Remove InferCtxt::freshener.

I have always found this confusingly named, because it creates a new
freshener rather than returning an existing one. We can remove it and
just use `TypeFreshener::new()` at the two call sites, avoiding this
confusion.

compiler/rustc_infer/src/infer/mod.rs

@@ -638,7 +638,7 @@ impl<'tcx> InferCtxt<'tcx> {
     }
 
     pub fn freshen<T: TypeFoldable<TyCtxt<'tcx>>>(&self, t: T) -> T {
-        t.fold_with(&mut self.freshener())
+        t.fold_with(&mut TypeFreshener::new(self))
     }
 
     /// Returns the origin of the type variable identified by `vid`.
@@ -658,10 +658,6 @@ impl<'tcx> InferCtxt<'tcx> {
         }
     }
 
-    pub fn freshener<'b>(&'b self) -> TypeFreshener<'b, 'tcx> {
-        freshen::TypeFreshener::new(self)
-    }
-
     pub fn unresolved_variables(&self) -> Vec<Ty<'tcx>> {
         let mut inner = self.inner.borrow_mut();
         let mut vars: Vec<Ty<'_>> = inner

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

@@ -195,7 +195,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
         SelectionContext {
             infcx,
-            freshener: infcx.freshener(),
+            freshener: TypeFreshener::new(infcx),
             intercrate_ambiguity_causes: None,
             query_mode: TraitQueryMode::Standard,
         }