Store def_id_to_hir_id as variant in hir_owner.

If hir_owner is Owner(_), the LocalDefId is pointing to an owner, so the ItemLocalId is 0.
If the HIR node does not exist, we store Phantom.
Otherwise, we store the HirId associated to the LocalDefId.

Author: cjgillot

compiler/rustc_ast_lowering/src/item.rs

@@ -105,12 +105,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> T {
         let old_len = self.in_scope_lifetimes.len();
 
-        let parent_generics =
-            match self.owners[parent_hir_id].as_ref().unwrap().node().expect_item().kind {
-                hir::ItemKind::Impl(hir::Impl { ref generics, .. })
-                | hir::ItemKind::Trait(_, _, ref generics, ..) => generics.params,
-                _ => &[],
-            };
+        let parent_generics = match self.owners[parent_hir_id].unwrap().node().expect_item().kind {
+            hir::ItemKind::Impl(hir::Impl { ref generics, .. })
+            | hir::ItemKind::Trait(_, _, ref generics, ..) => generics.params,
+            _ => &[],
+        };
         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
             hir::GenericParamKind::Lifetime { .. } => Some(param.name.normalize_to_macros_2_0()),
             _ => None,
@@ -480,6 +479,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             .node_id_to_hir_id
                             .insert(new_node_id, hir::HirId::make_owner(new_id));
                         debug_assert!(_old.is_none());
+                        self.owners.ensure_contains_elem(new_id, || hir::MaybeOwner::Phantom);
+                        let _old = std::mem::replace(
+                            &mut self.owners[new_id],
+                            hir::MaybeOwner::NonOwner(hir::HirId::make_owner(new_id)),
+                        );
+                        debug_assert!(matches!(_old, hir::MaybeOwner::Phantom));
                         continue;
                     };
                     let ident = *ident;

compiler/rustc_ast_lowering/src/lib.rs

@@ -98,7 +98,7 @@ struct LoweringContext<'a, 'hir: 'a> {
     arena: &'hir Arena<'hir>,
 
     /// The items being lowered are collected here.
-    owners: IndexVec<LocalDefId, Option<hir::OwnerInfo<'hir>>>,
+    owners: IndexVec<LocalDefId, hir::MaybeOwner<&'hir hir::OwnerInfo<'hir>>>,
     /// Bodies inside the owner being lowered.
     bodies: Vec<(hir::ItemLocalId, &'hir hir::Body<'hir>)>,
     /// Attributes inside the owner being lowered.
@@ -291,7 +291,8 @@ pub fn lower_crate<'a, 'hir>(
 ) -> &'hir hir::Crate<'hir> {
     let _prof_timer = sess.prof.verbose_generic_activity("hir_lowering");
 
-    let owners = IndexVec::from_fn_n(|_| None, resolver.definitions().def_index_count());
+    let owners =
+        IndexVec::from_fn_n(|_| hir::MaybeOwner::Phantom, resolver.definitions().def_index_count());
     LoweringContext {
         sess,
         resolver,
@@ -402,19 +403,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         let hir_hash = self.compute_hir_hash();
 
-        let mut def_id_to_hir_id = IndexVec::default();
-
-        for (node_id, hir_id) in self.node_id_to_hir_id.into_iter_enumerated() {
-            if let Some(def_id) = self.resolver.opt_local_def_id(node_id) {
-                if def_id_to_hir_id.len() <= def_id.index() {
-                    def_id_to_hir_id.resize(def_id.index() + 1, None);
-                }
-                def_id_to_hir_id[def_id] = hir_id;
-            }
-        }
-
-        self.resolver.definitions().init_def_id_to_hir_id_mapping(def_id_to_hir_id);
-
         let krate = hir::Crate { owners: self.owners, hir_hash };
         self.arena.alloc(krate)
     }
@@ -427,7 +415,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             .owners
             .iter_enumerated()
             .filter_map(|(def_id, info)| {
-                let info = info.as_ref()?;
+                let info = info.as_owner()?;
                 let def_path_hash = definitions.def_path_hash(def_id);
                 Some((def_path_hash, info))
             })
@@ -469,8 +457,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.current_hir_id_owner = current_owner;
         self.item_local_id_counter = current_local_counter;
 
-        let _old = self.owners.insert(def_id, info);
-        debug_assert!(_old.is_none());
+        self.owners.ensure_contains_elem(def_id, || hir::MaybeOwner::Phantom);
+        self.owners[def_id] = hir::MaybeOwner::Owner(self.arena.alloc(info));
 
         def_id
     }
@@ -488,6 +476,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     None
                 } else {
                     let def_id = self.resolver.opt_local_def_id(node_id)?;
+                    self.owners.ensure_contains_elem(def_id, || hir::MaybeOwner::Phantom);
+                    if let o @ hir::MaybeOwner::Phantom = &mut self.owners[def_id] {
+                        // Do not override a `MaybeOwner::Owner` that may already here.
+                        *o = hir::MaybeOwner::NonOwner(hir_id);
+                    }
                     Some((hir_id.local_id, def_id))
                 }
             })

compiler/rustc_hir/src/definitions.rs

@@ -7,7 +7,6 @@
 pub use crate::def_id::DefPathHash;
 use crate::def_id::{CrateNum, DefIndex, LocalDefId, StableCrateId, CRATE_DEF_INDEX, LOCAL_CRATE};
 use crate::def_path_hash_map::DefPathHashMap;
-use crate::hir;
 
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::StableHasher;
@@ -101,13 +100,6 @@ impl DefPathTable {
 pub struct Definitions {
     table: DefPathTable,
 
-    /// Only [`LocalDefId`]s for items and item-like are HIR owners.
-    /// The associated `HirId` has a `local_id` of `0`.
-    /// Generic parameters and closures are also assigned a `LocalDefId` but are not HIR owners.
-    /// Their `HirId`s are defined by their position while lowering the enclosing owner.
-    // FIXME(cjgillot) Some `LocalDefId`s from `use` items are dropped during lowering and lack a `HirId`.
-    pub(super) def_id_to_hir_id: IndexVec<LocalDefId, Option<hir::HirId>>,
-
     /// Item with a given `LocalDefId` was defined during macro expansion with ID `ExpnId`.
     expansions_that_defined: FxHashMap<LocalDefId, ExpnId>,
 
@@ -322,12 +314,6 @@ impl Definitions {
         })
     }
 
-    #[inline]
-    #[track_caller]
-    pub fn local_def_id_to_hir_id(&self, id: LocalDefId) -> hir::HirId {
-        self.def_id_to_hir_id[id].unwrap()
-    }
-
     /// Adds a root definition (no parent) and a few other reserved definitions.
     pub fn new(stable_crate_id: StableCrateId, crate_span: Span) -> Definitions {
         let key = DefKey {
@@ -354,7 +340,6 @@ impl Definitions {
 
         Definitions {
             table,
-            def_id_to_hir_id: Default::default(),
             expansions_that_defined: Default::default(),
             def_id_to_span,
             stable_crate_id,
@@ -406,20 +391,6 @@ impl Definitions {
         def_id
     }
 
-    /// Initializes the `LocalDefId` to `HirId` mapping once it has been generated during
-    /// AST to HIR lowering.
-    pub fn init_def_id_to_hir_id_mapping(
-        &mut self,
-        mapping: IndexVec<LocalDefId, Option<hir::HirId>>,
-    ) {
-        assert!(
-            self.def_id_to_hir_id.is_empty(),
-            "trying to initialize `LocalDefId` <-> `HirId` mappings twice"
-        );
-
-        self.def_id_to_hir_id = mapping;
-    }
-
     pub fn expansion_that_defined(&self, id: LocalDefId) -> ExpnId {
         self.expansions_that_defined.get(&id).copied().unwrap_or_else(ExpnId::root)
     }
@@ -431,7 +402,7 @@ impl Definitions {
     }
 
     pub fn iter_local_def_id(&self) -> impl Iterator<Item = LocalDefId> + '_ {
-        self.def_id_to_hir_id.iter_enumerated().map(|(k, _)| k)
+        self.table.def_path_hashes.indices().map(|local_def_index| LocalDefId { local_def_index })
     }
 
     #[inline(always)]

compiler/rustc_hir/src/hir.rs

@@ -711,6 +711,15 @@ pub struct OwnerNodes<'tcx> {
     pub local_id_to_def_id: SortedMap<ItemLocalId, LocalDefId>,
 }
 
+impl<'tcx> OwnerNodes<'tcx> {
+    pub fn node(&self) -> OwnerNode<'tcx> {
+        use rustc_index::vec::Idx;
+        let node = self.nodes[ItemLocalId::new(0)].as_ref().unwrap().node;
+        let node = node.as_owner().unwrap(); // Indexing must ensure it is an OwnerNode.
+        node
+    }
+}
+
 /// Full information resulting from lowering an AST node.
 #[derive(Debug, HashStable_Generic)]
 pub struct OwnerInfo<'hir> {
@@ -728,10 +737,39 @@ pub struct OwnerInfo<'hir> {
 impl<'tcx> OwnerInfo<'tcx> {
     #[inline]
     pub fn node(&self) -> OwnerNode<'tcx> {
-        use rustc_index::vec::Idx;
-        let node = self.nodes.nodes[ItemLocalId::new(0)].as_ref().unwrap().node;
-        let node = node.as_owner().unwrap(); // Indexing must ensure it is an OwnerNode.
-        node
+        self.nodes.node()
+    }
+}
+
+#[derive(Copy, Clone, Debug, HashStable_Generic)]
+pub enum MaybeOwner<T> {
+    Owner(T),
+    NonOwner(HirId),
+    /// Used as a placeholder for unused LocalDefId.
+    Phantom,
+}
+
+impl<T> MaybeOwner<T> {
+    pub fn as_owner(self) -> Option<T> {
+        match self {
+            MaybeOwner::Owner(i) => Some(i),
+            MaybeOwner::NonOwner(_) | MaybeOwner::Phantom => None,
+        }
+    }
+
+    pub fn map<U>(self, f: impl FnOnce(T) -> U) -> MaybeOwner<U> {
+        match self {
+            MaybeOwner::Owner(i) => MaybeOwner::Owner(f(i)),
+            MaybeOwner::NonOwner(hir_id) => MaybeOwner::NonOwner(hir_id),
+            MaybeOwner::Phantom => MaybeOwner::Phantom,
+        }
+    }
+
+    pub fn unwrap(self) -> T {
+        match self {
+            MaybeOwner::Owner(i) => i,
+            MaybeOwner::NonOwner(_) | MaybeOwner::Phantom => panic!("Not a HIR owner"),
+        }
     }
 }
 
@@ -743,7 +781,7 @@ impl<'tcx> OwnerInfo<'tcx> {
 /// [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/hir.html
 #[derive(Debug)]
 pub struct Crate<'hir> {
-    pub owners: IndexVec<LocalDefId, Option<OwnerInfo<'hir>>>,
+    pub owners: IndexVec<LocalDefId, MaybeOwner<&'hir OwnerInfo<'hir>>>,
     pub hir_hash: Fingerprint,
 }
 

compiler/rustc_hir/src/hir_id.rs

@@ -30,6 +30,11 @@ impl HirId {
         if self.local_id.index() == 0 { Some(self.owner) } else { None }
     }
 
+    #[inline]
+    pub fn is_owner(self) -> bool {
+        self.local_id.index() == 0
+    }
+
     #[inline]
     pub fn make_owner(owner: LocalDefId) -> Self {
         Self { owner, local_id: ItemLocalId::from_u32(0) }