[EXPERIMENT] Rewrite closure-of-async to async-closure

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 +1638,16 @@ 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, Ord, Eq, PartialOrd, Debug)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
 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 ()|`.

compiler/rustc_ast_lowering/src/expr.rs

@@ -219,18 +219,65 @@ 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(
+                            _,
+                            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,
+                                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 {

compiler/rustc_hir_typeck/src/closure.rs

@@ -14,7 +14,7 @@ use rustc_middle::span_bug;
 use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
 use rustc_middle::ty::{self, GenericArgs, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
 use rustc_span::def_id::LocalDefId;
-use rustc_span::Span;
+use rustc_span::{Span, DUMMY_SP};
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::error_reporting::traits::ArgKind;
 use rustc_trait_selection::traits;
@@ -563,25 +563,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return None;
         };
 
+        let mut return_ty = None;
+
         // FIXME: We may want to elaborate here, though I assume this will be exceedingly rare.
         for bound in self.obligations_for_self_ty(return_vid) {
             if let Some(ret_projection) = bound.predicate.as_projection_clause()
                 && let Some(ret_projection) = ret_projection.no_bound_vars()
                 && self.tcx.is_lang_item(ret_projection.def_id(), LangItem::FutureOutput)
             {
-                let sig = projection.rebind(self.tcx.mk_fn_sig(
-                    input_tys,
-                    ret_projection.term.expect_type(),
-                    false,
-                    hir::Safety::Safe,
-                    Abi::Rust,
-                ));
-
-                return Some(ExpectedSig { cause_span, sig });
+                return_ty = Some(ret_projection.term.expect_type());
             }
         }
 
-        None
+        let sig = projection.rebind(self.tcx.mk_fn_sig(
+            input_tys,
+            return_ty.unwrap_or_else(|| self.next_ty_var(cause_span.unwrap_or(DUMMY_SP))),
+            false,
+            hir::Safety::Safe,
+            Abi::Rust,
+        ));
+
+        return Some(ExpectedSig { cause_span, sig });
     }
 
     fn sig_of_closure(

tests/ui/async-await/async-closures/without-precise-captures-we-are-powerless.stderr

@@ -56,6 +56,11 @@ LL |     outlives::<'a>(call_once(c));
 LL |
 LL |     let c = async move || { println!("{}", *x.0); };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ move out of `x` occurs here
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL |     let c = async || { println!("{}", *x.0); }.clone();
+   |                                               ++++++++
 
 error[E0597]: `c` does not live long enough
   --> $DIR/without-precise-captures-we-are-powerless.rs:33:20

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`.

tests/ui/async-await/issue-69446-fnmut-capture.stderr

@@ -1,19 +1,16 @@
-error: captured variable cannot escape `FnMut` closure body
-  --> $DIR/issue-69446-fnmut-capture.rs:19:17
+error: async closure does not implement `FnMut` because it captures state from its environment
+  --> $DIR/issue-69446-fnmut-capture.rs:19:9
    |
-LL |       let mut x = Foo;
-   |           ----- variable defined here
-LL |       bar(move || async {
-   |  _______________-_^
-   | |               |
-   | |               inferred to be a `FnMut` closure
-LL | |         x.foo();
-   | |         - variable captured here
-LL | |     });
-   | |_____^ returns an `async` block that contains a reference to a captured variable, which then escapes the closure body
+LL |     bar(move || async {
+   |     --- ^^^^^^^
+   |     |
+   |     required by a bound introduced by this call
    |
-   = note: `FnMut` closures only have access to their captured variables while they are executing...
-   = note: ...therefore, they cannot allow references to captured variables to escape
+note: required by a bound in `bar`
+  --> $DIR/issue-69446-fnmut-capture.rs:12:25
+   |
+LL | async fn bar<T>(_: impl FnMut() -> T)
+   |                         ^^^^^^^^^^^^ required by this bound in `bar`
 
 error: aborting due to 1 previous error
 

tests/ui/async-await/issue-70935-complex-spans.stderr

@@ -13,6 +13,7 @@ note: required because it appears within the type `NotSync`
 LL | struct NotSync(PhantomData<*mut ()>);
    |        ^^^^^^^
    = note: required for `&NotSync` to implement `Send`
+   = note: required because it appears within the type `(&NotSync,)`
 note: required because it's used within this closure
   --> $DIR/issue-70935-complex-spans.rs:19:13
    |
@@ -46,6 +47,7 @@ note: required because it appears within the type `NotSync`
 LL | struct NotSync(PhantomData<*mut ()>);
    |        ^^^^^^^
    = note: required for `&NotSync` to implement `Send`
+   = note: required because it appears within the type `(&NotSync,)`
 note: required because it's used within this closure
   --> $DIR/issue-70935-complex-spans.rs:19:13
    |

tests/ui/impl-trait/opaque-cast-field-access-in-future.stderr

@@ -1,3 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/opaque-cast-field-access-in-future.rs:7:14
+   |
+LL |         &mut foo.bar;
+   |              ^^^ cannot infer type
+
 error[E0283]: type annotations needed
   --> $DIR/opaque-cast-field-access-in-future.rs:22:17
    |
@@ -6,6 +12,7 @@ LL | fn run() -> Foo<impl Future<Output = ()>> {
    |
    = note: cannot satisfy `_: Future`
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0283`.
+Some errors have detailed explanations: E0282, E0283.
+For more information about an error, try `rustc --explain E0282`.