Make evaluate_obligation not succeed unconditionally if it registered new hidden types for opaque types

Author: oli-obk

compiler/rustc_infer/src/infer/mod.rs

@@ -891,6 +891,10 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             .region_constraints_added_in_snapshot(&snapshot.undo_snapshot)
     }
 
+    pub fn opaque_types_added_in_snapshot(&self, snapshot: &CombinedSnapshot<'a, 'tcx>) -> bool {
+        self.inner.borrow().undo_log.opaque_types_in_snapshot(&snapshot.undo_snapshot)
+    }
+
     pub fn add_given(&self, sub: ty::Region<'tcx>, sup: ty::RegionVid) {
         self.inner.borrow_mut().unwrap_region_constraints().add_given(sub, sup);
     }

compiler/rustc_infer/src/infer/undo_log.rs

@@ -185,6 +185,10 @@ impl<'tcx> InferCtxtUndoLogs<'tcx> {
         })
     }
 
+    pub(crate) fn opaque_types_in_snapshot(&self, s: &Snapshot<'tcx>) -> bool {
+        self.logs[s.undo_len..].iter().any(|log| matches!(log, UndoLog::OpaqueTypes(..)))
+    }
+
     pub(crate) fn region_constraints(
         &self,
     ) -> impl Iterator<Item = &'_ region_constraints::UndoLog<'tcx>> + Clone {

compiler/rustc_middle/src/traits/select.rs

@@ -174,6 +174,10 @@ pub enum SelectionCandidate<'tcx> {
 pub enum EvaluationResult {
     /// Evaluation successful.
     EvaluatedToOk,
+    /// Evaluation successful, but need to rerun because opaque types got
+    /// hidden types assigned without it being known whether the opaque types
+    /// are within their defining scope
+    EvaluatedToOkModuloOpaqueTypes,
     /// Evaluation successful, but there were unevaluated region obligations.
     EvaluatedToOkModuloRegions,
     /// Evaluation is known to be ambiguous -- it *might* hold for some
@@ -252,9 +256,11 @@ impl EvaluationResult {
 
     pub fn may_apply(self) -> bool {
         match self {
-            EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToUnknown => {
-                true
-            }
+            EvaluatedToOkModuloOpaqueTypes
+            | EvaluatedToOk
+            | EvaluatedToOkModuloRegions
+            | EvaluatedToAmbig
+            | EvaluatedToUnknown => true,
 
             EvaluatedToErr | EvaluatedToRecur => false,
         }
@@ -264,7 +270,11 @@ impl EvaluationResult {
         match self {
             EvaluatedToUnknown | EvaluatedToRecur => true,
 
-            EvaluatedToOk | EvaluatedToOkModuloRegions | EvaluatedToAmbig | EvaluatedToErr => false,
+            EvaluatedToOkModuloOpaqueTypes
+            | EvaluatedToOk
+            | EvaluatedToOkModuloRegions
+            | EvaluatedToAmbig
+            | EvaluatedToErr => false,
         }
     }
 }

compiler/rustc_middle/src/ty/flags.rs

@@ -207,11 +207,6 @@ impl FlagComputation {
 
             &ty::FnDef(_, substs) => {
                 self.add_substs(substs);
-                // HACK(#98608, oli-obk): Function items with opaque types in their signature will
-                // end up not having the HAS_TY_OPAQUE flag set, causing `evaluate_obligation` to
-                // optimistically assume the function item matches any signature. See documentation
-                // on `HAS_FREE_LOCAL_NAMES` for details.
-                self.add_flags(TypeFlags::HAS_TY_OPAQUE);
             }
 
             &ty::FnPtr(fn_sig) => self.bound_computation(fn_sig, |computation, fn_sig| {

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -777,6 +777,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             Ok(
                 EvaluationResult::EvaluatedToOk
                 | EvaluationResult::EvaluatedToOkModuloRegions
+                | EvaluationResult::EvaluatedToOkModuloOpaqueTypes
                 | EvaluationResult::EvaluatedToAmbig,
             ) => {}
             _ => return false,

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

@@ -394,6 +394,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Err(_) => return Ok(EvaluatedToErr),
             }
 
+            if self.infcx.opaque_types_added_in_snapshot(snapshot) {
+                return Ok(result.max(EvaluatedToOkModuloOpaqueTypes));
+            }
+
             match self.infcx.region_constraints_added_in_snapshot(snapshot) {
                 None => Ok(result),
                 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),

src/test/ui/type-alias-impl-trait/issue-53398-cyclic-types.rs

@@ -3,7 +3,7 @@
 type Foo = impl Fn() -> Foo;
 
 fn foo() -> Foo {
-    foo //~ ERROR: overflow evaluating the requirement `<fn() -> Foo {foo} as FnOnce<()>>::Output == fn() -> Foo {foo}`
+    foo //~ ERROR: overflow evaluating the requirement `fn() -> Foo {foo}: Sized`
 }
 
 fn main() {}

src/test/ui/type-alias-impl-trait/issue-53398-cyclic-types.stderr

@@ -1,8 +1,10 @@
-error[E0275]: overflow evaluating the requirement `<fn() -> Foo {foo} as FnOnce<()>>::Output == fn() -> Foo {foo}`
+error[E0275]: overflow evaluating the requirement `fn() -> Foo {foo}: Sized`
   --> $DIR/issue-53398-cyclic-types.rs:6:5
    |
 LL |     foo
    |     ^^^
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_53398_cyclic_types`)
 
 error: aborting due to previous error