Auto merge of #108006 - cjgillot:def-impl, r=oli-obk

Avoid accessing HIR when it can be avoided

Experiment to see if it helps some incremental cases.

Will be rebased once https://github.com/rust-lang/rust/pull/107942 gets merged.

r? `@ghost`

2 files changed, 44 insertions, 74 deletions

diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 83b8988ceca..bbe4e67977c 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -1191,28 +1191,14 @@ impl<'v> RootCollector<'_, 'v> {
     fn process_item(&mut self, id: hir::ItemId) {
         match self.tcx.def_kind(id.owner_id) {
             DefKind::Enum | DefKind::Struct | DefKind::Union => {
-                let item = self.tcx.hir().item(id);
-                match item.kind {
-                    hir::ItemKind::Enum(_, ref generics)
-                    | hir::ItemKind::Struct(_, ref generics)
-                    | hir::ItemKind::Union(_, ref generics) => {
-                        if generics.params.is_empty() {
-                            if self.mode == MonoItemCollectionMode::Eager {
-                                debug!(
-                                    "RootCollector: ADT drop-glue for {}",
-                                    self.tcx.def_path_str(item.owner_id.to_def_id())
-                                );
-
-                                let ty = Instance::new(
-                                    item.owner_id.to_def_id(),
-                                    InternalSubsts::empty(),
-                                )
-                                .ty(self.tcx, ty::ParamEnv::reveal_all());
-                                visit_drop_use(self.tcx, ty, true, DUMMY_SP, self.output);
-                            }
-                        }
-                    }
-                    _ => bug!(),
+                if self.mode == MonoItemCollectionMode::Eager
+                    && self.tcx.generics_of(id.owner_id).count() == 0
+                {
+                    debug!("RootCollector: ADT drop-glue for `{id:?}`",);
+
+                    let ty =
+                        self.tcx.bound_type_of(id.owner_id.to_def_id()).no_bound_vars().unwrap();
+                    visit_drop_use(self.tcx, ty, true, DUMMY_SP, self.output);
                 }
             }
             DefKind::GlobalAsm => {
@@ -1238,10 +1224,9 @@ impl<'v> RootCollector<'_, 'v> {
                     collect_const_value(self.tcx, val, &mut self.output);
                 }
             }
-            DefKind::Impl => {
+            DefKind::Impl { .. } => {
                 if self.mode == MonoItemCollectionMode::Eager {
-                    let item = self.tcx.hir().item(id);
-                    create_mono_items_for_default_impls(self.tcx, item, self.output);
+                    create_mono_items_for_default_impls(self.tcx, id, self.output);
                 }
             }
             DefKind::Fn => {
@@ -1326,66 +1311,51 @@ fn item_requires_monomorphization(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     generics.requires_monomorphization(tcx)
 }
 
+#[instrument(level = "debug", skip(tcx, output))]
 fn create_mono_items_for_default_impls<'tcx>(
     tcx: TyCtxt<'tcx>,
-    item: &'tcx hir::Item<'tcx>,
+    item: hir::ItemId,
     output: &mut MonoItems<'tcx>,
 ) {
-    match item.kind {
-        hir::ItemKind::Impl(ref impl_) => {
-            if matches!(impl_.polarity, hir::ImplPolarity::Negative(_)) {
-                return;
-            }
+    let polarity = tcx.impl_polarity(item.owner_id);
+    if matches!(polarity, ty::ImplPolarity::Negative) {
+        return;
+    }
 
-            for param in impl_.generics.params {
-                match param.kind {
-                    hir::GenericParamKind::Lifetime { .. } => {}
-                    hir::GenericParamKind::Type { .. } | hir::GenericParamKind::Const { .. } => {
-                        return;
-                    }
-                }
-            }
+    if tcx.generics_of(item.owner_id).own_requires_monomorphization() {
+        return;
+    }
 
-            debug!(
-                "create_mono_items_for_default_impls(item={})",
-                tcx.def_path_str(item.owner_id.to_def_id())
-            );
+    let Some(trait_ref) = tcx.impl_trait_ref(item.owner_id) else {
+        return;
+    };
 
-            if let Some(trait_ref) = tcx.impl_trait_ref(item.owner_id) {
-                let trait_ref = trait_ref.subst_identity();
+    let trait_ref = trait_ref.subst_identity();
 
-                let param_env = ty::ParamEnv::reveal_all();
-                let trait_ref = tcx.normalize_erasing_regions(param_env, trait_ref);
-                let overridden_methods = tcx.impl_item_implementor_ids(item.owner_id);
-                for method in tcx.provided_trait_methods(trait_ref.def_id) {
-                    if overridden_methods.contains_key(&method.def_id) {
-                        continue;
-                    }
+    let param_env = ty::ParamEnv::reveal_all();
+    let trait_ref = tcx.normalize_erasing_regions(param_env, trait_ref);
+    let overridden_methods = tcx.impl_item_implementor_ids(item.owner_id);
+    for method in tcx.provided_trait_methods(trait_ref.def_id) {
+        if overridden_methods.contains_key(&method.def_id) {
+            continue;
+        }
 
-                    if tcx.generics_of(method.def_id).own_requires_monomorphization() {
-                        continue;
-                    }
+        if tcx.generics_of(method.def_id).own_requires_monomorphization() {
+            continue;
+        }
 
-                    let substs =
-                        InternalSubsts::for_item(tcx, method.def_id, |param, _| match param.kind {
-                            GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
-                            GenericParamDefKind::Type { .. }
-                            | GenericParamDefKind::Const { .. } => {
-                                trait_ref.substs[param.index as usize]
-                            }
-                        });
-                    let instance =
-                        ty::Instance::expect_resolve(tcx, param_env, method.def_id, substs);
-
-                    let mono_item = create_fn_mono_item(tcx, instance, DUMMY_SP);
-                    if mono_item.node.is_instantiable(tcx) && should_codegen_locally(tcx, &instance)
-                    {
-                        output.push(mono_item);
-                    }
-                }
+        let substs = InternalSubsts::for_item(tcx, method.def_id, |param, _| match param.kind {
+            GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
+            GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
+                trait_ref.substs[param.index as usize]
             }
+        });
+        let instance = ty::Instance::expect_resolve(tcx, param_env, method.def_id, substs);
+
+        let mono_item = create_fn_mono_item(tcx, instance, DUMMY_SP);
+        if mono_item.node.is_instantiable(tcx) && should_codegen_locally(tcx, &instance) {
+            output.push(mono_item);
         }
-        _ => bug!(),
     }
 }
 
diff --git a/compiler/rustc_monomorphize/src/polymorphize.rs b/compiler/rustc_monomorphize/src/polymorphize.rs
index aba842817ef..207ad332c22 100644
--- a/compiler/rustc_monomorphize/src/polymorphize.rs
+++ b/compiler/rustc_monomorphize/src/polymorphize.rs
@@ -172,7 +172,7 @@ fn mark_used_by_default_parameters<'tcx>(
         | DefKind::Field
         | DefKind::LifetimeParam
         | DefKind::GlobalAsm
-        | DefKind::Impl => {
+        | DefKind::Impl { .. } => {
             for param in &generics.params {
                 debug!(?param, "(other)");
                 if let ty::GenericParamDefKind::Lifetime = param.kind {