Add bitmask i{N <8} -> u8 impls

...and copy the notes for why they're legal.

Author: workingjubilee

crates/core_simd/src/intrinsics.rs

@@ -130,6 +130,14 @@ extern "platform-intrinsic" {
     pub(crate) fn simd_reduce_xor<T, U>(x: T) -> U;
 
     // truncate integer vector to bitmask
+    // `fn simd_bitmask(vector) -> unsigned integer` takes a vector of integers and
+    // returns either an unsigned integer or array of `u8`.
+    // Every element in the vector becomes a single bit in the returned bitmask.
+    // If the vector has less than 8 lanes, a u8 is returned with zeroed trailing bits.
+    // The bit order of the result depends on the byte endianness. LSB-first for little
+    // endian and MSB-first for big endian.
+    //
+    // UB if called on a vector with values other than 0 and -1.
     #[allow(unused)]
     pub(crate) fn simd_bitmask<T, U>(x: T) -> U;
 

crates/core_simd/src/masks/to_bitmask.rs

@@ -50,6 +50,9 @@ macro_rules! impl_integer_intrinsic {
 }
 
 impl_integer_intrinsic! {
+    unsafe impl ToBitMask<BitMask=u8> for Mask<_, 1>
+    unsafe impl ToBitMask<BitMask=u8> for Mask<_, 2>
+    unsafe impl ToBitMask<BitMask=u8> for Mask<_, 4>
     unsafe impl ToBitMask<BitMask=u8> for Mask<_, 8>
     unsafe impl ToBitMask<BitMask=u16> for Mask<_, 16>
     unsafe impl ToBitMask<BitMask=u32> for Mask<_, 32>