std.mem.sliceTo: Return slice with sentinel from unbounded pointers

Commit dec1163fbb removed sentinels from the returned slice for C
pointers. Since C pointers have no bounds, we know that it'll keep
scanning until it finds `end` (or crash trying). The same is also true
of many-item pointers without a sentinel (e.g. `[*]T`), so I added
support for those too.

lib/std/mem.zig

@@ -913,8 +913,9 @@ fn SliceTo(comptime T: type, comptime end: std.meta.Elem(T)) type {
         .pointer => |ptr_info| {
             const Elem = std.meta.Elem(T);
             const have_sentinel: bool = switch (ptr_info.size) {
-                .one, .slice, .many => if (std.meta.sentinel(T)) |s| s == end else false,
+                .one, .slice => if (std.meta.sentinel(T)) |s| s == end else false,
-                .c => false,
+                .many => if (std.meta.sentinel(T)) |s| s == end else true,
+                .c => true,
             };
             return @Pointer(.slice, .{
                 .@"const" = ptr_info.is_const,
@@ -961,8 +962,15 @@ test sliceTo {
         try testing.expectEqualSlices(u16, array[0..2], sliceTo(&array, 3));
         try testing.expectEqualSlices(u16, array[0..2], sliceTo(array[0..3], 3));
 
+        const many_ptr = @as([*]u16, @ptrCast(&array));
+        try testing.expectEqualSlices(u16, array[0..2], sliceTo(many_ptr, 3));
+        try testing.expectEqual([:3]u16, @TypeOf(sliceTo(many_ptr, 3)));
+
         const sentinel_ptr = @as([*:5]u16, @ptrCast(&array));
         try testing.expectEqualSlices(u16, array[0..2], sliceTo(sentinel_ptr, 3));
+        try testing.expectEqual([]u16, @TypeOf(sliceTo(sentinel_ptr, 3)));
+        try testing.expectEqualSlices(u16, array[0..4], sliceTo(sentinel_ptr, 5));
+        try testing.expectEqual([:5]u16, @TypeOf(sliceTo(sentinel_ptr, 5)));
         try testing.expectEqualSlices(u16, array[0..4], sliceTo(sentinel_ptr, 99));
 
         const optional_sentinel_ptr = @as(?[*:5]u16, @ptrCast(&array));
@@ -971,6 +979,7 @@ test sliceTo {
 
         const c_ptr = @as([*c]u16, &array);
         try testing.expectEqualSlices(u16, array[0..2], sliceTo(c_ptr, 3));
+        try testing.expectEqual([:3]u16, @TypeOf(sliceTo(c_ptr, 3)));
 
         const slice: []u16 = &array;
         try testing.expectEqualSlices(u16, array[0..2], sliceTo(slice, 3));
@@ -1015,6 +1024,10 @@ fn lenSliceTo(ptr: anytype, comptime end: std.meta.Elem(@TypeOf(ptr))) usize {
                 var i: usize = 0;
                 while (ptr[i] != end and ptr[i] != s) i += 1;
                 return i;
+            } else {
+                var i: usize = 0;
+                while (ptr[i] != end) i += 1;
+                return i;
             },
             .c => {
                 assert(ptr != null);