Sublime Text is great with detecting function definitions and allowing to jump into them.
However, with ST working with a C library is impossible without the documentation. You always need to write out a function before jumping into it and finding out what it does. Auto complete only works once it's in the file.
With "Goto Symbol in Project" I already see what I want, autocompleting to that would make Sublime go from great to god damn awesome for me.
There are no good alternatives for C plugin wise to auto-complete to a function, that hasn't been written in the file before: SublimeCodeIntel doesn't support C, all LSP plugins are wonky and installing a huge LSP Server just to autocomplete to some functions is painful. The list is already there, inside vanilla ST.
Asking in ST's suppoort IRC ended with people saying there is no way but LSPs. Thus for me ST is almost awesome...