For me, it's alignment that makes proportional fonts absolutely unusable.
I gave up alignment in my code many years ago, back when I was still using monospaced fonts. I gave it up because I was really tired of fiddling with spaces when I changed a name in code like this:
myType someFunctionName(someType someVariable,
someOtherType someOtherVariable,
int foo) {
...
}
myType someFunctionName(
someType someVariable,
someOtherType someOtherVariable,
int foo
) {
...
}
As a bonus, the overall width of my code was often much less, as in this example.
Later I discovered that using this style, my code looked the same in a proportional font as it did in a monospaced font. So I switched to a proportional font and never looked back. But the code is still just as readable in a monospaced font.
It does seem like the programming environment and language must be designed for it to work nicely, though. I've tried proportional fonts in several environments since then and it's always sucked for one reason or another.
Possibly Go is structured enough for proportional fonts to cleanly work though. Haven't tried it yet; I use emacs and last time I tried proportional fonts in that it was a joke. (And that was just straight-up English, no programming at all.)
I suppose I should in fairness observe this was actually a long time ago. Since then emacs has switched to XFT; I haven't tried it since then, because well, as I mentioned most languages don't work very well with it either. But perhaps I will try it in a Go buffer here.
(A few minutes later.) Hey, this isn't so bad. I'm not sure everybody would appreciate me sticking with it since it makes it hard to tell when you've got a "long line", but it's certainly radically better than I remember from the last time I tried this.
For those who may want to play at home, I use the Droid series of fonts for my programming and I just eval-region'ed this:
(custom-set-faces
'(default ((t (:stipple nil :background "#ffffff"
:foreground "#141312" :inverse-video nil :box nil
:strike-through nil :overline nil :underline nil
:slant normal :weight normal :height 105
:width normal :family "Droid Sans")))))
Go's attempt to auto-align struct name and types isn't working so well, though. (Fortunately, gofmt takes care of it so I still can't be wrong.)
Further follow-up: You don't realize how much you're subconsciously using the "shape" of the code to guide you until you change it and suddenly feel lost, like I'm in a fresh code base I've never seen before, despite the fact I'm actually in something I've more-or-less written from scratch over the past six weeks, which is just about the maximum level of familiarity you can have with a piece of code.