failed to install component: 'rustfmt-preview.. detected conflict: '"share/doc/rustfmt/README.md"'

[FauxFaux]

I've managed to confuse rustup on this one machine. I believe I may have run rustup component add while rustup update was running concurrently.

My default toolchain is nightly. rustup managed rustfmt is in ~/.cargo/bin:

% ls ~/.cargo/bin       
cargo  cargo-clippy  cargo-fmt  clippy-driver  git-fmt  rls  rustc 
rustdoc  rustfmt  rustfmt-format-diff  rust-gdb  rust-lldb  rustup

rustfmt won't start because it's expecting a different toolchain, because rustup update doesn't update it.

% rustup component list --toolchain=nightly | fgrep default
cargo-x86_64-unknown-linux-gnu (default)
rust-docs-x86_64-unknown-linux-gnu (default)
rust-std-x86_64-unknown-linux-gnu (default)
rustc-x86_64-unknown-linux-gnu (default)

% rustfmt
~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustfmt: error while loading shared libraries:
   libsyntax-59d13492ed717eeb.so: cannot open shared object file: No such file or directory

I can't re-add it, due to a conflict at a weird path:

% rustup component add rustfmt-preview --toolchain=nightly
info: downloading component 'rustfmt-preview'
info: installing component 'rustfmt-preview'
info: rolling back changes
error: failed to install component: 'rustfmt-preview-x86_64-unknown-linux-gnu', detected conflict:
   '"share/doc/rustfmt/README.md"'

The file does exist:

% file ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/share/doc/rustfmt/README.md
../README.md: ASCII text, with very long lines

I could personally resolve this by blowing away ~/.cargo and ~/.rustup, or possibly just by uninstalling and re-installing the nightly toolchain, but I feel the errors or consistency could be improved?

[Diggsey]

Dup of #988