RE: How to put default-directory into kill-ring?

[Drew Adams]

> is there any function to do this?

M-x cd RET C-x DEL C-g

`cd' changes directory, and the default is `default-directory'.
`C-x DEL' is `backward-kill-sentence', which kills the dir
in the minibuffer.  You can alternatively use `C-a C-k' to do
this.  (DEL is Backspace.)

If you have `insert-default-directory' = nil then  do `M-n'
after `M-x cd RET', to get the default dir in the minibuffer.

And yes, if you do this often, you could simply define a
command and key for it.