fix infinite recursion on complex traits

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

@@ -59,6 +59,7 @@ use rustc_middle::ty::print::with_no_trimmed_paths;
 
 mod candidate_assembly;
 mod confirmation;
+mod only_fresh_differ;
 
 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
 pub enum IntercrateAmbiguityCause {
@@ -1123,29 +1124,24 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // unbound variable. When we evaluate this trait-reference, we
         // will unify `$0` with `Vec<$1>` (for some fresh variable
         // `$1`), on the condition that `$1 : Eq`. We will then wind
-        // up with many candidates (since that are other `Eq` impls
+        // up with many candidates (since there are other `Eq` impls
         // that apply) and try to winnow things down. This results in
         // a recursive evaluation that `$1 : Eq` -- as you can
         // imagine, this is just where we started. To avoid that, we
-        // check for unbound variables and return an ambiguous (hence possible)
-        // match if we've seen this trait before.
+        // check if the stack contains an obligation that is identical
+        // up to fresh variables.
         //
         // This suffices to allow chains like `FnMut` implemented in
         // terms of `Fn` etc, but we could probably make this more
         // precise still.
-        let unbound_input_types =
-            stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
-
-        if unbound_input_types
-            && stack.iter().skip(1).any(|prev| {
-                stack.obligation.param_env == prev.obligation.param_env
-                    && self.match_fresh_trait_refs(
-                        stack.fresh_trait_pred,
-                        prev.fresh_trait_pred,
-                        prev.obligation.param_env,
-                    )
-            })
-        {
+        if stack.iter().skip(1).any(|prev| {
+            stack.obligation.param_env == prev.obligation.param_env
+                && self.is_repetition(
+                    stack.fresh_trait_pred,
+                    prev.fresh_trait_pred,
+                    prev.obligation.param_env,
+                )
+        }) {
             debug!("evaluate_stack --> unbound argument, recursive --> giving up",);
             return Ok(EvaluatedToUnknown);
         }
@@ -2457,14 +2453,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ///////////////////////////////////////////////////////////////////////////
     // Miscellany
 
-    fn match_fresh_trait_refs(
+    fn is_repetition(
         &self,
-        previous: ty::PolyTraitPredicate<'tcx>,
-        current: ty::PolyTraitPredicate<'tcx>,
+        new: ty::PolyTraitPredicate<'tcx>,
+        old: ty::PolyTraitPredicate<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> bool {
+        let mut matcher = only_fresh_differ::FreshDiffer::new(self.tcx(), param_env);
+        if matcher.relate(new, old).is_ok() {
+            return true;
+        }
+
         let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
-        matcher.relate(previous, current).is_ok()
+
+        new.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh())
+            && matcher.relate(new, old).is_ok()
     }
 
     fn push_stack<'o>(

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

@@ -0,0 +1,116 @@
+use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
+use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
+
+/// Requires that the types can be unified by substituting fresh variables
+/// for fresh variable only.
+pub struct FreshDiffer<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> FreshDiffer<'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Self {
+        Self { tcx, param_env }
+    }
+}
+
+impl<'tcx> TypeRelation<'tcx> for FreshDiffer<'tcx> {
+    fn tag(&self) -> &'static str {
+        "FreshDiffer"
+    }
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn intercrate(&self) -> bool {
+        false
+    }
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.param_env
+    }
+    fn a_is_expected(&self) -> bool {
+        true
+    } // irrelevant
+
+    fn mark_ambiguous(&mut self) {
+        bug!()
+    }
+
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        _: ty::Variance,
+        _: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        self.relate(a, b)
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn regions(
+        &mut self,
+        a: ty::Region<'tcx>,
+        b: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        Ok(a)
+    }
+
+    #[instrument(skip(self), level = "debug")]
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        if a == b {
+            return Ok(a);
+        }
+
+        match (a.kind(), b.kind()) {
+            (&ty::Infer(ty::FreshTy(_)), &ty::Infer(ty::FreshTy(_)))
+            | (&ty::Infer(ty::FreshIntTy(_)), &ty::Infer(ty::FreshIntTy(_)))
+            | (&ty::Infer(ty::FreshFloatTy(_)), &ty::Infer(ty::FreshFloatTy(_))) => Ok(a),
+
+            (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
+                Err(TypeError::Sorts(relate::expected_found(self, a, b)))
+            }
+
+            (&ty::Error(_), _) | (_, &ty::Error(_)) => Ok(self.tcx().ty_error()),
+
+            _ => relate::super_relate_tys(self, a, b),
+        }
+    }
+
+    fn consts(
+        &mut self,
+        a: ty::Const<'tcx>,
+        b: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        debug!("{}.consts({:?}, {:?})", self.tag(), a, b);
+        if a == b {
+            return Ok(a);
+        }
+
+        match (a.kind(), b.kind()) {
+            (_, ty::ConstKind::Infer(InferConst::Fresh(_))) => {
+                return Ok(a);
+            }
+
+            (ty::ConstKind::Infer(_), _) | (_, ty::ConstKind::Infer(_)) => {
+                return Err(TypeError::ConstMismatch(relate::expected_found(self, a, b)));
+            }
+
+            _ => {}
+        }
+
+        relate::super_relate_consts(self, a, b)
+    }
+
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        b: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>,
+    {
+        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
+    }
+}