Function check_file_offset 

pub fn check_file_offset(
    file_offset: &FileOffset,
    size: usize,
) -> Result<(), MmapRegionError>

Checks if a mapping of size bytes fits at the provided file_offset.

For a borrowed FileOffset and size, this function checks whether the mapping does not extend past EOF, and that adding the size to the file offset does not lead to overflow.