Folding revamp.

This commit makes type folding more like the way chalk does it.

Currently, `TypeFoldable` has `fold_with` and `super_fold_with` methods.
- `fold_with` is the standard entry point, and defaults to calling
  `super_fold_with`.
- `super_fold_with` does the actual work of traversing a type.
- For a few types of interest (`Ty`, `Region`, etc.) `fold_with` instead
  calls into a `TypeFolder`, which can then call back into
  `super_fold_with`.

With the new approach, `TypeFoldable` has `fold_with` and
`TypeSuperFoldable` has `super_fold_with`.
- `fold_with` is still the standard entry point, *and* it does the
  actual work of traversing a type, for all types except types of
  interest.
- `super_fold_with` is only implemented for the types of interest.

Benefits of the new model.
- I find it easier to understand. The distinction between types of
  interest and other types is clearer, and `super_fold_with` doesn't
  exist for most types.
- With the current model is easy to get confused and implement a
  `super_fold_with` method that should be left defaulted. (Some of the
  precursor commits fixed such cases.)
- With the current model it's easy to call `super_fold_with` within
  `TypeFolder` impls where `fold_with` should be called. The new
  approach makes this mistake impossible, and this commit fixes a number
  of such cases.
- It's potentially faster, because it avoids the `fold_with` ->
  `super_fold_with` call in all cases except types of interest. A lot of
  the time the compile would inline those away, but not necessarily
  always.

compiler/rustc_trait_selection/src/opaque_types.rs

@@ -7,7 +7,7 @@ use rustc_hir::OpaqueTyOrigin;
 use rustc_infer::infer::error_reporting::unexpected_hidden_region_diagnostic;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt as _};
 use rustc_infer::traits::{Obligation, ObligationCause, TraitEngine};
-use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts};
 use rustc_middle::ty::{self, OpaqueHiddenType, OpaqueTypeKey, ToPredicate, Ty, TyCtxt};
 use rustc_span::Span;

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -6,7 +6,7 @@ use super::*;
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::InferCtxt;
 use crate::traits::project::ProjectAndUnifyResult;
-use rustc_middle::ty::fold::TypeFolder;
+use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{Region, RegionVid, Term};
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};

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

@@ -27,7 +27,7 @@ use rustc_infer::traits::TraitEngine;
 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::error::ExpectedFound;
-use rustc_middle::ty::fold::TypeFolder;
+use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{
     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
 };

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -18,7 +18,9 @@ use rustc_errors::{FatalError, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
-use rustc_middle::ty::{self, EarlyBinder, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{
+    self, EarlyBinder, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor,
+};
 use rustc_middle::ty::{Predicate, ToPredicate};
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;

compiler/rustc_trait_selection/src/traits/project.rs

@@ -29,7 +29,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
 use rustc_middle::traits::select::OverflowError;
-use rustc_middle::ty::fold::{MaxUniverse, TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{MaxUniverse, TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, EarlyBinder, Term, ToPredicate, Ty, TyCtxt};
 use rustc_span::symbol::sym;
@@ -514,7 +514,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                             self.selcx.infcx().report_overflow_error(&obligation, true);
                         }
 
-                        let substs = substs.super_fold_with(self);
+                        let substs = substs.fold_with(self);
                         let generic_ty = self.tcx().bound_type_of(def_id);
                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
                         self.depth += 1;
@@ -531,8 +531,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                 // placeholders (see branch below). *Also*, we know that we can
                 // register an obligation to *later* project, since we know
                 // there won't be bound vars there.
-
-                let data = data.super_fold_with(self);
+                let data = data.fold_with(self);
                 let normalized_ty = if self.eager_inference_replacement {
                     normalize_projection_type(
                         self.selcx,
@@ -581,7 +580,7 @@ impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
                 let infcx = self.selcx.infcx();
                 let (data, mapped_regions, mapped_types, mapped_consts) =
                     BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
-                let data = data.super_fold_with(self);
+                let data = data.fold_with(self);
                 let normalized_ty = opt_normalize_projection_type(
                     self.selcx,
                     self.param_env,
@@ -671,7 +670,7 @@ impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
             universe_indices,
         };
 
-        let value = value.super_fold_with(&mut replacer);
+        let value = value.fold_with(&mut replacer);
 
         (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
     }
@@ -794,7 +793,7 @@ impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
             universe_indices,
             current_index: ty::INNERMOST,
         };
-        value.super_fold_with(&mut replacer)
+        value.fold_with(&mut replacer)
     }
 }
 

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -12,7 +12,7 @@ use rustc_data_structures::sso::SsoHashMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::traits::Normalized;
 use rustc_middle::mir;
-use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder};
+use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
 
@@ -205,7 +205,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                     Reveal::UserFacing => ty.try_super_fold_with(self),
 
                     Reveal::All => {
-                        let substs = substs.try_super_fold_with(self)?;
+                        let substs = substs.try_fold_with(self)?;
                         let recursion_limit = self.tcx().recursion_limit();
                         if !recursion_limit.value_within_limit(self.anon_depth) {
                             let obligation = Obligation::with_depth(
@@ -242,7 +242,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 // we don't need to replace them with placeholders (see branch below).
 
                 let tcx = self.infcx.tcx;
-                let data = data.try_super_fold_with(self)?;
+                let data = data.try_fold_with(self)?;
 
                 let mut orig_values = OriginalQueryValues::default();
                 // HACK(matthewjasper) `'static` is special-cased in selection,
@@ -281,7 +281,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                         &mut self.universes,
                         data,
                     );
-                let data = data.try_super_fold_with(self)?;
+                let data = data.try_fold_with(self)?;
 
                 let mut orig_values = OriginalQueryValues::default();
                 // HACK(matthewjasper) `'static` is special-cased in selection,
@@ -334,7 +334,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
         &mut self,
         constant: mir::ConstantKind<'tcx>,
     ) -> Result<mir::ConstantKind<'tcx>, Self::Error> {
-        let constant_kind = match constant {
+        Ok(match constant {
             mir::ConstantKind::Ty(c) => {
                 let const_folded = c.try_fold_with(self)?;
                 match const_folded.val() {
@@ -347,8 +347,6 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                 }
             }
             mir::ConstantKind::Val(_, _) => constant.try_super_fold_with(self)?,
-        };
-
-        Ok(constant_kind)
+        })
     }
 }

compiler/rustc_trait_selection/src/traits/structural_match.rs

@@ -6,7 +6,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::lang_items::LangItem;
 use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt, TypeFoldable, TypeVisitor};
+use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor};
 use rustc_span::Span;
 use std::ops::ControlFlow;