Not sure how to render it best but it is useful info. I'm thinking maybe make the last char of the identifier of a mutable member in some distinct different colour. That may not work well for single char local variable names though. FSharp has had this for a while.
@JohanLarsson can you show the picture from f#?
@ReedCopsey can you help us out with a screenie?
An alternative could be to animate opacity/visibility for mutables so that they flicker like broken fluorescent lamps :)
I would like an option that changes the look of usages at or after a possible reassignment has occurred, but not before that point.
Screenshot of Roslyn code that I stole from @alrz, demonstrating a ReSharper style option:

(What I'd prefer is no underline in if (workingDirectory == null) but underlines everywhere below it in the file.)
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Screenshot of Roslyn code that I stole from @alrz, demonstrating a ReSharper style option:
(What I'd prefer is no underline in
if (workingDirectory == null)but underlines everywhere below it in the file.)