add some debug asserts; won't normally run but may save someone a headache one day

Author: folkertdev

library/core/src/intrinsics/simd.rs

@@ -29,6 +29,7 @@ pub unsafe fn simd_insert<T, U>(_x: T, _idx: u32, _val: U) -> T {
 #[rustc_nounwind]
 #[cfg(not(bootstrap))]
 pub unsafe fn simd_insert_dyn<T, U>(mut x: T, idx: u32, val: U) -> T {
+    debug_assert!((idx as usize) < core::mem::size_of::<T>() / core::mem::size_of::<U>());
     // SAFETY: `idx` must be in-bounds
     unsafe { (&mut x as *mut T as *mut U).add(idx as usize).write(val) }
     x
@@ -61,6 +62,7 @@ pub unsafe fn simd_extract<T, U>(_x: T, _idx: u32) -> U {
 #[rustc_nounwind]
 #[cfg(not(bootstrap))]
 pub unsafe fn simd_extract_dyn<T, U>(x: T, idx: u32) -> U {
+    debug_assert!((idx as usize) < core::mem::size_of::<T>() / core::mem::size_of::<U>());
     // SAFETY: `idx` must be in-bounds
     unsafe { (&x as *const T as *const U).add(idx as usize).read() }
 }