Fix cases where std accidentally relied on inline(never)

2 files changed, 25 insertions, 3 deletions

diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 71b3754fac8..feceff10f95 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -176,6 +176,7 @@ use rustc_middle::mir::visit::Visitor as MirVisitor;
 use rustc_middle::mir::{self, Location};
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::adjustment::{CustomCoerceUnsized, PointerCoercion};
+use rustc_middle::ty::layout::ValidityRequirement;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{
     self, AssocKind, GenericParamDefKind, Instance, InstanceDef, Ty, TyCtxt, TypeFoldable,
@@ -923,6 +924,21 @@ fn visit_instance_use<'tcx>(
         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) {
+            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));
+            }
+        }
+    }
+
     match instance.def {
         ty::InstanceDef::Virtual(..) | ty::InstanceDef::Intrinsic(_) => {
             if !is_direct_call {
diff --git a/compiler/rustc_monomorphize/src/partitioning.rs b/compiler/rustc_monomorphize/src/partitioning.rs
index c2b307910e4..d33e5a46be2 100644
--- a/compiler/rustc_monomorphize/src/partitioning.rs
+++ b/compiler/rustc_monomorphize/src/partitioning.rs
@@ -212,11 +212,17 @@ where
     let cgu_name_cache = &mut FxHashMap::default();
 
     for mono_item in mono_items {
-        // Handle only root items directly here. Inlined items are handled at
-        // the bottom of the loop based on reachability.
+        // Handle only root (GloballyShared) items directly here. Inlined (LocalCopy) items
+        // are handled at the bottom of the loop based on reachability, with one exception.
+        // The #[lang = "start"] item is the program entrypoint, so there are no calls to it in MIR.
+        // So even if its mode is LocalCopy, we need to treat it like a root.
         match mono_item.instantiation_mode(cx.tcx) {
             InstantiationMode::GloballyShared { .. } => {}
-            InstantiationMode::LocalCopy => continue,
+            InstantiationMode::LocalCopy => {
+                if Some(mono_item.def_id()) != cx.tcx.lang_items().start_fn() {
+                    continue;
+                }
+            }
         }
 
         let characteristic_def_id = characteristic_def_id_of_mono_item(cx.tcx, mono_item);