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Any chance your tweaks are documented/posted/pastebinned anywhere? I love sublimeclang but it does seriously bog things down sometimes. I haven't really looked in to the choke points myself, though.
Auto-complete certainly isn't trivial, but it doesn't necessarily have to be inefficient either.
The problem with the current status quo is that there are multiple plugins (as you pointed out) with different behavior, using different mechanisms, and which aren't particularly easy to extend for new use cases.
What Sublime Text needs is a well designed API that will make adding proper completion possible. It _is_ possible to do this. You don't even necessarily need an IDE-level of integration. For example, Chocolat.app, a new text editor in the vein of Textmate/Sublime Text provides such an API here:
anyone used or heard of this plugin?
This great plugin gives proper introspective autocomplete for Python https://github.com/svaiter/SublimeJEDI
To help the developer implement this efficiently, it would be helpful for us to show some mockups of how we want this to look and exactly what/how the tooltip should contain inside of it in what contexts.
Also, the ability to have plugins provide the autocompletion text and do the parsing like SublimeJEDI posted above sounds excellent so that Sublime can do this correctly on a language-specific plugin parser basis.