address review

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -325,6 +325,13 @@ pub(crate) fn clone_project(&self, elem: PlaceElem<'tcx>) -> Self {
         }
     }
 
+    /// Similar to `Place::ty` but needed during mir building.
+    ///
+    /// Applies the projections in the `PlaceBuilder` to the base
+    /// type.
+    ///
+    /// Fallible as the root of this place may be an upvar for
+    /// which no base type can be determined.
     pub fn try_compute_ty<D>(
         &self,
         local_decls: &D,

compiler/rustc_mir_build/src/build/matches/util.rs

@@ -54,10 +54,20 @@ pub(crate) fn field_match_pairs_tuple_struct<'pat>(
                             let variant_idx = opt_variant_idx.unwrap();
                             adt_def.variant(variant_idx).fields[field_idx].ty(self.tcx, substs)
                         }
-                        ty::Adt(adt_def, substs) => {
-                            adt_def.all_fields().collect::<Vec<_>>()[field_idx].ty(self.tcx, substs)
-                        }
-                        ty::Tuple(elems) => elems.to_vec()[field_idx],
+                        ty::Adt(adt_def, substs) => adt_def
+                            .all_fields()
+                            .nth(field_idx)
+                            .unwrap_or_else(|| {
+                                bug!(
+                                    "expected to take field idx {:?} of fields of {:?}",
+                                    field_idx,
+                                    adt_def
+                                )
+                            })
+                            .ty(self.tcx, substs),
+                        ty::Tuple(elems) => elems.iter().nth(field_idx).unwrap_or_else(|| {
+                            bug!("expected to take field idx {:?} of {:?}", field_idx, elems)
+                        }),
                         _ => bug!(
                             "no field available, place_ty: {:#?}, kind: {:?}",
                             place_ty,