Save a bit on empty item trees by deduplicating them

crates/hir-def/src/db.rs

@@ -80,9 +80,11 @@ pub trait InternDatabase: SourceDatabase {
 
 #[salsa::query_group(DefDatabaseStorage)]
 pub trait DefDatabase: InternDatabase + ExpandDatabase + Upcast<dyn ExpandDatabase> {
+    /// Whether to expand procedural macros during name resolution.
     #[salsa::input]
     fn expand_proc_attr_macros(&self) -> bool;
 
+    /// Computes an [`ItemTree`] for the given file or macro expansion.
     #[salsa::invoke(ItemTree::file_item_tree_query)]
     fn file_item_tree(&self, file_id: HirFileId) -> Arc<ItemTree>;
 
@@ -96,6 +98,7 @@ pub trait DefDatabase: InternDatabase + ExpandDatabase + Upcast<dyn ExpandDataba
     #[salsa::invoke(DefMap::block_def_map_query)]
     fn block_def_map(&self, block: BlockId) -> Arc<DefMap>;
 
+    /// Turns a MacroId into a MacroDefId, describing the macro's definition post name resolution.
     fn macro_def(&self, m: MacroId) -> MacroDefId;
 
     // region:data

crates/hir-def/src/item_tree.rs

@@ -48,6 +48,7 @@ use either::Either;
 use hir_expand::{attrs::RawAttrs, name::Name, ExpandTo, HirFileId, InFile};
 use intern::Interned;
 use la_arena::{Arena, Idx, IdxRange, RawIdx};
+use once_cell::sync::OnceCell;
 use rustc_hash::FxHashMap;
 use smallvec::SmallVec;
 use span::{AstIdNode, FileAstId, SyntaxContextId};
@@ -100,6 +101,7 @@ pub struct ItemTree {
 impl ItemTree {
     pub(crate) fn file_item_tree_query(db: &dyn DefDatabase, file_id: HirFileId) -> Arc<ItemTree> {
         let _p = tracing::info_span!("file_item_tree_query", ?file_id).entered();
+        static EMPTY: OnceCell<Arc<ItemTree>> = OnceCell::new();
 
         let syntax = db.parse_or_expand(file_id);
 
@@ -131,19 +133,48 @@ impl ItemTree {
         if let Some(attrs) = top_attrs {
             item_tree.attrs.insert(AttrOwner::TopLevel, attrs);
         }
+        if item_tree.data.is_none() && item_tree.top_level.is_empty() && item_tree.attrs.is_empty()
+        {
+            EMPTY
+                .get_or_init(|| {
+                    Arc::new(ItemTree {
+                        top_level: SmallVec::new_const(),
+                        attrs: FxHashMap::default(),
+                        data: None,
+                    })
+                })
+                .clone()
+        } else {
             item_tree.shrink_to_fit();
             Arc::new(item_tree)
         }
+    }
 
     pub(crate) fn block_item_tree_query(db: &dyn DefDatabase, block: BlockId) -> Arc<ItemTree> {
+        let _p = tracing::info_span!("block_item_tree_query", ?block).entered();
+        static EMPTY: OnceCell<Arc<ItemTree>> = OnceCell::new();
+
         let loc = block.lookup(db);
         let block = loc.ast_id.to_node(db.upcast());
 
         let ctx = lower::Ctx::new(db, loc.ast_id.file_id);
         let mut item_tree = ctx.lower_block(&block);
+        if item_tree.data.is_none() && item_tree.top_level.is_empty() && item_tree.attrs.is_empty()
+        {
+            EMPTY
+                .get_or_init(|| {
+                    Arc::new(ItemTree {
+                        top_level: SmallVec::new_const(),
+                        attrs: FxHashMap::default(),
+                        data: None,
+                    })
+                })
+                .clone()
+        } else {
             item_tree.shrink_to_fit();
             Arc::new(item_tree)
         }
+    }
 
     /// Returns an iterator over all items located at the top level of the `HirFileId` this
     /// `ItemTree` was created from.

crates/hir-def/src/lib.rs

@@ -977,6 +977,14 @@ impl GenericDefId {
             _ => None,
         }
     }
+
+    pub fn from_callable(db: &dyn DefDatabase, def: CallableDefId) -> GenericDefId {
+        match def {
+            CallableDefId::FunctionId(f) => f.into(),
+            CallableDefId::StructId(s) => s.into(),
+            CallableDefId::EnumVariantId(e) => e.lookup(db).parent.into(),
+        }
+    }
 }
 
 impl From<AssocItemId> for GenericDefId {
@@ -1019,16 +1027,6 @@ impl CallableDefId {
     }
 }
 
-impl GenericDefId {
-    pub fn from(db: &dyn DefDatabase, def: CallableDefId) -> GenericDefId {
-        match def {
-            CallableDefId::FunctionId(f) => f.into(),
-            CallableDefId::StructId(s) => s.into(),
-            CallableDefId::EnumVariantId(e) => e.lookup(db).parent.into(),
-        }
-    }
-}
-
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub enum AttrDefId {
     ModuleId(ModuleId),

crates/hir-ty/src/chalk_db.rs

@@ -915,7 +915,7 @@ fn type_alias_associated_ty_value(
 
 pub(crate) fn fn_def_datum_query(db: &dyn HirDatabase, fn_def_id: FnDefId) -> Arc<FnDefDatum> {
     let callable_def: CallableDefId = from_chalk(db, fn_def_id);
-    let generic_def = GenericDefId::from(db.upcast(), callable_def);
+    let generic_def = GenericDefId::from_callable(db.upcast(), callable_def);
     let generic_params = generics(db.upcast(), generic_def);
     let (sig, binders) = db.callable_item_signature(callable_def).into_value_and_skipped_binders();
     let bound_vars = generic_params.bound_vars_subst(db, DebruijnIndex::INNERMOST);
@@ -946,7 +946,8 @@ pub(crate) fn fn_def_datum_query(db: &dyn HirDatabase, fn_def_id: FnDefId) -> Ar
 
 pub(crate) fn fn_def_variance_query(db: &dyn HirDatabase, fn_def_id: FnDefId) -> Variances {
     let callable_def: CallableDefId = from_chalk(db, fn_def_id);
-    let generic_params = generics(db.upcast(), GenericDefId::from(db.upcast(), callable_def));
+    let generic_params =
+        generics(db.upcast(), GenericDefId::from_callable(db.upcast(), callable_def));
     Variances::from_iter(
         Interner,
         std::iter::repeat(chalk_ir::Variance::Invariant).take(generic_params.len()),

crates/hir-ty/src/chalk_ext.rs

@@ -188,7 +188,7 @@ impl TyExt for Ty {
     fn as_generic_def(&self, db: &dyn HirDatabase) -> Option<GenericDefId> {
         match *self.kind(Interner) {
             TyKind::Adt(AdtId(adt), ..) => Some(adt.into()),
-            TyKind::FnDef(callable, ..) => Some(GenericDefId::from(
+            TyKind::FnDef(callable, ..) => Some(GenericDefId::from_callable(
                 db.upcast(),
                 db.lookup_intern_callable_def(callable.into()),
             )),

crates/hir-ty/src/db.rs

@@ -80,8 +80,32 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     #[salsa::cycle(crate::consteval::const_eval_discriminant_recover)]
     fn const_eval_discriminant(&self, def: EnumVariantId) -> Result<i128, ConstEvalError>;
 
+    #[salsa::invoke(crate::method_resolution::lookup_impl_method_query)]
+    fn lookup_impl_method(
+        &self,
+        env: Arc<TraitEnvironment>,
+        func: FunctionId,
+        fn_subst: Substitution,
+    ) -> (FunctionId, Substitution);
+
     // endregion:mir
 
+    #[salsa::invoke(crate::layout::layout_of_adt_query)]
+    #[salsa::cycle(crate::layout::layout_of_adt_recover)]
+    fn layout_of_adt(
+        &self,
+        def: AdtId,
+        subst: Substitution,
+        env: Arc<TraitEnvironment>,
+    ) -> Result<Arc<Layout>, LayoutError>;
+
+    #[salsa::invoke(crate::layout::layout_of_ty_query)]
+    #[salsa::cycle(crate::layout::layout_of_ty_recover)]
+    fn layout_of_ty(&self, ty: Ty, env: Arc<TraitEnvironment>) -> Result<Arc<Layout>, LayoutError>;
+
+    #[salsa::invoke(crate::layout::target_data_layout_query)]
+    fn target_data_layout(&self, krate: CrateId) -> Result<Arc<TargetDataLayout>, Arc<str>>;
+
     #[salsa::invoke(crate::lower::ty_query)]
     #[salsa::cycle(crate::lower::ty_recover)]
     fn ty(&self, def: TyDefId) -> Binders<Ty>;
@@ -104,30 +128,6 @@ pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
     #[salsa::invoke(crate::lower::field_types_query)]
     fn field_types(&self, var: VariantId) -> Arc<ArenaMap<LocalFieldId, Binders<Ty>>>;
 
-    #[salsa::invoke(crate::layout::layout_of_adt_query)]
-    #[salsa::cycle(crate::layout::layout_of_adt_recover)]
-    fn layout_of_adt(
-        &self,
-        def: AdtId,
-        subst: Substitution,
-        env: Arc<TraitEnvironment>,
-    ) -> Result<Arc<Layout>, LayoutError>;
-
-    #[salsa::invoke(crate::layout::layout_of_ty_query)]
-    #[salsa::cycle(crate::layout::layout_of_ty_recover)]
-    fn layout_of_ty(&self, ty: Ty, env: Arc<TraitEnvironment>) -> Result<Arc<Layout>, LayoutError>;
-
-    #[salsa::invoke(crate::layout::target_data_layout_query)]
-    fn target_data_layout(&self, krate: CrateId) -> Result<Arc<TargetDataLayout>, Arc<str>>;
-
-    #[salsa::invoke(crate::method_resolution::lookup_impl_method_query)]
-    fn lookup_impl_method(
-        &self,
-        env: Arc<TraitEnvironment>,
-        func: FunctionId,
-        fn_subst: Substitution,
-    ) -> (FunctionId, Substitution);
-
     #[salsa::invoke(crate::lower::callable_item_sig)]
     fn callable_item_signature(&self, def: CallableDefId) -> PolyFnSig;
 

crates/hir-ty/src/display.rs

@@ -988,7 +988,7 @@ impl HirDisplay for Ty {
                 f.end_location_link();
 
                 if parameters.len(Interner) > 0 {
-                    let generic_def_id = GenericDefId::from(db.upcast(), def);
+                    let generic_def_id = GenericDefId::from_callable(db.upcast(), def);
                     let generics = generics(db.upcast(), generic_def_id);
                     let (parent_len, self_param, type_, const_, impl_, lifetime) =
                         generics.provenance_split();

crates/hir-ty/src/infer/expr.rs

@@ -1896,7 +1896,7 @@ impl InferenceContext<'_> {
         if let TyKind::FnDef(fn_def, parameters) = callable_ty.kind(Interner) {
             let def: CallableDefId = from_chalk(self.db, *fn_def);
             let generic_predicates =
-                self.db.generic_predicates(GenericDefId::from(self.db.upcast(), def));
+                self.db.generic_predicates(GenericDefId::from_callable(self.db.upcast(), def));
             for predicate in generic_predicates.iter() {
                 let (predicate, binders) = predicate
                     .clone()