Better recursive async fn error message.

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

src/librustc_typeck/check/mod.rs

@@ -1325,8 +1325,23 @@ fn check_union(tcx: TyCtxt<'_>, id: hir::HirId, span: Span) {
     check_packed(tcx, span, def_id);
 }
 
-fn check_opaque<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, substs: SubstsRef<'tcx>, span: Span) {
+fn check_opaque<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def_id: DefId,
+    substs: SubstsRef<'tcx>,
+    span: Span,
+    origin: &hir::ExistTyOrigin
+) {
     if let Err(partially_expanded_type) = tcx.try_expand_impl_trait_type(def_id, substs) {
+        if let hir::ExistTyOrigin::AsyncFn = origin {
+            struct_span_err!(
+                tcx.sess, span, E0733,
+                "recursion in an `async fn` requires boxing",
+            )
+            .span_label(span, "an `async fn` cannot invoke itself directly")
+            .note("a recursive `async fn` must be rewritten to return a boxed future.")
+            .emit();
+        } else {
             let mut err = struct_span_err!(
                 tcx.sess, span, E0720,
                 "opaque type expands to a recursive type",
@@ -1339,6 +1354,7 @@ fn check_opaque<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, substs: SubstsRef<'tcx>,
             }
             err.emit();
         }
+    }
 }
 
 pub fn check_item_type<'tcx>(tcx: TyCtxt<'tcx>, it: &'tcx hir::Item) {
@@ -1387,11 +1403,11 @@ pub fn check_item_type<'tcx>(tcx: TyCtxt<'tcx>, it: &'tcx hir::Item) {
         hir::ItemKind::Union(..) => {
             check_union(tcx, it.hir_id, it.span);
         }
-        hir::ItemKind::Existential(..) => {
+        hir::ItemKind::Existential(hir::ExistTy{origin, ..}) => {
             let def_id = tcx.hir().local_def_id(it.hir_id);
 
             let substs = InternalSubsts::identity_for_item(tcx, def_id);
-            check_opaque(tcx, def_id, substs, it.span);
+            check_opaque(tcx, def_id, substs, it.span, &origin);
         }
         hir::ItemKind::Ty(..) => {
             let def_id = tcx.hir().local_def_id(it.hir_id);

src/librustc_typeck/error_codes.rs

@@ -4765,7 +4765,53 @@ assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
 ```
 "##,
 
+E0733: r##"
+Recursion in an `async fn` requires boxing. For example, this will not compile:
+
+```edition2018,compile_fail,E0733
+#![feature(async_await)]
+async fn foo(n: usize) {
+    if n > 0 {
+        foo(n - 1).await;
+    }
 }
+```
+
+To achieve async recursion, the `async fn` needs to be desugared
+such that the `Future` is explicit in the return type:
+
+```edition2018,compile_fail,E0720
+# #![feature(async_await)]
+use std::future::Future;
+fn foo_desugered(n: usize) -> impl Future<Output = ()> {
+    async move {
+        if n > 0 {
+            foo_desugered(n - 1).await;
+        }
+    }
+}
+```
+
+Finally, the future is wrapped in a pinned box:
+
+```edition2018
+# #![feature(async_await)]
+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;
+        }
+    })
+}
+```
+
+The `Box<...>` ensures that the result is of known size,
+and the pin is required to keep it in the same place in memory.
+"##,
+
+}  // (end of detailed error messages)
 
 register_diagnostics! {
 //  E0035, merged into E0087/E0089

src/test/ui/async-await/recursive-async-impl-trait-type.stderr

@@ -1,11 +1,11 @@
-error[E0720]: opaque type expands to a recursive type
+error[E0733]: recursion in an `async fn` requires boxing
   --> $DIR/recursive-async-impl-trait-type.rs:7:40
    |
 LL | async fn recursive_async_function() -> () {
-   |                                        ^^ expands to a recursive type
+   |                                        ^^ an `async fn` cannot invoke itself directly
    |
-   = note: expanded type is `std::future::GenFuture<[static generator@$DIR/recursive-async-impl-trait-type.rs:7:43: 9:2 {impl std::future::Future, ()}]>`
+   = note: a recursive `async fn` must be rewritten to return a boxed future.
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0720`.
+For more information about this error, try `rustc --explain E0733`.