Fix Rustdoc ICE when checking blanket impls

Fixes #55001, #54744

Previously, SelectionContext would unconditionally cache the selection
result for an obligation. This worked fine for most users of
SelectionContext, but it caused an issue when used by Rustdoc's blanket
impl finder.

The issue occured when SelectionContext chose a ParamCandidate which
contained inference variables. Since inference variables can change
between calls to select(), it's not safe to cache the selection result -
the chosen candidate might not be applicable for future results, leading
to an ICE when we try to run confirmation.

This commit prevents SelectionContext from caching any ParamCandidate
that contains inference variables. This should always be completely
safe, as trait selection should never depend on a particular result
being cached.

I've also added some extra debug!() statements, which I found helpful in
tracking down this bug.

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() {}