Fix: autocomplete hang in rust-mode

[jmpage]

Related to #1372, it appears that super-save doesn't play nicely with LSP features in rust-mode either.

My particular issue occurs when using autocomplete in rust. Typing "B" in the following snippet:

Example(B)

Results in emacs hanging as it saves the buffer while LSP mode is attempting to display autocompletions. This seems to cause an infinite loop.

I narrowed the issue down to super-save and confirmed that turning it off resolves the issue.

This PR re-implements the fix from #1373 by turning off super-save for rust-mode.

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