Expand macros in attributes

[SimonSapin]

Something like this would be nice for code generation in Cargo. (See rust-lang/cargo#824.)

#[path = concat!(env!("OUT_DIR"), "/hello.rs")]
mod foo;

But it currently doesn’t parse:

a.rs:1:10: 1:16 error: unexpected token: `concat`
a.rs:1 #[path = concat!(env!("OUT_DIR"), "/hello.rs")]
                ^~~~~~

Can we make this work? I have a hack in #18810 (comment) to work around it, but it’s ugly.

CC @alexcrichton @eddyb

[SimonSapin]

@eddyb on IRC:

it should be easy

[dwrensha]

This would be quite convenient.

[eddyb]

@SimonSapin you quoted me out of context, you get to write the RFC for it.
Though I have to wonder how mod foo; will work, right now the parser expands it right as it parses the initial AST.

[SimonSapin]

Is it possible to do this backward-compatibly after 1.0?

[huonw]

Is it possible to do this backward-compatibly after 1.0?

Should be fine, yes.

[SirVer]

Future searchers: I stumbled on this issue as I created a generated module in build.rs and #18810 contained a work around that helped me.

[Mark-Simulacrum]

Closing. If someone wants to pursue this, please follow the RFC process here https://github.com/rust-lang/rfcs#before-creating-an-rfc; the change is major enough to warrant this.

[pierzchalski]

Is anyone working on this, or is there an RFC in the pipeline? If not, I'm willing to start work on one.
CC @Mark-Simulacrum

[Mark-Simulacrum]

To my knowledge no one is working on this, but asking on internals.rust-lang.org is likely a good start.

[mjbshaw]

@pierzchalski Have you gotten a chance to start an RFC for this?

[pierzchalski]

@mjbshaw there didn't seem to be a lot of interest on the pre-RFC I wrote, but that might have been because I wrote it in the middle of the impl period. I might write up the RFC itself some time this/next week and see how well it goes.