Explain how to install rustfmt

[mbyio]

As a new Rust user, I was very confused about why Rustfmt wasn't working. I enabled it to run automatically on save, but if you don't have Rustfmt installed then it just echo's the source file to the terminal - no error messages or anything. So it would be really helpful if you could either add an error message or a note in the docs telling people to install rustfmt using cargo install rustfmt. Thanks!

[grangoku]

@michael-younkin thanks for this. I was confused as a new user as well. I use vim-plug so I just changed my rust import to Plug 'rust-lang/rust.vim', { 'for': [ 'rust' ], 'do': 'cargo install rustfmt' }

[rofrol]

I think it should be rather

Plug 'rust-lang/rust.vim', { 'for': [ 'rust' ], 'do': 'rustup component add rustfmt' }
let g:rustfmt_autosave = 1
let g:rustfmt_options = '--edition 2018'

We need to --edition, otherwise rustfmt will use 2015 edition.

#368 (comment)

[da-x]

Mentioned rustfmt installation in README.md. Plugin currently defaults to '--edition 2018' in absence of rustfmt.toml.