Hint that VecDeque binary search results are in bounds

library/alloc/src/collections/vec_deque/mod.rs

@@ -3314,13 +3314,22 @@ impl<T, A: Allocator> VecDeque<T, A> {
         let (front, back) = self.as_slices();
         let cmp_back = back.first().map(|elem| f(elem));
 
-        if let Some(Ordering::Equal) = cmp_back {
+        let result = if let Some(Ordering::Equal) = cmp_back {
             Ok(front.len())
         } else if let Some(Ordering::Less) = cmp_back {
             back.binary_search_by(f).map(|idx| idx + front.len()).map_err(|idx| idx + front.len())
         } else {
             front.binary_search_by(f)
+        };
+
+        match result {
+            // SAFETY: `index` points at an element of one of the two slices.
+            Ok(index) => unsafe { core::hint::assert_unchecked(index < self.len()) },
+            // SAFETY: An insertion point in either slice is at most one past the deque's end.
+            Err(index) => unsafe { core::hint::assert_unchecked(index <= self.len()) },
         }
+
+        result
     }
 
     /// Binary searches this `VecDeque` with a key extraction function.

tests/codegen-llvm/vec-deque-binary-search-index-no-bound-check.rs

@@ -0,0 +1,35 @@
+//@ compile-flags: -Copt-level=3
+#![crate_type = "lib"]
+
+use std::collections::VecDeque;
+
+unsafe extern "C" {
+    safe fn vec_deque_binary_search_insertion_point_out_of_bounds();
+}
+
+// Make sure no bounds check is emitted when indexing with a successful search result.
+
+// CHECK-LABEL: @vec_deque_binary_search_index_no_bounds_check
+#[no_mangle]
+pub fn vec_deque_binary_search_index_no_bounds_check(deque: &VecDeque<u8>) -> u8 {
+    // CHECK-NOT: expect_failed
+    if let Ok(index) = deque.binary_search_by(|element| element.cmp(&b'\\')) {
+        deque[index]
+    } else {
+        42
+    }
+}
+
+// An unsuccessful search result is a valid insertion point.
+
+// CHECK-LABEL: @vec_deque_binary_search_insertion_point_in_bounds
+#[no_mangle]
+pub fn vec_deque_binary_search_insertion_point_in_bounds(deque: &VecDeque<u8>) {
+    // CHECK-NOT: call void @vec_deque_binary_search_insertion_point_out_of_bounds
+    // CHECK: ret void
+    if let Err(index) = deque.binary_search_by(|element| element.cmp(&b'\\')) {
+        if index > deque.len() {
+            vec_deque_binary_search_insertion_point_out_of_bounds()
+        }
+    }
+}