test(tvix/eval): Add attr merge benchmarks

Add a quick couple of benchmarks for merging attribute sets, large and
small.

Change-Id: I26940a9cf4e0d30e3d9eb07a7b8c366ca4072ca3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6286
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: grfn <grfn@gws.fyi>

tvix/eval/benches/eval.rs

@@ -1,9 +1,25 @@
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use itertools::Itertools;
 use tvix_eval::interpret;
 
 fn eval_literals(c: &mut Criterion) {
     c.bench_function("int", |b| b.iter(|| black_box(interpret("42", None))));
 }
 
-criterion_group!(benches, eval_literals);
+fn eval_merge_attrs(c: &mut Criterion) {
+    c.bench_function("merge small attrs", |b| {
+        b.iter(|| black_box(interpret("{ a = 1; b = 2; } // { c = 3; }", None)))
+    });
+
+    c.bench_function("merge large attrs with small attrs", |b| {
+        let large_attrs = format!(
+            "{{{}}}",
+            (0..10000).map(|n| format!("a{n} = {n};")).join(" ")
+        );
+        let expr = format!("{large_attrs} // {{ c = 3; }}");
+        b.iter(move || black_box(interpret(&expr, None)))
+    });
+}
+
+criterion_group!(benches, eval_literals, eval_merge_attrs);
 criterion_main!(benches);