Simplify shallow resolver to just fold ty/consts

compiler/rustc_borrowck/src/type_check/relate_tys.rs

@@ -437,7 +437,7 @@ impl<'bccx, 'tcx> TypeRelation<'tcx> for NllTypeRelating<'_, 'bccx, 'tcx> {
         a: ty::Const<'tcx>,
         b: ty::Const<'tcx>,
     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
-        let a = self.type_checker.infcx.shallow_resolve(a);
+        let a = self.type_checker.infcx.shallow_resolve_const(a);
         assert!(!a.has_non_region_infer(), "unexpected inference var {:?}", a);
         assert!(!b.has_non_region_infer(), "unexpected inference var {:?}", b);
 

compiler/rustc_infer/src/infer/canonical/canonicalizer.rs

@@ -802,7 +802,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         const_var: ty::Const<'tcx>,
     ) -> ty::Const<'tcx> {
         debug_assert!(
-            !self.infcx.is_some_and(|infcx| const_var != infcx.shallow_resolve(const_var))
+            !self.infcx.is_some_and(|infcx| const_var != infcx.shallow_resolve_const(const_var))
         );
         let var = self.canonical_var(info, const_var.into());
         ty::Const::new_bound(self.tcx, self.binder_index, var, self.fold_ty(const_var.ty()))

compiler/rustc_infer/src/infer/mod.rs

@@ -1253,19 +1253,76 @@ impl<'tcx> InferCtxt<'tcx> {
         }
     }
 
-    /// Resolve any type variables found in `value` -- but only one
-    /// level. So, if the variable `?X` is bound to some type
-    /// `Foo<?Y>`, then this would return `Foo<?Y>` (but `?Y` may
-    /// itself be bound to a type).
-    ///
-    /// Useful when you only need to inspect the outermost level of
-    /// the type and don't care about nested types (or perhaps you
-    /// will be resolving them as well, e.g. in a loop).
-    pub fn shallow_resolve<T>(&self, value: T) -> T
-    where
-        T: TypeFoldable<TyCtxt<'tcx>>,
-    {
-        value.fold_with(&mut ShallowResolver { infcx: self })
+    pub fn shallow_resolve(&self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        if let ty::Infer(v) = ty.kind() { self.fold_infer_ty(*v).unwrap_or(ty) } else { ty }
+    }
+
+    // This is separate from `shallow_resolve` to keep that method small and inlinable.
+    #[inline(never)]
+    fn fold_infer_ty(&self, v: InferTy) -> Option<Ty<'tcx>> {
+        match v {
+            ty::TyVar(v) => {
+                // Not entirely obvious: if `typ` is a type variable,
+                // it can be resolved to an int/float variable, which
+                // can then be recursively resolved, hence the
+                // recursion. Note though that we prevent type
+                // variables from unifying to other type variables
+                // directly (though they may be embedded
+                // structurally), and we prevent cycles in any case,
+                // so this recursion should always be of very limited
+                // depth.
+                //
+                // Note: if these two lines are combined into one we get
+                // dynamic borrow errors on `self.inner`.
+                let known = self.inner.borrow_mut().type_variables().probe(v).known();
+                known.map(|t| self.shallow_resolve(t))
+            }
+
+            ty::IntVar(v) => self
+                .inner
+                .borrow_mut()
+                .int_unification_table()
+                .probe_value(v)
+                .map(|v| v.to_type(self.tcx)),
+
+            ty::FloatVar(v) => self
+                .inner
+                .borrow_mut()
+                .float_unification_table()
+                .probe_value(v)
+                .map(|v| v.to_type(self.tcx)),
+
+            ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => None,
+        }
+    }
+
+    pub fn shallow_resolve_const(&self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        match ct.kind() {
+            ty::ConstKind::Infer(infer_ct) => match infer_ct {
+                InferConst::Var(vid) => self
+                    .inner
+                    .borrow_mut()
+                    .const_unification_table()
+                    .probe_value(vid)
+                    .known()
+                    .unwrap_or(ct),
+                InferConst::EffectVar(vid) => self
+                    .inner
+                    .borrow_mut()
+                    .effect_unification_table()
+                    .probe_value(vid)
+                    .known()
+                    .unwrap_or(ct),
+                InferConst::Fresh(_) => ct,
+            },
+            ty::ConstKind::Param(_)
+            | ty::ConstKind::Bound(_, _)
+            | ty::ConstKind::Placeholder(_)
+            | ty::ConstKind::Unevaluated(_)
+            | ty::ConstKind::Value(_)
+            | ty::ConstKind::Error(_)
+            | ty::ConstKind::Expr(_) => ct,
+        }
     }
 
     pub fn root_var(&self, var: ty::TyVid) -> ty::TyVid {
@@ -1782,89 +1839,6 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for InferenceLiteralEraser<'tcx> {
     }
 }
 
-struct ShallowResolver<'a, 'tcx> {
-    infcx: &'a InferCtxt<'tcx>,
-}
-
-impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for ShallowResolver<'a, 'tcx> {
-    fn interner(&self) -> TyCtxt<'tcx> {
-        self.infcx.tcx
-    }
-
-    /// If `ty` is a type variable of some kind, resolve it one level
-    /// (but do not resolve types found in the result). If `typ` is
-    /// not a type variable, just return it unmodified.
-    #[inline]
-    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        if let ty::Infer(v) = ty.kind() { self.fold_infer_ty(*v).unwrap_or(ty) } else { ty }
-    }
-
-    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
-        match ct.kind() {
-            ty::ConstKind::Infer(InferConst::Var(vid)) => self
-                .infcx
-                .inner
-                .borrow_mut()
-                .const_unification_table()
-                .probe_value(vid)
-                .known()
-                .unwrap_or(ct),
-            ty::ConstKind::Infer(InferConst::EffectVar(vid)) => self
-                .infcx
-                .inner
-                .borrow_mut()
-                .effect_unification_table()
-                .probe_value(vid)
-                .known()
-                .unwrap_or(ct),
-            _ => ct,
-        }
-    }
-}
-
-impl<'a, 'tcx> ShallowResolver<'a, 'tcx> {
-    // This is separate from `fold_ty` to keep that method small and inlinable.
-    #[inline(never)]
-    fn fold_infer_ty(&mut self, v: InferTy) -> Option<Ty<'tcx>> {
-        match v {
-            ty::TyVar(v) => {
-                // Not entirely obvious: if `typ` is a type variable,
-                // it can be resolved to an int/float variable, which
-                // can then be recursively resolved, hence the
-                // recursion. Note though that we prevent type
-                // variables from unifying to other type variables
-                // directly (though they may be embedded
-                // structurally), and we prevent cycles in any case,
-                // so this recursion should always be of very limited
-                // depth.
-                //
-                // Note: if these two lines are combined into one we get
-                // dynamic borrow errors on `self.inner`.
-                let known = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
-                known.map(|t| self.fold_ty(t))
-            }
-
-            ty::IntVar(v) => self
-                .infcx
-                .inner
-                .borrow_mut()
-                .int_unification_table()
-                .probe_value(v)
-                .map(|v| v.to_type(self.infcx.tcx)),
-
-            ty::FloatVar(v) => self
-                .infcx
-                .inner
-                .borrow_mut()
-                .float_unification_table()
-                .probe_value(v)
-                .map(|v| v.to_type(self.infcx.tcx)),
-
-            ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => None,
-        }
-    }
-}
-
 impl<'tcx> TypeTrace<'tcx> {
     pub fn span(&self) -> Span {
         self.cause.span

compiler/rustc_infer/src/infer/relate/combine.rs

@@ -155,8 +155,8 @@ impl<'tcx> InferCtxt<'tcx> {
             return Ok(a);
         }
 
-        let a = self.shallow_resolve(a);
-        let b = self.shallow_resolve(b);
+        let a = self.shallow_resolve_const(a);
+        let b = self.shallow_resolve_const(b);
 
         // We should never have to relate the `ty` field on `Const` as it is checked elsewhere that consts have the
         // correct type for the generic param they are an argument for. However there have been a number of cases

compiler/rustc_infer/src/infer/resolve.rs

@@ -12,29 +12,27 @@ use rustc_middle::ty::{self, Const, InferConst, Ty, TyCtxt, TypeFoldable};
 /// useful for printing messages etc but also required at various
 /// points for correctness.
 pub struct OpportunisticVarResolver<'a, 'tcx> {
-    // The shallow resolver is used to resolve inference variables at every
-    // level of the type.
-    shallow_resolver: crate::infer::ShallowResolver<'a, 'tcx>,
+    infcx: &'a InferCtxt<'tcx>,
 }
 
 impl<'a, 'tcx> OpportunisticVarResolver<'a, 'tcx> {
     #[inline]
     pub fn new(infcx: &'a InferCtxt<'tcx>) -> Self {
-        OpportunisticVarResolver { shallow_resolver: crate::infer::ShallowResolver { infcx } }
+        OpportunisticVarResolver { infcx }
     }
 }
 
 impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for OpportunisticVarResolver<'a, 'tcx> {
     fn interner(&self) -> TyCtxt<'tcx> {
-        TypeFolder::interner(&self.shallow_resolver)
+        self.infcx.tcx
     }
 
     #[inline]
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         if !t.has_non_region_infer() {
             t // micro-optimize -- if there is nothing in this type that this fold affects...
         } else {
-            let t = self.shallow_resolver.fold_ty(t);
+            let t = self.infcx.shallow_resolve(t);
             t.super_fold_with(self)
         }
     }
@@ -43,7 +41,7 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for OpportunisticVarResolver<'a, 'tcx> {
         if !ct.has_non_region_infer() {
             ct // micro-optimize -- if there is nothing in this const that this fold affects...
         } else {
-            let ct = self.shallow_resolver.fold_const(ct);
+            let ct = self.infcx.shallow_resolve_const(ct);
             ct.super_fold_with(self)
         }
     }
@@ -160,7 +158,7 @@ impl<'a, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for FullTypeResolver<'a, 'tcx> {
         if !c.has_infer() {
             Ok(c) // micro-optimize -- if there is nothing in this const that this fold affects...
         } else {
-            let c = self.infcx.shallow_resolve(c);
+            let c = self.infcx.shallow_resolve_const(c);
             match c.kind() {
                 ty::ConstKind::Infer(InferConst::Var(vid)) => {
                     return Err(FixupError::UnresolvedConst(vid));

compiler/rustc_trait_selection/src/solve/normalize.rs

@@ -203,7 +203,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for NormalizationFolder<'_, 'tcx> {
     #[instrument(level = "debug", skip(self), ret)]
     fn try_fold_const(&mut self, ct: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
         let infcx = self.at.infcx;
-        debug_assert_eq!(ct, infcx.shallow_resolve(ct));
+        debug_assert_eq!(ct, infcx.shallow_resolve_const(ct));
         if !ct.has_aliases() {
             return Ok(ct);
         }

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -501,7 +501,7 @@ fn plug_infer_with_placeholders<'tcx>(
         }
 
         fn visit_const(&mut self, ct: ty::Const<'tcx>) {
-            let ct = self.infcx.shallow_resolve(ct);
+            let ct = self.infcx.shallow_resolve_const(ct);
             if ct.is_ct_infer() {
                 let Ok(InferOk { value: (), obligations }) =
                     self.infcx.at(&ObligationCause::dummy(), ty::ParamEnv::empty()).eq(

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -162,7 +162,7 @@ fn pred_known_to_hold_modulo_regions<'tcx>(
             let errors = ocx.select_all_or_error();
             match errors.as_slice() {
                 // Only known to hold if we did no inference.
-                [] => infcx.shallow_resolve(goal) == goal,
+                [] => infcx.resolve_vars_if_possible(goal) == goal,
 
                 errors => {
                     debug!(?errors);

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

@@ -1165,8 +1165,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
-        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
-        match self_ty.skip_binder().kind() {
+        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
+        match self_ty.kind() {
             ty::Alias(..)
             | ty::Dynamic(..)
             | ty::Error(_)
@@ -1317,7 +1317,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &PolyTraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
+        let self_ty = self.infcx.resolve_vars_if_possible(obligation.self_ty());
 
         match self_ty.skip_binder().kind() {
             ty::FnPtr(_) => candidates.vec.push(BuiltinCandidate { has_nested: false }),

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

@@ -157,10 +157,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
         let tcx = self.tcx();
 
-        let trait_predicate = self.infcx.shallow_resolve(obligation.predicate);
         let placeholder_trait_predicate =
-            self.infcx.enter_forall_and_leak_universe(trait_predicate).trait_ref;
-        let placeholder_self_ty = placeholder_trait_predicate.self_ty();
+            self.infcx.enter_forall_and_leak_universe(obligation.predicate).trait_ref;
+        let placeholder_self_ty = self.infcx.shallow_resolve(placeholder_trait_predicate.self_ty());
         let candidate_predicate = self
             .for_each_item_bound(
                 placeholder_self_ty,
@@ -422,7 +421,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
         debug!(?obligation, "confirm_auto_impl_candidate");
 
-        let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
+        let self_ty = obligation.predicate.self_ty().map_bound(|ty| self.infcx.shallow_resolve(ty));
         let types = self.constituent_types_for_ty(self_ty)?;
         Ok(self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types))
     }
@@ -1378,7 +1377,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let drop_trait = self.tcx().require_lang_item(LangItem::Drop, None);
 
         let tcx = self.tcx();
-        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
+        let self_ty = obligation.self_ty().map_bound(|ty| self.infcx.shallow_resolve(ty));
 
         let mut nested = vec![];
         let cause = obligation.derived_cause(BuiltinDerivedObligation);

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

@@ -571,7 +571,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             )?;
             // If the predicate has done any inference, then downgrade the
             // result to ambiguous.
-            if this.infcx.shallow_resolve(goal) != goal {
+            if this.infcx.resolve_vars_if_possible(goal) != goal {
                 result = result.max(EvaluatedToAmbig);
             }
             Ok(result)
@@ -1774,9 +1774,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             // that means that we must have newly inferred something about the GAT.
             // We should give up in that case.
             if !generics.params.is_empty()
-                && obligation.predicate.args[generics.parent_count..]
-                    .iter()
-                    .any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
+                && obligation.predicate.args[generics.parent_count..].iter().any(|&p| {
+                    p.has_non_region_infer() && self.infcx.resolve_vars_if_possible(p) != p
+                })
             {
                 ProjectionMatchesProjection::Ambiguous
             } else {

compiler/rustc_trait_selection/src/traits/util.rs

@@ -649,7 +649,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
     }
 
     fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
-        let ct = self.infcx.shallow_resolve(ct);
+        let ct = self.infcx.shallow_resolve_const(ct);
         if let ty::ConstKind::Placeholder(p) = ct.kind() {
             let replace_var = self.mapped_consts.get(&p);
             match replace_var {

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -44,7 +44,7 @@ pub fn obligations<'tcx>(
         GenericArgKind::Const(ct) => {
             match ct.kind() {
                 ty::ConstKind::Infer(_) => {
-                    let resolved = infcx.shallow_resolve(ct);
+                    let resolved = infcx.shallow_resolve_const(ct);
                     if resolved == ct {
                         // No progress.
                         return None;