Don't ICE on pending obligations from deep normalization in a loop

compiler/rustc_trait_selection/src/traits/normalize.rs

@@ -77,7 +77,15 @@ impl<'tcx> At<'_, 'tcx> {
                 .into_value_registering_obligations(self.infcx, &mut *fulfill_cx);
             let errors = fulfill_cx.select_all_or_error(self.infcx);
             let value = self.infcx.resolve_vars_if_possible(value);
-            if errors.is_empty() { Ok(value) } else { Err(errors) }
+            if errors.is_empty() {
+                Ok(value)
+            } else {
+                // Drop pending obligations, since deep normalization may happen
+                // in a loop and we don't want to trigger the assertion on the next
+                // iteration due to pending ambiguous obligations we've left over.
+                let _ = fulfill_cx.collect_remaining_errors(self.infcx);
+                Err(errors)
+            }
         }
     }
 }

tests/ui/traits/deep-norm-pending.rs

@@ -0,0 +1,24 @@
+trait Foo {
+    type Assoc;
+}
+
+trait Bar {
+    fn method() -> impl Sized;
+    //~^ ERROR the trait bound `T: Foo` is not satisfied
+}
+impl<T> Bar for T
+//~^ ERROR the trait bound `T: Foo` is not satisfied
+//~| ERROR the trait bound `T: Foo` is not satisfied
+where
+    <T as Foo>::Assoc: Sized,
+{
+    fn method() {}
+    //~^ ERROR the trait bound `T: Foo` is not satisfied
+    //~| ERROR the trait bound `T: Foo` is not satisfied
+    //~| ERROR the trait bound `T: Foo` is not satisfied
+    //~| ERROR the trait bound `T: Foo` is not satisfied
+    //~| ERROR the trait bound `T: Foo` is not satisfied
+    //~| ERROR the trait bound `T: Foo` is not satisfied
+}
+
+fn main() {}

tests/ui/traits/deep-norm-pending.stderr

@@ -0,0 +1,130 @@
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:15:5
+   |
+LL |     fn method() {}
+   |     ^^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:9:1
+   |
+LL | / impl<T> Bar for T
+LL | |
+LL | |
+LL | | where
+LL | |     <T as Foo>::Assoc: Sized,
+   | |_____________________________^ the trait `Foo` is not implemented for `T`
+   |
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:9:1
+   |
+LL | / impl<T> Bar for T
+LL | |
+LL | |
+LL | | where
+...  |
+LL | | }
+   | |_^ the trait `Foo` is not implemented for `T`
+   |
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:15:5
+   |
+LL |     fn method() {}
+   |     ^^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:15:5
+   |
+LL |     fn method() {}
+   |     ^^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+note: required for `T` to implement `Bar`
+  --> $DIR/deep-norm-pending.rs:9:9
+   |
+LL | impl<T> Bar for T
+   |         ^^^     ^
+...
+LL |     <T as Foo>::Assoc: Sized,
+   |                        ----- unsatisfied trait bound introduced here
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:15:5
+   |
+LL |     fn method() {}
+   |     ^^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:6:20
+   |
+LL |     fn method() -> impl Sized;
+   |                    ^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+note: required for `T` to implement `Bar`
+  --> $DIR/deep-norm-pending.rs:9:9
+   |
+LL | impl<T> Bar for T
+   |         ^^^     ^
+...
+LL |     <T as Foo>::Assoc: Sized,
+   |                        ----- unsatisfied trait bound introduced here
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:15:5
+   |
+LL |     fn method() {}
+   |     ^^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/deep-norm-pending.rs:15:8
+   |
+LL |     fn method() {}
+   |        ^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+help: consider further restricting type parameter `T` with trait `Foo`
+   |
+LL |     <T as Foo>::Assoc: Sized, T: Foo
+   |                               ++++++
+
+error: aborting due to 9 previous errors
+
+For more information about this error, try `rustc --explain E0277`.