simd_shuffle: require index argument to be a vector

Author: RalfJung

crates/core_simd/src/swizzle.rs

@@ -85,7 +85,7 @@ pub trait Swizzle<const N: usize> {
         LaneCount<N>: SupportedLaneCount,
         LaneCount<M>: SupportedLaneCount,
     {
-        // Safety: `vector` is a vector, and the index is a const array of u32.
+        // Safety: `vector` is a vector, and the index is a const vector of u32.
         unsafe {
             core::intrinsics::simd::simd_shuffle(
                 vector,
@@ -103,7 +103,11 @@ pub trait Swizzle<const N: usize> {
                         output[i] = index as u32;
                         i += 1;
                     }
-                    output
+
+                    // The index list needs to be returned as a vector.
+                    #[repr(simd)]
+                    struct SimdShuffleIdx<const LEN: usize>([u32; LEN]);
+                    SimdShuffleIdx(output)
                 },
             )
         }
@@ -121,7 +125,7 @@ pub trait Swizzle<const N: usize> {
         LaneCount<N>: SupportedLaneCount,
         LaneCount<M>: SupportedLaneCount,
     {
-        // Safety: `first` and `second` are vectors, and the index is a const array of u32.
+        // Safety: `first` and `second` are vectors, and the index is a const vector of u32.
         unsafe {
             core::intrinsics::simd::simd_shuffle(
                 first,
@@ -139,7 +143,11 @@ pub trait Swizzle<const N: usize> {
                         output[i] = index as u32;
                         i += 1;
                     }
-                    output
+
+                    // The index list needs to be returned as a vector.
+                    #[repr(simd)]
+                    struct SimdShuffleIdx<const LEN: usize>([u32; LEN]);
+                    SimdShuffleIdx(output)
                 },
             )
         }