Rollup merge of #148173 - tiif:fix-opaque-ice, r=BoxyUwU

Emit delayed bug during wfck for stranded opaque

Fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/235

## Problem

The fundamental issue here is ``OpaqueTypeCollector`` operates on ``rustc_middle::Ty``, but ``check_type_wf`` operates on HIR.

Since [check_type_wf](2f7620a5cc/compiler/rustc_hir_analysis/src/check/wfcheck.rs (L2262)) operates on HIR, it can see the stranded opaque and tries to infer it's hidden type. But ``OpaqueTypeCollector`` operates on ``rustc_middle::Ty``, so the ``OpaqueTypeCollector`` can no longer see a stranded opaque, hence its hidden type could not be inferred.

As a result, the tests ICE'ed at 34a8c7368c/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs (L253)

## Proposed solution

This PR detects stranded opaque types during wf check and emit a delayed bug for it.

## Alternative solution

`@BoxyUwU` and I had considered rewriting ``OpaqueTypeCollector`` to be a HIR visitor instead of a ``rustc_middle::Ty`` visitor, but we believe a HIR-based ``OpaqueTypeCollector`` will not work and might not worth the cost of rewriting.

## Acknowledgement

This PR is a joint effort with `@BoxyUwU` :3

compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -242,6 +242,16 @@ pub(super) fn find_opaque_ty_constraints_for_rpit<'tcx>(
     owner_def_id: LocalDefId,
     opaque_types_from: DefiningScopeKind,
 ) -> Ty<'tcx> {
+    // When an opaque type is stranded, its hidden type cannot be inferred
+    // so we should not continue.
+    if !tcx.opaque_types_defined_by(owner_def_id).contains(&def_id) {
+        let opaque_type_span = tcx.def_span(def_id);
+        let guar = tcx
+            .dcx()
+            .span_delayed_bug(opaque_type_span, "cannot infer type for stranded opaque type");
+        return Ty::new_error(tcx, guar);
+    }
+
     match opaque_types_from {
         DefiningScopeKind::HirTypeck => {
             let tables = tcx.typeck(owner_def_id);

tests/ui/traits/next-solver/opaques/stranded_opaque.rs

@@ -0,0 +1,50 @@
+//@ compile-flags: -Znext-solver
+#![feature(type_alias_impl_trait)]
+use std::future::Future;
+
+// Test for https://github.com/rust-lang/trait-system-refactor-initiative/issues/235
+
+// These are cases where an opaque types become "stranded" due to
+// some errors. Make sure we don't ICE in either case.
+
+// Case 1: `impl Send` is stranded
+fn foo() -> impl ?Future<Output = impl Send> {
+    //~^ ERROR bound modifier `?` can only be applied to `Sized`
+    //~| ERROR bound modifier `?` can only be applied to `Sized`
+    ()
+}
+
+// Case 2: `Assoc = impl Trait` is stranded
+trait Trait {}
+impl Trait for i32 {}
+
+fn produce() -> impl Trait<Assoc = impl Trait> {
+    //~^ ERROR associated type `Assoc` not found for `Trait`
+    //~| ERROR associated type `Assoc` not found for `Trait`
+    16
+}
+
+// Case 3: `impl Trait` is stranded
+fn ill_formed_string() -> String<impl Trait> {
+    //~^ ERROR struct takes 0 generic arguments but 1 generic argument was supplied
+   String::from("a string")
+}
+
+// Case 4: TAIT variant of Case 1 to 3
+type Foo = impl ?Future<Output = impl Send>;
+//~^ ERROR unconstrained opaque type
+//~| ERROR unconstrained opaque type
+//~| ERROR bound modifier `?` can only be applied to `Sized`
+//~| ERROR bound modifier `?` can only be applied to `Sized`
+
+type Produce =  impl Trait<Assoc = impl Trait>;
+//~^ ERROR unconstrained opaque type
+//~| ERROR unconstrained opaque type
+//~| ERROR associated type `Assoc` not found for `Trait`
+//~| ERROR associated type `Assoc` not found for `Trait`
+
+type IllFormedString =  String<impl Trait>;
+//~^ ERROR unconstrained opaque type
+//~| ERROR struct takes 0 generic arguments but 1 generic argument was supplied
+
+fn main() {}

tests/ui/traits/next-solver/opaques/stranded_opaque.stderr

@@ -0,0 +1,116 @@
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:11:18
+   |
+LL | fn foo() -> impl ?Future<Output = impl Send> {
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded_opaque.rs:21:28
+   |
+LL | fn produce() -> impl Trait<Assoc = impl Trait> {
+   |                            ^^^^^ associated type `Assoc` not found
+
+error[E0107]: struct takes 0 generic arguments but 1 generic argument was supplied
+  --> $DIR/stranded_opaque.rs:28:27
+   |
+LL | fn ill_formed_string() -> String<impl Trait> {
+   |                           ^^^^^^------------ help: remove the unnecessary generics
+   |                           |
+   |                           expected 0 generic arguments
+
+error[E0107]: struct takes 0 generic arguments but 1 generic argument was supplied
+  --> $DIR/stranded_opaque.rs:46:25
+   |
+LL | type IllFormedString =  String<impl Trait>;
+   |                         ^^^^^^------------ help: remove the unnecessary generics
+   |                         |
+   |                         expected 0 generic arguments
+
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:11:18
+   |
+LL | fn foo() -> impl ?Future<Output = impl Send> {
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded_opaque.rs:21:28
+   |
+LL | fn produce() -> impl Trait<Assoc = impl Trait> {
+   |                            ^^^^^ associated type `Assoc` not found
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: unconstrained opaque type
+  --> $DIR/stranded_opaque.rs:34:12
+   |
+LL | type Foo = impl ?Future<Output = impl Send>;
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same crate
+
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:34:17
+   |
+LL | type Foo = impl ?Future<Output = impl Send>;
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: bound modifier `?` can only be applied to `Sized`
+  --> $DIR/stranded_opaque.rs:34:17
+   |
+LL | type Foo = impl ?Future<Output = impl Send>;
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: unconstrained opaque type
+  --> $DIR/stranded_opaque.rs:34:34
+   |
+LL | type Foo = impl ?Future<Output = impl Send>;
+   |                                  ^^^^^^^^^
+   |
+   = note: `Foo` must be used in combination with a concrete type within the same crate
+
+error: unconstrained opaque type
+  --> $DIR/stranded_opaque.rs:40:17
+   |
+LL | type Produce =  impl Trait<Assoc = impl Trait>;
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `Produce` must be used in combination with a concrete type within the same crate
+
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded_opaque.rs:40:28
+   |
+LL | type Produce =  impl Trait<Assoc = impl Trait>;
+   |                            ^^^^^ associated type `Assoc` not found
+
+error[E0220]: associated type `Assoc` not found for `Trait`
+  --> $DIR/stranded_opaque.rs:40:28
+   |
+LL | type Produce =  impl Trait<Assoc = impl Trait>;
+   |                            ^^^^^ associated type `Assoc` not found
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: unconstrained opaque type
+  --> $DIR/stranded_opaque.rs:40:36
+   |
+LL | type Produce =  impl Trait<Assoc = impl Trait>;
+   |                                    ^^^^^^^^^^
+   |
+   = note: `Produce` must be used in combination with a concrete type within the same crate
+
+error: unconstrained opaque type
+  --> $DIR/stranded_opaque.rs:46:32
+   |
+LL | type IllFormedString =  String<impl Trait>;
+   |                                ^^^^^^^^^^
+   |
+   = note: `IllFormedString` must be used in combination with a concrete type within the same crate
+
+error: aborting due to 15 previous errors
+
+Some errors have detailed explanations: E0107, E0220.
+For more information about an error, try `rustc --explain E0107`.