Don't rely on upvars being assigned just because coroutine-closure kind is assigned

compiler/rustc_middle/src/ty/sty.rs

@@ -2231,7 +2231,7 @@ impl<'tcx> Ty<'tcx> {
     pub fn tuple_fields(self) -> &'tcx List<Ty<'tcx>> {
         match self.kind() {
             Tuple(args) => args,
-            _ => bug!("tuple_fields called on non-tuple"),
+            _ => bug!("tuple_fields called on non-tuple: {self:?}"),
         }
     }
 

compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs

@@ -292,7 +292,9 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_callable<'tcx>(
             let kind_ty = args.kind_ty();
             let sig = args.coroutine_closure_sig().skip_binder();
 
-            let coroutine_ty = if let Some(closure_kind) = kind_ty.to_opt_closure_kind() {
+            let coroutine_ty = if let Some(closure_kind) = kind_ty.to_opt_closure_kind()
+                && !args.tupled_upvars_ty().is_ty_var()
+            {
                 if !closure_kind.extends(goal_kind) {
                     return Err(NoSolution);
                 }
@@ -401,7 +403,9 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_async_callable<'tc
             let kind_ty = args.kind_ty();
             let sig = args.coroutine_closure_sig().skip_binder();
             let mut nested = vec![];
-            let coroutine_ty = if let Some(closure_kind) = kind_ty.to_opt_closure_kind() {
+            let coroutine_ty = if let Some(closure_kind) = kind_ty.to_opt_closure_kind()
+                && !args.tupled_upvars_ty().is_ty_var()
+            {
                 if !closure_kind.extends(goal_kind) {
                     return Err(NoSolution);
                 }

compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs

@@ -487,6 +487,11 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
             bug!();
         };
 
+        // Bail if the upvars haven't been constrained.
+        if tupled_upvars_ty.expect_ty().is_ty_var() {
+            return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS);
+        }
+
         let Some(closure_kind) = closure_fn_kind_ty.expect_ty().to_opt_closure_kind() else {
             // We don't need to worry about the self type being an infer var.
             return Err(NoSolution);

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1601,7 +1601,10 @@ fn confirm_closure_candidate<'cx, 'tcx>(
                 // If we know the kind and upvars, use that directly.
                 // Otherwise, defer to `AsyncFnKindHelper::Upvars` to delay
                 // the projection, like the `AsyncFn*` traits do.
-                let output_ty = if let Some(_) = kind_ty.to_opt_closure_kind() {
+                let output_ty = if let Some(_) = kind_ty.to_opt_closure_kind()
+                    // Fall back to projection if upvars aren't constrained
+                    && !args.tupled_upvars_ty().is_ty_var()
+                {
                     sig.to_coroutine_given_kind_and_upvars(
                         tcx,
                         args.parent_args(),
@@ -1731,7 +1734,10 @@ fn confirm_async_closure_candidate<'cx, 'tcx>(
 
             let term = match item_name {
                 sym::CallOnceFuture | sym::CallRefFuture => {
-                    if let Some(closure_kind) = kind_ty.to_opt_closure_kind() {
+                    if let Some(closure_kind) = kind_ty.to_opt_closure_kind()
+                        // Fall back to projection if upvars aren't constrained
+                        && !args.tupled_upvars_ty().is_ty_var()
+                    {
                         if !closure_kind.extends(goal_kind) {
                             bug!("we should not be confirming if the closure kind is not met");
                         }

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

@@ -400,39 +400,36 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
             }
             ty::CoroutineClosure(def_id, args) => {
+                let args = args.as_coroutine_closure();
                 let is_const = self.tcx().is_const_fn_raw(def_id);
-                match self.infcx.closure_kind(self_ty) {
-                    Some(closure_kind) => {
-                        let no_borrows = match self
-                            .infcx
-                            .shallow_resolve(args.as_coroutine_closure().tupled_upvars_ty())
-                            .kind()
-                        {
-                            ty::Tuple(tys) => tys.is_empty(),
-                            ty::Error(_) => false,
-                            _ => bug!("tuple_fields called on non-tuple"),
-                        };
-                        // A coroutine-closure implements `FnOnce` *always*, since it may
-                        // always be called once. It additionally implements `Fn`/`FnMut`
-                        // only if it has no upvars (therefore no borrows from the closure
-                        // that would need to be represented with a lifetime) and if the
-                        // closure kind permits it.
-                        // FIXME(async_closures): Actually, it could also implement `Fn`/`FnMut`
-                        // if it takes all of its upvars by copy, and none by ref. This would
-                        // require us to record a bit more information during upvar analysis.
-                        if no_borrows && closure_kind.extends(kind) {
-                            candidates.vec.push(ClosureCandidate { is_const });
-                        } else if kind == ty::ClosureKind::FnOnce {
-                            candidates.vec.push(ClosureCandidate { is_const });
-                        }
+                if let Some(closure_kind) = self.infcx.closure_kind(self_ty)
+                    // Ambiguity if upvars haven't been constrained yet
+                    && !args.tupled_upvars_ty().is_ty_var()
+                {
+                    let no_borrows = match args.tupled_upvars_ty().kind() {
+                        ty::Tuple(tys) => tys.is_empty(),
+                        ty::Error(_) => false,
+                        _ => bug!("tuple_fields called on non-tuple"),
+                    };
+                    // A coroutine-closure implements `FnOnce` *always*, since it may
+                    // always be called once. It additionally implements `Fn`/`FnMut`
+                    // only if it has no upvars (therefore no borrows from the closure
+                    // that would need to be represented with a lifetime) and if the
+                    // closure kind permits it.
+                    // FIXME(async_closures): Actually, it could also implement `Fn`/`FnMut`
+                    // if it takes all of its upvars by copy, and none by ref. This would
+                    // require us to record a bit more information during upvar analysis.
+                    if no_borrows && closure_kind.extends(kind) {
+                        candidates.vec.push(ClosureCandidate { is_const });
+                    } else if kind == ty::ClosureKind::FnOnce {
+                        candidates.vec.push(ClosureCandidate { is_const });
                     }
-                    None => {
-                        if kind == ty::ClosureKind::FnOnce {
-                            candidates.vec.push(ClosureCandidate { is_const });
-                        } else {
-                            // This stays ambiguous until kind+upvars are determined.
-                            candidates.ambiguous = true;
-                        }
+                } else {
+                    if kind == ty::ClosureKind::FnOnce {
+                        candidates.vec.push(ClosureCandidate { is_const });
+                    } else {
+                        // This stays ambiguous until kind+upvars are determined.
+                        candidates.ambiguous = true;
                     }
                 }
             }

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

@@ -0,0 +1,27 @@
+//@ edition: 2021
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+#![feature(async_closure)]
+
+fn constrain<T: async FnOnce()>(t: T) -> T {
+    t
+}
+
+fn call_once<T>(f: impl FnOnce() -> T) -> T {
+    f()
+}
+
+async fn async_call_once<T>(f: impl async FnOnce() -> T) -> T {
+    f().await
+}
+
+fn main() {
+    let c = constrain(async || {});
+    call_once(c);
+
+    let c = constrain(async || {});
+    async_call_once(c);
+}