Output as code string?

[jD91mZM2]

Hi! I've looked around and can't find anything for converting a syn object to a code string (like, fully working Rust code).
I'd like to use syn to modify source code. (Is that the wrong approach by the way?)

[dtolnay]

The quote crate can give you a string of any Syn data structure.

let v = /* ... */;
quote!(#v).to_string()

[jD91mZM2]

That seems to space-separate tokens rather than actually format them as runnable code. For example, use test::Test; becomes use test :: Test ;.

[dtolnay]

Space-separated tokens are runnable code. 😉

mod test { pub struct Test ; } use test :: Test ; fn main ( ) { let _ = Test ; }

There isn't currently a pretty-printer for Syn because we are focused mostly on the procedural macro use case where the tokens are handed straight back to the compiler, but you can pass the output through rustfmt if you need.

[jD91mZM2]

Alright! Is there any chance you could somehow preserve formatting through some optional feature or such?

[mystor]

@jD91mZM2 That would be the work of an external crate - it's outside the scope of syn (which only works with tokens).

[jD91mZM2]

Yeah but you would still need to alter the internal token enum to add whitespace as different variants?

[jD91mZM2]

Ooohhh I just noticed your dtolnay/proc-macro2#36 PR. If that somehow could be used in syn, that would be amazing!

[dtolnay]

Syn uses the same Span type as proc-macro2 so that PR should already be usable with Syn. You need to build with RUSTFLAGS='--cfg procmacro2_semver_exempt' because the API has not been figured out yet, but once you do, you get a start() and end() method on every span in the syntax tree giving you the line and column in the input string.

[jD91mZM2]

Awesome!

[jD91mZM2]

Can't seem to get it to work. Doesn't cfg work with cargo doc perhaps?

[dtolnay]

I don't think cargo doc picks it up but cargo build should work.

[jD91mZM2]

Ah, bummer. Thanks!