Add benchmark for projection caching issue

collector/benchmarks/README.md

@@ -100,6 +100,10 @@ programs.
 - **match-stress-exhaustive_patterns**: Contains code extracted from the `syn`
   crate to amplify the perf degradation caused by the `exhaustive_patterns`, as
   measured [here](https://github.com/rust-lang/rust/pull/79394).
+- **projection-caching**: A small program that causes extremely, deeply nested
+  types which stress the trait system's projection cache. Removing that cache
+  resulted in hours long compilations for some programs using futures,
+  actix-web and other libraries with similiarly nested type combinators.
 - **regression-31157**: A small program that caused a [large performance
   regression](https://github.com/rust-lang/rust/issues/31157) from the past.
 - **token-stream-stress**: Constructs a long token stream much like the `quote`

collector/benchmarks/projection-caching/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "projection-caching"
+version = "0.1.0"
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+futures = "0.3"
+
+[workspace]

collector/benchmarks/projection-caching/src/main.rs

@@ -0,0 +1,23 @@
+use futures::{stream, TryStreamExt};
+
+fn main() {
+    stream::empty::<Result<(), ()>>()
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ())
+        .inspect_ok(|_| ());
+}