Add combine based benchmark to catch exponential regression

Author: Markus Westerlind

collector/benchmarks/README.md

@@ -95,3 +95,4 @@ programs.
 - **wf-projection-stress-65510**: A stress test which showcases [quadratic
   behavior](https://github.com/rust-lang/rust/issues/65510) (in the number of
   associated type bounds).
+- **trait-stress-68666**: A stress test which constructs a parser in the same manner that `combine` does. Showcases [exponential behavior](https://github.com/rust-lang/rust/issues/68666) in trait selection.

collector/benchmarks/trait-stress/Cargo.lock

@@ -0,0 +1,10 @@
+[package]
+name = "combine"
+version = "0.1.0"
+authors = ["Markus Westerlind <[email protected]>"]
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+
+[workspace]

collector/benchmarks/trait-stress/Cargo.toml

@@ -0,0 +1,64 @@
+use std::marker::PhantomData;
+
+pub trait Parser<Input> {
+    type Output;
+
+    fn or<P2>(self, p: P2) -> Or<Self, P2>
+    where
+        Self: Sized,
+        P2: Parser<Input, Output = Self::Output>,
+    {
+        Or(self, p)
+    }
+}
+
+#[macro_export]
+macro_rules! choice {
+    ($first : expr) => {
+        $first
+    };
+    ($first : expr, $($rest : expr),+) => {
+        $first.or(choice!($($rest),+))
+    }
+}
+
+pub struct Or<P1, P2>(P1, P2);
+impl<Input, P1, P2> Parser<Input> for Or<P1, P2>
+where
+    P1: Parser<Input, Output = P2::Output>,
+    P2: Parser<Input>,
+{
+    type Output = P2::Output;
+}
+
+pub struct P<I, O>(PhantomData<fn(I) -> O>);
+impl<Input, O> Parser<Input> for P<Input, O> {
+    type Output = O;
+}
+
+pub fn token<I>(_: u8) -> impl Parser<I, Output = u8> {
+    P(PhantomData)
+}
+
+fn mcc_payload_item<I>() -> impl Parser<I, Output = u8> {
+    choice!(
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G'),
+        token(b'G')
+    )
+}
+
+fn main() {}