Insert tabs at beginning of line, spaces afterward

gistfile1.vim

function! TabOrSpace()
  let text = strpart(getline('.'), 0, col('.')-1)
  if text =~ '^\t*$'
    return "\<C-v>\<Tab>"
  else
    let l = strdisplaywidth(text)
    let n = float2nr(floor(l / &l:softtabstop) + 1) * &l:softtabstop
    return repeat(' ', n - l)
  endif
endfunction

imap <expr> <Tab> TabOrSpace()