[stdlib] Make pack_mask infer its return type

Signed-off-by: Yiwu Chen <210at85@gmail.com>

stdlib/src/memory/unsafe.mojo

@@ -67,10 +67,22 @@ fn bitcast[
     ](val.value)
 
 
+@always_inline("nodebug")
+fn _uint(n: Int) -> DType:
+    if n == 8:
+        return DType.uint8
+    elif n == 16:
+        return DType.uint16
+    elif n == 32:
+        return DType.uint32
+    else:
+        return DType.uint64
+
+
 @always_inline("nodebug")
 fn pack_mask[
     width: Int, //,
-    new_type: DType,
+    new_type: DType = _uint(width),
 ](val: SIMD[DType.bool, width]) -> Scalar[new_type]:
     """Packs a SIMD bool into an integer.
 

stdlib/src/utils/stringref.mojo

@@ -16,7 +16,7 @@
 from bit import count_trailing_zeros
 from builtin.dtype import _uint_type_of_width
 from collections.string import _atol, _isspace
-from memory import UnsafePointer, memcmp, bitcast, pack_mask
+from memory import UnsafePointer, memcmp, pack_mask
 from memory.memory import _memcmp_impl_unconstrained
 from utils import StringSlice
 from sys.ffi import c_char
@@ -634,7 +634,7 @@ fn _memchr[
 
     for i in range(0, vectorized_end, bool_mask_width):
         var bool_mask = source.load[width=bool_mask_width](i) == first_needle
-        var mask = pack_mask[_uint_type_of_width[bool_mask_width]()](bool_mask)
+        var mask = pack_mask(bool_mask)
         if mask:
             return source + int(i + count_trailing_zeros(mask))
 
@@ -678,7 +678,7 @@ fn _memmem[
         var eq_last = last_needle == last_block
 
         var bool_mask = eq_first & eq_last
-        var mask = pack_mask[_uint_type_of_width[bool_mask_width]()](bool_mask)
+        var mask = pack_mask(bool_mask)
 
         while mask:
             var offset = int(i + count_trailing_zeros(mask))