Auto merge of #97701 - compiler-errors:🅱-remove-arg-matrix, r=estebank

[beta] Revert arg matrix algorithm from `check_argument_types`

We decided in T-compiler meeting that this is best removed in beta, and reworked to be less ICE-y on nightly: https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202022-06-02/near/284755523

r? `@jackh726` `@estebank`

compiler/rustc_typeck/src/check/expr.rs

@@ -1416,13 +1416,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) {
         let tcx = self.tcx;
 
-        let expected_inputs =
-            self.expected_inputs_for_expected_output(span, expected, adt_ty, &[adt_ty]);
-        let adt_ty_hint = if let Some(expected_inputs) = expected_inputs {
-            expected_inputs.get(0).cloned().unwrap_or(adt_ty)
-        } else {
-            adt_ty
-        };
+        let adt_ty_hint = self
+            .expected_inputs_for_expected_output(span, expected, adt_ty, &[adt_ty])
+            .get(0)
+            .cloned()
+            .unwrap_or(adt_ty);
         // re-link the regions that EIfEO can erase.
         self.demand_eqtype(span, adt_ty_hint, adt_ty);
 

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -755,9 +755,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected_ret: Expectation<'tcx>,
         formal_ret: Ty<'tcx>,
         formal_args: &[Ty<'tcx>],
-    ) -> Option<Vec<Ty<'tcx>>> {
+    ) -> Vec<Ty<'tcx>> {
         let formal_ret = self.resolve_vars_with_obligations(formal_ret);
-        let ret_ty = expected_ret.only_has_type(self)?;
+        let Some(ret_ty) = expected_ret.only_has_type(self) else { return vec![]; };
 
         // HACK(oli-obk): This is a hack to keep RPIT and TAIT in sync wrt their behaviour.
         // Without it, the inference
@@ -779,7 +779,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 if let ty::subst::GenericArgKind::Type(ty) = ty.unpack() {
                     if let ty::Opaque(def_id, _) = *ty.kind() {
                         if self.infcx.opaque_type_origin(def_id, DUMMY_SP).is_some() {
-                            return None;
+                            return vec![];
                         }
                     }
                 }
@@ -820,7 +820,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
                 // Record all the argument types, with the substitutions
                 // produced from the above subtyping unification.
-                Ok(Some(formal_args.iter().map(|&ty| self.resolve_vars_if_possible(ty)).collect()))
+                Ok(formal_args.iter().map(|&ty| self.resolve_vars_if_possible(ty)).collect())
             })
             .unwrap_or_default();
         debug!(?formal_args, ?formal_ret, ?expect_args, ?expected_ret);