From e822937428e80bb9609a40b298d1c8b4e157c6ef Mon Sep 17 00:00:00 2001
From: Philippe-Cholet <44676486+Philippe-Cholet@users.noreply.github.com>
Date: Tue, 26 Mar 2024 11:19:27 +0100
Subject: [PATCH] Add invariant to VecDeque::pop_* that len < cap if pop
 successful

Similar to #114370 for VecDeque instead of Vec.
---
 .../alloc/src/collections/vec_deque/mod.rs    |  6 ++
 tests/codegen/vecdeque_pop_push.rs            | 75 +++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 tests/codegen/vecdeque_pop_push.rs

diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index 41adc2e79dc74..07b1b73c4f6a5 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -1614,6 +1614,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
             let old_head = self.head;
             self.head = self.to_physical_idx(1);
             self.len -= 1;
+            unsafe {
+                core::intrinsics::assume(self.len < self.capacity());
+            }
             Some(unsafe { self.buffer_read(old_head) })
         }
     }
@@ -1638,6 +1641,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
             None
         } else {
             self.len -= 1;
+            unsafe {
+                core::intrinsics::assume(self.len < self.capacity());
+            }
             Some(unsafe { self.buffer_read(self.to_physical_idx(self.len)) })
         }
     }
diff --git a/tests/codegen/vecdeque_pop_push.rs b/tests/codegen/vecdeque_pop_push.rs
new file mode 100644
index 0000000000000..c0c28a1193367
--- /dev/null
+++ b/tests/codegen/vecdeque_pop_push.rs
@@ -0,0 +1,75 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+use std::collections::VecDeque;
+
+#[no_mangle]
+// CHECK-LABEL: @noop_back(
+pub fn noop_back(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_back() {
+        v.push_back(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @noop_front(
+pub fn noop_front(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_front() {
+        v.push_front(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @move_byte_front_to_back(
+pub fn move_byte_front_to_back(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_front() {
+        v.push_back(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @move_byte_back_to_front(
+pub fn move_byte_back_to_front(v: &mut VecDeque<u8>) {
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: tail call void @llvm.assume
+    // CHECK-NOT: reserve_for_push
+    // CHECK-NOT: call
+    // CHECK: ret
+    if let Some(x) = v.pop_back() {
+        v.push_front(x);
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @push_back_byte(
+pub fn push_back_byte(v: &mut VecDeque<u8>) {
+    // CHECK: call {{.*}}reserve_for_push
+    v.push_back(3);
+}
+
+#[no_mangle]
+// CHECK-LABEL: @push_front_byte(
+pub fn push_front_byte(v: &mut VecDeque<u8>) {
+    // CHECK: call {{.*}}reserve_for_push
+    v.push_front(3);
+}