Rewrite closure-of-async to async-closure

Author: compiler-errors

Diff for: compiler/rustc_ast/src/ast.rs

@@ -1214,6 +1214,30 @@ impl Expr {
         }
     }
 
+    pub fn peel_uwu(&self) -> &Expr {
+        let mut expr = self;
+        loop {
+            match &expr.kind {
+                ExprKind::Block(blk, None) => {
+                    if blk.stmts.len() == 1
+                        && let StmtKind::Expr(blk) = &blk.stmts[0].kind
+                    {
+                        expr = blk;
+                    } else {
+                        break;
+                    }
+                }
+                ExprKind::Paren(paren) => {
+                    expr = paren;
+                }
+                _ => {
+                    break;
+                }
+            }
+        }
+        expr
+    }
+
     pub fn peel_parens(&self) -> &Expr {
         let mut expr = self;
         while let ExprKind::Paren(inner) = &expr.kind {
@@ -1614,15 +1638,15 @@ pub struct QSelf {
 }
 
 /// A capture clause used in closures and `async` blocks.
-#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic, Ord, PartialOrd)]
 pub enum CaptureBy {
+    /// `move` keyword was not specified.
+    Ref,
     /// `move |x| y + x`.
     Value {
         /// The span of the `move` keyword.
         move_kw: Span,
     },
-    /// `move` keyword was not specified.
-    Ref,
 }
 
 /// Closure lifetime binder, `for<'a, 'b>` in `for<'a, 'b> |_: &'a (), _: &'b ()|`.

Diff for: compiler/rustc_ast_lowering/src/expr.rs

@@ -218,18 +218,53 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         *fn_decl_span,
                         *fn_arg_span,
                     ),
-                    None => self.lower_expr_closure(
-                        binder,
-                        *capture_clause,
-                        e.id,
-                        hir_id,
-                        *constness,
-                        *movability,
-                        fn_decl,
-                        body,
-                        *fn_decl_span,
-                        *fn_arg_span,
-                    ),
+                    None => {
+                        let peeled = body.peel_uwu();
+                        if let ast::ExprKind::Gen(
+                            gen_capture_clause,
+                            block,
+                            gen_kind @ ast::GenBlockKind::Async,
+                            span,
+                        ) = &peeled.kind
+                        {
+                            let coroutine_kind = match gen_kind {
+                                GenBlockKind::Async => CoroutineKind::Async { span: *span, closure_id: peeled.node_id(), return_impl_trait_id: self.next_node_id() },
+                                GenBlockKind::Gen => CoroutineKind::Gen { span: *span, closure_id: peeled.node_id(), return_impl_trait_id: self.next_node_id() },
+                                GenBlockKind::AsyncGen => CoroutineKind::AsyncGen { span: *span, closure_id: peeled.node_id(), return_impl_trait_id: self.next_node_id() },
+                            };
+                            let id = self.next_node_id();
+                            self.lower_expr_coroutine_closure(
+                                binder,
+                                capture_clause.max(*gen_capture_clause),
+                                e.id,
+                                hir_id,
+                                coroutine_kind,
+                                fn_decl,
+                                &ast::Expr {
+                                    id,
+                                    span: *span,
+                                    kind: ExprKind::Block(block.clone(), None),
+                                    attrs: thin_vec![],
+                                    tokens: None,
+                                },
+                                *fn_decl_span,
+                                *fn_arg_span,
+                            )
+                        } else {
+                            self.lower_expr_closure(
+                                binder,
+                                *capture_clause,
+                                e.id,
+                                hir_id,
+                                *constness,
+                                *movability,
+                                fn_decl,
+                                body,
+                                *fn_decl_span,
+                                *fn_arg_span,
+                            )
+                        }
+                    }
                 },
                 ExprKind::Gen(capture_clause, block, genblock_kind, decl_span) => {
                     let desugaring_kind = match genblock_kind {

Diff for: tests/ui/async-await/async-closures/wrong-fn-kind.stderr

@@ -1,24 +1,3 @@
-error[E0525]: expected a closure that implements the `async Fn` trait, but this closure only implements `async FnOnce`
-  --> $DIR/wrong-fn-kind.rs:17:20
-   |
-LL |       needs_async_fn(move || async move {
-   |       -------------- -^^^^^^
-   |       |              |
-   |  _____|______________this closure implements `async FnOnce`, not `async Fn`
-   | |     |
-   | |     required by a bound introduced by this call
-LL | |
-LL | |         println!("{x}");
-   | |                    - closure is `async FnOnce` because it moves the variable `x` out of its environment
-LL | |     });
-   | |_____- the requirement to implement `async Fn` derives from here
-   |
-note: required by a bound in `needs_async_fn`
-  --> $DIR/wrong-fn-kind.rs:5:27
-   |
-LL | fn needs_async_fn(_: impl async Fn()) {}
-   |                           ^^^^^^^^^^ required by this bound in `needs_async_fn`
-
 error[E0596]: cannot borrow `x` as mutable, as it is a captured variable in a `Fn` closure
   --> $DIR/wrong-fn-kind.rs:9:20
    |
@@ -35,7 +14,6 @@ LL |
 LL |         x += 1;
    |         - mutable borrow occurs due to use of `x` in closure
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-Some errors have detailed explanations: E0525, E0596.
-For more information about an error, try `rustc --explain E0525`.
+For more information about this error, try `rustc --explain E0596`.