Should include! be recommended?

[SimonSapin]

Cargo recommends the include! macro to use generated code.

include!(concat!(env!("OUT_DIR"), "/hello.rs"))

However, it doesn’t seem to be documented, and its source comments say

This is generally a bad idea because it's going to behave unhygienically.

So, is include! the way forward, or should we find some other mechanism? Maybe make something like this work? (It currently gives error: unexpected token: concat``)

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

[alexcrichton]

This seems more relevant as a Rust bug than a Cargo bug in the sense that this is the only way I know of to do this sort of inclusion today. I'd personally love to have #[path = concat!(...)], but that would require language changes to the attribute syntax and this doesn't seem like a super-motivating example.

I suppose I would say, "I agree", but this isn't entirely Cargo's problem. If we had another solution it would be nice, but we need to develop on in rustc first. The answer may be adding another macro which does behave like mod (perhaps include_mod!) which would solve issues like rust-lang/rust#18810.

Note that the bit about being hygienic I think would mean that the entire output of include! would be unusable unless it specifically listed identifiers that it wanted to be reachable, and it's quite an old macro, so the comments may just be a little dated.

[cuviper]

I also noticed that include! causes the target to be rebuilt every time, regardless of whether any of the source files have actually changed. I thought at first this might be conservatism about what the compile-time env! contains, but even a hard-coded include! path does this.

[alexcrichton]

Due to the lack of other infrastructure in the compiler for doing an equivalent task, I'm going to close this issue.

[alexcrichton]

@cuviper sorry for taking awhile to get back to you, but you may have been running into #1162

[SimonSapin]

Is it possible to add support for #[path = concat!(...)] (rust-lang/rust#18849) later, backward-compatibly?

[alexcrichton]

Of course!