Don't do coroutine-closure-specific upvar analysis if tainted by errors

compiler/rustc_hir_typeck/src/upvar.rs

@@ -361,7 +361,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // For coroutine-closures, we additionally must compute the
         // `coroutine_captures_by_ref_ty` type, which is used to generate the by-ref
         // version of the coroutine-closure's output coroutine.
-        if let UpvarArgs::CoroutineClosure(args) = args {
+        if let UpvarArgs::CoroutineClosure(args) = args
+            // Don't do this if we are tainted by errors, because fallback causes us
+            // to fail to infer upvars for the outer coroutine-closure.
+            && self.tainted_by_errors().is_none()
+        {
             let closure_env_region: ty::Region<'_> = ty::Region::new_bound(
                 self.tcx,
                 ty::INNERMOST,

tests/ui/async-await/async-closures/dont-ice-when-body-tainted-by-errors.rs

@@ -0,0 +1,23 @@
+//@ edition: 2021
+
+#![feature(async_closure)]
+
+struct DropMe;
+
+trait Impossible {}
+fn trait_error<T: Impossible>() {}
+
+pub fn main() {
+    let b = DropMe;
+    let async_closure = async move || {
+        // Type error here taints the environment. This causes us to fallback all
+        // variables to `Error`. This means that when we compute the upvars for the
+        // *outer* coroutine-closure, we don't actually see any upvars since `MemCategorization`
+        // and `ExprUseVisitor`` will bail early when it sees error. This means
+        // that our underlying assumption that the parent and child captures are
+        // compatible ends up being broken, previously leading to an ICE.
+        trait_error::<()>();
+        //~^ ERROR the trait bound `(): Impossible` is not satisfied
+        let _b = b;
+    };
+}

tests/ui/async-await/async-closures/dont-ice-when-body-tainted-by-errors.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the trait bound `(): Impossible` is not satisfied
+  --> $DIR/dont-ice-when-body-tainted-by-errors.rs:19:23
+   |
+LL |         trait_error::<()>();
+   |                       ^^ the trait `Impossible` is not implemented for `()`
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/dont-ice-when-body-tainted-by-errors.rs:7:1
+   |
+LL | trait Impossible {}
+   | ^^^^^^^^^^^^^^^^
+note: required by a bound in `trait_error`
+  --> $DIR/dont-ice-when-body-tainted-by-errors.rs:8:19
+   |
+LL | fn trait_error<T: Impossible>() {}
+   |                   ^^^^^^^^^^ required by this bound in `trait_error`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.