Infra for "unit" benchmarking

crates/test_utils/src/bench_fixture.rs

@@ -0,0 +1,28 @@
+//! Generates large snippets of Rust code for usage in the benchmarks.
+
+use stdx::format_to;
+
+pub fn big_struct() -> String {
+    let n = 1_000;
+
+    let mut buf = "pub struct RegisterBlock {".to_string();
+    for i in 0..n {
+        format_to!(buf, "  /// Doc comment for {}.\n", i);
+        format_to!(buf, "  pub s{}: S{},\n", i, i);
+    }
+    buf.push_str("}\n\n");
+    for i in 0..n {
+        format_to!(
+            buf,
+            "
+
+#[repr(transparent)]
+struct S{} {{
+    field: u32,
+}}",
+            i
+        );
+    }
+
+    buf
+}

crates/test_utils/src/lib.rs

@@ -8,6 +8,7 @@
 
 #[macro_use]
 pub mod mark;
+pub mod bench_fixture;
 mod fixture;
 
 use std::{
@@ -16,6 +17,7 @@ use std::{
     path::PathBuf,
 };
 
+use profile::StopWatch;
 use serde_json::Value;
 use stdx::lines_with_ends;
 use text_size::{TextRange, TextSize};
@@ -406,3 +408,44 @@ pub fn format_diff(chunks: Vec<dissimilar::Chunk>) -> String {
     }
     buf
 }
+
+/// Utility for writing benchmark tests.
+///
+/// A benchmark test looks like this:
+///
+/// ```
+/// #[test]
+/// fn benchmark_foo() {
+///     if skip_slow_tests() { return; }
+///
+///     let data = bench_fixture::some_fixture();
+///     let analysis = some_setup();
+///
+///     let hash = {
+///         let _b = bench("foo");
+///         actual_work(analysis)
+///     };
+///     assert_eq!(hash, 92);
+/// }
+/// ```
+///
+/// * We skip benchmarks by default, to save time.
+///   Ideal benchmark time is 800 -- 1500 ms in debug.
+/// * We don't count preparation as part of the benchmark
+/// * The benchmark itself returns some kind of numeric hash.
+///   The hash is used as a sanity check that some code is actually run.
+///   Otherwise, it's too easy to win the benchmark by just doing nothing.
+pub fn bench(label: &'static str) -> impl Drop {
+    struct Bencher {
+        sw: StopWatch,
+        label: &'static str,
+    }
+
+    impl Drop for Bencher {
+        fn drop(&mut self) {
+            eprintln!("{}: {}", self.label, self.sw.elapsed())
+        }
+    }
+
+    Bencher { sw: StopWatch::start(), label }
+}