Remove some unused type folders.

I'm surprised the compiler doesn't warn about these. It appears having
an `impl` on a struct is enough to avoid a warning about it never being
constructed.

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -8,10 +8,7 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::print::with_forced_trimmed_paths;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::IntTypeExt;
-use rustc_middle::ty::{
-    self, ImplTraitInTraitData, IsSuggestable, Ty, TyCtxt, TypeFolder, TypeSuperFoldable,
-    TypeVisitableExt,
-};
+use rustc_middle::ty::{self, ImplTraitInTraitData, IsSuggestable, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::symbol::Ident;
 use rustc_span::{Span, DUMMY_SP};
 
@@ -874,28 +871,6 @@ fn infer_placeholder_type<'a>(
     item_ident: Ident,
     kind: &'static str,
 ) -> Ty<'a> {
-    // Attempts to make the type nameable by turning FnDefs into FnPtrs.
-    struct MakeNameable<'tcx> {
-        tcx: TyCtxt<'tcx>,
-    }
-
-    impl<'tcx> TypeFolder<TyCtxt<'tcx>> for MakeNameable<'tcx> {
-        fn interner(&self) -> TyCtxt<'tcx> {
-            self.tcx
-        }
-
-        fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-            let ty = match *ty.kind() {
-                ty::FnDef(def_id, substs) => {
-                    self.tcx.mk_fn_ptr(self.tcx.fn_sig(def_id).subst(self.tcx, substs))
-                }
-                _ => ty,
-            };
-
-            ty.super_fold_with(self)
-        }
-    }
-
     let ty = tcx.diagnostic_only_typeck(def_id).node_type(body_id.hir_id);
 
     // If this came from a free `const` or `static mut?` item,

compiler/rustc_hir_typeck/src/op.rs

@@ -12,9 +12,7 @@ use rustc_middle::ty::adjustment::{
     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
 };
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{
-    self, IsSuggestable, Ty, TyCtxt, TypeFolder, TypeSuperFoldable, TypeVisitableExt,
-};
+use rustc_middle::ty::{self, IsSuggestable, Ty, TyCtxt, TypeVisitableExt};
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{sym, Ident};
@@ -965,21 +963,3 @@ fn is_builtin_binop<'tcx>(lhs: Ty<'tcx>, rhs: Ty<'tcx>, op: hir::BinOp) -> bool
         }
     }
 }
-
-struct TypeParamEraser<'a, 'tcx>(&'a FnCtxt<'a, 'tcx>, Span);
-
-impl<'tcx> TypeFolder<TyCtxt<'tcx>> for TypeParamEraser<'_, 'tcx> {
-    fn interner(&self) -> TyCtxt<'tcx> {
-        self.0.tcx
-    }
-
-    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-        match ty.kind() {
-            ty::Param(_) => self.0.next_ty_var(TypeVariableOrigin {
-                kind: TypeVariableOriginKind::MiscVariable,
-                span: self.1,
-            }),
-            _ => ty.super_fold_with(self),
-        }
-    }
-}

compiler/rustc_middle/src/ty/sty.rs

@@ -7,8 +7,8 @@ use crate::ty::subst::{GenericArg, InternalSubsts, SubstsRef};
 use crate::ty::visit::ValidateBoundVars;
 use crate::ty::InferTy::*;
 use crate::ty::{
-    self, AdtDef, Discr, FallibleTypeFolder, Term, Ty, TyCtxt, TypeFlags, TypeFoldable,
-    TypeSuperFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
+    self, AdtDef, Discr, Term, Ty, TyCtxt, TypeFlags, TypeSuperVisitable, TypeVisitable,
+    TypeVisitableExt, TypeVisitor,
 };
 use crate::ty::{List, ParamEnv};
 use hir::def::DefKind;
@@ -1156,81 +1156,6 @@ where
     }
 }
 
-struct SkipBindersAt<'tcx> {
-    tcx: TyCtxt<'tcx>,
-    index: ty::DebruijnIndex,
-}
-
-impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for SkipBindersAt<'tcx> {
-    type Error = ();
-
-    fn interner(&self) -> TyCtxt<'tcx> {
-        self.tcx
-    }
-
-    fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, Self::Error>
-    where
-        T: ty::TypeFoldable<TyCtxt<'tcx>>,
-    {
-        self.index.shift_in(1);
-        let value = t.try_map_bound(|t| t.try_fold_with(self));
-        self.index.shift_out(1);
-        value
-    }
-
-    fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
-        if !ty.has_escaping_bound_vars() {
-            Ok(ty)
-        } else if let ty::Bound(index, bv) = *ty.kind() {
-            if index == self.index {
-                Err(())
-            } else {
-                Ok(self.interner().mk_bound(index.shifted_out(1), bv))
-            }
-        } else {
-            ty.try_super_fold_with(self)
-        }
-    }
-
-    fn try_fold_region(&mut self, r: ty::Region<'tcx>) -> Result<ty::Region<'tcx>, Self::Error> {
-        if !r.has_escaping_bound_vars() {
-            Ok(r)
-        } else if let ty::ReLateBound(index, bv) = r.kind() {
-            if index == self.index {
-                Err(())
-            } else {
-                Ok(self.interner().mk_re_late_bound(index.shifted_out(1), bv))
-            }
-        } else {
-            r.try_super_fold_with(self)
-        }
-    }
-
-    fn try_fold_const(&mut self, ct: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
-        if !ct.has_escaping_bound_vars() {
-            Ok(ct)
-        } else if let ty::ConstKind::Bound(index, bv) = ct.kind() {
-            if index == self.index {
-                Err(())
-            } else {
-                Ok(self.interner().mk_const(
-                    ty::ConstKind::Bound(index.shifted_out(1), bv),
-                    ct.ty().try_fold_with(self)?,
-                ))
-            }
-        } else {
-            ct.try_super_fold_with(self)
-        }
-    }
-
-    fn try_fold_predicate(
-        &mut self,
-        p: ty::Predicate<'tcx>,
-    ) -> Result<ty::Predicate<'tcx>, Self::Error> {
-        if !p.has_escaping_bound_vars() { Ok(p) } else { p.try_super_fold_with(self) }
-    }
-}
-
 /// Represents the projection of an associated type.
 ///
 /// For a projection, this would be `<Ty as Trait<...>>::N`.

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -9,7 +9,6 @@ use crate::infer::InferCtxt;
 use crate::traits::project::ProjectAndUnifyResult;
 use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_middle::mir::interpret::ErrorHandled;
-use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{ImplPolarity, Region, RegionVid};
 
@@ -851,23 +850,3 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         infcx.freshen(p)
     }
 }
-
-/// Replaces all ReVars in a type with ty::Region's, using the provided map
-pub struct RegionReplacer<'a, 'tcx> {
-    vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
-    tcx: TyCtxt<'tcx>,
-}
-
-impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for RegionReplacer<'a, 'tcx> {
-    fn interner(&self) -> TyCtxt<'tcx> {
-        self.tcx
-    }
-
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        (match *r {
-            ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
-            _ => None,
-        })
-        .unwrap_or_else(|| r.super_fold_with(self))
-    }
-}