Remove two fields from SubstFolder.

They're only used in error messages printed if there's an internal
compiler error, and the cost of maintaining them is high enough to show
up in profiles.

src/librustc_middle/ty/subst.rs

@@ -425,8 +425,7 @@ impl<'tcx, T: TypeFoldable<'tcx>> Subst<'tcx> for T {
         substs: &[GenericArg<'tcx>],
         span: Option<Span>,
     ) -> T {
-        let mut folder =
-            SubstFolder { tcx, substs, span, root_ty: None, ty_stack_depth: 0, binders_passed: 0 };
+        let mut folder = SubstFolder { tcx, substs, span, binders_passed: 0 };
         (*self).fold_with(&mut folder)
     }
 }
@@ -441,12 +440,6 @@ struct SubstFolder<'a, 'tcx> {
     /// The location for which the substitution is performed, if available.
     span: Option<Span>,
 
-    /// The root type that is being substituted, if available.
-    root_ty: Option<Ty<'tcx>>,
-
-    /// Depth of type stack
-    ty_stack_depth: usize,
-
     /// Number of region binders we have passed through while doing the substitution
     binders_passed: u32,
 }
@@ -478,9 +471,8 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
                         let span = self.span.unwrap_or(DUMMY_SP);
                         let msg = format!(
                             "Region parameter out of range \
-                             when substituting in region {} (root type={:?}) \
-                             (index={})",
-                            data.name, self.root_ty, data.index
+                             when substituting in region {} (index={})",
+                            data.name, data.index
                         );
                         span_bug!(span, "{}", msg);
                     }
@@ -495,25 +487,10 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
             return t;
         }
 
-        // track the root type we were asked to substitute
-        let depth = self.ty_stack_depth;
-        if depth == 0 {
-            self.root_ty = Some(t);
-        }
-        self.ty_stack_depth += 1;
-
-        let t1 = match t.kind {
+        match t.kind {
             ty::Param(p) => self.ty_for_param(p, t),
             _ => t.super_fold_with(self),
-        };
-
-        assert_eq!(depth + 1, self.ty_stack_depth);
-        self.ty_stack_depth -= 1;
-        if depth == 0 {
-            self.root_ty = None;
         }
-
-        t1
     }
 
     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
@@ -540,12 +517,11 @@ impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
                 span_bug!(
                     span,
                     "expected type for `{:?}` ({:?}/{}) but found {:?} \
-                     when substituting (root type={:?}) substs={:?}",
+                     when substituting, substs={:?}",
                     p,
                     source_ty,
                     p.index,
                     kind,
-                    self.root_ty,
                     self.substs,
                 );
             }
@@ -554,11 +530,10 @@ impl<'a, 'tcx> SubstFolder<'a, 'tcx> {
                 span_bug!(
                     span,
                     "type parameter `{:?}` ({:?}/{}) out of range \
-                     when substituting (root type={:?}) substs={:?}",
+                     when substituting, substs={:?}",
                     p,
                     source_ty,
                     p.index,
-                    self.root_ty,
                     self.substs,
                 );
             }