Omit Simd::cast during bootstrap

crates/core_simd/src/intrinsics.rs

@@ -41,6 +41,7 @@ extern "platform-intrinsic" {
     pub(crate) fn simd_cast<T, U>(x: T) -> U;
     /// follows Rust's `T as U` semantics, including saturating float casts
     /// which amounts to the same as `simd_cast` for many cases
+    #[cfg(not(bootstrap))]
     pub(crate) fn simd_as<T, U>(x: T) -> U;
 
     /// neg/fneg

crates/core_simd/src/vector.rs

@@ -100,6 +100,7 @@ where
     /// ```
     #[must_use]
     #[inline]
+    #[cfg(not(bootstrap))]
     pub fn cast<U: SimdElement>(self) -> Simd<U, LANES> {
         unsafe { intrinsics::simd_as(self) }
     }