compute: count fuel-test output through the container

A per-record `inspect` needs `&Container: IntoIterator`, and on macOS resolving
that recurses through `objc2`'s blanket impls until the trait solver overflows,
so `lint-macos` fails to compile the test. Counting through the container avoids
the resolution entirely.

The same test reads the container directly from the commit that makes FlatMap's
output columnar onward, so this only closes the window where it did not.

Author: antiguru

src/compute/src/render/flat_map.rs

@@ -330,6 +330,7 @@ mod tests {
     use mz_expr::MapFilterProject;
     use mz_repr::{Datum, ReprScalarType};
     use timely::dataflow::operators::Inspect;
+    use timely::dataflow::operators::InspectCore;
     use timely::dataflow::operators::capture::{Capture, Extract};
 
     use super::*;
@@ -370,7 +371,15 @@ mod tests {
                 let scope = stream.scope();
                 let (oks, _errs) =
                     flat_map_stage(stream, scope, exprs, func, mfp, Antichain::new(), budget);
-                oks.inspect(move |_| *sink.borrow_mut() += 1);
+                // Count through the container rather than per record. A
+                // per-record `inspect` needs `&Container: IntoIterator`, and
+                // resolving that on macOS recurses through `objc2`'s blanket
+                // impls until the trait solver overflows.
+                oks.inspect_container(move |event| {
+                    if let Ok((_time, data)) = event {
+                        *sink.borrow_mut() += data.len();
+                    }
+                });
                 input
             });