test(parallel): Add regression test for fn_sig cycle arity ICE #153391

Author: TKanX

tests/ui/parallel-rustc/fn-sig-cycle-ice-153391.rs

@@ -0,0 +1,17 @@
+// Regression test for #153391.
+//
+//@ edition:2024
+//@ compile-flags: -Z threads=16
+//@ compare-output-by-lines
+
+trait A {
+    fn g() -> B;
+    //~^ ERROR expected a type, found a trait
+}
+
+trait B {
+    fn bar(&self, x: &A);
+    //~^ ERROR expected a type, found a trait
+}
+
+fn main() {}

tests/ui/parallel-rustc/fn-sig-cycle-ice-153391.stderr

@@ -0,0 +1,30 @@
+error[E0782]: expected a type, found a trait
+   |
+LL |     fn bar(&self, x: &A);
+   |                       ^
+   |
+   = note: `A` is dyn-incompatible, otherwise a trait object could be used
+help: use a new generic type parameter, constrained by `A`
+   |
+LL -     fn bar(&self, x: &A);
+LL +     fn bar<T: A>(&self, x: &T);
+   |
+help: you can also use an opaque type, but users won't be able to specify the type parameter when calling the `fn`, having to rely exclusively on type inference
+   |
+LL |     fn bar(&self, x: &impl A);
+   |                       ++++
+
+error[E0782]: expected a type, found a trait
+  --> $DIR/fn-sig-cycle-ice-153391.rs:8:15
+   |
+LL |     fn g() -> B;
+   |               ^
+   |
+   |
+LL |     fn g() -> impl B;
+   |               ++++
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0782`.