Rustfmt: Document how to use rustfmt as a library

[jplatte]

Since there is a public API and there are various references to a rustfmt library in various issues here, it seems like it should be possible to use rustfmt as a library. However, most of the public API is not documented and I have not found a way of simply giving rustfmt some rust source code an having it format that.

[jesskfullwood]

I would appreciate this. This is the snippet I am using at the moment:

fn prettify_code(input: String) -> String {
    let mut buf = Vec::new();
    {
        let mut config = rustfmt_nightly::Config::default();
        config.set().emit_mode(rustfmt_nightly::EmitMode::Stdout);
        config.set().edition(rustfmt_nightly::Edition::Edition2018);
        let mut session = rustfmt_nightly::Session::new(config, Some(&mut buf));
        session.format(rustfmt_nightly::Input::Text(input)).unwrap();
    }
    String::from_utf8(buf).unwrap()
}

[max-sixty]

And to confirm, this isn't possible on stable at the moment?

Currently in insta we're shelling out. This is a bit hacky (e.g. can't guarantee versions / it's even installed), but simple & works.

If there were a way of linking rustfmt that would be an interesting alternative.

fn format_rust_expression(value: &str) -> Cow<'_, str> {
    const PREFIX: &str = "const x:() = ";
    const SUFFIX: &str = ";\n";
    if let Ok(mut proc) = Command::new("rustfmt")
        .arg("--emit=stdout")
        .arg("--edition=2018")
        .stdin(Stdio::piped())
        .stdout(Stdio::piped())
        .stderr(Stdio::null())
        .spawn()
    {
        {
            let stdin = proc.stdin.as_mut().unwrap();
            stdin.write_all(PREFIX.as_bytes()).unwrap();
            stdin.write_all(value.as_bytes()).unwrap();
            stdin.write_all(SUFFIX.as_bytes()).unwrap();
        }
        if let Ok(output) = proc.wait_with_output() {
            if output.status.success() {
                // slice between after the prefix and before the suffix
                // (currently 14 from the start and 2 before the end, respectively)
                let start = PREFIX.len() + 1;
                let end = output.stdout.len() - SUFFIX.len();
                return str::from_utf8(&output.stdout[start..end])
                    .unwrap()
                    .to_owned()
                    .into();
            }
        }
    }
    Cow::Borrowed(value)
}