I suspect most Guix developers use Emacs with the Geiser mode for enhanced
Scheme integration.

A segfault on "C-x u"? (you mean undo?)  Very strange.  Can you reproduce a
specific recipe?  Is it just in Guile files?  What minor modes are you running?
Is it terminal or graphical Emacs?

Note that if you have a US QWERTY layout, you might prefer "C-/" to undo.

-- 
Pierre Neidhardt
https://ambrevar.xyz/