Rollup merge of #127136 - compiler-errors:coroutine-closure-env-shim, r=oli-obk

Fix `FnMut::call_mut`/`Fn::call` shim for async closures that capture references

I adjusted async closures to be able to implement `Fn` and `FnMut` *even if* they capture references, as long as those references did not need to borrow data from the closure captures themselves. See #125259.

However, when I did this, I didn't actually relax an assertion in the `build_construct_coroutine_by_move_shim` shim code, which builds the `Fn`/`FnMut`/`FnOnce` implementations for async closures. Therefore, if we actually tried to *call* `FnMut`/`Fn` on async closures, it would ICE.

This PR adjusts this assertion to ensure that we only capture immutable references in closures if they implement `Fn`/`FnMut`. It also adds a bunch of tests and makes more of the async-closure tests into `build-pass` since we often care about these tests actually generating the right closure shims and stuff. I think it might be excessive to *always* use build-pass here, but 🤷 it's not that big of a deal.

Fixes #127019
Fixes #127012

r? oli-obk

compiler/rustc_codegen_ssa/src/mir/locals.rs

@@ -47,7 +47,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     let expected_ty = self.monomorphize(self.mir.local_decls[local].ty);
                     if expected_ty != op.layout.ty {
                         warn!(
-                            "Unexpected initial operand type: expected {expected_ty:?}, found {:?}.\
+                            "Unexpected initial operand type:\nexpected {expected_ty:?},\nfound    {:?}.\n\
                             See <https://github.com/rust-lang/rust/issues/114858>.",
                             op.layout.ty
                         );

compiler/rustc_mir_transform/src/shim.rs

@@ -1,18 +1,17 @@
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
+use rustc_index::{Idx, IndexVec};
 use rustc_middle::mir::*;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::GenericArgs;
 use rustc_middle::ty::{self, CoroutineArgs, CoroutineArgsExt, EarlyBinder, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
-use rustc_target::abi::{FieldIdx, VariantIdx, FIRST_VARIANT};
-
-use rustc_index::{Idx, IndexVec};
-
 use rustc_span::{source_map::Spanned, Span, DUMMY_SP};
+use rustc_target::abi::{FieldIdx, VariantIdx, FIRST_VARIANT};
 use rustc_target::spec::abi::Abi;
 
+use std::assert_matches::assert_matches;
 use std::fmt;
 use std::iter;
 
@@ -1020,21 +1019,19 @@ fn build_construct_coroutine_by_move_shim<'tcx>(
     receiver_by_ref: bool,
 ) -> Body<'tcx> {
     let mut self_ty = tcx.type_of(coroutine_closure_def_id).instantiate_identity();
+    let mut self_local: Place<'tcx> = Local::from_usize(1).into();
     let ty::CoroutineClosure(_, args) = *self_ty.kind() else {
         bug!();
     };
 
-    // We use `&mut Self` here because we only need to emit an ABI-compatible shim body,
-    // rather than match the signature exactly (which might take `&self` instead).
+    // We use `&Self` here because we only need to emit an ABI-compatible shim body,
+    // rather than match the signature exactly (which might take `&mut self` instead).
     //
-    // The self type here is a coroutine-closure, not a coroutine, and we never read from
-    // it because it never has any captures, because this is only true in the Fn/FnMut
-    // implementation, not the AsyncFn/AsyncFnMut implementation, which is implemented only
-    // if the coroutine-closure has no captures.
+    // We adjust the `self_local` to be a deref since we want to copy fields out of
+    // a reference to the closure.
     if receiver_by_ref {
-        // Triple-check that there's no captures here.
-        assert_eq!(args.as_coroutine_closure().tupled_upvars_ty(), tcx.types.unit);
-        self_ty = Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, self_ty);
+        self_local = tcx.mk_place_deref(self_local);
+        self_ty = Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, self_ty);
     }
 
     let poly_sig = args.as_coroutine_closure().coroutine_closure_sig().map_bound(|sig| {
@@ -1067,11 +1064,27 @@ fn build_construct_coroutine_by_move_shim<'tcx>(
         fields.push(Operand::Move(Local::from_usize(idx + 1).into()));
     }
     for (idx, ty) in args.as_coroutine_closure().upvar_tys().iter().enumerate() {
-        fields.push(Operand::Move(tcx.mk_place_field(
-            Local::from_usize(1).into(),
-            FieldIdx::from_usize(idx),
-            ty,
-        )));
+        if receiver_by_ref {
+            // The only situation where it's possible is when we capture immuatable references,
+            // since those don't need to be reborrowed with the closure's env lifetime. Since
+            // references are always `Copy`, just emit a copy.
+            assert_matches!(
+                ty.kind(),
+                ty::Ref(_, _, hir::Mutability::Not),
+                "field should be captured by immutable ref if we have an `Fn` instance"
+            );
+            fields.push(Operand::Copy(tcx.mk_place_field(
+                self_local,
+                FieldIdx::from_usize(idx),
+                ty,
+            )));
+        } else {
+            fields.push(Operand::Move(tcx.mk_place_field(
+                self_local,
+                FieldIdx::from_usize(idx),
+                ty,
+            )));
+        }
     }
 
     let source_info = SourceInfo::outermost(span);

compiler/rustc_symbol_mangling/src/legacy.rs

@@ -85,9 +85,13 @@ pub(super) fn mangle<'tcx>(
         }
         // FIXME(async_closures): This shouldn't be needed when we fix
         // `Instance::ty`/`Instance::def_id`.
-        ty::InstanceKind::ConstructCoroutineInClosureShim { .. }
-        | ty::InstanceKind::CoroutineKindShim { .. } => {
-            printer.write_str("{{fn-once-shim}}").unwrap();
+        ty::InstanceKind::ConstructCoroutineInClosureShim { receiver_by_ref, .. } => {
+            printer
+                .write_str(if receiver_by_ref { "{{by-move-shim}}" } else { "{{by-ref-shim}}" })
+                .unwrap();
+        }
+        ty::InstanceKind::CoroutineKindShim { .. } => {
+            printer.write_str("{{by-move-body-shim}}").unwrap();
         }
         _ => {}
     }

compiler/rustc_symbol_mangling/src/v0.rs

@@ -49,8 +49,15 @@ pub(super) fn mangle<'tcx>(
         ty::InstanceKind::ReifyShim(_, Some(ReifyReason::FnPtr)) => Some("reify_fnptr"),
         ty::InstanceKind::ReifyShim(_, Some(ReifyReason::Vtable)) => Some("reify_vtable"),
 
-        ty::InstanceKind::ConstructCoroutineInClosureShim { .. }
-        | ty::InstanceKind::CoroutineKindShim { .. } => Some("fn_once"),
+        // FIXME(async_closures): This shouldn't be needed when we fix
+        // `Instance::ty`/`Instance::def_id`.
+        ty::InstanceKind::ConstructCoroutineInClosureShim { receiver_by_ref: true, .. } => {
+            Some("by_move")
+        }
+        ty::InstanceKind::ConstructCoroutineInClosureShim { receiver_by_ref: false, .. } => {
+            Some("by_ref")
+        }
+        ty::InstanceKind::CoroutineKindShim { .. } => Some("by_move_body"),
 
         _ => None,
     };

compiler/rustc_ty_utils/src/abi.rs

@@ -127,9 +127,9 @@ fn fn_sig_for_fn_abi<'tcx>(
                     coroutine_kind = ty::ClosureKind::FnOnce;
 
                     // Implementations of `FnMut` and `Fn` for coroutine-closures
-                    // still take their receiver by (mut) ref.
+                    // still take their receiver by ref.
                     if receiver_by_ref {
-                        Ty::new_mut_ref(tcx, tcx.lifetimes.re_erased, coroutine_ty)
+                        Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, coroutine_ty)
                     } else {
                         coroutine_ty
                     }

src/tools/miri/tests/pass/async-closure.rs

@@ -1,7 +1,8 @@
 #![feature(async_closure, noop_waker, async_fn_traits)]
+#![allow(unused)]
 
 use std::future::Future;
-use std::ops::{AsyncFnMut, AsyncFnOnce};
+use std::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
 use std::pin::pin;
 use std::task::*;
 
@@ -17,6 +18,10 @@ pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
     }
 }
 
+async fn call(f: &mut impl AsyncFn(i32)) {
+    f(0).await;
+}
+
 async fn call_mut(f: &mut impl AsyncFnMut(i32)) {
     f(0).await;
 }
@@ -26,10 +31,10 @@ async fn call_once(f: impl AsyncFnOnce(i32)) {
 }
 
 async fn call_normal<F: Future<Output = ()>>(f: &impl Fn(i32) -> F) {
-    f(0).await;
+    f(1).await;
 }
 
-async fn call_normal_once<F: Future<Output = ()>>(f: impl FnOnce(i32) -> F) {
+async fn call_normal_mut<F: Future<Output = ()>>(f: &mut impl FnMut(i32) -> F) {
     f(1).await;
 }
 
@@ -39,14 +44,16 @@ pub fn main() {
         let mut async_closure = async move |a: i32| {
             println!("{a} {b}");
         };
+        call(&mut async_closure).await;
         call_mut(&mut async_closure).await;
         call_once(async_closure).await;
 
-        // No-capture closures implement `Fn`.
-        let async_closure = async move |a: i32| {
-            println!("{a}");
+        let b = 2i32;
+        let mut async_closure = async |a: i32| {
+            println!("{a} {b}");
         };
         call_normal(&async_closure).await;
-        call_normal_once(async_closure).await;
+        call_normal_mut(&mut async_closure).await;
+        call_once(async_closure).await;
     });
 }

src/tools/miri/tests/pass/async-closure.stdout

@@ -1,4 +1,6 @@
 0 2
+0 2
+1 2
+1 2
+1 2
 1 2
-0
-1

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#0}-{closure#0}.coroutine_by_move.0.panic-abort.mir

@@ -1,6 +1,6 @@
 // MIR for `main::{closure#0}::{closure#0}::{closure#0}` 0 coroutine_by_move
 
-fn main::{closure#0}::{closure#0}::{closure#0}(_1: {async closure body@$DIR/async_closure_shims.rs:42:53: 45:10}, _2: ResumeTy) -> ()
+fn main::{closure#0}::{closure#0}::{closure#0}(_1: {async closure body@$DIR/async_closure_shims.rs:53:53: 56:10}, _2: ResumeTy) -> ()
 yields ()
  {
     debug _task_context => _2;

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#0}-{closure#0}.coroutine_by_move.0.panic-unwind.mir

@@ -1,6 +1,6 @@
 // MIR for `main::{closure#0}::{closure#0}::{closure#0}` 0 coroutine_by_move
 
-fn main::{closure#0}::{closure#0}::{closure#0}(_1: {async closure body@$DIR/async_closure_shims.rs:42:53: 45:10}, _2: ResumeTy) -> ()
+fn main::{closure#0}::{closure#0}::{closure#0}(_1: {async closure body@$DIR/async_closure_shims.rs:53:53: 56:10}, _2: ResumeTy) -> ()
 yields ()
  {
     debug _task_context => _2;

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#0}.coroutine_closure_by_move.0.panic-abort.mir

@@ -1,10 +1,10 @@
 // MIR for `main::{closure#0}::{closure#0}` 0 coroutine_closure_by_move
 
-fn main::{closure#0}::{closure#0}(_1: {async closure@$DIR/async_closure_shims.rs:42:33: 42:52}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:42:53: 45:10} {
-    let mut _0: {async closure body@$DIR/async_closure_shims.rs:42:53: 45:10};
+fn main::{closure#0}::{closure#0}(_1: {async closure@$DIR/async_closure_shims.rs:53:33: 53:52}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:53:53: 56:10} {
+    let mut _0: {async closure body@$DIR/async_closure_shims.rs:53:53: 56:10};
 
     bb0: {
-        _0 = {coroutine@$DIR/async_closure_shims.rs:42:53: 45:10 (#0)} { a: move _2, b: move (_1.0: i32) };
+        _0 = {coroutine@$DIR/async_closure_shims.rs:53:53: 56:10 (#0)} { a: move _2, b: move (_1.0: i32) };
         return;
     }
 }

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#0}.coroutine_closure_by_move.0.panic-unwind.mir

@@ -1,10 +1,10 @@
 // MIR for `main::{closure#0}::{closure#0}` 0 coroutine_closure_by_move
 
-fn main::{closure#0}::{closure#0}(_1: {async closure@$DIR/async_closure_shims.rs:42:33: 42:52}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:42:53: 45:10} {
-    let mut _0: {async closure body@$DIR/async_closure_shims.rs:42:53: 45:10};
+fn main::{closure#0}::{closure#0}(_1: {async closure@$DIR/async_closure_shims.rs:53:33: 53:52}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:53:53: 56:10} {
+    let mut _0: {async closure body@$DIR/async_closure_shims.rs:53:53: 56:10};
 
     bb0: {
-        _0 = {coroutine@$DIR/async_closure_shims.rs:42:53: 45:10 (#0)} { a: move _2, b: move (_1.0: i32) };
+        _0 = {coroutine@$DIR/async_closure_shims.rs:53:53: 56:10 (#0)} { a: move _2, b: move (_1.0: i32) };
         return;
     }
 }

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}-{closure#0}.coroutine_by_move.0.panic-abort.mir

@@ -0,0 +1,47 @@
+// MIR for `main::{closure#0}::{closure#1}::{closure#0}` 0 coroutine_by_move
+
+fn main::{closure#0}::{closure#1}::{closure#0}(_1: {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10}, _2: ResumeTy) -> ()
+yields ()
+ {
+    debug _task_context => _2;
+    debug a => (_1.0: i32);
+    debug b => (*(_1.1: &i32));
+    let mut _0: ();
+    let _3: i32;
+    scope 1 {
+        debug a => _3;
+        let _4: &i32;
+        scope 2 {
+            debug a => _4;
+            let _5: &i32;
+            scope 3 {
+                debug b => _5;
+            }
+        }
+    }
+
+    bb0: {
+        StorageLive(_3);
+        _3 = (_1.0: i32);
+        FakeRead(ForLet(None), _3);
+        StorageLive(_4);
+        _4 = &_3;
+        FakeRead(ForLet(None), _4);
+        StorageLive(_5);
+        _5 = &(*(_1.1: &i32));
+        FakeRead(ForLet(None), _5);
+        _0 = const ();
+        StorageDead(_5);
+        StorageDead(_4);
+        StorageDead(_3);
+        drop(_1) -> [return: bb1, unwind: bb2];
+    }
+
+    bb1: {
+        return;
+    }
+
+    bb2 (cleanup): {
+        resume;
+    }
+}

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}-{closure#0}.coroutine_by_move.0.panic-unwind.mir

@@ -0,0 +1,47 @@
+// MIR for `main::{closure#0}::{closure#1}::{closure#0}` 0 coroutine_by_move
+
+fn main::{closure#0}::{closure#1}::{closure#0}(_1: {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10}, _2: ResumeTy) -> ()
+yields ()
+ {
+    debug _task_context => _2;
+    debug a => (_1.0: i32);
+    debug b => (*(_1.1: &i32));
+    let mut _0: ();
+    let _3: i32;
+    scope 1 {
+        debug a => _3;
+        let _4: &i32;
+        scope 2 {
+            debug a => _4;
+            let _5: &i32;
+            scope 3 {
+                debug b => _5;
+            }
+        }
+    }
+
+    bb0: {
+        StorageLive(_3);
+        _3 = (_1.0: i32);
+        FakeRead(ForLet(None), _3);
+        StorageLive(_4);
+        _4 = &_3;
+        FakeRead(ForLet(None), _4);
+        StorageLive(_5);
+        _5 = &(*(_1.1: &i32));
+        FakeRead(ForLet(None), _5);
+        _0 = const ();
+        StorageDead(_5);
+        StorageDead(_4);
+        StorageDead(_3);
+        drop(_1) -> [return: bb1, unwind: bb2];
+    }
+
+    bb1: {
+        return;
+    }
+
+    bb2 (cleanup): {
+        resume;
+    }
+}

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_move.0.panic-abort.mir

@@ -0,0 +1,10 @@
+// MIR for `main::{closure#0}::{closure#1}` 0 coroutine_closure_by_move
+
+fn main::{closure#0}::{closure#1}(_1: {async closure@$DIR/async_closure_shims.rs:62:33: 62:47}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10} {
+    let mut _0: {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10};
+
+    bb0: {
+        _0 = {coroutine@$DIR/async_closure_shims.rs:62:48: 65:10 (#0)} { a: move _2, b: move (_1.0: &i32) };
+        return;
+    }
+}

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_move.0.panic-unwind.mir

@@ -0,0 +1,10 @@
+// MIR for `main::{closure#0}::{closure#1}` 0 coroutine_closure_by_move
+
+fn main::{closure#0}::{closure#1}(_1: {async closure@$DIR/async_closure_shims.rs:62:33: 62:47}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10} {
+    let mut _0: {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10};
+
+    bb0: {
+        _0 = {coroutine@$DIR/async_closure_shims.rs:62:48: 65:10 (#0)} { a: move _2, b: move (_1.0: &i32) };
+        return;
+    }
+}

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_ref.0.panic-abort.mir

@@ -1,10 +1,10 @@
 // MIR for `main::{closure#0}::{closure#1}` 0 coroutine_closure_by_ref
 
-fn main::{closure#0}::{closure#1}(_1: &mut {async closure@$DIR/async_closure_shims.rs:49:29: 49:48}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:49:49: 51:10} {
-    let mut _0: {async closure body@$DIR/async_closure_shims.rs:49:49: 51:10};
+fn main::{closure#0}::{closure#1}(_1: &{async closure@$DIR/async_closure_shims.rs:62:33: 62:47}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10} {
+    let mut _0: {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10};
 
     bb0: {
-        _0 = {coroutine@$DIR/async_closure_shims.rs:49:49: 51:10 (#0)} { a: move _2 };
+        _0 = {coroutine@$DIR/async_closure_shims.rs:62:48: 65:10 (#0)} { a: move _2, b: ((*_1).0: &i32) };
         return;
     }
 }

tests/mir-opt/async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_ref.0.panic-unwind.mir

@@ -1,10 +1,10 @@
 // MIR for `main::{closure#0}::{closure#1}` 0 coroutine_closure_by_ref
 
-fn main::{closure#0}::{closure#1}(_1: &mut {async closure@$DIR/async_closure_shims.rs:49:29: 49:48}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:49:49: 51:10} {
-    let mut _0: {async closure body@$DIR/async_closure_shims.rs:49:49: 51:10};
+fn main::{closure#0}::{closure#1}(_1: &{async closure@$DIR/async_closure_shims.rs:62:33: 62:47}, _2: i32) -> {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10} {
+    let mut _0: {async closure body@$DIR/async_closure_shims.rs:62:48: 65:10};
 
     bb0: {
-        _0 = {coroutine@$DIR/async_closure_shims.rs:49:49: 51:10 (#0)} { a: move _2 };
+        _0 = {coroutine@$DIR/async_closure_shims.rs:62:48: 65:10 (#0)} { a: move _2, b: ((*_1).0: &i32) };
         return;
     }
 }

tests/mir-opt/async_closure_shims.rs

@@ -3,9 +3,10 @@
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 
 #![feature(async_closure, noop_waker, async_fn_traits)]
+#![allow(unused)]
 
 use std::future::Future;
-use std::ops::{AsyncFnMut, AsyncFnOnce};
+use std::ops::{AsyncFn, AsyncFnMut, AsyncFnOnce};
 use std::pin::pin;
 use std::task::*;
 
@@ -21,6 +22,10 @@ pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
     }
 }
 
+async fn call(f: &mut impl AsyncFn(i32)) {
+    f(0).await;
+}
+
 async fn call_mut(f: &mut impl AsyncFnMut(i32)) {
     f(0).await;
 }
@@ -33,9 +38,15 @@ async fn call_normal<F: Future<Output = ()>>(f: &impl Fn(i32) -> F) {
     f(1).await;
 }
 
+async fn call_normal_mut<F: Future<Output = ()>>(f: &mut impl FnMut(i32) -> F) {
+    f(1).await;
+}
+
 // EMIT_MIR async_closure_shims.main-{closure#0}-{closure#0}.coroutine_closure_by_move.0.mir
 // EMIT_MIR async_closure_shims.main-{closure#0}-{closure#0}-{closure#0}.coroutine_by_move.0.mir
 // EMIT_MIR async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_ref.0.mir
+// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#1}.coroutine_closure_by_move.0.mir
+// EMIT_MIR async_closure_shims.main-{closure#0}-{closure#1}-{closure#0}.coroutine_by_move.0.mir
 pub fn main() {
     block_on(async {
         let b = 2i32;
@@ -43,12 +54,17 @@ pub fn main() {
             let a = &a;
             let b = &b;
         };
+        call(&mut async_closure).await;
         call_mut(&mut async_closure).await;
         call_once(async_closure).await;
 
-        let async_closure = async move |a: i32| {
+        let b = 2i32;
+        let mut async_closure = async |a: i32| {
             let a = &a;
+            let b = &b;
         };
         call_normal(&async_closure).await;
+        call_normal_mut(&mut async_closure).await;
+        call_once(async_closure).await;
     });
 }

tests/ui/async-await/async-closures/constrained-but-no-upvars-yet.rs

@@ -1,5 +1,5 @@
 //@ edition: 2021
-//@ check-pass
+//@ build-pass
 //@ revisions: current next
 //@ ignore-compare-mode-next-solver (explicit revisions)
 //@[next] compile-flags: -Znext-solver

tests/ui/async-await/async-closures/force-move-due-to-actually-fnonce.rs

@@ -1,6 +1,6 @@
 //@ aux-build:block-on.rs
 //@ edition:2021
-//@ check-pass
+//@ build-pass
 
 #![feature(async_closure)]
 

tests/ui/async-await/async-closures/force-move-due-to-inferred-kind.rs

@@ -1,6 +1,6 @@
 //@ aux-build:block-on.rs
 //@ edition:2021
-//@ check-pass
+//@ build-pass
 
 #![feature(async_closure)]
 

tests/ui/async-await/async-closures/implements-fnmut.rs

@@ -1,4 +1,4 @@
-//@ check-pass
+//@ build-pass
 //@ edition: 2021
 
 // Demonstrates that an async closure may implement `FnMut` (not just `async FnMut`!)
@@ -9,9 +9,13 @@
 
 #![feature(async_closure)]
 
-fn main() {}
+fn main() {
+    hello(&Ty);
+}
 
-fn needs_fn_mut<T>(x: impl FnMut() -> T) {}
+fn needs_fn_mut<T>(mut x: impl FnMut() -> T) {
+    x();
+}
 
 fn hello(x: &Ty) {
     needs_fn_mut(async || { x.hello(); });

tests/ui/async-await/async-closures/signature-deduction.rs

@@ -1,4 +1,4 @@
-//@ check-pass
+//@ build-pass
 //@ edition: 2021
 
 #![feature(async_closure)]