Depend on the binary of another workspace package being built by cargo

Question

I have a workspace project with multiple packages. The two important ones are:

• flowc - which is a lib and a binary
• flowstdlib

flowc is a kind of compiler that I build as part of the project.

flowstdlib has a build script, that uses the flowc built binary to build that package (generate "code", files etc), so I need the flowc compiler ready when flowstdlib is to be built.

In cargo.toml of flowstdlib I define flowc as a build dependency:

[build-dependencies]
flowc = {path = "../flowc", version = "0.31.0" }`

(I've tried making it also a dependency, but no change)

in the build.rs of flowstdlib I look for it in the path, and if not found in the ../target/debug/flowc location:

    let flowc = if Path::new(env!("CARGO_MANIFEST_DIR")).join("../target/debug/flowc").exists() {
        "../target/debug/flowc"
    } else if Simpath::new("PATH").find_type("flowc", FileType::File).is_ok() {
        "flowc"
    } else {
        ""
    };

When I run the build, it looks like it's trying to build multiple packages at the same time in parallel:

   Compiling flowstdlib v0.31.0 (/Users/andrew/workspace/flow/flowstdlib)
   Compiling flowsamples v0.31.1 (/Users/andrew/workspace/flow/samples)
warning: Could not find `flowc` in $PATH or `target/debug`, so cannot build flowstdlib
error: failed to run custom build command for `flowsamples v0.31.1 (/Users/andrew/workspace/flow/samples)`

and the flowstdlib build fails as flowc binary is not built yet.

Since the build continues and eventually finishes building flowc, if I re-run the build, it will work the second time around (as flowc binary is now found).

So:

• it looks like a build-dependency does wait for the depended-on binary to be built (maybe it waits for the library to be built, hard to tell)

Question

How I can make the build of flowstdlib wait for the completion of the flowc binary? (without forcing a non-parallel build)

Answer 1

Pending the RFC landing, my workaround is to split the build into two commands (I'm using a Makefile to invoke them currently):

* cargo build -p flowc   # will complete the build of the flowc binary
* cargo build            # will build the entire workspace, including flowstdlib