Rollup merge of #92425 - calebzulawski:simd-cast, r=workingjubilee

Improve SIMD casts

* Allows `simd_cast` intrinsic to take `usize` and `isize`
* Adds `simd_as` intrinsic, which is the same as `simd_cast` except for saturating float-to-int conversions (matching the behavior of `as`).

cc `@workingjubilee`

src/test/ui/simd/intrinsic/generic-as.rs

@@ -0,0 +1,48 @@
+// run-pass
+
+#![feature(repr_simd, platform_intrinsics)]
+
+extern "platform-intrinsic" {
+    fn simd_as<T, U>(x: T) -> U;
+}
+
+#[derive(Copy, Clone)]
+#[repr(simd)]
+struct V<T>([T; 2]);
+
+fn main() {
+    unsafe {
+        let u = V::<u32>([u32::MIN, u32::MAX]);
+        let i: V<i16> = simd_as(u);
+        assert_eq!(i.0[0], u.0[0] as i16);
+        assert_eq!(i.0[1], u.0[1] as i16);
+    }
+
+    unsafe {
+        let f = V::<f32>([f32::MIN, f32::MAX]);
+        let i: V<i16> = simd_as(f);
+        assert_eq!(i.0[0], f.0[0] as i16);
+        assert_eq!(i.0[1], f.0[1] as i16);
+    }
+
+    unsafe {
+        let f = V::<f32>([f32::MIN, f32::MAX]);
+        let u: V<u8> = simd_as(f);
+        assert_eq!(u.0[0], f.0[0] as u8);
+        assert_eq!(u.0[1], f.0[1] as u8);
+    }
+
+    unsafe {
+        let f = V::<f64>([f64::MIN, f64::MAX]);
+        let i: V<isize> = simd_as(f);
+        assert_eq!(i.0[0], f.0[0] as isize);
+        assert_eq!(i.0[1], f.0[1] as isize);
+    }
+
+    unsafe {
+        let f = V::<f64>([f64::MIN, f64::MAX]);
+        let u: V<usize> = simd_as(f);
+        assert_eq!(u.0[0], f.0[0] as usize);
+        assert_eq!(u.0[1], f.0[1] as usize);
+    }
+}

src/test/ui/simd/intrinsic/generic-cast-pointer-width.rs

@@ -0,0 +1,21 @@
+// run-pass
+#![feature(repr_simd, platform_intrinsics)]
+
+extern "platform-intrinsic" {
+    fn simd_cast<T, U>(x: T) -> U;
+}
+
+#[derive(Copy, Clone)]
+#[repr(simd)]
+struct V<T>([T; 4]);
+
+fn main() {
+    let u = V::<usize>([0, 1, 2, 3]);
+    let uu32: V<u32> = unsafe { simd_cast(u) };
+    let ui64: V<i64> = unsafe { simd_cast(u) };
+
+    for (u, (uu32, ui64)) in u.0.iter().zip(uu32.0.iter().zip(ui64.0.iter())) {
+        assert_eq!(*u as u32, *uu32);
+        assert_eq!(*u as i64, *ui64);
+    }
+}