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Make dropping an empty BTreeMap free #161791

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Make dropping an empty BTreeMap free #161791

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BTree: make dropping an empty BTreeMap free
jakedrew Aug 25, 2026
e93ed22
Adding codegen for the empty BTree drop
jakedrew Aug 26, 2026
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6 changes: 5 additions & 1 deletion library/alloc/src/collections/btree/map.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -205,7 +205,11 @@ pub struct BTreeMap<
#[stable(feature = "btree_drop", since = "1.7.0")]
unsafe impl<#[may_dangle] K, #[may_dangle] V, A: Allocator + Clone> Drop for BTreeMap<K, V, A> {
fn drop(&mut self) {
drop(unsafe { ptr::read(self) }.into_iter())
if self.root.is_some() {
drop(unsafe { ptr::read(self) }.into_iter())
} else {
unsafe { ManuallyDrop::drop(&mut self.alloc) }
}
}
}

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15 changes: 15 additions & 0 deletions tests/codegen-llvm/btree-empty-drop.rs
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@@ -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-NOT: dying_next

@clarfonthey clarfonthey Aug 27, 2026 •
edited by rustbot
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I personally would rather this check for calls to dealloc rather than dying_next, since I this test could easily become worthless if this method is renamed.

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// CHECK: ret void
#[no_mangle]
pub fn drop_btree() {
let _ = BTreeMap::<(), ()>::new();
}
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