Rollup merge of #123662 - compiler-errors:no-upvars-yet, r=oli-obk

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

Previously, code relied on the implicit assumption that if a coroutine-closure's kind variable was constrained, then its upvars were also constrained. This is because we assign all of them at once at the end up upvar analysis.

However, there's another way that a coroutine-closure's kind can be constrained: from a signature hint in closure signature deduction. After #123350, we use these hints, which means the implicit assumption above no longer holds.

This PR adds the necessary checks so that we don't ICE.

r? oli-obk

5 files changed, 48 insertions, 36 deletions

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index d9e99bf07af..dd73f0f4a35 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/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:?}"),
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs b/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
index 8a96d810134..a778414d9d1 100644
--- a/compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
+++ b/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);
                 }
diff --git a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs b/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
index befde8f768a..ebf2a0d9621 100644
--- a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
+++ b/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);
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 8d04fd45940..a5483c5bbc0 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/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");
                         }
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index c1f340dfc7c..974e5ef0e16 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/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;
                     }
                 }
             }