add bitmask roundtrip test for vector length below 8

Author: RalfJung

crates/core_simd/tests/masks.rs

@@ -80,6 +80,18 @@ macro_rules! test_mask_api {
                 assert_eq!(bitmask, 0b1000001101001001);
                 assert_eq!(core_simd::Mask::<$type, 16>::from_bitmask(bitmask), mask);
             }
+
+            #[test]
+            fn roundtrip_bitmask_conversion_short() {
+                use core_simd::ToBitMask;
+                let values = [
+                    false, false, false, true,
+                ];
+                let mask = core_simd::Mask::<$type, 4>::from_array(values);
+                let bitmask = mask.to_bitmask();
+                assert_eq!(bitmask, 0b1000);
+                assert_eq!(core_simd::Mask::<$type, 4>::from_bitmask(bitmask), mask);
+            }
         }
     }
 }