Make dropping an empty BTreeMap free

library/alloc/src/collections/btree/map.rs

@@ -205,8 +205,14 @@ pub struct BTreeMap<
 #[stable(feature = "btree_drop", since = "1.7.0")]
 unsafe impl<#[may_dangle] K, #[may_dangle] V, A: AllocatorClone> Drop for BTreeMap<K, V, A> {
     fn drop(&mut self) {
-        // ignore-tidy-undocumented-unsafe
-        drop(unsafe { ptr::read(self) }.into_iter())
+        if self.root.is_some() {
+            // ignore-tidy-undocumented-unsafe
+            drop(unsafe { ptr::read(self) }.into_iter())
+        } else {
+            // SAFETY: With no root there are no nodes to free, so only the allocator needs
+            // dropping. `self` is not used after this, and `alloc` is dropped only here.
+            unsafe { ManuallyDrop::drop(&mut self.alloc) }
+        }
     }
 }
 

tests/codegen-llvm/btree-empty-drop.rs

@@ -0,0 +1,15 @@
+// Regression test for https://github.com/rust-lang/rust/issues/161375
+// Checking that an empty BTree's drop is optimized away.
+//@ compile-flags: -Copt-level=3
+
+#![crate_type = "lib"]
+
+use std::collections::BTreeMap;
+
+// CHECK-LABEL: @drop_btree(
+// CHECK-NEXT: {{.*}}:
+// CHECK-NEXT: ret void
+#[no_mangle]
+pub fn drop_btree() {
+    let _ = BTreeMap::<(), ()>::new();
+}