Add debug assertions to some unsafe functions

These debug assertions are all implemented only at runtime using
`const_eval_select`, and in the error path they execute
`intrinsics::abort` instead of being a normal debug assertion to
minimize the impact of these assertions on code size, when enabled.

Of all these changes, the bounds checks for unchecked indexing are
expected to be most impactful (case in point, they found a problem in
rustc).

1 files changed, 5 insertions, 1 deletions

diff --git a/library/core/src/num/nonzero.rs b/library/core/src/num/nonzero.rs
index 1ebd1c58f2b..b42b6a939c4 100644
--- a/library/core/src/num/nonzero.rs
+++ b/library/core/src/num/nonzero.rs
@@ -52,9 +52,13 @@ macro_rules! nonzero_integers {
                 #[$const_new_unchecked_stability]
                 #[must_use]
                 #[inline]
+                #[rustc_allow_const_fn_unstable(const_fn_fn_ptr_basics)] // required by assert_unsafe_precondition
                 pub const unsafe fn new_unchecked(n: $Int) -> Self {
                     // SAFETY: this is guaranteed to be safe by the caller.
-                    unsafe { Self(n) }
+                    unsafe {
+                        core::intrinsics::assert_unsafe_precondition!(n != 0);
+                        Self(n)
+                    }
                 }
 
                 /// Creates a non-zero if the given value is not zero.