Exposing a rust library to C code

[SirVer]

My use case is to swap out unsafe C with save rust implementations. To keep APIs in tact, I need to provide a C library that bridges to Rust. I was trying to do this:

package(default_visibility = ["//visibility:public"])

load(
    "@io_bazel_rules_rust//rust:rust.bzl",
    "rust_library",
)

rust_library(
    name = "greeter",
    srcs = ["lib.rs"],
)

cc_binary(
   name = "rust_from_c",
   srcs = ["hello_world.cc"],
   deps = [":greeter"],
)

with lib.rs:

#[no_mangle]
pub extern "C" fn greet(){
    println!("Hello from rust!");
}

and hello_world.cc:

void greet();
int main(int argc, char *argv[])
{
	greet();
	return 0;
}

This should conceptually work, but bazel refuses:

ERROR: /usr/src/app/rust_from_c/BUILD:18:14: in deps attribute of cc_library rule //rust_from_c:greeter: rust_library rule '//rust_from_c:greeter_rust' is misplaced here (expected cc_inc_library, cc_library, objc_library or cc_proto_library)

Is there a way to call rust code from C?

[SirVer]

@acmcarther I think you know most about these rules, right?

[acmcarther]

From my initial research:

https://doc.rust-lang.org/reference/linkage.html
You either want "staticlib" or "cdylib". Specifying crate types is enabled in #38, which was historically blocked on CI issues (I need to fix the PR urgently).

Once that is done, it is possible to pass this file output into the srcs of a cc_library or cc_binary rule. See the bazel docs for cc_library#srcs: https://docs.bazel.build/versions/master/be/c-cpp.html#cc_library.

This may not work (either because the providers from the rust_library aren't quite right, or because cc_library#srcs requires inputs to be statically known and not generated via another rule). In that case, we'd want to consider adding a cc_library_from_rust_library rule that reshapes the providers yielded by rust_library into a form that cc_library will accept as a dep.

EDIT: Clarified last sentence to indicate that we'd use the modified dependency as a "dep" not a "src".

[SirVer]

Thank you! So this is indeed blocked on #38. Also, having to state a rust_library as a src for a cc_library seems odd - from the semantics, dep would be the better keyword. But if src works I'll take it.

[mfarrugi]

Added an example of this here c_calling_rust, can tidy it up and create a pr after #38

Can't currently invoke the cc_library rule from rust_library like a macro can, so we could add a macro that calls rust_library and then `cc_library as in the example or figure out what it would take for cc_* rules to be more flexible in accepting native dependencies.

[SirVer]

awesome!

/cc @thomasschiwietz

[mfarrugi]

This is now possible and an example is pushed.
Dylibs are a little trickier but support for them on linux is pending.

[thomasschiwietz]

very cool!

[mfarrugi]

Resolved by #74