I use the current Emacs master branch and the marginalia package which
adds annotations to completion, e.g., the truncated value and docstrings
of variables when completing a variable name.  When I do C-h v l
(describe-variable, starting completion with initial input "l"), Emacs
immediately gets stuck, takes 100% CPU, and will hang there
indefinitely.

I've made a C and Lisp backtrace using GDB, attached to this report.