Happens a lot to me, as over the years, a lot of such ad-hoc yet reusable code gets written, and I can usually remember enough implementation details (e.g. which modules were used there, how the methods/classes were called and such), but going "grep" over the source dir takes a shitload of time.
Some things make it faster - ack or pss tools can scan only relevant things (like e.g. "grep ... **/*.py" will do in zsh), but these also run for minutes, as even simple "find" does - there're several django source trees in appengine sdk, php projects with 4k+ files inside, maybe even whole linux kernel source tree or two...
Traversing all these each time on regular fs to find something that can be rewritten in a few minutes will never be an option for me, but luckily there're cool post-fs projects like tmsu, which allow to transcend single-hierarchy-index limitation of a traditional unix fs in much more elegant and useful way than gazillion of symlinks and dentries.
tmsu allows to attach any tags to any files, then query these files back using a set of tags, which it does really fast using sqlite db and clever indexes there.
So, just tagging all the "*.py" files with "lang:py" will allow to:
% time tmsu files lang:py | grep myclass tmsu files lang:py 0.08s user 0.01s system 98% cpu 0.094 total grep --color=auto myclass 0.01s user 0.00s system 10% cpu 0.093 total
That's 0.1s instead of several minutes for all the python code in the development area on this machine.
I've added taggers for things that are immediately useful for me to tag files by - implementation language, code hosting (github, bitbucket, local project, as I sometimes remember that snippet was in some public tool), scm type (git, hg, bzr, svn), but it adding a new one is just a metter of writing a "Tagger" function, which, given the path and config, returns a list of string tags, plus they're only used if explicitly enabled in config.
Being a proper compiled language, Go allows to make the thing into a single static binary, which is quite neat, as I realized that I now have a tool to tag all the things everywhere - media files on servers' remote-fs'es, like music and movies, hundreds of configuration files by the app they belong to (think tmsu files daemon:apache to find/grep all the horrible ".htaccess" things and it's "*.conf" includes), distfiles by the os package name, etc... can be useful.
So, to paraphrase well-known meme, Tag All The Things! ;)