Update library/alloc/src/raw_vec.rs

Co-authored-by: scottmcm <[email protected]>

Author: kornelski

library/alloc/src/raw_vec.rs

@@ -291,10 +291,14 @@ impl<T, A: Allocator> RawVec<T, A> {
 
         if self.needs_to_grow(len, additional) {
             do_reserve_and_handle(self, len, additional);
-            unsafe {
-                // Inform the optimizer that the reservation has succeeded
-                core::intrinsics::assume(!self.needs_to_grow(len, additional));
-            }
+        }
+
+        // SAFETY: The call to `do_reserve_and_handle` ensured this
+        // (or it panicked) and thus the addition cannot overflow.
+        unsafe {
+            // Inform the optimizer that the reservation has succeeded
+            let min_cap = len.unchecked_add(additional);
+            core::intrinsics::assume(self.cap >= min_cap);
         }
     }