Last nits

compiler/rustc_error_codes/src/error_codes/E0733.md

@@ -10,48 +10,30 @@ async fn foo(n: usize) {
 }
 ```
 
-To perform async recursion, the `async fn` needs to be desugared such that the
-`Future` is explicit in the return type:
-
-```edition2018,compile_fail,E0733
-use std::future::Future;
-fn foo_desugared(n: usize) -> impl Future<Output = ()> {
-    async move {
-        if n > 0 {
-            foo_desugared(n - 1).await;
-        }
-    }
-}
-```
-
-Finally, the future is wrapped in a pinned box:
+The recursive invocation can be boxed:
 
 ```edition2018
-use std::future::Future;
-use std::pin::Pin;
-fn foo_recursive(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
-    Box::pin(async move {
-        if n > 0 {
-            foo_recursive(n - 1).await;
-        }
-    })
+async fn foo(n: usize) {
+    if n > 0 {
+        Box::pin(foo(n - 1)).await;
+    }
 }
 ```
 
 The `Box<...>` ensures that the result is of known size, and the pin is
 required to keep it in the same place in memory.
 
-Alternatively, the recursive call-site can be boxed:
+Alternatively, the body can be boxed:
 
 ```edition2018
 use std::future::Future;
 use std::pin::Pin;
-fn foo_recursive(n: usize) -> impl Future<Output = ()> {
-    async move {
+fn foo(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
+    Box::pin(async move {
         if n > 0 {
-            Box::pin(foo_recursive(n - 1)).await;
+            foo(n - 1).await;
         }
-    }
+    })
 }
 ```
 

compiler/rustc_middle/src/values.rs

@@ -13,6 +13,7 @@ use rustc_span::{ErrorGuaranteed, Span};
 
 use std::collections::VecDeque;
 use std::fmt::Write;
+use std::ops::ControlFlow;
 
 impl<'tcx> Value<TyCtxt<'tcx>> for Ty<'_> {
     fn from_cycle_error(tcx: TyCtxt<'tcx>, _: &CycleError, guar: ErrorGuaranteed) -> Self {
@@ -130,16 +131,34 @@ impl<'tcx> Value<TyCtxt<'tcx>> for ty::EarlyBinder<ty::Binder<'_, ty::FnSig<'_>>
     }
 }
 
+// Take a cycle of `Q` and try `try_cycle` on every permutation, falling back to `otherwise`.
+fn search_for_cycle_permutation<Q, T>(
+    cycle: &[Q],
+    try_cycle: impl Fn(&mut VecDeque<&Q>) -> ControlFlow<T, ()>,
+    otherwise: impl FnOnce() -> T,
+) -> T {
+    let mut cycle: VecDeque<_> = cycle.iter().collect();
+    for _ in 0..cycle.len() {
+        match try_cycle(&mut cycle) {
+            ControlFlow::Continue(_) => {
+                cycle.rotate_left(1);
+            }
+            ControlFlow::Break(t) => return t,
+        }
+    }
+
+    otherwise()
+}
+
 impl<'tcx, T> Value<TyCtxt<'tcx>> for Result<T, &'_ ty::layout::LayoutError<'_>> {
     fn from_cycle_error(
         tcx: TyCtxt<'tcx>,
         cycle_error: &CycleError,
         _guar: ErrorGuaranteed,
     ) -> Self {
-        let mut cycle: VecDeque<_> = cycle_error.cycle.iter().collect();
-
-        let guar = 'search: {
-            for _ in 0..cycle.len() {
+        let diag = search_for_cycle_permutation(
+            &cycle_error.cycle,
+            |cycle| {
                 if cycle[0].query.dep_kind == dep_kinds::layout_of
                     && let Some(def_id) = cycle[0].query.ty_def_id
                     && let Some(def_id) = def_id.as_local()
@@ -204,13 +223,16 @@ impl<'tcx, T> Value<TyCtxt<'tcx>> for Result<T, &'_ ty::layout::LayoutError<'_>>
                     ) {
                         diag.note("a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future");
                     }
-                    break 'search diag.emit();
+
+                    ControlFlow::Break(diag)
                 } else {
-                    cycle.rotate_left(1);
+                    ControlFlow::Continue(())
                 }
-            }
-            report_cycle(tcx.sess, cycle_error).emit()
-        };
+            },
+            || report_cycle(tcx.sess, cycle_error),
+        );
+
+        let guar = diag.emit();
 
         // tcx.arena.alloc cannot be used because we are not allowed to use &'tcx LayoutError under
         // min_specialization. Since this is an error path anyways, leaking doesn't matter (and really,