Conditional compilation based on crate_type

[BenTheElder]

Ideally we should be able to do something like this:
Cargo.toml:

.....
[lib]
path = "src/lib.rs"
crate-type = ["rlib","dylib"]
.....

src/lib.rs:

....
#[no_mangle]
#[cfg(crate_type="dylib")]
pub unsafe extern "C" fn .....

To allow only exporting the c api in the dylib or perhaps in the dylib and static lib and not in the rust code.
This would be consistent with being able to build multiple library formats but allow the unsafe c methods to not be exported to rust code.

I checked against: http://doc.rust-lang.org/reference.html#conditional-compilation
and did some experimentation myself, with no luck.

If anyone has any better suggestions, I'd love to hear them.

I'm looking to generate very similar rust and c libraries with similar apis where the c library is just a c style wrapper around the rust api, also written in rust. Ideally i'd keep this in a single source and conditionally define the c api's in the dylib for loading from various languages with c ffi's and exclude the c api from the rust lib.

[BenTheElder]

I've since rewritten all of the wrapper functions I intended to exclude from the rlib to be totally safe and documented them with their c equivalent definitions. I plan to just ensure the documentation notes that the wrapper functions are not intended to be used from rust and that the implementation defined mangled methods should be preferred.

I'm leaving this open for now, but feel free to close it. It might still be useful to have, but I've given up on it for now rather than complicate my project structure further (E.G. having a rust lib and a seperate wrapper lib), in favor of better documentation and safer code.

[Stebalien]

Triage: I believe this is fixed (see https://doc.rust-lang.org/reference.html#linkage).

[steveklabnik]

I believe so as well, and given that @BenTheElder found another way in the meantime, and nobody else has commented in a year, let's give it a close.

[ccll]

This is not working, at least on these versions as I tried:
rustc 1.17.0-nightly (a17e5e2 2017-02-20)
rustc 1.15.1 (021bd29 2017-02-08)
rustc 1.15.0 (10893a9 2017-01-19)
rustc 1.14.0 (e8a0123 2016-12-16)

Possibly a regression?

Details
I'm working on a Windows project, where one crate needs to be an DLL ("cdylib"), and the function named "DllMain" should be exported as a C function to do some initialization when the DLL is loaded. In the meanwhile this crate contains some type definitions used by other high-level crates, so this crate should be built to both "rlib" and "cdylib", and the "rlib" version should NOT export "DllMain" as the other high-level crates may themselves be an "cdylib" and has their own version of "DllMain" exported.
But as I added the conditional compilation as stated above, the function "DllMain" just disappears from the DLL's exported symbol table.

here is an shorter version of the snippet:

// lib.rs
#[no_mangle]
#[cfg(crate_type="cdylib")]
pub extern "stdcall" fn DllMain() {

}

# Cargo.toml
...
[lib]
crate_type = ["rlib", "cdylib"]
...

[tmccombs]

I don't see anything about this in the link to the documentation.

[nbigaouette-eai]

I can't find anything about this in the reference either.

Using #[cfg(crate_type="cdylib")] to include something only when cdylib crate is being built (with crate_type = ["rlib", "cdylib"] in Cargo.toml) does not work for me either.

I would like to do this to not have to expose an unsage API meant to be consumed by FFI/Python.

[dtolnay]

Reopening because I was looking for this today and it doesn't seem like it was ever implemented.

I don't think an RFC would be required for this.

[pheki]

I'm working on this! Can I be assigned?

[pheki]

I did implement this, but I'm not sure it is possible to be intuitive as rustc allows multiple crate types in the same session.

When you compile a crate with crate-type = ["rlib","cdylib"], cargo generates a single rustc call with both --crate-type rlib and --crate-type cdylib, which would make both #[cfg(crate_type="rlib")] and #[cfg(crate_type="cdylib")] compile...

I can create a PR anyone's interested.