Don't require rigid alias's trait to hold

Author: compiler-errors

compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

@@ -45,7 +45,6 @@ where
                             goal,
                             goal.predicate.alias,
                         );
-                        this.add_goal(GoalSource::AliasWellFormed, goal.with(cx, trait_ref));
                         this.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
                     })
                 })

tests/ui/traits/next-solver/coroutine-higher-ranked-rigid.rs

@@ -0,0 +1,33 @@
+//@ compile-flags: -Znext-solver
+//@ edition: 2024
+//@ check-pass
+
+// Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/177>.
+// Coroutines erase all free lifetimes from their interior types. In the future in `from_request`,
+// that means the `'r` lifetime in the type `<T as FromRequest<'r>>::Assoc`, which is in the
+// `dyn Future` type it's awaiting. Normalizing this associated type, with its free lifetimes
+// replaced, means proving `T: FromRequest<'!0>`, which definitely doesn't hold.
+
+// proving `T: Trait` holds when `<T as Trait>::Assoc` is rigid is not necessary for soundness,
+// at least not *yet*, and it's not even necessary for diagnostics since we have other special
+// casing for, e.g., AliasRelate goals failing in the BestObligation folder.
+
+// The old solver deals this by not checking that `T: Trait` holds when `<T as Trait>::Assoc` is
+// rigid. Introducing this additional requirement when projecting rigidly in the old solver
+// causes this (and tons of production crates) to fail. See the fallout from the crater run at:
+// <https://github.com/rust-lang/rust/pull/139763>.
+
+use std::{future::Future, pin::Pin};
+
+pub trait FromRequest<'r> {
+    type Assoc;
+    fn from_request() -> Pin<Box<dyn Future<Output = Self::Assoc> + Send>>;
+}
+
+fn test<'r, T: FromRequest<'r>>() -> Pin<Box<dyn Future<Output = ()> + Send>> {
+    Box::pin(async move {
+        T::from_request().await;
+    })
+}
+
+fn main() {}