fix: emit_serialized_unit_graph uses the configured shell

[aatifsyed]

I'm writing a compatibility layer between the cargo and meson build systems.
This change allows me to plumb that through better :)

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[epage]

What difference is this making for you?

[aatifsyed]

I'm using cargo-as-a-library to get the unit graph.

Before this change:

fn main() -> anyhow::Result<()> {
    match env::var_os("CARGO2MESON_CHILD").is_some() {
        // we're the child process, so run cargo so that it emits the unit graph
        true => {
            // ...
            let mut options = CompileOptions::new(&gctx, CompileMode::Build)?;
            options.build_config.unit_graph = true;
            cargo::ops::compile(&Workspace::new(&manifest_path, &gctx)?, &options)?;
        },
        // we're the parent process, so spawn ourselves and capture stdout
        false => {
            // ...
            let output = Command::new(env::current_exe()).env("CARGO2MESON_CHILD", "1") /* .arg(..) */ .output()?;
            serde_json::from_slice(&output.stdout)?;
            // ...
        }
    }
    Ok(())
}

After this change:

fn main() -> anyhow::Result<()> {
    // ...
    let shared_writer = SharedWriter::new(vec![]);
    let gtcx = GlobalContext::new(Shell::from_writer(Box::new(shared_writer.clone())), path, path);
    let mut options = CompileOptions::new(&gctx, CompileMode::Build)?;
    options.build_config.unit_graph = true;
    cargo::ops::compile(&Workspace::new(&manifest_path, &gctx)?, &options)?;
    serde_json::from_slice(&shared_writer.take())?;
    // ...
}

I won't have to workaround by spawning myself, and can use existing cargo mechanisms to grab the output :)

Happy to provide less abridged examples, but I hope the above gets the point across :)

[weihanglo]

The change itself looks reasonable.

Just remind that libcargo is provided as-is, so you may find it painful when a cargo version bump is needed 😆.

accidentally approved

[aatifsyed]

I've applied @weihanglo's great suggestion :)