From 638e49b0fcb0df56212e328689bbcce029cfc000 Mon Sep 17 00:00:00 2001
From: Lukas Wirth <me@lukaswirth.dev>
Date: Thu, 26 Jun 2025 14:02:59 +0200
Subject: [PATCH] Skip unnecessary `eq` work in `BodySourceMap`

---
 crates/hir-def/src/expr_store.rs       | 58 +++++++++++++++++++++-----
 crates/hir-def/src/expr_store/scope.rs |  2 +-
 2 files changed, 49 insertions(+), 11 deletions(-)

diff --git a/crates/hir-def/src/expr_store.rs b/crates/hir-def/src/expr_store.rs
index 85bd193223..51612f341a 100644
--- a/crates/hir-def/src/expr_store.rs
+++ b/crates/hir-def/src/expr_store.rs
@@ -93,7 +93,7 @@ pub type TypeSource = InFile<TypePtr>;
 pub type LifetimePtr = AstPtr<ast::Lifetime>;
 pub type LifetimeSource = InFile<LifetimePtr>;
 
-#[derive(Debug, Eq, PartialEq)]
+#[derive(Debug, PartialEq, Eq)]
 pub struct ExpressionStore {
     pub exprs: Arena<Expr>,
     pub pats: Arena<Pat>,
@@ -114,7 +114,7 @@ pub struct ExpressionStore {
     ident_hygiene: FxHashMap<ExprOrPatId, HygieneId>,
 }
 
-#[derive(Debug, Eq, PartialEq, Default)]
+#[derive(Debug, Eq, Default)]
 pub struct ExpressionStoreSourceMap {
     // AST expressions can create patterns in destructuring assignments. Therefore, `ExprSource` can also map
     // to `PatId`, and `PatId` can also map to `ExprSource` (the other way around is unaffected).
@@ -127,19 +127,20 @@ pub struct ExpressionStoreSourceMap {
     label_map: FxHashMap<LabelSource, LabelId>,
     label_map_back: ArenaMap<LabelId, LabelSource>,
 
-    binding_definitions: FxHashMap<BindingId, SmallVec<[PatId; 4]>>,
-
-    /// We don't create explicit nodes for record fields (`S { record_field: 92 }`).
-    /// Instead, we use id of expression (`92`) to identify the field.
-    field_map_back: FxHashMap<ExprId, FieldSource>,
-    pat_field_map_back: FxHashMap<PatId, PatFieldSource>,
-
     types_map_back: ArenaMap<TypeRefId, TypeSource>,
     types_map: FxHashMap<TypeSource, TypeRefId>,
 
     lifetime_map_back: ArenaMap<LifetimeRefId, LifetimeSource>,
     lifetime_map: FxHashMap<LifetimeSource, LifetimeRefId>,
 
+    binding_definitions:
+        ArenaMap<BindingId, SmallVec<[PatId; 2 * size_of::<usize>() / size_of::<PatId>()]>>,
+
+    /// We don't create explicit nodes for record fields (`S { record_field: 92 }`).
+    /// Instead, we use id of expression (`92`) to identify the field.
+    field_map_back: FxHashMap<ExprId, FieldSource>,
+    pat_field_map_back: FxHashMap<PatId, PatFieldSource>,
+
     template_map: Option<Box<FormatTemplate>>,
 
     pub expansions: FxHashMap<InFile<MacroCallPtr>, MacroCallId>,
@@ -149,6 +150,43 @@ pub struct ExpressionStoreSourceMap {
     pub diagnostics: Vec<ExpressionStoreDiagnostics>,
 }
 
+impl PartialEq for ExpressionStoreSourceMap {
+    fn eq(&self, other: &Self) -> bool {
+        // we only need to compare one of the two mappings
+        // as the other is a reverse mapping and thus will compare
+        // the same as normal mapping
+        let Self {
+            expr_map: _,
+            expr_map_back,
+            pat_map: _,
+            pat_map_back,
+            label_map: _,
+            label_map_back,
+            types_map_back,
+            types_map: _,
+            lifetime_map_back,
+            lifetime_map: _,
+            // If this changed, our pattern data must have changed
+            binding_definitions: _,
+            // If this changed, our expression data must have changed
+            field_map_back: _,
+            // If this changed, our pattern data must have changed
+            pat_field_map_back: _,
+            template_map,
+            expansions,
+            diagnostics,
+        } = self;
+        *expr_map_back == other.expr_map_back
+            && *pat_map_back == other.pat_map_back
+            && *label_map_back == other.label_map_back
+            && *types_map_back == other.types_map_back
+            && *lifetime_map_back == other.lifetime_map_back
+            && *template_map == other.template_map
+            && *expansions == other.expansions
+            && *diagnostics == other.diagnostics
+    }
+}
+
 /// The body of an item (function, const etc.).
 #[derive(Debug, Eq, PartialEq, Default)]
 pub struct ExpressionStoreBuilder {
@@ -698,7 +736,7 @@ impl ExpressionStoreSourceMap {
     }
 
     pub fn patterns_for_binding(&self, binding: BindingId) -> &[PatId] {
-        self.binding_definitions.get(&binding).map_or(&[], Deref::deref)
+        self.binding_definitions.get(binding).map_or(&[], Deref::deref)
     }
 
     pub fn node_label(&self, node: InFile<&ast::Label>) -> Option<LabelId> {
diff --git a/crates/hir-def/src/expr_store/scope.rs b/crates/hir-def/src/expr_store/scope.rs
index a46711c67e..2dd0b9bdb8 100644
--- a/crates/hir-def/src/expr_store/scope.rs
+++ b/crates/hir-def/src/expr_store/scope.rs
@@ -535,7 +535,7 @@ fn foo() {
 
         let resolved = scopes.resolve_name_in_scope(expr_scope, &name_ref.as_name()).unwrap();
         let pat_src = source_map
-            .pat_syntax(*source_map.binding_definitions[&resolved.binding()].first().unwrap())
+            .pat_syntax(*source_map.binding_definitions[resolved.binding()].first().unwrap())
             .unwrap();
 
         let local_name = pat_src.value.syntax_node_ptr().to_node(file.syntax());