@@ -54,6 +54,7 @@ fn memset_intrinsic<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 }
 
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
+    /// In the `Err` case, returns the instance that should be called instead.
     pub fn codegen_intrinsic_call(
         bx: &mut Bx,
         instance: ty::Instance<'tcx>,
@@ -61,7 +62,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         args: &[OperandRef<'tcx, Bx::Value>],
         llresult: Bx::Value,
         span: Span,
-    ) {
+    ) -> Result<(), ty::Instance<'tcx>> {
         let callee_ty = instance.ty(bx.tcx(), ty::ParamEnv::reveal_all());
 
         let ty::FnDef(def_id, fn_args) = *callee_ty.kind() else {
@@ -81,7 +82,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         let llval = match name {
             sym::abort => {
                 bx.abort();
-                return;
+                return Ok(());
             }
 
             sym::va_start => bx.va_start(args[0].immediate()),
@@ -150,7 +151,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     args[0].immediate(),
                     args[2].immediate(),
                 );
-                return;
+                return Ok(());
             }
             sym::write_bytes => {
                 memset_intrinsic(
@@ -161,7 +162,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     args[1].immediate(),
                     args[2].immediate(),
                 );
-                return;
+                return Ok(());
             }
 
             sym::volatile_copy_nonoverlapping_memory => {
@@ -174,7 +175,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     args[1].immediate(),
                     args[2].immediate(),
                 );
-                return;
+                return Ok(());
             }
             sym::volatile_copy_memory => {
                 copy_intrinsic(
@@ -186,7 +187,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     args[1].immediate(),
                     args[2].immediate(),
                 );
-                return;
+                return Ok(());
             }
             sym::volatile_set_memory => {
                 memset_intrinsic(
@@ -197,17 +198,17 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     args[1].immediate(),
                     args[2].immediate(),
                 );
-                return;
+                return Ok(());
             }
             sym::volatile_store => {
                 let dst = args[0].deref(bx.cx());
                 args[1].val.volatile_store(bx, dst);
-                return;
+                return Ok(());
             }
             sym::unaligned_volatile_store => {
                 let dst = args[0].deref(bx.cx());
                 args[1].val.unaligned_volatile_store(bx, dst);
-                return;
+                return Ok(());
             }
             sym::exact_div => {
                 let ty = arg_tys[0];
@@ -225,7 +226,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             name,
                             ty,
                         });
-                        return;
+                        return Ok(());
                     }
                 }
             }
@@ -245,7 +246,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             name,
                             ty: arg_tys[0],
                         });
-                        return;
+                        return Ok(());
                     }
                 }
             }
@@ -256,14 +257,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         span,
                         ty: arg_tys[0],
                     });
-                    return;
+                    return Ok(());
                 }
                 let Some((_width, signed)) = int_type_width_signed(ret_ty, bx.tcx()) else {
                     bx.tcx().dcx().emit_err(InvalidMonomorphization::FloatToIntUnchecked {
                         span,
                         ty: ret_ty,
                     });
-                    return;
+                    return Ok(());
                 };
                 if signed {
                     bx.fptosi(args[0].immediate(), llret_ty)
@@ -280,14 +281,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 }
             }
 
-            sym::const_allocate => {
-                // returns a null pointer at runtime.
-                bx.const_null(bx.type_ptr())
-            }
-
             sym::const_deallocate => {
                 // nop at runtime.
-                return;
+                return Ok(());
             }
 
             // This requires that atomic intrinsics follow a specific naming pattern:
@@ -350,10 +346,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             bx.store(val, dest.llval, dest.align);
                             let dest = result.project_field(bx, 1);
                             bx.store(success, dest.llval, dest.align);
-                            return;
                         } else {
-                            return invalid_monomorphization(ty);
+                            invalid_monomorphization(ty);
                         }
+                        return Ok(());
                     }
 
                     "load" => {
@@ -383,7 +379,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                 )
                             }
                         } else {
-                            return invalid_monomorphization(ty);
+                            invalid_monomorphization(ty);
+                            return Ok(());
                         }
                     }
 
@@ -399,10 +396,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                 val = bx.ptrtoint(val, bx.type_isize());
                             }
                             bx.atomic_store(val, ptr, parse_ordering(bx, ordering), size);
-                            return;
                         } else {
-                            return invalid_monomorphization(ty);
+                            invalid_monomorphization(ty);
                         }
+                        return Ok(());
                     }
 
                     "fence" => {
@@ -410,7 +407,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             parse_ordering(bx, ordering),
                             SynchronizationScope::CrossThread,
                         );
-                        return;
+                        return Ok(());
                     }
 
                     "singlethreadfence" => {
@@ -418,7 +415,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             parse_ordering(bx, ordering),
                             SynchronizationScope::SingleThread,
                         );
-                        return;
+                        return Ok(());
                     }
 
                     // These are all AtomicRMW ops
@@ -449,7 +446,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             }
                             bx.atomic_rmw(atom_op, ptr, val, parse_ordering(bx, ordering))
                         } else {
-                            return invalid_monomorphization(ty);
+                            invalid_monomorphization(ty);
+                            return Ok(());
                         }
                     }
                 }
@@ -458,7 +456,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             sym::nontemporal_store => {
                 let dst = args[0].deref(bx.cx());
                 args[1].val.nontemporal_store(bx, dst);
-                return;
+                return Ok(());
             }
 
             sym::ptr_guaranteed_cmp => {
@@ -493,8 +491,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
             _ => {
                 // Need to use backend-specific things in the implementation.
-                bx.codegen_intrinsic_call(instance, fn_abi, args, llresult, span);
-                return;
+                return bx.codegen_intrinsic_call(instance, fn_abi, args, llresult, span);
             }
         };
 
@@ -507,6 +504,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     .store(bx, result);
             }
         }
+        Ok(())
     }
 }
 
diff --git a/compiler/rustc_codegen_ssa/src/traits/intrinsic.rs b/compiler/rustc_codegen_ssa/src/traits/intrinsic.rs
index 450672fb941..502f0b3fcb5 100644
--- a/compiler/rustc_codegen_ssa/src/traits/intrinsic.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/intrinsic.rs
@@ -8,6 +8,8 @@ pub trait IntrinsicCallMethods<'tcx>: BackendTypes {
     /// Remember to add all intrinsics here, in `compiler/rustc_hir_analysis/src/check/mod.rs`,
     /// and in `library/core/src/intrinsics.rs`; if you need access to any LLVM intrinsics,
     /// add them to `compiler/rustc_codegen_llvm/src/context.rs`.
+    /// Returns `Err` if another instance should be called instead. This is used to invoke
+    /// intrinsic default bodies in case an intrinsic is not implemented by the backend.
     fn codegen_intrinsic_call(
         &mut self,
         instance: ty::Instance<'tcx>,
@@ -15,7 +17,7 @@ pub trait IntrinsicCallMethods<'tcx>: BackendTypes {
         args: &[OperandRef<'tcx, Self::Value>],
         llresult: Self::Value,
         span: Span,
-    );
+    ) -> Result<(), ty::Instance<'tcx>>;
 
     fn abort(&mut self);
     fn assume(&mut self, val: Self::Value);
diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index b0cf1d1656d..397112e8db7 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -363,7 +363,7 @@ pub fn check_intrinsic_type(
             ),
 
             sym::const_allocate => {
-                (0, 0, vec![tcx.types.usize, tcx.types.usize], Ty::new_mut_ptr(tcx, tcx.types.u8))
+                (0, 1, vec![tcx.types.usize, tcx.types.usize], Ty::new_mut_ptr(tcx, tcx.types.u8))
             }
             sym::const_deallocate => (
                 0,
diff --git a/compiler/rustc_mir_transform/src/instsimplify.rs b/compiler/rustc_mir_transform/src/instsimplify.rs
index 81cf31e6bf4..a9de37244c5 100644
--- a/compiler/rustc_mir_transform/src/instsimplify.rs
+++ b/compiler/rustc_mir_transform/src/instsimplify.rs
@@ -296,9 +296,9 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
         if args.is_empty() {
             return;
         }
-        let ty = args.type_at(0);
 
-        let known_is_valid = intrinsic_assert_panics(self.tcx, self.param_env, ty, intrinsic_name);
+        let known_is_valid =
+            intrinsic_assert_panics(self.tcx, self.param_env, args[0], intrinsic_name);
         match known_is_valid {
             // We don't know the layout or it's not validity assertion at all, don't touch it
             None => {}
@@ -317,10 +317,11 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
 fn intrinsic_assert_panics<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    ty: Ty<'tcx>,
+    arg: ty::GenericArg<'tcx>,
     intrinsic_name: Symbol,
 ) -> Option<bool> {
     let requirement = ValidityRequirement::from_intrinsic(intrinsic_name)?;
+    let ty = arg.expect_ty();
     Some(!tcx.check_validity_requirement((requirement, param_env.and(ty))).ok()?)
 }
 
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 149e4c2cb08..a9926820caf 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -956,19 +956,24 @@ fn visit_instance_use<'tcx>(
     if !should_codegen_locally(tcx, &instance) {
         return;
     }
-
-    // The intrinsics assert_inhabited, assert_zero_valid, and assert_mem_uninitialized_valid will
-    // be lowered in codegen to nothing or a call to panic_nounwind. So if we encounter any
-    // of those intrinsics, we need to include a mono item for panic_nounwind, else we may try to
-    // codegen a call to that function without generating code for the function itself.
     if let ty::InstanceDef::Intrinsic(def_id) = instance.def {
         let name = tcx.item_name(def_id);
         if let Some(_requirement) = ValidityRequirement::from_intrinsic(name) {
+            // The intrinsics assert_inhabited, assert_zero_valid, and assert_mem_uninitialized_valid will
+            // be lowered in codegen to nothing or a call to panic_nounwind. So if we encounter any
+            // of those intrinsics, we need to include a mono item for panic_nounwind, else we may try to
+            // codegen a call to that function without generating code for the function itself.
             let def_id = tcx.lang_items().get(LangItem::PanicNounwind).unwrap();
             let panic_instance = Instance::mono(tcx, def_id);
             if should_codegen_locally(tcx, &panic_instance) {
                 output.push(create_fn_mono_item(tcx, panic_instance, source));
             }
+        } else if tcx.has_attr(def_id, sym::rustc_intrinsic) {
+            // Codegen the fallback body of intrinsics with fallback bodies
+            let instance = ty::Instance::new(def_id, instance.args);
+            if should_codegen_locally(tcx, &instance) {
+                output.push(create_fn_mono_item(tcx, instance, source));
+            }
         }
     }
 
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index d56c3d3b6c1..3a178510e79 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -2368,18 +2368,6 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn ptr_guaranteed_cmp<T>(ptr: *const T, other: *const T) -> u8;
 
-    /// Allocates a block of memory at compile time.
-    /// At runtime, just returns a null pointer.
-    ///
-    /// # Safety
-    ///
-    /// - The `align` argument must be a power of two.
-    ///    - At compile time, a compile error occurs if this constraint is violated.
-    ///    - At runtime, it is not checked.
-    #[rustc_const_unstable(feature = "const_heap", issue = "79597")]
-    #[rustc_nounwind]
-    pub fn const_allocate(size: usize, align: usize) -> *mut u8;
-
     /// Deallocates a memory which allocated by `intrinsics::const_allocate` at compile time.
     /// At runtime, does nothing.
     ///
@@ -2594,6 +2582,22 @@ pub(crate) const unsafe fn debug_assertions() -> bool {
     cfg!(debug_assertions)
 }
 
+/// Allocates a block of memory at compile time.
+/// At runtime, just returns a null pointer.
+///
+/// # Safety
+///
+/// - The `align` argument must be a power of two.
+///    - At compile time, a compile error occurs if this constraint is violated.
+///    - At runtime, it is not checked.
+#[rustc_const_unstable(feature = "const_heap", issue = "79597")]
+#[rustc_nounwind]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
+    // const eval overrides this function, but runtime code should always just return null pointers.
+    crate::ptr::null_mut()
+}
+
 // Some functions are defined here because they accidentally got made
 // available in this module on stable. See <https://github.com/rust-lang/rust/issues/15702>.
 // (`transmute` also falls into this category, but it cannot be wrapped due to the