[incremental] introduce `ensure()` operation

[nikomatsakis]

As part of #45208, we would like to introduce a first-class ensure() operation. Invoking Q::ensure(K) for some query Q with key K is semantically equivalent to introducing a fresh query Q1 that has a return type of unit and a provider like:

fn Q1(tcx, key: K) {
    Q(tcx, key); // discard result
}

In other words, it executes the query Q but does not use the result. This is worthwhile because it avoids the need to load the result from disk (or execute the provider, if we are not storing the results of Q to disk).

In order to introduce ensure(), we want to modify the plumbing macros for the query infrastructure. I think we could add an accessor called ensure, somewhat like try_get. It would look something like this:

/// Ensures that the query's result for `key` is up-to-date,
/// without necessarily executing the query. In particular, if all the inputs
/// to the query are green, then the query will not be re-executed. If however
/// some inputs have changed, then the query will be re-executed, and its
/// results cached (after being compared against the hash of the previous results).
///
/// This function is particularly useful when executing passes for their side-effects --
/// e.g., in order to report errors for erroneous programs.
///
/// Because the query will not execute unless it has changed, and hence the result
/// may not actually be available, this function returns unit. 
pub fn ensure(
    tcx: TyCtxt<'a, $tcx, 'lcx>,
    key: $K)
{
    let dep_node = Self::to_dep_node(tcx, &key);

    // Ensuring an "input" query makes no sense, nor anonymous queries:
    assert!(!dep_node.kind.is_anon());
    assert!(!dep_node.kind.is_input());

    // if this node has no color, then try to mark it green
    if tcx.dep_graph.node_color(&dep_node).is_none() {
        // This is an adapted variant on [this code] -- the only difference is that it does not invoke
        // `load_from_disk_and_cache_in_memory` when done. It probably makes sense to extract
        // a helper function.
        //
        // [this code]: https://github.com/rust-lang/rust/blob/72d65019c789138f555c7cf7139508d2f9f0dffe/src/librustc/ty/maps/plumbing.rs#L313-L335
        if let Some(DepNodeColor::Green(dep_node_index)) = tcx.dep_graph.node_color(&dep_node) {
            profq_msg!(tcx, ProfileQueriesMsg::CacheHit);
            tcx.dep_graph.read_index(dep_node_index);
            return; 
        }

        debug!("ty::queries::{}::try_get_with(key={:?}) - running try_mark_green",
            stringify!($name),
            key);

        if let Some(dep_node_index) = tcx.dep_graph.try_mark_green(tcx, &dep_node) {
            debug_assert!(tcx.dep_graph.is_green(dep_node_index));
            profq_msg!(tcx, ProfileQueriesMsg::CacheHit);
            tcx.dep_graph.read_index(dep_node_index);
        return;
    }

    // If the node is not green, or we could not make it green,
    // just execute the query normally and discard the result:
    tcx.$name(key);
}

Next, just to use this function somewhere, we can convert the code in librustc_typeck to use ensure. Notably, change this line from self.tcx.typeck_tables_of(body_owner_def_id) to ty::maps::queries::typeck_tables_of::ensure(self.tcx, body_owner_def_id).

(This last step is not enough to skip type-checking, since there are other uses of typeck_tables_of, but it's a necessary precondition.)

[theotherjimmy]

Already working on this. Thanks for the extra detail!

[theotherjimmy]

@nikomatsakis I think there may be a few problems with the above code. I had to reorder the check for if the node is already green out of the check to see if the node has no color. Without that re-ordering, that check would always be false, and we would always execute green queries. Both are bad.