Because it's not in fact part of their project, despite their assertion that it is.
>But you can’t. cargo fmt doesn’t have an option for that. How is even possible for your format command to not let you format an individual file?
Because cargo is a tool for working with projects, not with individual source files.
>Problem is, I had never even heard of rustfmt.
Putting "rust how to format a file" into a search engine returns rustfmt in the top results. But in any case, the problem they needed to fix is to make that file a part of their project so that `cargo fmt` will format it, not work around that by running rustfmt manually. But alas they've convinced themselves that's not the problem.
>At this point I was just hopping mad. It knew that I wanted to wrap comments. But it refused, because this was “unstable”. It knew that I wanted to opt into experimental features, but refused again.
Whatever method they used to install nightly, they're not using it here. The error is coming from the non-nightly rustfmt that they were using originally.
>It would literally have been better if they had not shipped this wrap_comments feature at all. Instead they just keep dangling it in front of my face and snatching it away as soon as I try to use it. Because they hate me.
I'd be amused at this trolling if I didn't know the author is completely serious.