Make diverging_type_vars a Vec

It being a `Set` does not really make sense. You never really should do a `contains` on it, since you should normalize the tyvid to its root var first.

Author: JonathanBrouwer

compiler/rustc_hir_typeck/src/fallback.rs

@@ -211,7 +211,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         let diverging_roots: UnordSet<ty::TyVid> = self
             .diverging_type_vars
             .borrow()
-            .items()
+            .iter()
             .map(|&ty_id| self.shallow_resolve(Ty::new_var(self.tcx, ty_id)))
             .filter_map(|ty| ty.ty_vid())
             .map(|vid| self.root_var(vid))

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -322,7 +322,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             match a.kind {
                 Adjust::NeverToAny => {
                     if let ty::Infer(ty::TyVar(a_id)) = a.target.kind() {
-                        self.diverging_type_vars.borrow_mut().insert(*a_id);
+                        self.diverging_type_vars.borrow_mut().push(*a_id);
                         debug!("apply_adjustments: adding `{:?}` as diverging type var", a.target);
                     }
                 }

compiler/rustc_hir_typeck/src/typeck_root_ctxt.rs

@@ -1,7 +1,6 @@
 use std::cell::{Cell, RefCell};
 use std::ops::Deref;
 
-use rustc_data_structures::unord::UnordSet;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::{self as hir, HirId, HirIdMap};
 use rustc_infer::infer::{InferCtxt, InferOk, OpaqueTypeStorageEntries, TyCtxtInferExt};
@@ -66,7 +65,7 @@ pub(crate) struct TypeckRootCtxt<'tcx> {
     /// Whenever we introduce an adjustment from `!` into a type variable,
     /// we record that type variable here. This is later used to inform
     /// fallback. See the `fallback` module for details.
-    pub(super) diverging_type_vars: RefCell<UnordSet<TyVid>>,
+    pub(super) diverging_type_vars: RefCell<Vec<TyVid>>,
 }
 
 impl<'tcx> Deref for TypeckRootCtxt<'tcx> {