if the source dir for an mdbook contains the build output dir(?) we recurse infinitely

[Gankra]

failed to copy
./book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/FontAwesome/fonts/fontawesome-webfont.svg
to
book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/FontAwesome/fonts/fontawesome-webfont.svg

Reported to me over discord. I haven't looked fully into it and i'm not sure which copy is freaking out, but clearly something somewhere is trying to recursively copy a directory, and it's recursing into the directory where things are getting copied, so it just keeps digging itself deeper and deeper until something explodes. Apparently mdbook proper has some logic to avoid this.

[Gankra]

I believe the repro steps were to just make an mdbook in the root of your cargo project with src="./" which is a real thing mdbook makes very easy to do.

[kouhaidev]

nearly-minimal repro: https://gitlab.com/bssf/wiki/-/tree/oranda-repro

oranda build
↪ >o_o< INFO: Building components: changelog, mdbook
  × Couldn't build your mdbook at
  ├─▶ Rendering failed
  ├─▶ failed to copy `./book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/FontAwesome/fonts/fontawesome-
  │   webfont.svg` to `book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/
  │   book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/book/FontAwesome/fonts/fontawesome-
  │   webfont.svg`
  ╰─▶ File name too long (os error 63)