Auto merge of #107693 - petrochenkov:metable, r=oli-obk

rustc_metadata: Encode/decode some `LazyArrays` without an `Option`

and a couple of related changes, see individual commits.

Addresses comments in https://github.com/rust-lang/rust/pull/107166#discussion_r1083417124 and https://github.com/rust-lang/rust/pull/107166#discussion_r1083768417, cc `@cjgillot` `@oli-obk.`

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -654,7 +654,7 @@ impl<'a, 'tcx, T> Decodable<DecodeContext<'a, 'tcx>> for LazyValue<T> {
 impl<'a, 'tcx, T> Decodable<DecodeContext<'a, 'tcx>> for LazyArray<T> {
     fn decode(decoder: &mut DecodeContext<'a, 'tcx>) -> Self {
         let len = decoder.read_usize();
-        if len == 0 { LazyArray::empty() } else { decoder.read_lazy_array(len) }
+        if len == 0 { LazyArray::default() } else { decoder.read_lazy_array(len) }
     }
 }
 
@@ -864,7 +864,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                 .tables
                 .children
                 .get(self, index)
-                .unwrap_or_else(LazyArray::empty)
+                .expect("fields are not encoded for a variant")
                 .decode(self)
                 .map(|index| ty::FieldDef {
                     did: self.local_def_id(index),
@@ -896,7 +896,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                 .tables
                 .children
                 .get(self, item_id)
-                .unwrap_or_else(LazyArray::empty)
+                .expect("variants are not encoded for an enum")
                 .decode(self)
                 .filter_map(|index| {
                     let kind = self.def_kind(index);
@@ -1045,7 +1045,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .tables
             .fn_arg_names
             .get(self, id)
-            .unwrap_or_else(LazyArray::empty)
+            .expect("argument names not encoded for a function")
             .decode((self, sess))
             .nth(0)
             .map_or(false, |ident| ident.name == kw::SelfLower)
@@ -1060,7 +1060,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .tables
             .children
             .get(self, id)
-            .unwrap_or_else(LazyArray::empty)
+            .expect("associated items not encoded for an item")
             .decode((self, sess))
             .map(move |child_index| self.local_def_id(child_index))
     }
@@ -1068,13 +1068,12 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     fn get_associated_item(self, id: DefIndex, sess: &'a Session) -> ty::AssocItem {
         let name = self.item_name(id);
 
-        let kind = match self.def_kind(id) {
-            DefKind::AssocConst => ty::AssocKind::Const,
-            DefKind::AssocFn => ty::AssocKind::Fn,
-            DefKind::AssocTy => ty::AssocKind::Type,
+        let (kind, has_self) = match self.def_kind(id) {
+            DefKind::AssocConst => (ty::AssocKind::Const, false),
+            DefKind::AssocFn => (ty::AssocKind::Fn, self.get_fn_has_self_parameter(id, sess)),
+            DefKind::AssocTy => (ty::AssocKind::Type, false),
             _ => bug!("cannot get associated-item of `{:?}`", self.def_key(id)),
         };
-        let has_self = self.get_fn_has_self_parameter(id, sess);
         let container = self.root.tables.assoc_container.get(self, id).unwrap();
 
         ty::AssocItem {
@@ -1131,7 +1130,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .tables
             .children
             .get(self, id)
-            .unwrap_or_else(LazyArray::empty)
+            .expect("fields not encoded for a struct")
             .decode(self)
             .map(move |index| respan(self.get_span(index, sess), self.item_name(index)))
     }
@@ -1144,7 +1143,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .tables
             .children
             .get(self, id)
-            .unwrap_or_else(LazyArray::empty)
+            .expect("fields not encoded for a struct")
             .decode(self)
             .map(move |field_index| self.get_visibility(field_index))
     }
@@ -1159,7 +1158,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                 .tables
                 .inherent_impls
                 .get(self, id)
-                .unwrap_or_else(LazyArray::empty)
                 .decode(self)
                 .map(|index| self.local_def_id(index)),
         )
@@ -1174,7 +1172,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
                 .tables
                 .inherent_impls
                 .get(self, ty_index)
-                .unwrap_or_else(LazyArray::empty)
                 .decode(self)
                 .map(move |impl_index| (ty_def_id, self.local_def_id(impl_index)))
         })
@@ -1322,7 +1319,7 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
     ) -> DefPathHash {
         *def_path_hashes
             .entry(index)
-            .or_insert_with(|| self.root.tables.def_path_hashes.get(self, index).unwrap())
+            .or_insert_with(|| self.root.tables.def_path_hashes.get(self, index))
     }
 
     #[inline]

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -1,6 +1,7 @@
 use crate::creader::{CStore, LoadedMacro};
 use crate::foreign_modules;
 use crate::native_libs;
+use crate::rmeta::table::IsDefault;
 use crate::rmeta::AttrFlags;
 
 use rustc_ast as ast;
@@ -88,6 +89,14 @@ macro_rules! provide_one {
             }
         }
     };
+    ($tcx:ident, $def_id:ident, $other:ident, $cdata:ident, $name:ident => { table_defaulted_array }) => {
+        provide_one! {
+            $tcx, $def_id, $other, $cdata, $name => {
+                let lazy = $cdata.root.tables.$name.get($cdata, $def_id.index);
+                if lazy.is_default() { &[] } else { $tcx.arena.alloc_from_iter(lazy.decode(($cdata, $tcx))) }
+            }
+        }
+    };
     ($tcx:ident, $def_id:ident, $other:ident, $cdata:ident, $name:ident => { table_direct }) => {
         provide_one! {
             $tcx, $def_id, $other, $cdata, $name => {
@@ -187,10 +196,10 @@ impl IntoArgs for (CrateNum, SimplifiedType) {
 }
 
 provide! { tcx, def_id, other, cdata,
-    explicit_item_bounds => { table }
+    explicit_item_bounds => { table_defaulted_array }
     explicit_predicates_of => { table }
     generics_of => { table }
-    inferred_outlives_of => { table }
+    inferred_outlives_of => { table_defaulted_array }
     super_predicates_of => { table }
     type_of => { table }
     variances_of => { table }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -76,13 +76,13 @@ pub(super) struct EncodeContext<'a, 'tcx> {
     symbol_table: FxHashMap<Symbol, usize>,
 }
 
-/// If the current crate is a proc-macro, returns early with `LazyArray::empty()`.
+/// If the current crate is a proc-macro, returns early with `LazyArray::default()`.
 /// This is useful for skipping the encoding of things that aren't needed
 /// for proc-macro crates.
 macro_rules! empty_proc_macro {
     ($self:ident) => {
         if $self.is_proc_macro {
-            return LazyArray::empty();
+            return LazyArray::default();
         }
     };
 }
@@ -365,21 +365,31 @@ impl<'a, 'tcx> TyEncoder for EncodeContext<'a, 'tcx> {
     }
 }
 
-// Shorthand for `$self.$tables.$table.set($def_id.index, $self.lazy_value($value))`, which would
+// Shorthand for `$self.$tables.$table.set_some($def_id.index, $self.lazy_value($value))`, which would
 // normally need extra variables to avoid errors about multiple mutable borrows.
 macro_rules! record {
     ($self:ident.$tables:ident.$table:ident[$def_id:expr] <- $value:expr) => {{
         {
             let value = $value;
             let lazy = $self.lazy(value);
-            $self.$tables.$table.set($def_id.index, lazy);
+            $self.$tables.$table.set_some($def_id.index, lazy);
         }
     }};
 }
 
-// Shorthand for `$self.$tables.$table.set($def_id.index, $self.lazy_value($value))`, which would
+// Shorthand for `$self.$tables.$table.set_some($def_id.index, $self.lazy_value($value))`, which would
 // normally need extra variables to avoid errors about multiple mutable borrows.
 macro_rules! record_array {
+    ($self:ident.$tables:ident.$table:ident[$def_id:expr] <- $value:expr) => {{
+        {
+            let value = $value;
+            let lazy = $self.lazy_array(value);
+            $self.$tables.$table.set_some($def_id.index, lazy);
+        }
+    }};
+}
+
+macro_rules! record_defaulted_array {
     ($self:ident.$tables:ident.$table:ident[$def_id:expr] <- $value:expr) => {{
         {
             let value = $value;
@@ -467,13 +477,13 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             {
                 let def_key = self.lazy(table.def_key(def_index));
                 let def_path_hash = table.def_path_hash(def_index);
-                self.tables.def_keys.set(def_index, def_key);
+                self.tables.def_keys.set_some(def_index, def_key);
                 self.tables.def_path_hashes.set(def_index, def_path_hash);
             }
         } else {
             for (def_index, def_key, def_path_hash) in table.enumerated_keys_and_path_hashes() {
                 let def_key = self.lazy(def_key);
-                self.tables.def_keys.set(def_index, def_key);
+                self.tables.def_keys.set_some(def_index, def_key);
                 self.tables.def_path_hashes.set(def_index, *def_path_hash);
             }
         }
@@ -548,7 +558,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
             let on_disk_index: u32 =
                 on_disk_index.try_into().expect("cannot export more than U32_MAX files");
-            adapted.set(on_disk_index, self.lazy(source_file));
+            adapted.set_some(on_disk_index, self.lazy(source_file));
         }
 
         adapted.encode(&mut self.opaque)
@@ -1147,9 +1157,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         if state.is_doc_hidden {
             attr_flags |= AttrFlags::IS_DOC_HIDDEN;
         }
-        if !attr_flags.is_empty() {
-            self.tables.attr_flags.set_nullable(def_id.local_def_index, attr_flags);
-        }
+        self.tables.attr_flags.set(def_id.local_def_index, attr_flags);
     }
 
     fn encode_def_ids(&mut self) {
@@ -1161,7 +1169,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             let def_id = local_id.to_def_id();
             let def_kind = tcx.opt_def_kind(local_id);
             let Some(def_kind) = def_kind else { continue };
-            self.tables.opt_def_kind.set(def_id.index, def_kind);
+            self.tables.opt_def_kind.set_some(def_id.index, def_kind);
             let def_span = tcx.def_span(local_id);
             record!(self.tables.def_span[def_id] <- def_span);
             self.encode_attrs(local_id);
@@ -1192,9 +1200,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 record!(self.tables.generics_of[def_id] <- g);
                 record!(self.tables.explicit_predicates_of[def_id] <- self.tcx.explicit_predicates_of(def_id));
                 let inferred_outlives = self.tcx.inferred_outlives_of(def_id);
-                if !inferred_outlives.is_empty() {
-                    record_array!(self.tables.inferred_outlives_of[def_id] <- inferred_outlives);
-                }
+                record_defaulted_array!(self.tables.inferred_outlives_of[def_id] <- inferred_outlives);
             }
             if should_encode_type(tcx, local_id, def_kind) {
                 record!(self.tables.type_of[def_id] <- self.tcx.type_of(def_id));
@@ -1215,15 +1221,12 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 record!(self.tables.trait_impl_trait_tys[def_id] <- table);
             }
         }
+
         let inherent_impls = tcx.with_stable_hashing_context(|hcx| {
             tcx.crate_inherent_impls(()).inherent_impls.to_sorted(&hcx, true)
         });
-
-        for (def_id, implementations) in inherent_impls {
-            if implementations.is_empty() {
-                continue;
-            }
-            record_array!(self.tables.inherent_impls[def_id.to_def_id()] <- implementations.iter().map(|&def_id| {
+        for (def_id, impls) in inherent_impls {
+            record_defaulted_array!(self.tables.inherent_impls[def_id.to_def_id()] <- impls.iter().map(|def_id| {
                 assert!(def_id.is_local());
                 def_id.index
             }));
@@ -1264,14 +1267,14 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             };
             record!(self.tables.variant_data[variant.def_id] <- data);
 
-            self.tables.constness.set(variant.def_id.index, hir::Constness::Const);
+            self.tables.constness.set_some(variant.def_id.index, hir::Constness::Const);
             record_array!(self.tables.children[variant.def_id] <- variant.fields.iter().map(|f| {
                 assert!(f.did.is_local());
                 f.did.index
             }));
 
             if let Some((CtorKind::Fn, ctor_def_id)) = variant.ctor {
-                self.tables.constness.set(ctor_def_id.index, hir::Constness::Const);
+                self.tables.constness.set_some(ctor_def_id.index, hir::Constness::Const);
                 let fn_sig = tcx.fn_sig(ctor_def_id);
                 record!(self.tables.fn_sig[ctor_def_id] <- fn_sig);
                 // FIXME only encode signature for ctor_def_id
@@ -1332,9 +1335,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
     fn encode_explicit_item_bounds(&mut self, def_id: DefId) {
         debug!("EncodeContext::encode_explicit_item_bounds({:?})", def_id);
         let bounds = self.tcx.explicit_item_bounds(def_id);
-        if !bounds.is_empty() {
-            record_array!(self.tables.explicit_item_bounds[def_id] <- bounds);
-        }
+        record_defaulted_array!(self.tables.explicit_item_bounds[def_id] <- bounds);
     }
 
     fn encode_info_for_trait_item(&mut self, def_id: DefId) {
@@ -1342,16 +1343,16 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let tcx = self.tcx;
 
         let impl_defaultness = tcx.impl_defaultness(def_id.expect_local());
-        self.tables.impl_defaultness.set(def_id.index, impl_defaultness);
+        self.tables.impl_defaultness.set_some(def_id.index, impl_defaultness);
         let trait_item = tcx.associated_item(def_id);
-        self.tables.assoc_container.set(def_id.index, trait_item.container);
+        self.tables.assoc_container.set_some(def_id.index, trait_item.container);
 
         match trait_item.kind {
             ty::AssocKind::Const => {}
             ty::AssocKind::Fn => {
                 record_array!(self.tables.fn_arg_names[def_id] <- tcx.fn_arg_names(def_id));
-                self.tables.asyncness.set(def_id.index, tcx.asyncness(def_id));
-                self.tables.constness.set(def_id.index, hir::Constness::NotConst);
+                self.tables.asyncness.set_some(def_id.index, tcx.asyncness(def_id));
+                self.tables.constness.set_some(def_id.index, hir::Constness::NotConst);
             }
             ty::AssocKind::Type => {
                 self.encode_explicit_item_bounds(def_id);
@@ -1367,14 +1368,14 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let tcx = self.tcx;
 
         let ast_item = self.tcx.hir().expect_impl_item(def_id.expect_local());
-        self.tables.impl_defaultness.set(def_id.index, ast_item.defaultness);
+        self.tables.impl_defaultness.set_some(def_id.index, ast_item.defaultness);
         let impl_item = self.tcx.associated_item(def_id);
-        self.tables.assoc_container.set(def_id.index, impl_item.container);
+        self.tables.assoc_container.set_some(def_id.index, impl_item.container);
 
         match impl_item.kind {
             ty::AssocKind::Fn => {
                 let hir::ImplItemKind::Fn(ref sig, body) = ast_item.kind else { bug!() };
-                self.tables.asyncness.set(def_id.index, sig.header.asyncness);
+                self.tables.asyncness.set_some(def_id.index, sig.header.asyncness);
                 record_array!(self.tables.fn_arg_names[def_id] <- self.tcx.hir().body_param_names(body));
                 // Can be inside `impl const Trait`, so using sig.header.constness is not reliable
                 let constness = if self.tcx.is_const_fn_raw(def_id) {
@@ -1382,18 +1383,16 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 } else {
                     hir::Constness::NotConst
                 };
-                self.tables.constness.set(def_id.index, constness);
+                self.tables.constness.set_some(def_id.index, constness);
             }
             ty::AssocKind::Const | ty::AssocKind::Type => {}
         }
         if let Some(trait_item_def_id) = impl_item.trait_item_def_id {
-            self.tables.trait_item_def_id.set(def_id.index, trait_item_def_id.into());
+            self.tables.trait_item_def_id.set_some(def_id.index, trait_item_def_id.into());
         }
         if impl_item.kind == ty::AssocKind::Fn {
             record!(self.tables.fn_sig[def_id] <- tcx.fn_sig(def_id));
-            if tcx.is_intrinsic(def_id) {
-                self.tables.is_intrinsic.set_nullable(def_id.index, true);
-            }
+            self.tables.is_intrinsic.set(def_id.index, tcx.is_intrinsic(def_id));
         }
     }
 
@@ -1522,14 +1521,12 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
         match item.kind {
             hir::ItemKind::Fn(ref sig, .., body) => {
-                self.tables.asyncness.set(def_id.index, sig.header.asyncness);
+                self.tables.asyncness.set_some(def_id.index, sig.header.asyncness);
                 record_array!(self.tables.fn_arg_names[def_id] <- self.tcx.hir().body_param_names(body));
-                self.tables.constness.set(def_id.index, sig.header.constness);
+                self.tables.constness.set_some(def_id.index, sig.header.constness);
             }
             hir::ItemKind::Macro(ref macro_def, _) => {
-                if macro_def.macro_rules {
-                    self.tables.is_macro_rules.set_nullable(def_id.index, true);
-                }
+                self.tables.is_macro_rules.set(def_id.index, macro_def.macro_rules);
                 record!(self.tables.macro_definition[def_id] <- &*macro_def.body);
             }
             hir::ItemKind::Mod(ref m) => {
@@ -1537,20 +1534,20 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             }
             hir::ItemKind::OpaqueTy(ref opaque) => {
                 self.encode_explicit_item_bounds(def_id);
-                if matches!(opaque.origin, hir::OpaqueTyOrigin::TyAlias) {
-                    self.tables.is_type_alias_impl_trait.set_nullable(def_id.index, true);
-                }
+                self.tables
+                    .is_type_alias_impl_trait
+                    .set(def_id.index, matches!(opaque.origin, hir::OpaqueTyOrigin::TyAlias));
             }
             hir::ItemKind::Impl(hir::Impl { defaultness, constness, .. }) => {
-                self.tables.impl_defaultness.set(def_id.index, *defaultness);
-                self.tables.constness.set(def_id.index, *constness);
+                self.tables.impl_defaultness.set_some(def_id.index, *defaultness);
+                self.tables.constness.set_some(def_id.index, *constness);
 
                 let trait_ref = self.tcx.impl_trait_ref(def_id).map(ty::EarlyBinder::skip_binder);
                 if let Some(trait_ref) = trait_ref {
                     let trait_def = self.tcx.trait_def(trait_ref.def_id);
                     if let Ok(mut an) = trait_def.ancestors(self.tcx, def_id) {
                         if let Some(specialization_graph::Node::Impl(parent)) = an.nth(1) {
-                            self.tables.impl_parent.set(def_id.index, parent.into());
+                            self.tables.impl_parent.set_some(def_id.index, parent.into());
                         }
                     }
 
@@ -1564,7 +1561,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 }
 
                 let polarity = self.tcx.impl_polarity(def_id);
-                self.tables.impl_polarity.set(def_id.index, polarity);
+                self.tables.impl_polarity.set_some(def_id.index, polarity);
             }
             hir::ItemKind::Trait(..) => {
                 let trait_def = self.tcx.trait_def(def_id);
@@ -1601,9 +1598,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         }
         if let hir::ItemKind::Fn(..) = item.kind {
             record!(self.tables.fn_sig[def_id] <- tcx.fn_sig(def_id));
-            if tcx.is_intrinsic(def_id) {
-                self.tables.is_intrinsic.set_nullable(def_id.index, true);
-            }
+            self.tables.is_intrinsic.set(def_id.index, tcx.is_intrinsic(def_id));
         }
         if let hir::ItemKind::Impl { .. } = item.kind {
             if let Some(trait_ref) = self.tcx.impl_trait_ref(def_id) {
@@ -1650,7 +1645,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
             ty::Closure(_, substs) => {
                 let constness = self.tcx.constness(def_id.to_def_id());
-                self.tables.constness.set(def_id.to_def_id().index, constness);
+                self.tables.constness.set_some(def_id.to_def_id().index, constness);
                 record!(self.tables.fn_sig[def_id.to_def_id()] <- ty::EarlyBinder(substs.as_closure().sig()));
             }
 
@@ -1678,12 +1673,12 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         self.hygiene_ctxt.encode(
             &mut (&mut *self, &mut syntax_contexts, &mut expn_data_table, &mut expn_hash_table),
             |(this, syntax_contexts, _, _), index, ctxt_data| {
-                syntax_contexts.set(index, this.lazy(ctxt_data));
+                syntax_contexts.set_some(index, this.lazy(ctxt_data));
             },
             |(this, _, expn_data_table, expn_hash_table), index, expn_data, hash| {
                 if let Some(index) = index.as_local() {
-                    expn_data_table.set(index.as_raw(), this.lazy(expn_data));
-                    expn_hash_table.set(index.as_raw(), this.lazy(hash));
+                    expn_data_table.set_some(index.as_raw(), this.lazy(expn_data));
+                    expn_hash_table.set_some(index.as_raw(), this.lazy(hash));
                 }
             },
         );
@@ -1708,10 +1703,10 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             let spans = self.tcx.sess.parse_sess.proc_macro_quoted_spans();
             for (i, span) in spans.into_iter().enumerate() {
                 let span = self.lazy(span);
-                self.tables.proc_macro_quoted_spans.set(i, span);
+                self.tables.proc_macro_quoted_spans.set_some(i, span);
             }
 
-            self.tables.opt_def_kind.set(LOCAL_CRATE.as_def_id().index, DefKind::Mod);
+            self.tables.opt_def_kind.set_some(LOCAL_CRATE.as_def_id().index, DefKind::Mod);
             record!(self.tables.def_span[LOCAL_CRATE.as_def_id()] <- tcx.def_span(LOCAL_CRATE.as_def_id()));
             self.encode_attrs(LOCAL_CRATE.as_def_id().expect_local());
             let vis = tcx.local_visibility(CRATE_DEF_ID).map_id(|def_id| def_id.local_def_index);
@@ -1753,8 +1748,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 def_key.disambiguated_data.data = DefPathData::MacroNs(name);
 
                 let def_id = id.to_def_id();
-                self.tables.opt_def_kind.set(def_id.index, DefKind::Macro(macro_kind));
-                self.tables.proc_macro.set(def_id.index, macro_kind);
+                self.tables.opt_def_kind.set_some(def_id.index, DefKind::Macro(macro_kind));
+                self.tables.proc_macro.set_some(def_id.index, macro_kind);
                 self.encode_attrs(id);
                 record!(self.tables.def_keys[def_id] <- def_key);
                 record!(self.tables.def_ident_span[def_id] <- span);
@@ -1969,7 +1964,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 Linkage::Static => Some(LinkagePreference::RequireStatic),
             }));
         }
-        LazyArray::empty()
+        LazyArray::default()
     }
 
     fn encode_info_for_foreign_item(&mut self, def_id: DefId, nitem: &hir::ForeignItem<'_>) {
@@ -1979,22 +1974,20 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
 
         match nitem.kind {
             hir::ForeignItemKind::Fn(_, ref names, _) => {
-                self.tables.asyncness.set(def_id.index, hir::IsAsync::NotAsync);
+                self.tables.asyncness.set_some(def_id.index, hir::IsAsync::NotAsync);
                 record_array!(self.tables.fn_arg_names[def_id] <- *names);
                 let constness = if self.tcx.is_const_fn_raw(def_id) {
                     hir::Constness::Const
                 } else {
                     hir::Constness::NotConst
                 };
-                self.tables.constness.set(def_id.index, constness);
+                self.tables.constness.set_some(def_id.index, constness);
                 record!(self.tables.fn_sig[def_id] <- tcx.fn_sig(def_id));
             }
             hir::ForeignItemKind::Static(..) | hir::ForeignItemKind::Type => {}
         }
         if let hir::ForeignItemKind::Fn(..) = nitem.kind {
-            if tcx.is_intrinsic(def_id) {
-                self.tables.is_intrinsic.set_nullable(def_id.index, true);
-            }
+            self.tables.is_intrinsic.set(def_id.index, tcx.is_intrinsic(def_id));
         }
     }
 }

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -115,14 +115,16 @@ impl<T: ParameterizedOverTcx> ParameterizedOverTcx for LazyArray<T> {
     type Value<'tcx> = LazyArray<T::Value<'tcx>>;
 }
 
+impl<T> Default for LazyArray<T> {
+    fn default() -> LazyArray<T> {
+        LazyArray::from_position_and_num_elems(NonZeroUsize::new(1).unwrap(), 0)
+    }
+}
+
 impl<T> LazyArray<T> {
     fn from_position_and_num_elems(position: NonZeroUsize, num_elems: usize) -> LazyArray<T> {
         LazyArray { position, num_elems, _marker: PhantomData }
     }
-
-    fn empty() -> LazyArray<T> {
-        LazyArray::from_position_and_num_elems(NonZeroUsize::new(1).unwrap(), 0)
-    }
 }
 
 /// A list of lazily-decoded values, with the added capability of random access.
@@ -316,7 +318,7 @@ pub(crate) struct IncoherentImpls {
 /// Define `LazyTables` and `TableBuilders` at the same time.
 macro_rules! define_tables {
     (
-        - nullable: $($name1:ident: Table<$IDX1:ty, $T1:ty>,)+
+        - defaulted: $($name1:ident: Table<$IDX1:ty, $T1:ty>,)+
         - optional: $($name2:ident: Table<$IDX2:ty, $T2:ty>,)+
     ) => {
         #[derive(MetadataEncodable, MetadataDecodable)]
@@ -343,11 +345,15 @@ macro_rules! define_tables {
 }
 
 define_tables! {
-- nullable:
+- defaulted:
     is_intrinsic: Table<DefIndex, bool>,
     is_macro_rules: Table<DefIndex, bool>,
     is_type_alias_impl_trait: Table<DefIndex, bool>,
     attr_flags: Table<DefIndex, AttrFlags>,
+    def_path_hashes: Table<DefIndex, DefPathHash>,
+    explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Predicate<'static>, Span)>>,
+    inferred_outlives_of: Table<DefIndex, LazyArray<(ty::Clause<'static>, Span)>>,
+    inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
 
 - optional:
     attributes: Table<DefIndex, LazyArray<ast::Attribute>>,
@@ -360,12 +366,8 @@ define_tables! {
     lookup_const_stability: Table<DefIndex, LazyValue<attr::ConstStability>>,
     lookup_default_body_stability: Table<DefIndex, LazyValue<attr::DefaultBodyStability>>,
     lookup_deprecation_entry: Table<DefIndex, LazyValue<attr::Deprecation>>,
-    // As an optimization, a missing entry indicates an empty `&[]`.
-    explicit_item_bounds: Table<DefIndex, LazyArray<(ty::Predicate<'static>, Span)>>,
     explicit_predicates_of: Table<DefIndex, LazyValue<ty::GenericPredicates<'static>>>,
     generics_of: Table<DefIndex, LazyValue<ty::Generics>>,
-    // As an optimization, a missing entry indicates an empty `&[]`.
-    inferred_outlives_of: Table<DefIndex, LazyArray<(ty::Clause<'static>, Span)>>,
     super_predicates_of: Table<DefIndex, LazyValue<ty::GenericPredicates<'static>>>,
     type_of: Table<DefIndex, LazyValue<Ty<'static>>>,
     variances_of: Table<DefIndex, LazyArray<ty::Variance>>,
@@ -393,7 +395,6 @@ define_tables! {
     generator_kind: Table<DefIndex, LazyValue<hir::GeneratorKind>>,
     trait_def: Table<DefIndex, LazyValue<ty::TraitDef>>,
     trait_item_def_id: Table<DefIndex, RawDefId>,
-    inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
     expn_that_defined: Table<DefIndex, LazyValue<ExpnId>>,
     unused_generic_params: Table<DefIndex, LazyValue<UnusedGenericParams>>,
     params_in_repr: Table<DefIndex, LazyValue<BitSet<u32>>>,
@@ -403,7 +404,6 @@ define_tables! {
     // `DefPathTable` up front, since we may only ever use a few
     // definitions from any given crate.
     def_keys: Table<DefIndex, LazyValue<DefKey>>,
-    def_path_hashes: Table<DefIndex, DefPathHash>,
     proc_macro_quoted_spans: Table<usize, LazyValue<Span>>,
     generator_diagnostic_data: Table<DefIndex, LazyValue<GeneratorDiagnosticData<'static>>>,
     variant_data: Table<DefIndex, LazyValue<VariantData>>,

compiler/rustc_metadata/src/rmeta/table.rs

@@ -10,11 +10,51 @@ use rustc_span::hygiene::MacroKind;
 use std::marker::PhantomData;
 use std::num::NonZeroUsize;
 
+pub(super) trait IsDefault: Default {
+    fn is_default(&self) -> bool;
+}
+
+impl<T> IsDefault for Option<T> {
+    fn is_default(&self) -> bool {
+        self.is_none()
+    }
+}
+
+impl IsDefault for AttrFlags {
+    fn is_default(&self) -> bool {
+        self.is_empty()
+    }
+}
+
+impl IsDefault for bool {
+    fn is_default(&self) -> bool {
+        !self
+    }
+}
+
+impl IsDefault for u32 {
+    fn is_default(&self) -> bool {
+        *self == 0
+    }
+}
+
+impl<T> IsDefault for LazyArray<T> {
+    fn is_default(&self) -> bool {
+        self.num_elems == 0
+    }
+}
+
+impl IsDefault for DefPathHash {
+    fn is_default(&self) -> bool {
+        self.0 == Fingerprint::ZERO
+    }
+}
+
 /// Helper trait, for encoding to, and decoding from, a fixed number of bytes.
 /// Used mainly for Lazy positions and lengths.
 /// Unchecked invariant: `Self::default()` should encode as `[0; BYTE_LEN]`,
 /// but this has no impact on safety.
-pub(super) trait FixedSizeEncoding: Default {
+pub(super) trait FixedSizeEncoding: IsDefault {
     /// This should be `[u8; BYTE_LEN]`;
     /// Cannot use an associated `const BYTE_LEN: usize` instead due to const eval limitations.
     type ByteArray;
@@ -23,6 +63,8 @@ pub(super) trait FixedSizeEncoding: Default {
     fn write_to_bytes(self, b: &mut Self::ByteArray);
 }
 
+/// This implementation is not used generically, but for reading/writing
+/// concrete `u32` fields in `Lazy*` structures, which may be zero.
 impl FixedSizeEncoding for u32 {
     type ByteArray = [u8; 4];
 
@@ -58,7 +100,7 @@ macro_rules! fixed_size_enum {
             fn write_to_bytes(self, b: &mut [u8;1]) {
                 use $ty::*;
                 b[0] = match self {
-                    None => 0,
+                    None => unreachable!(),
                     $(Some($($pat)*) => 1 + ${index()},)*
                 }
             }
@@ -155,20 +197,18 @@ fixed_size_enum! {
 }
 
 // We directly encode `DefPathHash` because a `LazyValue` would incur a 25% cost.
-impl FixedSizeEncoding for Option<DefPathHash> {
+impl FixedSizeEncoding for DefPathHash {
     type ByteArray = [u8; 16];
 
     #[inline]
     fn from_bytes(b: &[u8; 16]) -> Self {
-        Some(DefPathHash(Fingerprint::from_le_bytes(*b)))
+        DefPathHash(Fingerprint::from_le_bytes(*b))
     }
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 16]) {
-        let Some(DefPathHash(fingerprint)) = self else {
-            panic!("Trying to encode absent DefPathHash.")
-        };
-        *b = fingerprint.to_le_bytes();
+        debug_assert!(!self.is_default());
+        *b = self.0.to_le_bytes();
     }
 }
 
@@ -179,17 +219,17 @@ impl FixedSizeEncoding for Option<RawDefId> {
     #[inline]
     fn from_bytes(b: &[u8; 8]) -> Self {
         let krate = u32::from_le_bytes(b[0..4].try_into().unwrap());
-        let index = u32::from_le_bytes(b[4..8].try_into().unwrap());
         if krate == 0 {
             return None;
         }
+        let index = u32::from_le_bytes(b[4..8].try_into().unwrap());
         Some(RawDefId { krate: krate - 1, index })
     }
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 8]) {
         match self {
-            None => *b = [0; 8],
+            None => unreachable!(),
             Some(RawDefId { krate, index }) => {
                 // CrateNum is less than `CrateNum::MAX_AS_U32`.
                 debug_assert!(krate < u32::MAX);
@@ -210,6 +250,7 @@ impl FixedSizeEncoding for AttrFlags {
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 1]) {
+        debug_assert!(!self.is_default());
         b[0] = self.bits();
     }
 }
@@ -224,6 +265,7 @@ impl FixedSizeEncoding for bool {
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 1]) {
+        debug_assert!(!self.is_default());
         b[0] = self as u8
     }
 }
@@ -242,9 +284,54 @@ impl<T> FixedSizeEncoding for Option<LazyValue<T>> {
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 4]) {
-        let position = self.map_or(0, |lazy| lazy.position.get());
+        match self {
+            None => unreachable!(),
+            Some(lazy) => {
+                let position = lazy.position.get();
+                let position: u32 = position.try_into().unwrap();
+                position.write_to_bytes(b)
+            }
+        }
+    }
+}
+
+impl<T> LazyArray<T> {
+    #[inline]
+    fn write_to_bytes_impl(self, b: &mut [u8; 8]) {
+        let ([position_bytes, meta_bytes],[])= b.as_chunks_mut::<4>() else { panic!() };
+
+        let position = self.position.get();
         let position: u32 = position.try_into().unwrap();
-        position.write_to_bytes(b)
+        position.write_to_bytes(position_bytes);
+
+        let len = self.num_elems;
+        let len: u32 = len.try_into().unwrap();
+        len.write_to_bytes(meta_bytes);
+    }
+
+    fn from_bytes_impl(position_bytes: &[u8; 4], meta_bytes: &[u8; 4]) -> Option<LazyArray<T>> {
+        let position = NonZeroUsize::new(u32::from_bytes(position_bytes) as usize)?;
+        let len = u32::from_bytes(meta_bytes) as usize;
+        Some(LazyArray::from_position_and_num_elems(position, len))
+    }
+}
+
+impl<T> FixedSizeEncoding for LazyArray<T> {
+    type ByteArray = [u8; 8];
+
+    #[inline]
+    fn from_bytes(b: &[u8; 8]) -> Self {
+        let ([position_bytes, meta_bytes],[])= b.as_chunks::<4>() else { panic!() };
+        if *meta_bytes == [0; 4] {
+            return Default::default();
+        }
+        LazyArray::from_bytes_impl(position_bytes, meta_bytes).unwrap()
+    }
+
+    #[inline]
+    fn write_to_bytes(self, b: &mut [u8; 8]) {
+        assert!(!self.is_default());
+        self.write_to_bytes_impl(b)
     }
 }
 
@@ -253,23 +340,16 @@ impl<T> FixedSizeEncoding for Option<LazyArray<T>> {
 
     #[inline]
     fn from_bytes(b: &[u8; 8]) -> Self {
-        let ([ref position_bytes, ref meta_bytes],[])= b.as_chunks::<4>() else { panic!() };
-        let position = NonZeroUsize::new(u32::from_bytes(position_bytes) as usize)?;
-        let len = u32::from_bytes(meta_bytes) as usize;
-        Some(LazyArray::from_position_and_num_elems(position, len))
+        let ([position_bytes, meta_bytes],[])= b.as_chunks::<4>() else { panic!() };
+        LazyArray::from_bytes_impl(position_bytes, meta_bytes)
     }
 
     #[inline]
     fn write_to_bytes(self, b: &mut [u8; 8]) {
-        let ([ref mut position_bytes, ref mut meta_bytes],[])= b.as_chunks_mut::<4>() else { panic!() };
-
-        let position = self.map_or(0, |lazy| lazy.position.get());
-        let position: u32 = position.try_into().unwrap();
-        position.write_to_bytes(position_bytes);
-
-        let len = self.map_or(0, |lazy| lazy.num_elems);
-        let len: u32 = len.try_into().unwrap();
-        len.write_to_bytes(meta_bytes);
+        match self {
+            None => unreachable!(),
+            Some(lazy) => lazy.write_to_bytes_impl(b),
+        }
     }
 }
 
@@ -289,20 +369,27 @@ impl<I: Idx, const N: usize, T> TableBuilder<I, Option<T>>
 where
     Option<T>: FixedSizeEncoding<ByteArray = [u8; N]>,
 {
-    pub(crate) fn set(&mut self, i: I, value: T) {
-        self.set_nullable(i, Some(value))
+    pub(crate) fn set_some(&mut self, i: I, value: T) {
+        self.set(i, Some(value))
     }
 }
 
 impl<I: Idx, const N: usize, T: FixedSizeEncoding<ByteArray = [u8; N]>> TableBuilder<I, T> {
-    pub(crate) fn set_nullable(&mut self, i: I, value: T) {
-        // FIXME(eddyb) investigate more compact encodings for sparse tables.
-        // On the PR @michaelwoerister mentioned:
-        // > Space requirements could perhaps be optimized by using the HAMT `popcnt`
-        // > trick (i.e. divide things into buckets of 32 or 64 items and then
-        // > store bit-masks of which item in each bucket is actually serialized).
-        self.blocks.ensure_contains_elem(i, || [0; N]);
-        value.write_to_bytes(&mut self.blocks[i]);
+    /// Sets the table value if it is not default.
+    /// ATTENTION: For optimization default values are simply ignored by this function, because
+    /// right now metadata tables never need to reset non-default values to default. If such need
+    /// arises in the future then a new method (e.g. `clear` or `reset`) will need to be introduced
+    /// for doing that explicitly.
+    pub(crate) fn set(&mut self, i: I, value: T) {
+        if !value.is_default() {
+            // FIXME(eddyb) investigate more compact encodings for sparse tables.
+            // On the PR @michaelwoerister mentioned:
+            // > Space requirements could perhaps be optimized by using the HAMT `popcnt`
+            // > trick (i.e. divide things into buckets of 32 or 64 items and then
+            // > store bit-masks of which item in each bucket is actually serialized).
+            self.blocks.ensure_contains_elem(i, || [0; N]);
+            value.write_to_bytes(&mut self.blocks[i]);
+        }
     }
 
     pub(crate) fn encode(&self, buf: &mut FileEncoder) -> LazyTable<I, T> {
@@ -331,10 +418,7 @@ where
         let start = self.position.get();
         let bytes = &metadata.blob()[start..start + self.encoded_size];
         let (bytes, []) = bytes.as_chunks::<N>() else { panic!() };
-        match bytes.get(i.index()) {
-            Some(bytes) => FixedSizeEncoding::from_bytes(bytes),
-            None => FixedSizeEncoding::from_bytes(&[0; N]),
-        }
+        bytes.get(i.index()).map_or_else(Default::default, FixedSizeEncoding::from_bytes)
     }
 
     /// Size of the table in entries, including possible gaps.

compiler/rustc_span/src/def_id.rs

@@ -119,6 +119,12 @@ impl DefPathHash {
     }
 }
 
+impl Default for DefPathHash {
+    fn default() -> Self {
+        DefPathHash(Fingerprint::ZERO)
+    }
+}
+
 impl Borrow<Fingerprint> for DefPathHash {
     #[inline]
     fn borrow(&self) -> &Fingerprint {

compiler/rustc_ty_utils/src/assoc.rs

@@ -22,14 +22,17 @@ fn associated_item_def_ids(tcx: TyCtxt<'_>, def_id: DefId) -> &[DefId] {
         hir::ItemKind::Impl(ref impl_) => tcx.arena.alloc_from_iter(
             impl_.items.iter().map(|impl_item_ref| impl_item_ref.id.owner_id.to_def_id()),
         ),
-        hir::ItemKind::TraitAlias(..) => &[],
         _ => span_bug!(item.span, "associated_item_def_ids: not impl or trait"),
     }
 }
 
 fn associated_items(tcx: TyCtxt<'_>, def_id: DefId) -> ty::AssocItems<'_> {
-    let items = tcx.associated_item_def_ids(def_id).iter().map(|did| tcx.associated_item(*did));
-    ty::AssocItems::new(items)
+    if tcx.is_trait_alias(def_id) {
+        ty::AssocItems::new(Vec::new())
+    } else {
+        let items = tcx.associated_item_def_ids(def_id).iter().map(|did| tcx.associated_item(*did));
+        ty::AssocItems::new(items)
+    }
 }
 
 fn impl_item_implementor_ids(tcx: TyCtxt<'_>, impl_id: DefId) -> FxHashMap<DefId, DefId> {