I'm seeing the same problem on Windows:

  $ rustup component add rustfmt
  error: toolchain 'stable-x86_64-pc-windows-msvc' does not contain component 'rustfmt' for target 'x86_64-pc-windows-msvc'

I looked for how to uninstall rust, and found that "the book" says that the command 'rustup self uninstall' will uninstall both rust and rustup. So I figured, what the heck, I'll start from scratch. I ran this command, uninstalling everything. Then I re-downloaded rustup-init.exe, and then ran it, reinstalling rust. I even then did 'rustup self update' and 'rustup update' for good measure.

I still get the same error when trying to install rustfmt. What else is there to do if completely uninstalling and reinstalling doesn't work?