Support formatting async/await code

[nsavch]

Currently if you try to save async/await code you get a rustfmt error. In order to fix it you need to pass the argument "--edition 2018" to it (cargo fmt does it automatically). Unfortunately rustic dosen't allow to customize rustfmt options, only the binary path. Would be cool if there was a variable for customizing rustfmt options.

Thanks for your work, the mode is quite cool overall.

[brotzeit]

Thanks for the bug report. I will take a look at it. But this feels like it should be handled by rustfmt similarly to cargo fmt. Do you know if this works with rust-mode ?

[nsavch]

Cargo fmt just adds the "--edition 2018" argument to all rustfmt calls:

project » cargo fmt -v                                                                     
[bin (2018)] "/project/src/main.rs"
rustfmt --edition 2018 /project/src/main.rs

The original rust-mode fails to format with the same error as rustic.

[nsavch]

Actually I found a workaround. You can create a .rustfmt.toml file and specify 2018 edition there. After that rustic auto format works again.

See example rustfmt.toml file here - https://github.com/rust-lang/rustfmt/blob/master/rustfmt.toml

[brotzeit]

Ok. Let's leave this issue open for others.

[brotzeit]

I added a note in the readme.