Add many-assoc-items benchmark

Author: ecstatic-morse

collector/benchmarks/README.md

@@ -79,6 +79,9 @@ programs.
   performance](https://github.com/rust-lang/rust/issues/46449) in the past.
 - **issue-58319**: A small program that caused [poor
   performance](https://github.com/rust-lang/rust/issues/58319) in the past.
+- **many-assoc-items**: Contains a struct with many associated items, which
+  caused [quadratic behavior](https://github.com/rust-lang/rust/issues/68957)
+  in the past.
 - **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/many-assoc-items/Cargo.lock

@@ -0,0 +1,6 @@
+[package]
+name = "many-assoc-items"
+version = "0.1.0"
+edition = "2018"
+
+[workspace]