From d11e979bb3060e88af27d94292b2bf46a35f419e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Sat, 13 Jun 2026 23:34:16 +0000
Subject: [PATCH] `impl [const] Default for BTreeMap`

---
 library/alloc/src/collections/btree/map.rs         | 3 ++-
 library/alloctests/lib.rs                          | 1 +
 tests/ui/traits/const-traits/const-traits-alloc.rs | 3 +++
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index 7d994359ed48b..0a1f7738632c1 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -2594,7 +2594,8 @@ impl<K: Hash, V: Hash, A: Allocator + Clone> Hash for BTreeMap<K, V, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<K, V> Default for BTreeMap<K, V> {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+const impl<K, V> Default for BTreeMap<K, V> {
     /// Creates an empty `BTreeMap`.
     fn default() -> BTreeMap<K, V> {
         BTreeMap::new()
diff --git a/library/alloctests/lib.rs b/library/alloctests/lib.rs
index 590f559e0d524..83b017b7625b9 100644
--- a/library/alloctests/lib.rs
+++ b/library/alloctests/lib.rs
@@ -20,6 +20,7 @@
 #![feature(const_alloc_error)]
 #![feature(const_cmp)]
 #![feature(const_convert)]
+#![feature(const_default)]
 #![feature(const_destruct)]
 #![feature(const_heap)]
 #![feature(const_option_ops)]
diff --git a/tests/ui/traits/const-traits/const-traits-alloc.rs b/tests/ui/traits/const-traits/const-traits-alloc.rs
index 4dfec2f77f1ba..fe882dcf59629 100644
--- a/tests/ui/traits/const-traits/const-traits-alloc.rs
+++ b/tests/ui/traits/const-traits/const-traits-alloc.rs
@@ -5,5 +5,8 @@
 const STRING: String = Default::default();
 // alloc::vec
 const VEC: Vec<()> = Default::default();
+// alloc::collections::btree::map::BTreeMap
+use std::collections::BTreeMap;
+const BTREE: BTreeMap<(), ()> = Default::default();
 
 fn main() {}