Auto merge of rust-lang#117703 - compiler-errors:recursive-async, r=<try>

Support async recursive calls (as long as they have indirection)

Before rust-lang#101692, we stored coroutine witness types directly inside of the coroutine. That means that a coroutine could not contain itself (as a witness field) without creating a cycle in the type representation of the coroutine, which we detected with the `OpaqueTypeExpander`, which is used to detect cycles when expanding opaque types after that are inferred to contain themselves.

After `-Zdrop-tracking-mir` was stabilized, we no longer store these generator witness fields directly, but instead behind a def-id based query. That means there is no technical obstacle in the compiler preventing coroutines from containing themselves per se, other than the fact that for a coroutine to have a non-infinite layout, it must contain itself wrapped in a layer of allocation indirection (like a `Box`).

This means that it should be valid for this code to work:

```
async fn async_fibonacci(i: u32) -> u32 {
    if i == 0 || i == 1 {
        i
    } else {
        Box::pin(async_fibonacci(i - 1)).await
          + Box::pin(async_fibonacci(i - 2)).await
    }
}
```

Whereas previously, you'd need to coerce the future to `Pin<Box<dyn Future<Output = ...>>` before `await`ing it, to prevent the async's desugared coroutine from containing itself across as await point.

This PR does two things:
1. Remove the behavior from `OpaqueTypeExpander` where it intentionally fetches and walks through the coroutine's witness fields. This was kept around after `-Zdrop-tracking-mir` was stabilized so we would not be introducing new stable behavior, and to preserve the much better diagnostics of async recursion compared to a layout error.
2. Reworks the way we report layout errors having to do with coroutines, to make up for the diagnostic regressions introduced by (1.). We actually do even better now, pointing out the call sites of the recursion!

Author: bors

compiler/rustc_ast_lowering/src/expr.rs

@@ -792,8 +792,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
         // this name to identify what is being awaited by a suspended async functions.
         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
-        let (awaitee_pat, awaitee_pat_hid) =
-            self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
+        let (awaitee_pat, awaitee_pat_hid) = self.pat_ident_binding_mode(
+            gen_future_span,
+            awaitee_ident,
+            hir::BindingAnnotation::MUT,
+        );
 
         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 

compiler/rustc_hir/src/hir.rs

@@ -1529,6 +1529,15 @@ pub enum CoroutineKind {
     Coroutine,
 }
 
+impl CoroutineKind {
+    pub fn is_fn_like(self) -> bool {
+        matches!(
+            self,
+            CoroutineKind::Async(CoroutineSource::Fn) | CoroutineKind::Gen(CoroutineSource::Fn)
+        )
+    }
+}
+
 impl fmt::Display for CoroutineKind {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {