fudge infer vars in cause code intentionally

compiler/rustc_infer/src/traits/mod.rs

@@ -37,6 +37,8 @@ use crate::infer::InferCtxt;
 #[derive(Clone, TypeFoldable, TypeVisitable)]
 pub struct Obligation<'tcx, T> {
     /// The reason we have to prove this thing.
+    /// FIXME: we shouldn't ignore the cause but instead change the affected visitors
+    /// to only visit predicates manually.
     #[type_foldable(identity)]
     #[type_visitable(ignore)]
     pub cause: ObligationCause<'tcx>,

compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs

@@ -177,7 +177,11 @@ fn find_best_leaf_obligation<'tcx>(
                 )
                 .break_value()
                 .ok_or(())
+                // walk around the fact that the cause in `Obligation` is ignored by folders so that
+                // we can properly fudge the infer vars in cause code.
+                .map(|o| (o.cause.clone(), o))
         })
+        .map(|(cause, o)| PredicateObligation { cause, ..o })
         .unwrap_or(obligation);
     deeply_normalize_for_diagnostics(infcx, obligation.param_env, obligation)
 }

tests/ui/traits/error-reporting/leaking-vars-in-cause-code-1.rs

@@ -0,0 +1,43 @@
+//@ compile-flags: -Znext-solver
+//@ edition: 2024
+//
+// A regression test for the ICE variant in trait-system-refactor-initiative#245.
+// We'll meet regions that're already popped off when using parent predicate in cause code.
+// `cause` in `Obligation` is ignored by folders/visitors.
+// In this case, `fudge_inference_if_ok` doesn't fudge a region var in cause code.
+//
+// The old solver doesn't trigger ICE because regions in the predicate are replaced with
+// placeholders when checking generator witness. Besides, the old solver doesn't eagerly
+// resolves vars before canonicalizing the predicate in `predicate_must_hold_modulo_regions`.
+
+trait AsyncFn: Send + 'static {
+    type Fut: Future<Output = ()> + Send;
+
+    fn call(&self) -> Self::Fut;
+}
+
+async fn wrap_call<P: AsyncFn + ?Sized>(filter: &P) {
+    filter.call().await;
+}
+
+fn get_boxed_fn() -> Box<DynAsyncFnBoxed> {
+    todo!()
+}
+
+async fn cursed_fut() {
+    wrap_call(get_boxed_fn().as_ref()).await;
+}
+
+fn observe_fut_not_send() {
+    assert_send(cursed_fut());
+    //~^ ERROR: `dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` cannot be shared between threads safely [E0277]
+}
+
+fn assert_send<T: Send>(t: T) -> T {
+    t
+}
+
+pub type BoxFuture<'a, T> = std::pin::Pin<Box<dyn Future<Output = T> + Send + 'a>>;
+type DynAsyncFnBoxed = dyn AsyncFn<Fut = BoxFuture<'static, ()>>;
+
+fn main() {}

tests/ui/traits/error-reporting/leaking-vars-in-cause-code-1.stderr

@@ -0,0 +1,35 @@
+error[E0277]: `dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` cannot be shared between threads safely
+  --> $DIR/leaking-vars-in-cause-code-1.rs:32:17
+   |
+LL |     assert_send(cursed_fut());
+   |     ----------- ^^^^^^^^^^^^ `dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` cannot be shared between threads safely
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Sync` is not implemented for `dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>`
+   = note: required for `&dyn AsyncFn<Fut = Pin<Box<dyn Future<Output = ()> + Send>>>` to implement `Send`
+note: required because it's used within this `async` fn body
+  --> $DIR/leaking-vars-in-cause-code-1.rs:19:53
+   |
+LL |   async fn wrap_call<P: AsyncFn + ?Sized>(filter: &P) {
+   |  _____________________________________________________^
+LL | |     filter.call().await;
+LL | | }
+   | |_^
+note: required because it's used within this `async` fn body
+  --> $DIR/leaking-vars-in-cause-code-1.rs:27:23
+   |
+LL |   async fn cursed_fut() {
+   |  _______________________^
+LL | |     wrap_call(get_boxed_fn().as_ref()).await;
+LL | | }
+   | |_^
+note: required by a bound in `assert_send`
+  --> $DIR/leaking-vars-in-cause-code-1.rs:36:19
+   |
+LL | fn assert_send<T: Send>(t: T) -> T {
+   |                   ^^^^ required by this bound in `assert_send`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/error-reporting/leaking-vars-in-cause-code-2.rs

@@ -0,0 +1,31 @@
+//@ compile-flags: -Znext-solver
+
+// The `cause` in `Obligation` is ignored by type folders. So infer vars in cause code is not
+// fudged.
+// Check the comments of
+// `leaking-vars-in-cause-code-1.rs` for more details.
+trait Trait<T> {}
+struct A<T>(T);
+struct B<T>(T);
+
+trait IncompleteGuidance {}
+
+impl<T> Trait<()> for A<T>
+where
+    T: IncompleteGuidance,
+{
+}
+
+impl<T, U> Trait<()> for B<T>
+//~^ ERROR: the type parameter `U` is not constrained by the impl trait, self type, or predicates
+where
+    A<T>: Trait<U>,
+{
+}
+
+fn impls_trait<T: Trait<()>>() {}
+
+fn main() {
+    impls_trait::<B<()>>();
+    //~^ ERROR: the trait bound `(): IncompleteGuidance` is not satisfied
+}

tests/ui/traits/error-reporting/leaking-vars-in-cause-code-2.stderr

@@ -0,0 +1,37 @@
+error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/leaking-vars-in-cause-code-2.rs:19:9
+   |
+LL | impl<T, U> Trait<()> for B<T>
+   |         ^ unconstrained type parameter
+
+error[E0277]: the trait bound `(): IncompleteGuidance` is not satisfied
+  --> $DIR/leaking-vars-in-cause-code-2.rs:29:19
+   |
+LL |     impls_trait::<B<()>>();
+   |                   ^^^^^ the trait `IncompleteGuidance` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/leaking-vars-in-cause-code-2.rs:11:1
+   |
+LL | trait IncompleteGuidance {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^
+note: required for `A<()>` to implement `Trait<()>`
+  --> $DIR/leaking-vars-in-cause-code-2.rs:13:9
+   |
+LL | impl<T> Trait<()> for A<T>
+   |         ^^^^^^^^^     ^^^^
+LL | where
+LL |     T: IncompleteGuidance,
+   |        ------------------ unsatisfied trait bound introduced here
+   = note: 1 redundant requirement hidden
+   = note: required for `B<()>` to implement `Trait<()>`
+note: required by a bound in `impls_trait`
+  --> $DIR/leaking-vars-in-cause-code-2.rs:26:19
+   |
+LL | fn impls_trait<T: Trait<()>>() {}
+   |                   ^^^^^^^^^ required by this bound in `impls_trait`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0207, E0277.
+For more information about an error, try `rustc --explain E0207`.