Use prettyplease for codegen formatting

[malobre]

I'm currently working on #1932 where I need to format generated code, it is currently done by checking if rustfmt is installed and calling its binary:

tools/xtask/src/lib.rs

Lines 52 to 61 in 7991631

	fn reformat(text: impl std::fmt::Display) -> Result<String> {
	let _e = pushenv("RUSTUP_TOOLCHAIN", "stable");
	ensure_rustfmt()?;
	let stdout = run!(
	"rustfmt --config fn_single_line=true";
	<text.to_string().as_bytes()
	)?;
	let preamble = "Generated file, do not edit by hand, see `xtask/src/codegen`";
	Ok(format!("//! {}\n\n{}\n", preamble, stdout))
	}

D. Tolnay just published a new crate called prettyplease made for this use-case (pretty-printing generated code).

According to prettyplease readme:

• it's faster than rustfmt
• it only depends on syn & proc-macro2, which we already use
• it can be ergonomically called, unlike rustfmt

Thoughts ?

[ematipico]

Oh, so this crate is a formatter for syn? I wouldn't mind using it for our code generation. Considering that syn, quote and now prettyplease are maintained by the same author, I think the integration between them might be better compared to rustfmt.

[malobre]

Yep, it exposes a single function prettyplease::unparse which takes a syn::File.

In my case I had a TokenStream from proc-macro2, which can be easily converted:

let pretty = prettyplease::unparse(&syn::parse2::<syn::File>(tokens)?);

[yassere]

I haven't looked closely at prettyplease, but I certainly like the idea of using a purpose-built library by dtolnay rather than shelling out to rustfmt for our codegen.