File clone checking on APFS: https://stackoverflow.com/a/65241088/149482.
On Windows, file cloning appears to be possible with https://learn.microsoft.com/en-us/windows/win32/api/winioctl/ni-winioctl-fsctl_duplicate_extents_to_file. The destination file is opened, then the regions of the source file are specified and the clone occurs. It might not be available on NTFS.
For filesystems where cloning is not supported, it should be possible to "fall back" to synchronously reading while a transaction is open, or providing an API to copy out or lock values more efficiently than is usually available.
Here's a list of support https://www.ctrl.blog/entry/file-cloning.html.
Solaris will need this https://docs.oracle.com/cd/E88353_01/html/E37843/reflink-3c.html.
cargo test on Linux can be done by creating an XFS filesystem in a file and mounting that. Symlink the tmp dir in the repo root, and set the TMPDIR environment variable to point into the XFS filesystem mount directory.