diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 5981b5031c649..23ffedd97e057 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -606,14 +606,14 @@ fn try_inlining<'tcx, I: Inliner<'tcx>>(
         ty::EarlyBinder::bind(callee_body.clone()),
     ) else {
         debug!("failed to normalize callee body");
-        return Err("implementation limitation");
+        return Err("implementation limitation -- could not normalize callee body");
     };
 
     // Normally, this shouldn't be required, but trait normalization failure can create a
     // validation ICE.
     if !validate_types(tcx, inliner.typing_env(), &callee_body, &caller_body).is_empty() {
         debug!("failed to validate callee body");
-        return Err("implementation limitation");
+        return Err("implementation limitation -- callee body failed validation");
     }
 
     // Check call signature compatibility.
@@ -622,16 +622,14 @@ fn try_inlining<'tcx, I: Inliner<'tcx>>(
     let output_type = callee_body.return_ty();
     if !util::sub_types(tcx, inliner.typing_env(), output_type, destination_ty) {
         trace!(?output_type, ?destination_ty);
-        debug!("failed to normalize return type");
-        return Err("implementation limitation");
+        return Err("implementation limitation -- return type mismatch");
     }
     if callsite.fn_sig.abi() == ExternAbi::RustCall {
-        // FIXME: Don't inline user-written `extern "rust-call"` functions,
-        // since this is generally perf-negative on rustc, and we hope that
-        // LLVM will inline these functions instead.
-        if callee_body.spread_arg.is_some() {
-            return Err("user-written rust-call functions");
-        }
+        /* match callsite.callee.def {
+            InstanceKind::Item(def_id) if tcx.is_closure_like(def_id) => {}
+            InstanceKind::FnPtrShim(..) | InstanceKind::ClosureOnceShim { .. } => {}
+            _ => return Err("not inlining non-builtin call"),
+        } */
 
         let (self_arg, arg_tuple) = match &args[..] {
             [arg_tuple] => (None, arg_tuple),
@@ -642,12 +640,17 @@ fn try_inlining<'tcx, I: Inliner<'tcx>>(
         let self_arg_ty = self_arg.map(|self_arg| self_arg.node.ty(&caller_body.local_decls, tcx));
 
         let arg_tuple_ty = arg_tuple.node.ty(&caller_body.local_decls, tcx);
-        let ty::Tuple(arg_tuple_tys) = *arg_tuple_ty.kind() else {
-            bug!("Closure arguments are not passed as a tuple");
+        let arg_tys = if callee_body.spread_arg.is_some() {
+            std::slice::from_ref(&arg_tuple_ty)
+        } else {
+            let ty::Tuple(arg_tuple_tys) = *arg_tuple_ty.kind() else {
+                bug!("Closure arguments are not passed as a tuple");
+            };
+            arg_tuple_tys.as_slice()
         };
 
         for (arg_ty, input) in
-            self_arg_ty.into_iter().chain(arg_tuple_tys).zip(callee_body.args_iter())
+            self_arg_ty.into_iter().chain(arg_tys.iter().copied()).zip(callee_body.args_iter())
         {
             let input_type = callee_body.local_decls[input].ty;
             if !util::sub_types(tcx, inliner.typing_env(), input_type, arg_ty) {
@@ -663,7 +666,7 @@ fn try_inlining<'tcx, I: Inliner<'tcx>>(
             if !util::sub_types(tcx, inliner.typing_env(), input_type, arg_ty) {
                 trace!(?arg_ty, ?input_type);
                 debug!("failed to normalize argument type");
-                return Err("implementation limitation");
+                return Err("implementation limitation -- arg mismatch");
             }
         }
     }
@@ -693,13 +696,13 @@ fn check_mir_is_available<'tcx, I: Inliner<'tcx>>(
             // won't cause cycles on this.
             if !inliner.tcx().is_mir_available(callee_def_id) {
                 debug!("item MIR unavailable");
-                return Err("implementation limitation");
+                return Err("implementation limitation -- MIR unavailable");
             }
         }
         // These have no own callable MIR.
         InstanceKind::Intrinsic(_) | InstanceKind::Virtual(..) => {
             debug!("instance without MIR (intrinsic / virtual)");
-            return Err("implementation limitation");
+            return Err("implementation limitation -- cannot inline intrinsic");
         }
 
         // FIXME(#127030): `ConstParamHasTy` has bad interactions with
@@ -709,7 +712,7 @@ fn check_mir_is_available<'tcx, I: Inliner<'tcx>>(
         // substituted.
         InstanceKind::DropGlue(_, Some(ty)) if ty.has_type_flags(TypeFlags::HAS_CT_PARAM) => {
             debug!("still needs substitution");
-            return Err("implementation limitation");
+            return Err("implementation limitation -- HACK for dropping polymorphic type");
         }
 
         // This cannot result in an immediate cycle since the callee MIR is a shim, which does
@@ -1060,8 +1063,7 @@ fn make_call_args<'tcx, I: Inliner<'tcx>>(
 
         closure_ref_arg.chain(tuple_tmp_args).collect()
     } else {
-        // FIXME(edition_2024): switch back to a normal method call.
-        <_>::into_iter(args)
+        args.into_iter()
             .map(|a| create_temp_if_necessary(inliner, a.node, callsite, caller_body, return_block))
             .collect()
     }
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index c3f5806e1aa90..354e7aaf3228d 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -1968,6 +1968,7 @@ impl<T: ?Sized, A: Allocator> LegacyReceiver for Box<T, A> {}
 impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
     type Output = <F as FnOnce<Args>>::Output;
 
+    #[inline(never)]
     extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
         <F as FnOnce<Args>>::call_once(*self, args)
     }
@@ -1975,6 +1976,7 @@ impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F
 
 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
 impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
+    #[inline(never)]
     extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
         <F as FnMut<Args>>::call_mut(self, args)
     }
@@ -1982,6 +1984,7 @@ impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F,
 
 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
 impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
+    #[inline(never)]
     extern "rust-call" fn call(&self, args: Args) -> Self::Output {
         <F as Fn<Args>>::call(self, args)
     }