Reverse folder hierarchy

#91318 introduced a trait for infallible folders distinct from the fallible version.  For some reason (completely unfathomable to me now that I look at it with fresh eyes), the infallible trait was a supertrait of the fallible one: that is, all fallible folders were required to also be infallible.  Moreover the `Error` associated type was defined on the infallible trait!  It's so absurd that it has me questioning whether I was entirely sane.

This trait reverses the hierarchy, so that the fallible trait is a supertrait of the infallible one: all infallible folders are required to also be fallible (which is a trivial blanket implementation).  This of course makes much more sense!  It also enables the `Error` associated type to sit on the fallible trait, where it sensibly belongs.

There is one downside however: folders expose a `tcx` accessor method.  Since the blanket fallible implementation for infallible folders only has access to a generic `F: TypeFolder`, we need that trait to expose such an accessor to which we can delegate.  Alternatively it's possible to extract that accessor into a separate `HasTcx` trait (or similar) that would then be a supertrait of both the fallible and infallible folder traits: this would ensure that there's only one unambiguous `tcx` method, at the cost of a little additional boilerplate.  If desired, I can submit that as a separate PR.

r? @jackh726

compiler/rustc_infer/src/infer/resolve.rs

@@ -92,7 +92,7 @@ fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
                     .borrow_mut()
                     .unwrap_region_constraints()
                     .opportunistic_resolve_var(rid);
-                self.tcx().reuse_or_mk_region(r, ty::ReVar(resolved))
+                TypeFolder::tcx(self).reuse_or_mk_region(r, ty::ReVar(resolved))
             }
             _ => r,
         }
@@ -179,15 +179,13 @@ struct FullTypeResolver<'a, 'tcx> {
     infcx: &'a InferCtxt<'a, 'tcx>,
 }
 
-impl<'a, 'tcx> TypeFolder<'tcx> for FullTypeResolver<'a, 'tcx> {
+impl<'a, 'tcx> FallibleTypeFolder<'tcx> for FullTypeResolver<'a, 'tcx> {
     type Error = FixupError<'tcx>;
 
     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
         self.infcx.tcx
     }
-}
 
-impl<'a, 'tcx> FallibleTypeFolder<'tcx> for FullTypeResolver<'a, 'tcx> {
     fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         if !t.needs_infer() {
             Ok(t) // micro-optimize -- if there is nothing in this type that this fold affects...

compiler/rustc_middle/src/ty/fold.rs

@@ -241,58 +241,37 @@ fn super_fold_with<F: TypeFolder<'tcx, Error = !>>(self, folder: &mut F) -> Self
 /// a blanket implementation of [`FallibleTypeFolder`] will defer to
 /// the infallible methods of this trait to ensure that the two APIs
 /// are coherent.
-pub trait TypeFolder<'tcx>: Sized {
-    type Error = !;
-
+pub trait TypeFolder<'tcx>: FallibleTypeFolder<'tcx, Error = !> {
     fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
 
     fn fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Binder<'tcx, T>
     where
         T: TypeFoldable<'tcx>,
-        Self: TypeFolder<'tcx, Error = !>,
     {
         t.super_fold_with(self)
     }
 
-    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx>
-    where
-        Self: TypeFolder<'tcx, Error = !>,
-    {
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
         t.super_fold_with(self)
     }
 
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx>
-    where
-        Self: TypeFolder<'tcx, Error = !>,
-    {
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
         r.super_fold_with(self)
     }
 
-    fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx>
-    where
-        Self: TypeFolder<'tcx, Error = !>,
-    {
+    fn fold_const(&mut self, c: ty::Const<'tcx>) -> ty::Const<'tcx> {
         c.super_fold_with(self)
     }
 
-    fn fold_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ty::Unevaluated<'tcx>
-    where
-        Self: TypeFolder<'tcx, Error = !>,
-    {
+    fn fold_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ty::Unevaluated<'tcx> {
         uv.super_fold_with(self)
     }
 
-    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx>
-    where
-        Self: TypeFolder<'tcx, Error = !>,
-    {
+    fn fold_predicate(&mut self, p: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
         p.super_fold_with(self)
     }
 
-    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx>
-    where
-        Self: TypeFolder<'tcx, Error = !>,
-    {
+    fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'tcx> {
         bug!("most type folders should not be folding MIR datastructures: {:?}", c)
     }
 }
@@ -304,7 +283,11 @@ fn fold_mir_const(&mut self, c: mir::ConstantKind<'tcx>) -> mir::ConstantKind<'t
 /// A blanket implementation of this trait (that defers to the relevant
 /// method of [`TypeFolder`]) is provided for all infallible folders in
 /// order to ensure the two APIs are coherent.
-pub trait FallibleTypeFolder<'tcx>: TypeFolder<'tcx> {
+pub trait FallibleTypeFolder<'tcx>: Sized {
+    type Error;
+
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
+
     fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, Self::Error>
     where
         T: TypeFoldable<'tcx>,
@@ -350,8 +333,14 @@ fn try_fold_mir_const(
 // delegates to infallible methods to ensure coherence.
 impl<'tcx, F> FallibleTypeFolder<'tcx> for F
 where
-    F: TypeFolder<'tcx, Error = !>,
+    F: TypeFolder<'tcx>,
 {
+    type Error = !;
+
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+        TypeFolder::tcx(self)
+    }
+
     fn try_fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Result<Binder<'tcx, T>, Self::Error>
     where
         T: TypeFoldable<'tcx>,

compiler/rustc_middle/src/ty/normalize_erasing_regions.rs

@@ -228,15 +228,13 @@ fn try_normalize_generic_arg_after_erasing_regions(
     }
 }
 
-impl<'tcx> TypeFolder<'tcx> for TryNormalizeAfterErasingRegionsFolder<'tcx> {
+impl<'tcx> FallibleTypeFolder<'tcx> for TryNormalizeAfterErasingRegionsFolder<'tcx> {
     type Error = NormalizationError<'tcx>;
 
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.tcx
     }
-}
 
-impl<'tcx> FallibleTypeFolder<'tcx> for TryNormalizeAfterErasingRegionsFolder<'tcx> {
     fn try_fold_ty(&mut self, ty: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
         match self.try_normalize_generic_arg_after_erasing_regions(ty.into()) {
             Ok(t) => Ok(t.expect_ty()),

compiler/rustc_middle/src/ty/subst.rs

@@ -705,7 +705,7 @@ fn shift_vars_through_binders<T: TypeFoldable<'tcx>>(&self, val: T) -> T {
             return val;
         }
 
-        let result = ty::fold::shift_vars(self.tcx(), val, self.binders_passed);
+        let result = ty::fold::shift_vars(TypeFolder::tcx(self), val, self.binders_passed);
         debug!("shift_vars: shifted result = {:?}", result);
 
         result

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

@@ -12,7 +12,7 @@
 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, TypeSuperFoldable};
+use rustc_middle::ty::fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable};
 use rustc_middle::ty::subst::Subst;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
 
@@ -162,15 +162,13 @@ struct QueryNormalizer<'cx, 'tcx> {
     universes: Vec<Option<ty::UniverseIndex>>,
 }
 
-impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
+impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
     type Error = NoSolution;
 
     fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
         self.infcx.tcx
     }
-}
 
-impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
     fn try_fold_binder<T: TypeFoldable<'tcx>>(
         &mut self,
         t: ty::Binder<'tcx, T>,