Rollup merge of #70367 - nikomatsakis:issue-69307, r=Aaron1011

save/restore `pessimistic_yield` when entering bodies

This flag is used to make the execution order around `+=` operators
pessimistic. Failure to save/restore the flag was causing independent
async blocks to effect one another, leading to strange ICEs and failed
assumptions.

Fixes #69307

r? @Zoxc

src/test/ui/async-await/issues/issue-69307-nested.rs

@@ -0,0 +1,30 @@
+// Regression test for #69307
+//
+// Having a `async { .. foo.await .. }` block appear inside of a `+=`
+// expression was causing an ICE due to a failure to save/restore
+// state in the AST numbering pass when entering a nested body.
+//
+// check-pass
+// edition:2018
+
+fn block_on<F>(_: F) -> usize {
+    0
+}
+
+fn main() {}
+
+async fn bar() {
+    let mut sum = 0;
+    sum += {
+        block_on(async {
+            baz().await;
+            let mut inner = 1;
+            inner += block_on(async {
+                baz().await;
+                0
+            })
+        })
+    };
+}
+
+async fn baz() {}

src/test/ui/async-await/issues/issue-69307.rs

@@ -0,0 +1,23 @@
+// Regression test for #69307
+//
+// Having a `async { .. foo.await .. }` block appear inside of a `+=`
+// expression was causing an ICE due to a failure to save/restore
+// state in the AST numbering pass when entering a nested body.
+//
+// check-pass
+// edition:2018
+
+fn block_on<F>(_: F) -> usize {
+    0
+}
+
+fn main() {}
+
+async fn bar() {
+    let mut sum = 0;
+    sum += block_on(async {
+        baz().await;
+    });
+}
+
+async fn baz() {}