Auto merge of #98641 - lcnr:mir-dropck, r=oli-obk

fully move dropck to mir

r? `@oli-obk`

compiler/rustc_borrowck/src/type_check/liveness/mod.rs

@@ -1,11 +1,11 @@
+use itertools::{Either, Itertools};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::mir::{Body, Local};
 use rustc_middle::ty::{RegionVid, TyCtxt};
-use std::rc::Rc;
-
 use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
 use rustc_mir_dataflow::move_paths::MoveData;
 use rustc_mir_dataflow::ResultsCursor;
+use std::rc::Rc;
 
 use crate::{
     constraints::OutlivesConstraintSet,
@@ -46,7 +46,8 @@ pub(super) fn generate<'mir, 'tcx>(
         &typeck.borrowck_context.universal_regions,
         &typeck.borrowck_context.constraints.outlives_constraints,
     );
-    let live_locals = compute_live_locals(typeck.tcx(), &free_regions, &body);
+    let (relevant_live_locals, boring_locals) =
+        compute_relevant_live_locals(typeck.tcx(), &free_regions, &body);
     let facts_enabled = use_polonius || AllFacts::enabled(typeck.tcx());
 
     let polonius_drop_used = if facts_enabled {
@@ -57,48 +58,44 @@ pub(super) fn generate<'mir, 'tcx>(
         None
     };
 
-    if !live_locals.is_empty() || facts_enabled {
-        trace::trace(
-            typeck,
-            body,
-            elements,
-            flow_inits,
-            move_data,
-            live_locals,
-            polonius_drop_used,
-        );
-    }
+    trace::trace(
+        typeck,
+        body,
+        elements,
+        flow_inits,
+        move_data,
+        relevant_live_locals,
+        boring_locals,
+        polonius_drop_used,
+    );
 }
 
-// The purpose of `compute_live_locals` is to define the subset of `Local`
+// The purpose of `compute_relevant_live_locals` is to define the subset of `Local`
 // variables for which we need to do a liveness computation. We only need
 // to compute whether a variable `X` is live if that variable contains
 // some region `R` in its type where `R` is not known to outlive a free
 // region (i.e., where `R` may be valid for just a subset of the fn body).
-fn compute_live_locals<'tcx>(
+fn compute_relevant_live_locals<'tcx>(
     tcx: TyCtxt<'tcx>,
     free_regions: &FxHashSet<RegionVid>,
     body: &Body<'tcx>,
-) -> Vec<Local> {
-    let live_locals: Vec<Local> = body
-        .local_decls
-        .iter_enumerated()
-        .filter_map(|(local, local_decl)| {
+) -> (Vec<Local>, Vec<Local>) {
+    let (boring_locals, relevant_live_locals): (Vec<_>, Vec<_>) =
+        body.local_decls.iter_enumerated().partition_map(|(local, local_decl)| {
             if tcx.all_free_regions_meet(&local_decl.ty, |r| {
                 free_regions.contains(&r.to_region_vid())
             }) {
-                None
+                Either::Left(local)
             } else {
-                Some(local)
+                Either::Right(local)
             }
-        })
-        .collect();
+        });
 
     debug!("{} total variables", body.local_decls.len());
-    debug!("{} variables need liveness", live_locals.len());
+    debug!("{} variables need liveness", relevant_live_locals.len());
     debug!("{} regions outlive free regions", free_regions.len());
 
-    live_locals
+    (relevant_live_locals, boring_locals)
 }
 
 /// Computes all regions that are (currently) known to outlive free

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -41,12 +41,13 @@ pub(super) fn trace<'mir, 'tcx>(
     elements: &Rc<RegionValueElements>,
     flow_inits: &mut ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
     move_data: &MoveData<'tcx>,
-    live_locals: Vec<Local>,
+    relevant_live_locals: Vec<Local>,
+    boring_locals: Vec<Local>,
     polonius_drop_used: Option<Vec<(Local, Location)>>,
 ) {
     debug!("trace()");
 
-    let local_use_map = &LocalUseMap::build(&live_locals, elements, body);
+    let local_use_map = &LocalUseMap::build(&relevant_live_locals, elements, body);
 
     let cx = LivenessContext {
         typeck,
@@ -61,10 +62,12 @@ pub(super) fn trace<'mir, 'tcx>(
     let mut results = LivenessResults::new(cx);
 
     if let Some(drop_used) = polonius_drop_used {
-        results.add_extra_drop_facts(drop_used, live_locals.iter().copied().collect())
+        results.add_extra_drop_facts(drop_used, relevant_live_locals.iter().copied().collect())
     }
 
-    results.compute_for_all_locals(live_locals);
+    results.compute_for_all_locals(relevant_live_locals);
+
+    results.dropck_boring_locals(boring_locals);
 }
 
 /// Contextual state for the type-liveness generator.
@@ -133,8 +136,8 @@ impl<'me, 'typeck, 'flow, 'tcx> LivenessResults<'me, 'typeck, 'flow, 'tcx> {
         }
     }
 
-    fn compute_for_all_locals(&mut self, live_locals: Vec<Local>) {
-        for local in live_locals {
+    fn compute_for_all_locals(&mut self, relevant_live_locals: Vec<Local>) {
+        for local in relevant_live_locals {
             self.reset_local_state();
             self.add_defs_for(local);
             self.compute_use_live_points_for(local);
@@ -157,19 +160,37 @@ impl<'me, 'typeck, 'flow, 'tcx> LivenessResults<'me, 'typeck, 'flow, 'tcx> {
         }
     }
 
+    // Runs dropck for locals whose liveness isn't relevant. This is
+    // necessary to eagerly detect unbound recursion during drop glue computation.
+    fn dropck_boring_locals(&mut self, boring_locals: Vec<Local>) {
+        for local in boring_locals {
+            let local_ty = self.cx.body.local_decls[local].ty;
+            let drop_data = self.cx.drop_data.entry(local_ty).or_insert_with({
+                let typeck = &mut self.cx.typeck;
+                move || LivenessContext::compute_drop_data(typeck, local_ty)
+            });
+
+            drop_data.dropck_result.report_overflows(
+                self.cx.typeck.infcx.tcx,
+                self.cx.body.local_decls[local].source_info.span,
+                local_ty,
+            );
+        }
+    }
+
     /// Add extra drop facts needed for Polonius.
     ///
     /// Add facts for all locals with free regions, since regions may outlive
     /// the function body only at certain nodes in the CFG.
     fn add_extra_drop_facts(
         &mut self,
         drop_used: Vec<(Local, Location)>,
-        live_locals: FxHashSet<Local>,
+        relevant_live_locals: FxHashSet<Local>,
     ) {
         let locations = IntervalSet::new(self.cx.elements.num_points());
 
         for (local, location) in drop_used {
-            if !live_locals.contains(&local) {
+            if !relevant_live_locals.contains(&local) {
                 let local_ty = self.cx.body.local_decls[local].ty;
                 if local_ty.has_free_regions() {
                     self.cx.add_drop_live_facts_for(local, local_ty, &[location], &locations);

compiler/rustc_middle/src/query/mod.rs

@@ -1825,7 +1825,8 @@ rustc_queries! {
         remap_env_constness
     }
 
-    /// Do not call this query directly: invoke `infcx.at().dropck_outlives()` instead.
+    /// Do not call this query directly:
+    /// invoke `DropckOutlives::new(dropped_ty)).fully_perform(typeck.infcx)` instead.
     query dropck_outlives(
         goal: CanonicalTyGoal<'tcx>
     ) -> Result<

compiler/rustc_typeck/src/check/dropck.rs

@@ -8,8 +8,6 @@ use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::util::IgnoreRegions;
 use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
 use rustc_span::Span;
-use rustc_trait_selection::traits::query::dropck_outlives::AtExt;
-use rustc_trait_selection::traits::ObligationCause;
 
 /// This function confirms that the `Drop` implementation identified by
 /// `drop_impl_did` is not any more specialized than the type it is
@@ -234,18 +232,14 @@ fn ensure_drop_predicates_are_implied_by_item_defn<'tcx>(
 /// This function is not only checking that the dropck obligations are met for
 /// the given type, but it's also currently preventing non-regular recursion in
 /// types from causing stack overflows (dropck_no_diverge_on_nonregular_*.rs).
+///
+/// FIXME: Completely rip out dropck and regionck.
 pub(crate) fn check_drop_obligations<'a, 'tcx>(
-    rcx: &mut RegionCtxt<'a, 'tcx>,
-    ty: Ty<'tcx>,
-    span: Span,
-    body_id: hir::HirId,
+    _rcx: &mut RegionCtxt<'a, 'tcx>,
+    _ty: Ty<'tcx>,
+    _span: Span,
+    _body_id: hir::HirId,
 ) {
-    debug!("check_drop_obligations typ: {:?}", ty);
-
-    let cause = &ObligationCause::misc(span, body_id);
-    let infer_ok = rcx.infcx.at(cause, rcx.fcx.param_env).dropck_outlives(ty);
-    debug!("dropck_outlives = {:#?}", infer_ok);
-    rcx.fcx.register_infer_ok_obligations(infer_ok);
 }
 
 // This is an implementation of the TypeRelation trait with the

src/test/ui/dropck/dropck_no_diverge_on_nonregular_1.rs

@@ -23,5 +23,4 @@ enum FingerTree<T:'static> {
 fn main() {
     let ft = //~ ERROR overflow while adding drop-check rules for FingerTree
         FingerTree::Single(1);
-    //~^ ERROR overflow while adding drop-check rules for FingerTree
 }

src/test/ui/dropck/dropck_no_diverge_on_nonregular_1.stderr

@@ -6,13 +6,5 @@ LL |     let ft =
    |
    = note: overflowed on FingerTree<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<i32>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
 
-error[E0320]: overflow while adding drop-check rules for FingerTree<i32>
-  --> $DIR/dropck_no_diverge_on_nonregular_1.rs:25:9
-   |
-LL |         FingerTree::Single(1);
-   |         ^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: overflowed on FingerTree<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<i32>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 

src/test/ui/dropck/dropck_no_diverge_on_nonregular_2.rs

@@ -22,5 +22,4 @@ enum FingerTree<T:'static> {
 fn main() {
     let ft = //~ ERROR overflow while adding drop-check rules for FingerTree
         FingerTree::Single(1);
-    //~^ ERROR overflow while adding drop-check rules for FingerTree
 }

src/test/ui/dropck/dropck_no_diverge_on_nonregular_2.stderr

@@ -6,13 +6,5 @@ LL |     let ft =
    |
    = note: overflowed on FingerTree<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<i32>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
 
-error[E0320]: overflow while adding drop-check rules for FingerTree<i32>
-  --> $DIR/dropck_no_diverge_on_nonregular_2.rs:24:9
-   |
-LL |         FingerTree::Single(1);
-   |         ^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: overflowed on FingerTree<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<i32>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 

src/test/ui/dropck/dropck_no_diverge_on_nonregular_3.rs

@@ -31,6 +31,5 @@ enum Wrapper<T:'static> {
 fn main() {
     let w = //~ ERROR overflow while adding drop-check rules for Option
         Some(Wrapper::Simple::<u32>);
-    //~^ ERROR overflow while adding drop-check rules for Option
-    //~| ERROR overflow while adding drop-check rules for Wrapper
+    //~^ ERROR overflow while adding drop-check rules for Wrapper
 }

src/test/ui/dropck/dropck_no_diverge_on_nonregular_3.stderr

@@ -6,14 +6,6 @@ LL |     let w =
    |
    = note: overflowed on FingerTree<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<u32>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
 
-error[E0320]: overflow while adding drop-check rules for Option<Wrapper<u32>>
-  --> $DIR/dropck_no_diverge_on_nonregular_3.rs:33:9
-   |
-LL |         Some(Wrapper::Simple::<u32>);
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: overflowed on FingerTree<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<u32>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
-
 error[E0320]: overflow while adding drop-check rules for Wrapper<u32>
   --> $DIR/dropck_no_diverge_on_nonregular_3.rs:33:14
    |
@@ -22,5 +14,5 @@ LL |         Some(Wrapper::Simple::<u32>);
    |
    = note: overflowed on FingerTree<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<Node<u32>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 

src/test/ui/infinite/infinite-struct.rs

@@ -1,6 +1,5 @@
 struct Take(Take);
 //~^ ERROR has infinite size
-//~| ERROR cycle detected
 
 // check that we don't hang trying to find the tail of a recursive struct (#79437)
 fn foo() -> Take {

src/test/ui/infinite/infinite-struct.stderr

@@ -11,16 +11,6 @@ help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `Take` repre
 LL | struct Take(Box<Take>);
    |             ++++    +
 
-error[E0391]: cycle detected when computing drop-check constraints for `Take`
-  --> $DIR/infinite-struct.rs:1:1
-   |
-LL | struct Take(Take);
-   | ^^^^^^^^^^^
-   |
-   = note: ...which immediately requires computing drop-check constraints for `Take` again
-   = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst }, value: Take } }`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0072, E0391.
-For more information about an error, try `rustc --explain E0072`.
+For more information about this error, try `rustc --explain E0072`.

src/test/ui/infinite/infinite-tag-type-recursion.rs

@@ -1,5 +1,4 @@
 enum MList { Cons(isize, MList), Nil }
 //~^ ERROR recursive type `MList` has infinite size
-//~| ERROR cycle detected when computing drop-check constraints for `MList`
 
 fn main() { let a = MList::Cons(10, MList::Cons(11, MList::Nil)); }

src/test/ui/infinite/infinite-tag-type-recursion.stderr

@@ -11,16 +11,6 @@ help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `MList` repr
 LL | enum MList { Cons(isize, Box<MList>), Nil }
    |                          ++++     +
 
-error[E0391]: cycle detected when computing drop-check constraints for `MList`
-  --> $DIR/infinite-tag-type-recursion.rs:1:1
-   |
-LL | enum MList { Cons(isize, MList), Nil }
-   | ^^^^^^^^^^
-   |
-   = note: ...which immediately requires computing drop-check constraints for `MList` again
-   = note: cycle used when computing dropck types for `Canonical { max_universe: U0, variables: [], value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: [], reveal: UserFacing, constness: NotConst }, value: MList } }`
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0072, E0391.
-For more information about an error, try `rustc --explain E0072`.
+For more information about this error, try `rustc --explain E0072`.

src/test/ui/recursion/issue-38591-non-regular-dropck-recursion.rs

@@ -1,18 +1,14 @@
-// Dropck shouldn't hit a recursion limit from checking `S<u32>` since it has
-// no free regions or type parameters.
-// Codegen however, has to error for the infinitely many `drop_in_place`
-// functions it has been asked to create.
-
-// build-fail
-// normalize-stderr-test: ".nll/" -> "/"
-// compile-flags: -Zmir-opt-level=0
+// `S` is infinitely recursing so it's not possible to generate a finite
+// drop impl (ignoring polymorphization).
+//
+// Dropck should therefore detect that this is the case and eagerly error.
 
 struct S<T> {
     t: T,
     s: Box<S<fn(u: T)>>,
 }
 
-fn f(x: S<u32>) {}
+fn f(x: S<u32>) {} //~ ERROR overflow while adding drop-check rules for S<u32>
 
 fn main() {
     // Force instantiation.