How to pretty print Syn AST?

I'm trying to use syn to create an AST from a Rust file and then using quote to write it to another. However, when I write it, it puts extra spaces between everything.

Note that the example below is just to demonstrate the minimum reproducible problem I'm having. I realize that if I just wanted to copy the code over I could copy the file but it doesn't fit my case and I need to use an AST.

pub fn build_file() {
    let current_dir = std::env::current_dir().expect("Unable to get current directory");
    let rust_file = std::fs::read_to_string(current_dir.join("src").join("lib.rs")).expect("Unable to read rust file");
    let ast = syn::parse_file(&rust_file).expect("Unable to create AST from rust file");

    match std::fs::write("src/utils.rs", quote::quote!(#ast).to_string());
}

The file that it creates an AST of is this:

#[macro_use]
extern crate foo;
mod test;
fn init(handle: foo::InitHandle) {
    handle.add_class::<Test::test>();
}

What it outputs is this:

# [macro_use] extern crate foo ; mod test ; fn init (handle : foo :: InitHandle) { handle . add_class :: < Test :: test > () ; }

I've even tried running it through rustfmt after writing it to the file like so:

utils::write_file("src/utils.rs", quote::quote!(#ast).to_string());

match std::process::Command::new("cargo").arg("fmt").output() {
    Ok(_v) => (),
    Err(e) => std::process::exit(1),
}

But it doesn't seem to make any difference.

Solution

The quote crate is not really concerned with pretty printing the generated code. You can run it through rustfmt, you just have to execute rustfmt src/utils.rs or cargo fmt -- src/utils.rs.

use std::fs;
use std::io;
use std::path::Path;
use std::process::Command;

fn write_and_fmt<P: AsRef<Path>, S: ToString>(path: P, code: S) -> io::Result<()> {
    fs::write(&path, code.to_string())?;

    Command::new("rustfmt")
        .arg(path.as_ref())
        .spawn()?
        .wait()?;

    Ok(())
}

Now you can just execute:

write_and_fmt("src/utils.rs", quote::quote!(#ast)).expect("unable to save or format");

See also "Any interest in a pretty-printing crate for Syn?" on the Rust forum.