Is there a way to use syn with stable Rust?

[emk]

This is a great library!

With syntex, it was possible to support stable Rust via build.rs and source-code processing. Is there a way to do that with syn?

[dtolnay]

Syn itself compiles fine on stable, in fact serde_codegen depends on it on stable. Here is the relevant code.

• Use syntex to register a custom derive
• Receive a syntex AST in your registered function
• Use pprust to print it to a string
• Parse it to a syn AST
• Expand it using the same codepath that proc_macro uses
• Print the expanded syn AST to a string
• Create a syntex parse session and parse it
• Return the expanded syntex AST

There are a few missing pieces before it can be used without syntex but I have been making steady progress on those and expect to have it ready in the next week or two. The biggest blocker was being able to parse the full Rust syntax other than just structs and enums. That is largely done, for example this 6000 line file copied from the rustc source code can be parsed and printed back such that the original and the printed one are parsed identically by syntex.

If you have time to help out, the remaining missing pieces in order of importance are:

• Porting over syntex's custom-derive registry and derive-finding code
• Implementing a workaround for this parsing ambiguity in Rust
• Integrating an optional dependency on a pretty-printer (probably syntex)
• Floating point literals Implement non-string literals in attributes #10
• Hexadecimal, octal, binary literals and literals containing underscores Implement fancy integer literals #34

The pretty-printer is because debugging will be much easier if your build.rs produces formatted code rather than a single massive line. I don't want to be in the business of maintaining a pretty printer myself, so we need to use a dependency on an existing one that can be disabled once things work.

[dtolnay]

I implemented the biggest missing piece in #55. I will try to make it through the remaining items quickly.

let mut registry = Registry::new();
registry.add_derive("Serialize", expand_serialize);
registry.add_derive("Deserialize", expand_deserialize);
registry.expand_file(src, dst).unwrap();

fn expand_serialize(input: MacroInput) -> Result<Expanded, String> { /* ... */ }
fn expand_deserialize(input: MacroInput) -> Result<Expanded, String> { /* ... */ }

[dtolnay]

All the pieces have been implemented. I need to write documentation and example code and cut a release.

[emk]

This is just fantastic! I have a couple of crates that will benefit from this.

[dtolnay]

I released 0.10.0 with a custom-derive registry that is able to expand custom derives on stable Rust.