Disabling racer-mode does not disable eldoc

[Wilfred]

This annoying as it makes it harder to disable eldoc on machines that don't have racer installed.

The problem is that eldoc-documentation-function is never reset.

[ideasman42]

For some reason on one of my projects racer is very slow and eldoc makes it near unusable,

As a workaround this works:

(add-hook
 'rust-mode-hook
 (lambda ()
   (racer-mode)
   (setq-local eldoc-documentation-function #'ignore)  ;; <-- override eldoc function!

[Wilfred]

@ideasman42 could you run M-x racer-debug immediately after eldoc has been slow? This should give you enough information to report a racer bug.

[ideasman42]

Submitted report #82