identity uses are ok, even if there are no defining uses

Author: lcnr

compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs

@@ -503,7 +503,16 @@ pub(crate) fn apply_computed_concrete_opaque_types<'tcx>(
     let mut errors = Vec::new();
     for &(key, hidden_type) in opaque_types {
         let Some(expected) = get_concrete_opaque_type(concrete_opaque_types, key.def_id) else {
-            assert!(tcx.use_typing_mode_borrowck(), "non-defining use in defining scope");
+            if !tcx.use_typing_mode_borrowck() {
+                if let ty::Alias(ty::Opaque, alias_ty) = hidden_type.ty.kind()
+                    && alias_ty.def_id == key.def_id.to_def_id()
+                    && alias_ty.args == key.args
+                {
+                    continue;
+                } else {
+                    unreachable!("non-defining use in defining scope");
+                }
+            }
             errors.push(DeferredOpaqueTypeError::NonDefiningUseInDefiningScope {
                 span: hidden_type.span,
                 opaque_type_key: key,

tests/ui/impl-trait/non-defining-uses/ice-issue-146191.current.stderr

@@ -0,0 +1,29 @@
+error[E0277]: the trait bound `(): ReturnsSend` is not satisfied
+  --> $DIR/ice-issue-146191.rs:6:52
+   |
+LL | fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
+   |                                                    ^^^^^^^^^^^^^^^^ the trait `ReturnsSend` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/ice-issue-146191.rs:14:1
+   |
+LL | trait ReturnsSend {}
+   | ^^^^^^^^^^^^^^^^^
+note: required by a bound in an opaque type
+  --> $DIR/ice-issue-146191.rs:6:57
+   |
+LL | fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
+   |                                                         ^^^^^^^^^^^
+
+error[E0733]: recursion in an async block requires boxing
+  --> $DIR/ice-issue-146191.rs:8:5
+   |
+LL |     async { create_complex_future().await }
+   |     ^^^^^   ----------------------------- recursive call here
+   |
+   = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0277, E0733.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/impl-trait/non-defining-uses/ice-issue-146191.next.stderr

@@ -0,0 +1,17 @@
+error[E0282]: type annotations needed
+  --> $DIR/ice-issue-146191.rs:8:5
+   |
+LL |     async { create_complex_future().await }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
+
+error[E0282]: type annotations needed
+  --> $DIR/ice-issue-146191.rs:8:5
+   |
+LL |     async { create_complex_future().await }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0282`.

tests/ui/impl-trait/non-defining-uses/ice-issue-146191.rs

@@ -0,0 +1,15 @@
+//@ edition: 2024
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+
+fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
+    //[current]~^ ERROR the trait bound `(): ReturnsSend` is not satisfied
+    async { create_complex_future().await }
+    //[current]~^ ERROR recursion in an async block requires
+    //[next]~^^ ERROR type annotations needed
+    //[next]~| ERROR type annotations needed
+}
+
+trait ReturnsSend {}
+fn main() {}