Rework try_overloaded_call_traits to not prefer taking async closure …

…by move

compiler/rustc_hir_typeck/src/callee.rs

@@ -183,16 +183,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     kind: TypeVariableOriginKind::TypeInference,
                     span: callee_expr.span,
                 });
+                // We may actually receive a coroutine back whose kind is different
+                // from the closure that this dispatched from. This is because when
+                // we have no captures, we automatically implement `FnOnce`. This
+                // impl forces the closure kind to `FnOnce` i.e. `u8`.
+                let kind_ty = self.next_ty_var(TypeVariableOrigin {
+                    kind: TypeVariableOriginKind::TypeInference,
+                    span: callee_expr.span,
+                });
                 let call_sig = self.tcx.mk_fn_sig(
                     [coroutine_closure_sig.tupled_inputs_ty],
                     coroutine_closure_sig.to_coroutine(
                         self.tcx,
                         closure_args.parent_args(),
-                        // Inherit the kind ty of the closure, since we're calling this
-                        // coroutine with the most relaxed `AsyncFn*` trait that we can.
-                        // We don't necessarily need to do this here, but it saves us
-                        // computing one more infer var that will get constrained later.
-                        closure_args.kind_ty(),
+                        kind_ty,
                         self.tcx.coroutine_for_closure(def_id),
                         tupled_upvars_ty,
                     ),
@@ -263,14 +267,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Try the options that are least restrictive on the caller first.
         for (opt_trait_def_id, method_name, borrow) in [
             (self.tcx.lang_items().fn_trait(), Ident::with_dummy_span(sym::call), true),
-            (self.tcx.lang_items().fn_mut_trait(), Ident::with_dummy_span(sym::call_mut), true),
-            (self.tcx.lang_items().fn_once_trait(), Ident::with_dummy_span(sym::call_once), false),
             (self.tcx.lang_items().async_fn_trait(), Ident::with_dummy_span(sym::async_call), true),
+            (self.tcx.lang_items().fn_mut_trait(), Ident::with_dummy_span(sym::call_mut), true),
             (
                 self.tcx.lang_items().async_fn_mut_trait(),
                 Ident::with_dummy_span(sym::async_call_mut),
                 true,
             ),
+            (self.tcx.lang_items().fn_once_trait(), Ident::with_dummy_span(sym::call_once), false),
             (
                 self.tcx.lang_items().async_fn_once_trait(),
                 Ident::with_dummy_span(sym::async_call_once),

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -336,11 +336,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let is_const = self.tcx().is_const_fn_raw(def_id);
                 match self.infcx.closure_kind(self_ty) {
                     Some(closure_kind) => {
-                        let no_borrows = self
+                        let no_borrows = match self
                             .infcx
                             .shallow_resolve(args.as_coroutine_closure().tupled_upvars_ty())
-                            .tuple_fields()
-                            .is_empty();
+                            .kind()
+                        {
+                            ty::Tuple(tys) => tys.is_empty(),
+                            ty::Error(_) => false,
+                            _ => bug!("tuple_fields called on non-tuple"),
+                        };
                         if no_borrows && closure_kind.extends(kind) {
                             candidates.vec.push(ClosureCandidate { is_const });
                         } else if kind == ty::ClosureKind::FnOnce {

tests/ui/async-await/async-closures/def-path.stderr

@@ -5,11 +5,11 @@ LL |     let x = async || {};
    |                      -- the expected `async` closure body
 LL |
 LL |     let () = x();
-   |         ^^   --- this expression has type `{static main::{closure#0}::{closure#0}<?7t> upvar_tys=?15t witness=?6t}`
+   |         ^^   --- this expression has type `{static main::{closure#0}::{closure#0}<?16t> upvar_tys=?15t witness=?6t}`
    |         |
    |         expected `async` closure body, found `()`
    |
-   = note: expected `async` closure body `{static main::{closure#0}::{closure#0}<?7t> upvar_tys=?15t witness=?6t}`
+   = note: expected `async` closure body `{static main::{closure#0}::{closure#0}<?16t> upvar_tys=?15t witness=?6t}`
                          found unit type `()`
 
 error: aborting due to 1 previous error

tests/ui/async-await/async-closures/is-not-fn.rs

@@ -5,8 +5,5 @@
 fn main() {
     fn needs_fn(x: impl FnOnce()) {}
     needs_fn(async || {});
-    //~^ ERROR expected a `FnOnce()` closure, found `{coroutine-closure@
-    // FIXME(async_closures): This should explain in more detail how async fns don't
-    // implement the regular `Fn` traits. Or maybe we should just fix it and make them
-    // when there are no upvars or whatever.
+    //~^ ERROR expected `{coroutine-closure@is-not-fn.rs:7:14}` to be a closure that returns `()`
 }

tests/ui/async-await/async-closures/is-not-fn.stderr

@@ -1,13 +1,13 @@
-error[E0277]: expected a `FnOnce()` closure, found `{coroutine-closure@$DIR/is-not-fn.rs:7:14: 7:22}`
+error[E0271]: expected `{coroutine-closure@is-not-fn.rs:7:14}` to be a closure that returns `()`, but it returns `{async closure body@$DIR/is-not-fn.rs:7:23: 7:25}`
   --> $DIR/is-not-fn.rs:7:14
    |
 LL |     needs_fn(async || {});
-   |     -------- ^^^^^^^^^^^ expected an `FnOnce()` closure, found `{coroutine-closure@$DIR/is-not-fn.rs:7:14: 7:22}`
+   |     -------- ^^^^^^^^^^^ expected `()`, found `async` closure body
    |     |
    |     required by a bound introduced by this call
    |
-   = help: the trait `FnOnce<()>` is not implemented for `{coroutine-closure@$DIR/is-not-fn.rs:7:14: 7:22}`
-   = note: wrap the `{coroutine-closure@$DIR/is-not-fn.rs:7:14: 7:22}` in a closure with no arguments: `|| { /* code */ }`
+   = note:         expected unit type `()`
+           found `async` closure body `{async closure body@$DIR/is-not-fn.rs:7:23: 7:25}`
 note: required by a bound in `needs_fn`
   --> $DIR/is-not-fn.rs:6:25
    |
@@ -16,4 +16,4 @@ LL |     fn needs_fn(x: impl FnOnce()) {}
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0271`.

tests/ui/async-await/async-closures/move-consuming-capture.stderr

@@ -5,12 +5,15 @@ LL |         let x = async move || {
    |             - move occurs because `x` has type `{coroutine-closure@$DIR/move-consuming-capture.rs:13:17: 13:30}`, which does not implement the `Copy` trait
 ...
 LL |         x().await;
-   |         --- `x` moved due to this method call
+   |         --- `x` moved due to this call
 LL |         x().await;
    |         ^ value used here after move
    |
-note: `async_call_once` takes ownership of the receiver `self`, which moves `x`
-  --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
+note: this value implements `FnOnce`, which causes it to be moved when called
+  --> $DIR/move-consuming-capture.rs:16:9
+   |
+LL |         x().await;
+   |         ^
 
 error: aborting due to 1 previous error
 

tests/ui/async-await/async-fn/dyn-pos.stderr

@@ -8,6 +8,9 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
    |
    = note: the trait cannot be made into an object because it contains the generic associated type `CallFuture`
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFn` for this new enum and using it instead:
+             &F
+             std::boxed::Box<F, A>
 
 error[E0038]: the trait `AsyncFnMut` cannot be made into an object
   --> $DIR/dyn-pos.rs:5:16
@@ -19,6 +22,10 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
    |
    = note: the trait cannot be made into an object because it contains the generic associated type `CallMutFuture`
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFnMut` for this new enum and using it instead:
+             &F
+             &mut F
+             std::boxed::Box<F, A>
 
 error[E0038]: the trait `AsyncFn` cannot be made into an object
   --> $DIR/dyn-pos.rs:5:16
@@ -30,6 +37,9 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
    |
    = note: the trait cannot be made into an object because it contains the generic associated type `CallFuture`
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFn` for this new enum and using it instead:
+             &F
+             std::boxed::Box<F, A>
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0038]: the trait `AsyncFnMut` cannot be made into an object
@@ -42,6 +52,10 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
    |
    = note: the trait cannot be made into an object because it contains the generic associated type `CallMutFuture`
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFnMut` for this new enum and using it instead:
+             &F
+             &mut F
+             std::boxed::Box<F, A>
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0038]: the trait `AsyncFn` cannot be made into an object
@@ -54,6 +68,9 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
    |
    = note: the trait cannot be made into an object because it contains the generic associated type `CallFuture`
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFn` for this new enum and using it instead:
+             &F
+             std::boxed::Box<F, A>
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0038]: the trait `AsyncFnMut` cannot be made into an object
@@ -66,6 +83,10 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
   --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
    |
    = note: the trait cannot be made into an object because it contains the generic associated type `CallMutFuture`
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFnMut` for this new enum and using it instead:
+             &F
+             &mut F
+             std::boxed::Box<F, A>
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error[E0038]: the trait `AsyncFn` cannot be made into an object
@@ -81,6 +102,9 @@ note: for a trait to be "object safe" it needs to allow building a vtable to all
   ::: $SRC_DIR/core/src/ops/async_function.rs:LL:COL
    |
    = note: the trait cannot be made into an object because it contains the generic associated type `CallMutFuture`
+   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFn` for this new enum and using it instead:
+             &F
+             std::boxed::Box<F, A>
 
 error: aborting due to 7 previous errors