Fix Rustdoc ICE when checking blanket impls

src/librustc/infer/combine.rs

@@ -251,6 +251,7 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
                   dir: RelationDir)
                   -> RelateResult<'tcx, Generalization<'tcx>>
     {
+        debug!("generalize(ty={:?}, for_vid={:?}, dir={:?}", ty, for_vid, dir);
         // Determine the ambient variance within which `ty` appears.
         // The surrounding equation is:
         //
@@ -273,8 +274,15 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
             root_ty: ty,
         };
 
-        let ty = generalize.relate(&ty, &ty)?;
+        let ty = match generalize.relate(&ty, &ty) {
+            Ok(ty) => ty,
+            Err(e) => {
+                debug!("generalize: failure {:?}", e);
+                return Err(e);
+            }
+        };
         let needs_wf = generalize.needs_wf;
+        debug!("generalize: success {{ {:?}, {:?} }}", ty, needs_wf);
         Ok(Generalization { ty, needs_wf })
     }
 }

src/librustc/infer/equate.rs

@@ -74,6 +74,9 @@ impl<'combine, 'infcx, 'gcx, 'tcx> TypeRelation<'infcx, 'gcx, 'tcx>
         let infcx = self.fields.infcx;
         let a = infcx.type_variables.borrow_mut().replace_if_possible(a);
         let b = infcx.type_variables.borrow_mut().replace_if_possible(b);
+
+        debug!("{}.tys: replacements ({:?}, {:?})", self.tag(), a, b);
+
         match (&a.sty, &b.sty) {
             (&ty::Infer(TyVar(a_id)), &ty::Infer(TyVar(b_id))) => {
                 infcx.type_variables.borrow_mut().equate(a_id, b_id);

src/librustc/traits/select.rs

@@ -1522,6 +1522,33 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             .map(|v| v.get(tcx))
     }
 
+    /// Determines whether can we safely cache the result
+    /// of selecting an obligation. This is almost always 'true',
+    /// except when dealing with certain ParamCandidates.
+    ///
+    /// Ordinarily, a ParamCandidate will contain no inference variables,
+    /// since it was usually produced directly from a DefId. However,
+    /// certain cases (currently only librustdoc's blanket impl finder),
+    /// a ParamEnv may be explicitly constructed with inference types.
+    /// When this is the case, we do *not* want to cache the resulting selection
+    /// candidate. This is due to the fact that it might not always be possible
+    /// to equate the obligation's trait ref and the candidate's trait ref,
+    /// if more constraints end up getting added to an inference variable.
+    ///
+    /// Because of this, we always want to re-run the full selection
+    /// process for our obligation the next time we see it, since
+    /// we might end up picking a different SelectionCandidate (or none at all)
+    fn can_cache_candidate(&self,
+        result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>
+     ) -> bool {
+        match result {
+            Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => {
+                !trait_ref.skip_binder().input_types().any(|t| t.walk().any(|t_| t_.is_ty_infer()))
+            },
+            _ => true
+        }
+    }
+
     fn insert_candidate_cache(
         &mut self,
         param_env: ty::ParamEnv<'tcx>,
@@ -1531,6 +1558,14 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
     ) {
         let tcx = self.tcx();
         let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref;
+
+        if !self.can_cache_candidate(&candidate) {
+            debug!("insert_candidate_cache(trait_ref={:?}, candidate={:?} -\
+                    candidate is not cacheable", trait_ref, candidate);
+            return;
+
+        }
+
         if self.can_use_global_caches(param_env) {
             if let Err(Overflow) = candidate {
                 // Don't cache overflow globally; we only produce this

src/librustdoc/clean/blanket_impl.rs

@@ -50,6 +50,7 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
         name: Option<String>,
     ) -> Vec<Item>
     where F: Fn(DefId) -> Def {
+        debug!("get_blanket_impls(def_id={:?}, ...)", def_id);
         let mut impls = Vec::new();
         if self.cx
             .tcx
@@ -78,6 +79,8 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
             }
             self.cx.tcx.for_each_relevant_impl(trait_def_id, ty, |impl_def_id| {
                 self.cx.tcx.infer_ctxt().enter(|infcx| {
+                    debug!("get_blanet_impls: Considering impl for trait '{:?}' {:?}",
+                           trait_def_id, impl_def_id);
                     let t_generics = infcx.tcx.generics_of(impl_def_id);
                     let trait_ref = infcx.tcx.impl_trait_ref(impl_def_id)
                                              .expect("Cannot get impl trait");
@@ -104,8 +107,8 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
                         drop(obligations);
 
                         debug!(
-                            "invoking predicate_may_hold: {:?}",
-                            trait_ref,
+                            "invoking predicate_may_hold: param_env={:?}, trait_ref={:?}, ty={:?}",
+                             param_env, trait_ref, ty
                         );
                         let may_apply = match infcx.evaluate_obligation(
                             &traits::Obligation::new(
@@ -117,6 +120,10 @@ impl<'a, 'tcx, 'rcx, 'cstore> BlanketImplFinder <'a, 'tcx, 'rcx, 'cstore> {
                             Ok(eval_result) => eval_result.may_apply(),
                             Err(traits::OverflowError) => true, // overflow doesn't mean yes *or* no
                         };
+                        debug!("get_blanket_impls: found applicable impl: {}\
+                               for trait_ref={:?}, ty={:?}",
+                               may_apply, trait_ref, ty);
+
                         if !may_apply {
                             return
                         }

src/test/rustdoc/issue-55001.rs

@@ -0,0 +1,31 @@
+// Regression test for issue #55001. Previously, we would incorrectly
+// cache certain trait selection results when checking for blanket impls,
+// resulting in an ICE when we tried to confirm the cached ParamCandidate
+// against an obligation.
+
+pub struct DefaultAllocator;
+pub struct Standard;
+pub struct Inner;
+
+pub trait Rand {}
+
+pub trait Distribution<T> {}
+pub trait Allocator<N> {}
+
+impl<T> Rand for T where Standard: Distribution<T> {}
+
+impl<A> Distribution<Point<A>> for Standard
+where
+DefaultAllocator: Allocator<A>,
+Standard: Distribution<A> {}
+
+impl Distribution<Inner> for Standard {}
+
+
+pub struct Point<N>
+where DefaultAllocator: Allocator<N>
+{
+    field: N
+}
+
+fn main() {}