On the other hand, I don't think I've ever seen a proportional font designed specifically for programming in. Like, with the same "make sure 1/l and 0/O look different" considerations programmers' fonts get, but proportional.
You should check out Input, which is exactly that. I've been using it for a couple of months, can definitely recommend it. The only issue I've run into is that certain features in Sublime Text don't work so well with proportional fonts, such as indent guides.
I've been using Trebuchet MS for coding, and it does a good job on this: all the confusing pairs are easily distinguished. For example, the lowercase "l" has a little rounded hook at the bottom to it doesn't look like a capital "I". It's not perfect, but I've been enjoying using it.
Before that I used Georgia, which is also fairly good in this area. I stopped using Georgia because it rendered poorly with too-thin stems in Windows 8.1 on a MacBook Pro Retina. Looked fine in OSX and on my ThinkPad's 145 DPI display.
The most common concerns are a result of the font being monospace.
In most proportional fonts, 1 has a hat, and l is a vertical line; and 0 is a thin, squared off oval; while O is almost a circle. The characters become similar in monospaced typefaces so that they can fill the rectangle they're supposed to fill.
I and l are often indistinguishable; but there's no shortage of fonts that put the extra strokes onto I [eg: Verdana, Tahoma, every serif font...]
I've used "Untyped" and "Trim" from this collection[0], and they were plenty readable. They take care of ambiguous characters and provide a wider space character than general-purpose proportional fonts.
Serious question: what kind of codebases are you working on where there tokens in which it's not immediately obvious from the context whether they contain digits or numbers? I have never seen such a situation.
