-Z dump-dep-graph does not generate output

[gftea]

A simple test setup, run on WSL2 on windows 11.

cargo new hello
cd hello
cargo clean; cargo +nightly rustc -- -Zdump-dep-graph

There is no file generated in current directory, nor in the /tmp folder

[cjgillot]

You need to pass -Zquery-dep-graph too. We should probably add a diagnostic.

[gftea]

OK, now it works with cargo clean; cargo +nightly rustc -- -Zquery-dep-graph -Zdump-dep-graph

but, now I have below codes, the generated dep-graph does not have any info about foo() function. Why is that?

fn foo() {
    let x = "foo";
    println!("{}", x);
}
fn main() {
    foo();
}

[cjgillot]

The generated graph only records dependencies between queries. It used to have both the query name and its argument. I removed that a few months back to have much terser dumps, easier to use.

[gftea]

I see, thanks! Why not let -Zdump-dep-graph implicitly enable query-dep-graph?

[cjgillot]

For the silliest reason: nobody implemented it 😄.
If you want to do it, please go ahead.

[gftea]

@cjgillot after reading the codes, it seems would be better to hint the missing flag if query-dep-graph is not given.

In build_dep_graph(), record_graph only true when query-dep-graph is true.

pub fn build_dep_graph(...) {
    ....
    Some(DepGraph::new(
        &sess.prof,
        prev_graph,
        prev_work_products,
        encoder,
        sess.opts.unstable_opts.query_dep_graph,
        sess.opts.unstable_opts.incremental_info,
    ))

In assert_dep_graph(), would be good to hint to user if missing query-dep-graph flag.

pub fn assert_dep_graph(tcx: TyCtxt<'_>) {
    tcx.dep_graph.with_ignore(|| {
        if tcx.sess.opts.unstable_opts.dump_dep_graph {
            tcx.dep_graph.with_query(dump_graph);
        }

        if !tcx.sess.opts.unstable_opts.query_dep_graph {
            // --> Hint about the missing option ?
            return;
        }

and maybe better to have early error before runninig compiler?

fn run_compiler(
    at_args: &[String],
    callbacks: &mut (dyn Callbacks + Send),
    file_loader: Option<Box<dyn FileLoader + Send + Sync>>,
    make_codegen_backend: Option<
        Box<dyn FnOnce(&config::Options) -> Box<dyn CodegenBackend> + Send>,
    >,
) -> interface::Result<()> {
    let args = args::arg_expand_all(at_args);

    let Some(matches) = handle_options(&args) else { return Ok(()) };

    let sopts = config::build_session_options(&matches);
    // check dump-dep-graph and query-dep-graph here?

[cjgillot]

Inside build_session_options may even be better. There are already a few diagnostics given in that function for option collisions.