assert_unsafe_precondition!(length <= capacity) in Vec::from_raw_parts

This hopefully helps to catch cases where these arguments are
accidentally swapped.

Author: GoldsteinE

library/alloc/src/lib.rs

@@ -156,6 +156,7 @@
 #![feature(try_trait_v2)]
 #![feature(try_with_capacity)]
 #![feature(tuple_trait)]
+#![feature(ub_checks)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
 #![feature(unwrap_infallible)]

library/alloc/src/vec/mod.rs

@@ -944,6 +944,14 @@ impl<T, A: Allocator> Vec<T, A> {
     #[inline]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub unsafe fn from_raw_parts_in(ptr: *mut T, length: usize, capacity: usize, alloc: A) -> Self {
+        assert_unsafe_precondition!(
+            check_library_ub,
+            "Vec::from_raw_parts requires that length is less than or equal to capacity",
+            (
+                length: usize = length,
+                capacity: usize = capacity,
+            ) => length <= capacity,
+        );
         unsafe { Vec { buf: RawVec::from_raw_parts_in(ptr, capacity, alloc), len: length } }
     }