C extensions `tags` update

[bruno-]

Hi, I'm playing with ruby C extensions + browsing ruby source (also related to https://github.com/tpope/rbenv-ctags/pull/5) and I notice an issue: after tags are used once to jump to ruby source or include dir, vim's tags option looses tags for ruby C source.

This is a bummer, because the majority of C functions or macros "exposed" to users via ruby.h almost always delegate to some internal ruby functions.. So in most cases a single tags jump is not enough.

I compared how this works when editing ruby files and then making a tags jump to ruby std lib. In this case tags option is handled nicely.

This PR contains a proof of concept solution for the problem: if a user opens a ruby C extension file an autocmd is registered (once). While vim's cwd is inside a ruby project, path and tags options for C files will be updated.

Feedback is appreciated!

[tpope]

I did something vaguely similar in fireplace.vim and it was a huge pain. My advice would be not to activate all of rake.vim; copy 'path' and 'tags' only. Something vaguely like seems like it would work while being pretty conservative:

au FileType c,cpp
      \ if !empty(getbufvar('#', 'rake_root')) && getbufvar('#', '&filetype') =~# '^\%(c\|cpp\)$' |
      \   let &l:path = getbufvar('#', '&path') |
      \   let &l:tags = getbufvar('#', '&tags') |
      \ endif

[bruno-]

Thanks for the pointer, the PR is updated.

Please note that aside from copying l:path and l:tags we also have to carry around the b:rake_root variable in ruby source C files.

Other than that I've tested this and things work.

[tpope]

Yeah I don't like the sound of that. After that rbenv-ctags request is ready I'll play with it more myself.

[bruno-]

All right, thanks for that.