3 files changed, 43 insertions, 11 deletions
diff --git a/crates/hir-def/src/item_scope.rs b/crates/hir-def/src/item_scope.rs
index 3ed321d189d..2001fb29a9e 100644
--- a/crates/hir-def/src/item_scope.rs
+++ b/crates/hir-def/src/item_scope.rs
@@ -334,10 +334,6 @@ impl ItemScope {
         )
     }
 
-    pub(crate) fn collect_legacy_macros(&self) -> FxHashMap<Name, SmallVec<[MacroId; 1]>> {
-        self.legacy_macros.clone()
-    }
-
     /// Marks everything that is not a procedural macro as private to `this_module`.
     pub(crate) fn censor_non_proc_macros(&mut self, this_module: ModuleId) {
         self.types
diff --git a/crates/hir-def/src/nameres/collector.rs b/crates/hir-def/src/nameres/collector.rs
index 97481ea7df7..62fb3c7882c 100644
--- a/crates/hir-def/src/nameres/collector.rs
+++ b/crates/hir-def/src/nameres/collector.rs
@@ -3,7 +3,7 @@
 //! `DefCollector::collect` contains the fixed-point iteration loop which
 //! resolves imports and expands macros.
 
-use std::{iter, mem};
+use std::{cmp::Ordering, iter, mem};
 
 use base_db::{CrateId, Dependency, Edition, FileId};
 use cfg::{CfgExpr, CfgOptions};
@@ -1928,9 +1928,13 @@ impl ModCollector<'_, '_> {
         let modules = &mut def_map.modules;
         let res = modules.alloc(ModuleData::new(origin, vis));
         modules[res].parent = Some(self.module_id);
-        for (name, mac) in modules[self.module_id].scope.collect_legacy_macros() {
-            for &mac in &mac {
-                modules[res].scope.define_legacy_macro(name.clone(), mac);
+
+        if let Some((target, source)) = Self::borrow_modules(modules.as_mut(), res, self.module_id)
+        {
+            for (name, macs) in source.scope.legacy_macros() {
+                for &mac in macs {
+                    target.scope.define_legacy_macro(name.clone(), mac);
+                }
             }
         }
         modules[self.module_id].children.insert(name.clone(), res);
@@ -2226,14 +2230,40 @@ impl ModCollector<'_, '_> {
     }
 
     fn import_all_legacy_macros(&mut self, module_id: LocalModuleId) {
-        let macros = self.def_collector.def_map[module_id].scope.collect_legacy_macros();
-        for (name, macs) in macros {
+        let Some((source, target)) = Self::borrow_modules(self.def_collector.def_map.modules.as_mut(), module_id, self.module_id) else {
+            return
+        };
+
+        for (name, macs) in source.scope.legacy_macros() {
             macs.last().map(|&mac| {
-                self.def_collector.define_legacy_macro(self.module_id, name.clone(), mac)
+                target.scope.define_legacy_macro(name.clone(), mac);
             });
         }
     }
 
+    /// Mutably borrow two modules at once, retu
+    fn borrow_modules(
+        modules: &mut [ModuleData],
+        a: LocalModuleId,
+        b: LocalModuleId,
+    ) -> Option<(&mut ModuleData, &mut ModuleData)> {
+        let a = a.into_raw().into_u32() as usize;
+        let b = b.into_raw().into_u32() as usize;
+
+        let (a, b) = match a.cmp(&b) {
+            Ordering::Equal => return None,
+            Ordering::Less => {
+                let (prefix, b) = modules.split_at_mut(b);
+                (&mut prefix[a], &mut b[0])
+            }
+            Ordering::Greater => {
+                let (prefix, a) = modules.split_at_mut(a);
+                (&mut a[0], &mut prefix[b])
+            }
+        };
+        Some((a, b))
+    }
+
     fn is_cfg_enabled(&self, cfg: &CfgExpr) -> bool {
         self.def_collector.cfg_options.check(cfg) != Some(false)
     }
diff --git a/lib/la-arena/src/lib.rs b/lib/la-arena/src/lib.rs
index 5107f294394..f39c3a3e4ca 100644
--- a/lib/la-arena/src/lib.rs
+++ b/lib/la-arena/src/lib.rs
@@ -451,6 +451,12 @@ impl<T> Arena<T> {
     }
 }
 
+impl<T> AsMut<[T]> for Arena<T> {
+    fn as_mut(&mut self) -> &mut [T] {
+        self.data.as_mut()
+    }
+}
+
 impl<T> Default for Arena<T> {
     fn default() -> Arena<T> {
         Arena { data: Vec::new() }