Add invariant to Vec::pop that len < cap if pop successful

Fixes: https://github.com/rust-lang/rust/issues/114334

2 files changed, 25 insertions, 0 deletions

diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 35015238e6e..3b12c1bee0b 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -1956,6 +1956,7 @@ impl<T, A: Allocator> Vec<T, A> {
         } else {
             unsafe {
                 self.len -= 1;
+                core::intrinsics::assume(self.len < self.capacity());
                 Some(ptr::read(self.as_ptr().add(self.len())))
             }
         }
diff --git a/tests/codegen/vec_pop_push_noop.rs b/tests/codegen/vec_pop_push_noop.rs
new file mode 100644
index 00000000000..8bc7b68a816
--- /dev/null
+++ b/tests/codegen/vec_pop_push_noop.rs
@@ -0,0 +1,24 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+// CHECK-LABEL: @noop(
+pub fn noop(v: &mut Vec<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() {
+        v.push(x)
+    }
+}
+
+#[no_mangle]
+// CHECK-LABEL: @push_byte(
+pub fn push_byte(v: &mut Vec<u8>) {
+    // CHECK: call {{.*}}reserve_for_push
+    v.push(3);
+}